Essential Reproduction E-Book

Martin H. Johnson

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Serie: essentials
• Sprache: Englisch

Seitenzahl: 1318

Veröffentlichungsjahr: 2018

Beschreibung

Essential Reproduction provides an accessible account of the fundamentals of reproduction within the context of cutting-edge knowledge and examples of its application. The eighth edition of this internationally best-selling title provides a multidisciplinary approach integrating anatomy, physiology, genetics, behaviour, biochemistry, molecular biology and clinical science, to give thorough coverage of the study of mammalian reproduction.

Key features:

• Contains discussion of the latest on conceptual, informational and applied aspects of reproduction
• New pedagogical features such as clinical case studies at the end of each chapter
• Better use of boxed material to improve separation of narrative text from ancillary information
• Highlighted key words for ease of reference relate to summary of key points
• Introduction now split into two sections
• Expanded content in Fetal challenges, and Society and reproduction
• Substantial rearrangement and updating in Making sperm, Controlling fertility, and Restoring fertility

Leseprobe

Table of Contents

Cover

Title Page

Contributors

Preface

How to use this book

Acknowledgements

About the companion website

Part 1: Introduction

CHAPTER 1: What is reproduction?

Reproductive strategies

Reproductive life cycles

Conclusions

CHAPTER 2: The infrastructure of reproduction

The reproductive hardware

The brain, hypothalamus and pituitary

Reproductive messengers

General features of reproductive hormones

Conclusions

Part 2: Making women and men

CHAPTER 3: Sex

The genetic determinant of sex is located on the Y chromosome

When, where and how does Sry act?

Sexual differentiation within the gonads

Sexually dimorphic somatic differentiation depends on the endocrine activity of the fetal testis

Conclusions

CHAPTER 4: Sexual maturation

Prepubertal development

Puberty and adolescence

A distinctive pattern of hormonal changes underlies puberty

The central nervous system plays a key role in the onset of puberty

Conclusions

CHAPTER 5: Gender

Gender is a system of classification based on sex

Gender stereotypes and gender identities

The brain and behavioural dimorphism

Might the underlying basis for sex and gender differences in humans be hormonal?

Gender development may form part of social learning in humans

Gender and reproduction

Conclusions

CHAPTER 6: Sexual selection

Sexuality

Eliciting male sexual interest and arousal

Female sexual behaviour

The brain, hormones and sexual behaviour

Social factors influence sexual behaviour in higher primates

Selecting sexual partners

Conclusions

Part 3: Preparing for pregnancy

CHAPTER 7: Making sperm

Testicular organization

Spermatogenesis has three main phases

Spermatogenesis is highly organized both temporally and spatially

Conclusions

CHAPTER 8: Men

Testicular hormones

Spermatogenesis is dependent on endocrine support

Testicular hormones modulate the output of pituitary hormones

Androgens play an essential role in the fecund male

Conclusions

CHAPTER 9: Making eggs

The adult ovary consists of follicles and interstitial tissue

The follicle is the fundamental reproductive element of the ovary

Ovulation

The corpus luteum is the postovulatory ‘follicle’

Conclusions

CHAPTER 10: Women

Follicular development and the ovarian cycle

Ovarian hormones regulate gonadotrophin secretion

Feedback by steroid hormones and the inhibins regulates the menstrual cycle

Positive and negative feedback are mediated at the level of both the hypothalamus and pituitary

The ovarian cycle in relation to the oestrous and menstrual cycles

Conclusions

Part 4: Making an embryo

CHAPTER 11: Sperm and eggs

Spermatozoa require a period of epididymal maturation

Semen is made up of spermatozoa and seminal plasma

Coition

Gamete transport through the female genital tract

Conclusions

CHAPTER 12: Fertilization

Spermatozoa gain their full fertilizing capacity in the female tract

Penetrating the egg investments

Spermatozoal–oocyte interactions

Fertilization completion

Anomalous fertilization

Conclusions

CHAPTER 13: Initiating pregnancy

The preimplantation conceptus

Implantation

The molecular conversations at implantation

Summary

The prolongation of luteal life

Conclusions

Part 5: Maintaining a pregnancy

CHAPTER 14: Supporting the embryo and fetus

Nutritional strategies

Endocrine support strategies

Conclusions

CHAPTER 15: Growing the fetus

Patterns of fetal growth

Placental transport

Conclusions

CHAPTER 16: Fetal challenges

Pregnancy loss

Fetomaternal immune relations

Fetal hypoxia

Developmental programming

Conclusions

Part 6: A new individual

CHAPTER 17: Preparing for birth

Fetal systems develop and mature in preparation for postnatal life

Fetal hormones orchestrate development and preparations for birth

Conclusions

CHAPTER 18: Giving birth

Parturition

Labour

Endocrine control of parturition

Atypical births

Conclusions

CHAPTER 19: Lactation

Lactation

The milk ejection reflex (MER)

Fertility is reduced during lactation

Cessation of lactation

Milk

Conclusions

CHAPTER 20: Postnatal care

Patterns of maternal behaviour vary amongst mammals

Genetics and maternal care

Conclusions

Part 7: Manipulating reproduction

CHAPTER 21: Controlling fertility

Fertility, fecundibility and fecundity

Artificial control of fertility

Pregnancy termination

Conclusions

CHAPTER 22: Restoring fertility

Causes of subfertility

Approaches to subfertility treatment

Assisted reproductive technologies (ART)

Conclusions

CHAPTER 23: Society and reproduction

Social constraints on fertility

Biological constraints on fertility

Society and the infertile

Reproduction, sexuality and ethics

Conclusions

Index

End User License Agreement

List of Tables

Chapter 02

Table 2.1 Principal properties of natural progestagens and their receptors

Table 2.2 Principal properties of natural androgens

Table 2.3 Principal properties of natural oestrogens

Table 2.4 Properties of human gonadotrophins

Table 2.5 Properties of the human somatomammotrophic polypeptides

Table 2.6 Half‐lives of some hormones in the blood

Table 2.7 Steroid‐binding proteins in human plasma

Chapter 03

Table 3.1 Effect of human chromosome constitution on the development of the gonad

Chapter 05

Table 5.1 Sex and gender: commonly used distinguishing descriptors

Chapter 06

Table 6.1 A classification system for sexualities

Chapter 07

Table 7.1 Kinetics of spermatogenesis

Chapter 08

Table 8.1 Relative size of principal male accessory sex glands

Chapter 09

Table 9.1 Duration of phases of follicular development in the non‐pregnant animal

Table 9.2 Human follicular development

Table 9.3 Luteotrophic/antiluteolytic hormones for the non‐pregnant corpus luteum of different species

Chapter 10

Table 10.1 Ovarian cyclicity in different species

Chapter 11

Table 11.1 Maturational changes to spermatozoa in the epididymis

Table 11.2 Composition of ejaculate of man and domestic animals

Table 11.3 Estimates of the survival of viable fertile gametes in the female genital tract

Chapter 12

Table 12.1 Incidence of chromosomal abnormalities in spontaneously aborted human conceptions

Chapter 13

Table 13.1 Times (in days) after ovulation at which various developmental and maternal events occur

Table 13.2 Comparison of the durations (in days post‐fertilization) of the main developmental phases in five mammals (modified from Johnson and Selwood, 1996)

Table 13.3 Classification of implantation and placental forms in several species

Chapter 14

Table 14.1 Dependence of pregnancy on maternal ovarian and pituitary function in various species

Table 14.2 Plasma levels of various steroids in women during late pregnancy

Table 14.3 Major sites of hormone synthesis during established pregnancy: a comparison of different species

Chapter 15

Table 15.1 O

2

and CO

2

composition of human maternal and fetal blood

Table 15.2 Compositions of amniotic fluid in early and late pregnancy and of the full‐term maternal and fetal serum

Chapter 16

Table 16.1 Adult diseases that have been associated with suboptimal intrauterine conditions

Chapter 17

Table 17.1 Summary of functions of the glucocorticosteroids secreted by the fetal adrenal cortex towards term (for details see text)

Chapter 19

Table 19.1 Summary of events leading to full lactation in the human

Table 19.2 Some contents of human mature milk

Chapter 21

Table 21.1 Reported pregnancy rates and patterns of usage for various forms of contraception

Table 21.2 The wide variation in teenage pregnancies across the world (%)

Table 21.3 Condom use

Table 21.4 Steroid‐based contraceptives for women

Chapter 22

Table 22.1 Steroidogenesis in the postmenopausal woman. Change from resting plasma concentrations (%)

Table 22.2 WHO (2013) characteristics associated with ‘normal’ and ‘subfertile’ semen

Chapter 23

Table 23.1 Proposed genetic markers for some of the most common and high impact reproductive conditions to affect men and women

Table 23.2 Data from a randomized control trial comparing elective single transfer (eSET) followed where unsuccessful by a frozen embryo transfer (FrET) versus double (DET) embryo transfer

List of Illustrations

Chapter 01

Figure 1.1 Mitosis and meiosis in human cells. Each human cell contains 23 pairs of homologous chromosomes, making 46 chromosomes in total. Each set of 23 chromosomes is called a

haploid

set. When a cell has two complete sets, it is described as being

diploid

. In this figure, we show at the top a single schematized human cell with just two of the 23 homologous pairs of chromosomes illustrated, each being individually colour‐coded. Between divisions, the cell is in

interphase

, during which it grows and duplicates both its

centriole

and the DNA in each of its chromosomes. As a result of the DNA replication, each chromosome consists of two identical

chromatids

joined at the

centromere

. Interphase chromosomes are not readily visible, being long, thin and decondensed (but are shown in this figure in a more condensed form for simplicity of representation). Lower left panel: In

mitotic prophase

, the two chromatids become distinctly visible under the light microscope as each shortens and thickens by a spiralling contraction; at the end of prophase the

nucleoli and nuclear membrane

break down. In

mitotic metaphase

, microtubules form a

mitotic spindle

between the two centrioles and the chromosomes lie on its

equator

. In

mitotic anaphase

, the centromere of each chromosome splits and the two chromatids in each chromosome migrate to opposite poles of the spindle (

karyokinesis

). During

mitotic telophase

division of the cytoplasm into two daughters (known as

cytokinesis

) along with breakdown of the spindle and the reformation of nuclear membranes and nucleoli occurs, as does the decondensation of chromosomes so that they are no longer visible under the light microscope. Two genetically identical daughter cells now exist where one existed before. Mitosis is a non‐sexual or vegetative form of reproduction. Lower right panel:

Meiosis

involves two sequential divisions. The

first meiotic prophase (prophase 1)

is lengthy and can be divided into several sequential steps: (1)

leptotene

chromosomes are long and thin; (2) during

zygotene

, homologous pairs of chromosomes from each haploid set come to lie side by side along parts of their length; (3) in

pachytene

, chromosomes start to thicken and shorten and become more closely associated in pairs along their entire length at which time

synapsis, crossing over

and

chromatid exchange

take place and nucleoli disappear; (4) in

diplotene

and

diakinesis

, chromosomes shorten further and show evidence of being closely linked to their homologue at the

chiasmata

where crossing over and the reciprocal exchange of DNA sequences have occurred, giving a looped or cross‐shaped appearance. In

meiotic metaphase 1

, the nuclear membrane breaks down, and homologous pairs of chromosomes align on the equator of the spindle. In

meiotic anaphase 1

, homologous chromosomes move in opposite directions. In

meiotic telophase 1

, cytokinesis occurs; the nuclear membrane may re‐form temporarily, although this does not always happen, yielding two daughter cells each with half the number of chromosomes (only one member of each homologous pair), but each chromosome consisting of two genetically unique chromatids (because of the crossing‐over at chiasmata). In the

second meiotic division

, these chromatids then separate much as in mitosis, to yield a total of four haploid offspring from the original cell, each containing only one complete set of chromosomes. Due to chromatid exchange and the random segregation of homologous chromosomes, each haploid cell is genetically unique. At fertilization, two haploid cells will come together to yield a new diploid zygote.

Figure 1.2 Numbers of ovarian germ cells during the life of a human female from conception. Note the steady rise early in fetal life followed by a precipitous decline prior to birth and shortly afterwards.

Figure 1.3 Cartoon to show differences between mammalian and frog eggs. From bottom up: frog and mouse; the relative sizes of each egg (diameters); the numbers recoverable from each at a single ovulation; the transition achieved in the first 24 hours after fertilization. Note that human eggs are of very similar size to mouse and elephant eggs, despite giving rise to very different‐sized animals; all three are much smaller than frog eggs. This is because frog eggs must carry with them most of what they need to transform into swimming tadpoles that can then feed themselves, something that they do very rapidly! Mammalian eggs in contrast gain their nutrients for growth from the mother: largely through the placenta (see Chapters 14 and 15).

Figure 1.4 Female fecundity is ‘time limited’. Rates of fertility (red range) and childlessness (blue bars) by age of woman. The fertility rate data were collected from populations of married women who reported that no efforts were made to limit their fertility. These data approximate to a measure of fecundity by age in humans. Note the steep decline from 35 years. (The range reflects the different circumstances of the populations, drawn from different parts of the world.) The histograms show the proportions of women remaining childless after first marriage at the ages indicated despite continuing attempts to deliver a child. Note again the sharp rise above 35 years, implying a fall in fecundity from this time onwards until the menopause at around age 50 years.

Figure 1.5 The generative life cycle is shown in red and indicates the pluripotent lineage, whereas the somatic life cycle is in blue and ensures the survival and physical transmission of the germ cells across generations.

Figure 1.6 Schematic summary of the procedure for somatic cell nuclear transfer (SCNT) in sheep or mice. A differentiated cell is cultured and its division cycle arrested by removal of nutrients (G0 stage). A

karyoplast

(the nucleus with a small amount of cytoplasm and cell membrane surrounding it) is then prepared from the quiescent cell. It is placed next to an unfertilized oocyte from which its own genetic material has been removed by suction. A fusogenic signal is then given. The nucleus and enucleated oocyte fuse and initiate cleavage. The cleaving conceptus is placed into the mouse or ewe uterus and a viable offspring may result.

Figure 1.7 Summary of mechanisms available for genetic imprinting. Two sorts of epigenetic modification can leave an imprint. (a) The direct methylation of some, but not all, cytosines (see CpG island in lower part of panel) within the DNA sequence itself. This methylation then blocks access to transcriptional machinery – lower part of (a). Once initiated, this methylation pattern can be copied at each round of DNA replication as long as the maintenance methylase is present, and it is thus heritable through many mitoses. (b) A second sort of epigenetic modification is seen in the histone isotypes present in the chromatin surrounding the promoter region of the genes, as well as in their post‐translational modification (e.g. Ac = acetylation). Depending on the chromatin structure, the DNA can be organized in a ‘loose’ or open state, and so is accessible to the transcriptional machinery and available for expression, or packed tightly and repressed. See also Box 7.3 for discussion of this type of modification during chromatin reorganization during spermatogenesis. (c) Quite complex interactions may occur during development between these two types of epigenetic modification and associated transcriptional proteins. However, these are early days in the science of epigenesis and much remains to be understood (see Chapters 16 and 20 for further discussion of the importance of epigenetic imprinting in health and disease).

Chapter 02

Figure 2.1 A schematic view of the main anatomical structures associated with the reproductive system. The hypothalamic region (H) sits at the base of the brain and is connected to the pituitary gland (P). The pituitary gland communicates hormonally with a range of reproductive organs and tissues including the breasts (see Chapter 19), the uterus and cervix (see Chapters 9, 10 and 18), and the gonads: testis and ovary (see Chapters 4, 7 and 8). The gonads themselves in turn communicate hormonally with the pituitary and the brain (see Chapters 8 and 10 for details). The gonads also influence hormonally multiple sites in the internal and external genitalia (see Chapters 3, 4, 8, 10, 18 and 19).

Figure 2.2 A sagittal section through the human brain with the pituitary and pineal glands attached. Note the comparatively small size of the hypothalamus and its rather compressed dimensions ventrally. The pineal is attached by its stalk to the epithalamus (habenula region) and lies above the midbrain colliculi. The third ventricle is a midline slit‐like structure, which has been opened up by the midline cut exposing the medial surface of the brain. The thalamus (above) and the hypothalamus (below) form one wall (the right in this view) of the third ventricle, which are better viewed in Figure 2.4.

Figure 2.3 A highly schematic and enlarged view of the human hypothalamus and pituitary. Note the portal capillary system (red) derived from the superior hypophyseal artery and running from the median eminence/arcuate nucleus region of the hypothalamus above to the anterior lobe of the pituitary below. The anatomically and functionally well‐defined paraventricular, ventromedial and arcuate nuclei contain the cell bodies of parvocellular neurons whose axons (blue) terminate in close association with the portal capillaries. Parvocellular neurons also arise in the less well‐defined anterior hypothalamic/preoptic areas continuum and send axons to the portal plexus. Magnocellular neurons (green) are also located in the paraventricular nuclei, with a second group in the supraoptic nuclei, and send axons along the infundibulum to the posterior pituitary.

Figure 2.4 Three coronal sections at different anterior–posterior levels of the human hypothalamus (consult Figure 2.3 to construct the planes of sections). (a) Through the optic chiasm, note the third ventricle in the midline flanked by the anterior paraventricular (magnocellular) nuclei, which, together with the laterally placed supraoptic nuclei, synthesize oxytocin and vasopressin. The latter are then transported along the hypothalamo–hypophyseal tract (axons of neurons with cell bodies in these nuclei) to the posterior pituitary. The region of the suprachiasmatic nuclei and the anterior hypothalamic–preoptic area (AHA–POA) are also shown. (b) Through the infundibulum and median eminence, and showing the relationship between the parvocellular arcuate and ventromedial nuclei (VMN). The capillary loops of the portal plexus are found in this region. (c) Through the level of the mammillary bodies and showing the mammillary nuclear complex. The area labelled LAT HYP in all three sections is the lateral hypothalamus, and is composed of many nerve fibres ascending (largely aminergic) from the brainstem and descending from the rostral limbic and olfactory areas. This pathway represents a major input/output system for the more medially placed hypothalamic nuclei.

Figure 2.5 (a) Schematic representation of the magnocellular neurosecretory system. Neurons in the supraoptic and paraventricular nuclei send their axons via the hypothalamo–hypophyseal tract and infundibulum to the neurohypophysis, where terminals lie in association with capillary walls, the site of neurosecretion. (b) Schematic representation of the parvocellular GnRH neurosecretory system. Neurons in the medial preoptic and anterior hypothalamic areas, and in the arcuate nucleus send axons down to the portal vessels in the external layer (palisade zone) of the median eminence, where neurosecretion occurs.

Figure 2.6 (a) GnRH detected by immunofluorescence in the median eminence (middle region). Note the very dense network of GnRH neuron terminals in the lateral region (lateral palisade zone), but no fluorescent cell bodies are visible. (b) Tyrosine hydroxylase immunofluorescence in an adjacent section through the median eminence. The dense terminal fluorescence in the external layer largely represents dopamine (although noradrenaline‐containing terminals will also be labelled by this procedure). Large dopamine‐containing cell bodies are clearly visible in the arcuate nucleus.

Figure 2.7 Basic structure of the cholesterol molecule. Each of the 27 carbon atoms is assigned a number and each ring a letter. The individual carbon atoms are simply carrying hydrogen atoms unless otherwise indicated (e.g. C‐1 and C‐19 are –CH

2

– and –CH

3

residues, whereas C‐3 is –CHOH–). Cholesterol is converted to pregnenolone by cleavage of the terminal six carbons, leaving a steroid nucleus of 21 carbons. The conversion occurs on the inner mitochondrial membrane and requires NADPH, oxygen and cytochrome P‐450. Pregnenolone is then converted to the sex steroids in the adjacent smooth endoplasmic reticulum.

Figure 2.8 Pathway of interconversion of steroids. Some of the principal enzymes involved are indicated, although the scheme is very simplified; many of them have multiple isoforms. Note that extensive interconversions are possible; however, in any given tissue, absence of certain enzymes will mean that only parts of the matrix will be completed. Clearly, the enzymes present will therefore determine which steroids can and cannot be made from which available substrates. The members of the four major classes of sex steroids are shown with distinct colour bars: progestagens, red; androgens, green; oestrogens, yellow; and corticosteroids, blue. Structurally, natural progestagens are characterized by 21 carbons (C

21

steroids), a double bond between C4 and C5, a β‐acetyl at C17 and a β‐methyl at C13. Natural androgens are characterized by 19 carbons (C19 steroids); strong androgenic activity is associated with a β‐hydroxylated C17 and a ketone structure (–C = O) at C3. Natural oestrogens have 18 carbons (C18 steroids), an aromatized A ring hydroxylated at C3, and a β‐hydroxyl group at C17. Note that in steroid terminology the suffixes denote the following: ‐ol = hydroxyl group; ‐diol = two hydroxyl groups; ‐one = ketone group; ‐dione = two ketone groups. An unsaturated –C–C‐link is indicated by ‐ene; two such links are indicated by ‐diene. Note also that aromatase denotes 19‐hydroxysteroid dehydrogenase + C‐10,19 lyase.

Figure 2.9 Receptor activation and second messengers. Ligands can influence cell function, and ultimately nuclear gene expression, in a variety of ways. Here we summarize in a very simplified form a very complex and rapidly evolving field of study. In doing so we refer to some ligands that will appear later in the text. (1) Steroid hormones, being lipid soluble, pass freely into cell nuclei where they bind receptors (R), displacing associated stabilizing proteins, such as HSP90, leading to receptor activation by phosphorylation; the activated complex can then bind to steroid‐specific response elements (SREs) in the DNA and to transcription factors (TF) to activate steroid‐specific genes. (2) Some ligands (prolactin, growth hormone, placental lactogen, LIF, leptin, TNFα) cross‐link two receptor chains (as hetero‐ or homo‐dimers). The cross‐linked complex then activates the Jak‐Stat cytosolic kinase cascade, resulting in either the phosphorylation of TFs in the cytoplasm and their translocation to the nucleus, or the translocation of the kinases themselves to the nucleus where they phosphorylate and activate TFs. Hormonal ligands that act in this way can cross‐link overlapping spectra of receptor monomers to induce overlapping spectra of downstream cascades, so accounting for some of the redundancy seen amongst peptide ligands. (3) Other ligands, including growth factors of the EGF (EGF and HB‐EGF), insulin (insulin, IGF 1 and 2, relaxin) and TGFβ 9 (Amh, activin, inhibin and BMPs) families bind to, and multimerize, receptors that

are themselves kinases

(K). Multimerization leads to kinase activation and both autophosphorylation of the receptor (–P) and phosphorylation of cytoplasmic factors (CF–P), and thereby to a cascade of kinase activity culminating in TF activation. Again, there is some overlap of downstream kinases and their targets both within this class of ligand–receptor interaction and between this class and class 2 above. The EGF and insulin family receptors are tyrosine kinases and are dimerized on ligand binding. Their activation then triggers numerous downstream second messenger pathways including phospholipase Cγ, ras/MAP kinases, and multiple STAT isoforms. The TGFβ family members form a complex with two type II receptors and then recruit two type I receptors, all four receptors having serine/threonine kinase activity. The activated kinases phosphorylate the receptors themselves and the intracellular signalling Smad proteins. The activated Smads translocate to the nucleus to bind with co‐factors to response elements in cytokine target gene promoters. (4) Some ligands (LH, FSH, CG, GnRH, oxytocin, arginine vasopressin) bind to receptors which then associate with G proteins (G). G proteins can then act in at least two ways: (i) to stimulate phospholipase Cβ (PLC‐β) to hydrolyze phosphatidylinositol phosphate (PIP

2

) to 1,4,5‐triphosphate (IP

3

) and diacylglycerol (DAG), which release Ca

2+

and activate protein kinase C (PKC) respectively; or (ii) to modulate activity of adenyl cyclase (AC) and so the output of cAMP. (5) Some ligands (EGF) may activate phospholipase Cγ (PLC‐γ) directly, without G‐protein mediation.

Figure 2.10 (a) Biosynthetic pathway (from top downwards) and molecular structures of the catecholamines: dopamine, noradrenaline and adrenaline.

Dopamine

is released from terminals in the hypothalamus and reaches the anterior pituitary where it is an important modulator of prolactin secretion (see Box 2.3). (b) Structure of the indolamine = melatonin. Melatonin is synthesized from serotonin (5‐hydroxytryptamine) in the

pineal gland

and is under environmental control. It is released into the bloodstream and exerts important actions within the hypothalamus in order to regulate reproductive activity in seasonally breeding mammals (see Box 4.3).

Figure 2.11 GnRH is a decapeptide derived by cleavage from a larger precursor called prepro‐GnRH. The upper blue bar shows the structure of the human cDNA for prepro‐GnRH (molecular weight 10 000), which comprises the decapeptide GnRH preceded by a signal sequence of 23 amino acids and followed by a Gly–Lys–Arg sequence necessary for enzymatic processing and C‐terminal amidation of GnRH. The C‐terminal region of the precursor is occupied by a further 56 amino acids that constitute the so‐called GnRH‐associated peptide or

GAP

, the function of which, if any, is unknown. The amino acid sequence of the GnRH is shown in the lower part of the figure. Recently, a second GnRH gene has been identified (

GnRH‐II

) differing at three amino acids from

GnRH‐I

(pGlu–His–Trp–Ser–

His

–Gly–

Trp–Tyr

–Pro–Gly). There is a suggestion that it may preferentially release FSH, but the evidence is unclear.

Figure 2.12 Oxytocin, like GnRH, is also derived from a larger precursor (a) consisting of (central yellow bar): a leader sequence (S); the oxytocin nonapeptide sequence (OT); a Gly–Lys–Arg linker sequence, serving the same function as described for GnRH in the legend to Figure 2.11, and a neurophysin sequence. Neurophysin is an intracellular protein able to bind oxytocin, and in this way the hormone is packaged for transport along the axon from the hypothalamus to the terminals of the neurons in the posterior pituitary. Neurophysin is released, along with oxytocin, into the bloodstream, but it has no clear function in the body. (b) The nine amino acids comprising oxytocin contain a hexapeptide ‘ring’ and a tripeptide ‘tail’. (c) A schematized structure of a hypothalamic neurosecretory neuron indicating the cellular location of the various synthetic and processing stages that result in the release of oxytocin (OT) and neurophysin (NP) from neurohypophyseal terminals.

Chapter 03

Figure 3.1 Karyotypes of two mitotic human cells: one male and one female. Each cell was placed in colchicine, a drug that arrested them in mitotic metaphase when the chromosomes were condensed and clearly visible (see Figure 1.1). The chromosomes were stained and then classified according to the so‐called ‘

Denver’ system

. The 44 autosomes (22 pairs of homologues) are grossly similar in size in each sex, but the pair of sex chromosomes are distinguishable by size, being XX (both large) in the female and XY (one large, one small) in the male. After meiotic division, all four female cells (only two shown) contain one X chromosome : the homogametic sex. In contrast, two of the male cells each contain an X chromosome and two contain a Y chromosome: the heterogametic sex.

Figure 3.2 A 3‐week human embryo showing: (a) the origin of the primordial germ cells and (b) the route of their migration. Section Q is the plane of transverse section through the lumbar region shown at 4 weeks in (c). In (d) the same plane of section is shown at 5 weeks of development: the ‘indifferent gonad’ stage.

Figure 3.3 Testicular development during (a) week 8 and (b) weeks 16–20 of human embryo–fetal life. (a) The primitive sex cords proliferate into the medulla, establish contact with the mesonephric medullary cords of the rete testis blastema and become separated from the coelomic epithelium by the

tunica albuginea

(fibrous connective tissue), which eventually forms the

testicular capsule

. (b) Note the horseshoe shape of the seminiferous cords and their continuity with the rete testis cords. The vasa efferentia, derived from the mesonephric tubules, connect the seminiferous cords with the Wolffian duct. Comparable diagrams of ovarian development around (c) week 7 and (d) weeks 20–24 of development. (c) The primitive sex cords are less well organized and more cortical, while medullary mesonephric cords are absent or degenerate. The cortical coelomic epithelial cells condense around the arriving primordial germ cells to yield primordial follicles, as shown in (d). In the absence of medullary cords and a persistent

rete ovarii

, no communication is established with the mesonephric tubules. Hence, in the adult, oocytes are shed from the surface of the ovary and are not transported by tubules to the oviduct (compare with the male, see Chapters 9 and 11).

Figure 3.4 Sections through immature (a) testis and (b) ovary, both at the same magnification. Note in (a) that each tubule is surrounded by a basement membrane (BM), and within the tubule there is no lumen (T) and a relatively homogeneous‐looking set of cells comprising a very few spermatogonial stem cells and mostly Sertoli cells, as seen here. Note in (b) that each oocyte (O) is relatively large and contains within it a distinctive nucleus (the germinal vesicle, GV). The ooctye is surrounded by a thin layer of follicle granulosa cells (FC) to form the primordial follicle.

Figure 3.5 Differentiation of the internal genitalia in the human male (left) and female (right) at: (a) week 6; (b) the fourth month; and (c) the time of descent of the testis and ovary. Note the paramesonephric Müllerian and mesonephric Wolffian ducts are present in both sexes early on, the former eventually regressing in the male and persisting in the female, and vice versa. The

appendix testis

and

utriculus prostaticus

in the male, and

epoophoron, paroophoron

and

Gartner’s cyst

in the female are thought to be remnants of the degenerated Müllerian and Wolffian ducts, respectively.

Figure 3.6 Differentiation of the external genitalia in the human female (left) and male (right) from common primordia shown at: (a) 4 weeks; and (b) 6 weeks. (c) In the female, the

labia minora

form from the urethral folds and the genital tubercle elongates to form the

clitoris

. (d) Subsequent changes by the fifth month are more pronounced in the male, with enlargement of the genital tubercle to form the

glans penis

and fusion of the urethral folds to enclose the urethral tube and form the

shaft of the penis

(the genital swellings probably also contribute cells to the shaft). (e) The definitive external genitalia of the female at birth. (f) The definitive external genitalia of the male at birth.

Figure 3.7 (a) External genitalia of an XY adult with complete androgen insensitivity syndrome (testicular feminization). Affected individuals are phenotypic females with normal external genitalia, breasts and a female gender identity. On examination, these women are found to have an XY chromosome constitution, abdominal testes and blood levels of androgens in the male range. Examination of the usual androgen target tissues of these women reveals an absence or deficiency of androgen receptors. (b) The external genitalia from XX girls with adrenogenital syndrome show varying degrees of masculinization, from an enlarged clitoris to development of a small penis and (empty) scrotum. Ovaries are present internally. The adrenal cortex has inappropriately secreted androgens at the expense of glucocorticoids during fetal life and directed development of the genitalia along the male line. For example, not uncommon genetic deficiencies in the fetal adrenal gland are reduced activity of 21α‐hydroxylase (which converts 17α‐hydroxyprogesterone to 11‐desoxycortisol; Figure 2.8) or of 11β‐hydroxylase (which converts 11‐desoxycortisol to cortisol). In either case, there is a resulting deficiency of corticosteroids, which leads to further compensatory stimulation of the corticosteroid biosynthetic path (congenital adrenal hyperplasia), which in turn leads to the accumulation of high levels of 17α‐hydroxyprogesterone. This steroid is then converted by 17,20‐desmolase to androgen, which masculinizes female fetuses. Clearly, the more severe cases could lead to sex assignment as a boy or to indecision. Conversely, genetic deficiency of 17,20‐desmolase itself results in depressed androgen output, and the failure of male fetuses to masculinize (see Chapter 5, Gender stereotypes and gender identities).

Chapter 04

Figure 4.1 (a–c) Parasagittal sections and (d, e) front view of the testis migration through a developing male abdomen. The initial retroperitoneal, abdominal position of the testis shifts pelvically between 10 and 15 weeks (a to b), extending the blood supply (and Wolffian duct derivatives, not shown) as the gubernaculum shortens and the suspensory ligament, shown in (d) connecting the testis to the posterior abdominal wall, lengthens and then regresses (e). A musculofascial layer (b) evaginates into the scrotal swelling accompanied by peritoneal membrane, forming the

processus vaginalis

. Between weeks 25 and 28 of pregnancy in the human (c), the testis migrates over the pubic bone behind the processus vaginalis (which then wraps around it forming a double‐layered sac), reaching the scrotum by weeks 35–40. The fascia and peritoneum become closely apposed above the testis, obliterating the peritoneal cavity and leaving only a

tunica vaginalis

(c) around the testis. The fascial layers, the obliterated stem of the processus vaginalis, the vas deferens and testicular vessels and nerves form the

spermatic cord

(c). (d, e) show the extended course ultimately taken by the testicular vessels and vas deferens.

Figure 4.2 Section through an adult human testis to show: (a) general structure; (b) arterial supply; and (c) venous drainage.

Figure 4.3 Circulating FSH and LH levels in the plasma (averaged from weekly collections from three male rhesus monkeys) over the first 3 years of life (0, day of birth to puberty onset). LH and FSH pulses occur during the first 20 weeks or so of life when plasma levels are in the adult range. Thereafter secretion ceases and levels remain low or undetectable through childhood and the juvenile stages until about week 120. The first visible change is an increase in FSH secretion followed soon after by increasing pulsatile secretion of LH. These early gonadotrophin changes are mirrored in androgen elevation early on.

Figure 4.4 Growth velocity curves for boys and girls. Note the later time of ‘take‐off’ in boys, which generally ensures a greater height at the start of the adolescent growth spurt. Also note that average peak height velocity is 9 cm/year for girls and 10 cm/year for boys.

Figure 4.5 Stages of female breast development (regulated by oestrogen): 1, preadolescent stage during which the papilla (nipple) alone is elevated; 2, breast bud stage in which the papilla and breast are elevated as a small mound and the areolar area increases; 3, continued enlargement of the breast and areola, but without separation in their contours; pigmentation increases; 4, further breast enlargement but with the papilla and areola projecting above the breast contour; 5, mature stage in which the areola has become recessed, and forms a smooth contour with the rest of the breast – only the papilla is elevated. Classification of this stage is independent of breast size, which is determined principally by genetic and nutritional factors.

Figure 4.6 Stages of pubic hair development in girls (regulated by adrenal androgen): 1, no pubic hair is visible; 2, sparse growth of long, downy hair which is only slightly curled and situated primarily along the labia; 3, appearance of coarser, curlier and often darker hair; 4, hair spreads to cover labia; 5, hair spreads more over the junction of the pubes and is now adult in type but not quantity; no spread to the medial surface of the thighs; 6, adult stage in which the classical ‘inverse triangle’ of pubic hair distribution is seen, with additional spread to the medial surface of the thighs. Pubic hair maturation is accompanied by apocrine odour development, and skin oiliness and acne.

Figure 4.7 Stages of external genitalia development in boys (regulated by androgens): 1, preadolescent stage during which penis, testes and scrotum are of similar size and proportion as in early childhood; 2, scrotum and testes have enlarged; texture of the scrotal skin has also changed and become slightly reddened; pubic hair is at the base of the penis and downy; 3, testes and scrotum have grown further but now the penis has increased in size: first in length and then in breadth; facial hair appears for the first time on the upper lip and cheeks, and pubic hair is longer and more extensively distributed; 4, further enlargement of the testes, scrotum (which has darkened in colour) and penis; the glans penis has now begun to develop; 5, adult stage; facial hair has now extended to lower lip and chin. Hair on the chest, back, abdomen and more of the face is genetically variable and starts appearing 3 or so years after stage 5 is achieved.

Figure 4.8 Schematic of five boys all aged 14 years illustrating marked individual differences in physical maturation at the same chronological age.

Figure 4.9 Summary of the sequence of events during puberty in (a) girls and (b) boys. The figures below each symbol represent the range of ages within which each event may begin and end. The figures within each symbol refer to the stages illustrated in Figures 4.5–4.7.

Figure 4.10 Plasma LH concentrations over a 24‐hour period in: (a) a prepubertal girl (9 years); (b) an early pubertal boy (15 years); (c) a late pubertal boy (16 years); and (d) a young adult male. The sleep pattern for each nocturnal sleep period is depicted in the top left‐hand corner of each graph (REM, rapid eye movement or ‘paradoxical’ sleep). Note the marked daily rhythm in (b), with sleep‐augmented LH secretion, and the overall higher LH concentrations in (d) compared with (a), but no clear daily rhythm in either. Note that prior to the juvenile quiescent phase in higher primates, low frequency pulsing of GnRH and gonadotrophins is observed, but is switched off at the end of the infant phase, and, interestingly, is switched off for longer and more completely in boys than in girls.

Figure 4.11 Plasma concentrations of gonadotrophins and steroid hormones during various pubertal stages in (a) girls and (b) boys. Bone age is assessed by examining radiographs of the hand, knee and elbow, for comparison with standards of maturation in a normal population. It is an index of physical maturation, and better correlated with the development of secondary sexual characteristics than chronological age.

Figure 4.12 Induction of premature menstrual cycles in an immature female rhesus monkey by the infusion of GnRH (1 mg/min for 6 minutes once every hour) shown by the horizontal bar (day 0–110); levels of LH, SH, oestradiol and progesterone were undetectable in blood samples prior to GnRH infusions, but rose to produce a full menstrual cycle each with a period of menstrual flow (M). Cessation of GnRH infusions was followed by prompt re‐entry into a non‐cyclic, prepubertal state.

Figure 4.13 Schematic diagram to show some of the postulated neurochemical interactions that may control GnRH secretion. In primates, GnRH neurons lie in the medial preoptic area (blue) and arcuate nucleus (purple) and project to the portal vessels in the median eminence, especially to the lateral palisade zone (see Figures 2.4–2.6). γ‐Aminobutyric acid (GABA) neurons (yellow) in the preoptic and mediobasal hypothalamic areas and opioid β‐endorphin neurons (green) richly innervate the medial preoptic area GnRH‐containing neurons to influence output of GnRH. Dopamine neurons (mauve) in the arcuate nucleus modulate prolactin release (and may affect GnRH output, but this is controversial). Noradrenergic and adrenergic neurons (black) in the medulla oblongata project to the medial anterior hypothalamus and preoptic area, and may also participate in the regulation of GnRH secretion (see Box 4.2).

Figure 4.14 Secular trend towards an earlier age at menarche in girls from Western Europe and the USA.

Figure 4.15 Age plotted against body weight in three populations of girls in 1835, 1895 and 1947. Note the constant weights at initiation of the growth spurt (30 kg) and at menarche (47 kg).

Chapter 05

Figure 5.1 Sex‐dependent behaviour patterns in male and female rats. Note the immobile lordosis posture shown by the receptive female, which enables the male to mount and achieve intromissions, which will result in ejaculation. Receptivity and lordosis are shown predominantly by females; mounting, intromission and ejaculation patterns of behaviour are shown predominantly by males.

Figure 5.2 Photomicrographs of coronal sections through the preoptic area of three 21‐day‐old gerbils (

Meriones unguiculatus

). Section (a) is taken from a male, (b) from a female and (c) from a female treated neonatally with androgens (testosterone propionate: 50 mg on the day of birth and 50 mg the next day). The sexually dimorphic area in the gerbil (SDA) can be divided into several regions: medial (mSDA); lateral (lSDA); and pars compacta (SDApc). The SDA differs between males and females in a number of aspects: prominence (not necessarily size); acetylcholinesterase histochemistry; steroid binding; and various other neurochemical characteristics, but most obviously in the presence or absence of the SDApc. Thus, the SDApc is virtually never found in females (compare a with b). Note that in females treated neonatally with testosterone, there is a clear SDApc (compare c with b). These pictures provide clear evidence of the impact of hormones during a critical period of early life on the differentiation of this part of the brain. The medial preoptic area in general is closely involved with the regulation of sexual behaviour (see Chapter 6), and some progress has been made in relating specific aspects of sexual behaviour to subdivisions of the SDA. It is also important to note that such sex differences in the structure of the preoptic area are found in many species, from rats to humans, but the precise details of the dimorphism vary considerably.

Figure 5.3 Frequency of ‘rough‐and‐tumble play’ during the first, second and third years of life of a male rhesus monkey (red circles), female rhesus monkey (blue squares) and female rhesus monkey treated prenatally with androgens (green circles). Note that males display this behaviour at a higher frequency than females and that androgenized females are intermediate.

Figure 5.4 Sexually dimorphic patterns of mounting behaviour in young rhesus monkeys. (a) Early in life, both males and females show immature mounts by standing on the cage floor. (b) During development, males show progressively more mature mounts in which they clasp the female’s calves so that she supports his weight entirely. Androgenized females display more of the latter type of mature mounts than do untreated females.

Figure 5.5 (a, b) Two coronal sections through the hypothalamus from a presumed heterosexual male to show the four INAH nuclei. In (a) INAH1 and 2 are seen, with INAH3 emerging within the area of the medial preoptic/anterior hypothalamic nucleus (MP‐AHN). INAH3 and 4 are prominent in the posterior section (b), close to a cell group comprising the anterior hypothalamic nucleus (AH) and the bed nucleus of the stria terminalis (ST) (PVN, paraventricular nucleus; oc, optic chiasm; SON, supraoptic nucleus; v, third ventricle). (c) A summary of data comparing INAH3 in men and women; * significant differences. The study also compared these data with those for the brains of 14 homosexual men, with values of 0.096 mm

3

, 0.069 mm

3

/g, 18792/mm

3

and 1831 respectively – none differing significantly from heterosexual men.

Figure 5.6 The scatter of sizes of the central bed nucleus of the stria terminalis (cBST) in human adult males, females and trans males (male‐to‐female transgendered). Error bars ± SEM. Note the sex difference and that the trans male nuclear volume is closer to the female pattern.

Figure 5.7 Mean values for morphometric volumes of different brain features in developing males (blue) and females (red) between the ages 6 and 20 years (upper and lower 95% confidence intervals shown). Curves a, b, c and f differ significantly in height and shape; curve d differs in height only.

Chapter 06

Figure 6.1 Effects of castration and testosterone replacement on patterns of ejaculatory behaviour in the male rat. Note that appreciable levels of the behaviour persist for some weeks after castration and that a comparable time is required for restoration of the behaviour following the subcutaneous implantation of testosterone.

Figure 6.2 Differential testosterone requirements for ‘maintaining’ ejaculatory behaviour in newly castrated rats and ‘restoring’ the same behaviour in long‐term castrates. Note that more testosterone propionate was required in the latter group; treatment, as in Figure 6.1, was given for several weeks to effect this change.

Figure 6.3 The effects of testosterone replacement in a hypogonadal man aged 40 years and castrated 1 year earlier for testicular neoplasia. (Red bars = sexual activity to orgasmic ejaculation; white bars = incomplete sexual activity). Note that about 3 weeks after stopping testosterone treatment, sexual activity, sexual thoughts and energy all declined, but never ceased entirely. There was no response to placebo, but a response within 1 or 2 weeks of restarting testosterone treatment. As with studies on animals, the nature of the stimulus and the response must be considered carefully when interpreting these data. An interesting issue in studies of sexual behaviour in men is whether erections occur as a result of sexual arousal, or whether they also contribute to its development. Is penile erection a response or a stimulus? Erections may be such an important indicator to men of sexual excitement that a decrease in erectile ability may contribute to a much higher threshold for sexual arousal and hence decreased sexual activity. A clear separation of genital and motivational responsiveness to androgens in hypogonadal men has simply not been achieved, but actions at both sites, as in rats, are probable.

Figure 6.4 Erectile response (measured as increase in penile diameter) to an erotic film and fantasy in hypogonadal men with and without testosterone replacement. The hypogonadal men did not differ from controls in their response to the film, but their response to fantasy was significantly lower than that for controls when they were androgen deficient, but improved with testosterone replacement. The latency of their erectile response was significantly reduced after hormone replacement. **

P

 < 0.01.

Figure 6.5 (a) Sexual interactions in rhesus monkeys, measured as mounts by the male, between male: female pairs during the menstrual cycle. Note the high levels of interaction, which peak at mid‐cycle and fall thereafter. The premenstrual rise is characteristic. This pattern of interaction seems to be more due to fluctuating interest in the female by the male than vice versa, as is revealed more clearly in (b), which summarizes the results of experiments demonstrating that: (A) in ovariectomized females, male sexual activity is low, as is male acceptance of sexual invitations by females; (B) treating females with oestradiol, by subcutaneous injections, increases both these parameters of the males’ behaviour; (C) the effect is lost if the oestradiol is smeared, as a cream, on the female’s perineum; (D) if it is placed in the vagina, an effect identical to that seen in (B) occurs. These data indicate that oestradiol increases sexual attractiveness by an action on the vagina.

Figure 6.6 Summary of effects on three male rhesus monkeys of a synthetic mixture of aliphatic acids applied to the perineal sexual skin of an ovariectomized female. Mounting behaviour is markedly stimulated during treatment, and ejaculations (E) occur consistently. Withdrawal of the mixture is followed by a reversal of these changes in the male sexual behaviour.

Figure 6.7 A block diagram summarizing the results of experiments on adrenal androgens and sexual behaviours in ovariectomized female rhesus monkeys receiving oestradiol benzoate throughout the experiment to ensure their attractiveness to the males. (a) Controls. (b) Removal of endogenously secreted androgens from the adrenal by suppression with dexamethasone or adrenalectomy with glucocorticoid replacement, caused large decreases in proceptive behaviour and initiation of sexual interaction by the female, and smaller decreases in receptive behaviour. (c) These decreases were reversed by treatment with testosterone propionate or androstenedione.

Figure 6.8 Plot of intercourse frequency averaged for 68 women (aged 24–35 years and having ligated tubes or IUCDs) relative to day of menstrual cycle (Day 0 = ovulation; 171 menstrual cycles analyzed). Solid line shows 3‐day moving average and dashed line shows mean values for each day. Horizontal line shows mean overall frequency.

Figure 6.9 The effects on women who had undergone ovariectomy of treatment with oestradiol, oestradiol plus androgen or no hormone. Levels of sexual desire, the frequency of sexual thoughts and level of sexual arousal are all significantly greater in the combined treatment group, indicating the impact of androgenic steroids on sexuality in women.

Figure 6.10 (a) The effects of testosterone implanted in the CNS on the sexual behaviour of castrated male rats. Testosterone placed in the preoptic–anterior hypothalamic continuum induced high levels of ejaculatory patterns, comparable to those observed following systemic treatment with the hormone. Testosterone placed in the posterior hypothalamus or elsewhere in the brain had no significant effect. Important controls were provided by the fact that cholesterol had no effect on sexual activity in any site, so arguing for some degree of hormonal specificity. More recently it has been shown that, at least in rats, oestradiol has similar effects. (b) Sites in the hypothalamus at which testosterone‐induced increases in sexual behaviour in male rats. AHA, anterior hypothalamic area; ARC, arcuate nucleus; DMN, dorsomedial nucleus; OT and OCH, optic tract and chiasm; POA, preoptic area; PVN, paraventricular nucleus; SCH, suprachiasmatic nucleus; VMN, ventromedial nucleus.

Figure 6.11 (a) A block diagram summarizing the results of experiments in which testosterone propionate was implanted in the CNS of androgen‐deprived female rhesus monkeys. All females were ovariectomized and received injections of oestradiol benzoate throughout the experiment, so that they remained attractive to males. Adrenalectomy was followed, as shown in Figure 6.7, by decreased levels of proceptive and receptive behaviour (compare A with B). These changes in sexual activity were reversed by placing testosterone in the anterior hypothalamus (C) but not in the posterior hypothalamus (D). Cholesterol in the anterior hypothalamus was without effect (E). (b) Diagram of a sagittal section of the rhesus monkey’s brain to show sites in the anterior hypothalamic area (♀) where testosterone implants reversed the behavioural effects of adrenalectomy in females (C).

Figure 6.12 Changes in sexual and aggressive behaviour and plasma testosterone levels in both a dominant and a subordinate male talapoin monkey following a 12‐week period in isolation, a 6‐week period in an all‐male group, and then a 7‐week period in a social group with oestradiol‐treated females. Note that only the dominant male’s plasma testosterone increases in the social group, and that only he is sexually active and being aggressive towards, but not receiving aggression from, other males. The subordinate male receives, but does not give, aggression and withdraws more frequently, especially when females are in the group. A, attacks; T, threats (both measures of aggressive behaviour); W, withdrawals (a measure of submissive behaviour). (Mounts and ejaculations are both measures of sexual behaviour.)

Figure 6.13 Sexual and aggressive behaviour received by dominant and subordinate female talapoin monkeys in a social group, and changes in plasma LH and prolactin levels when challenged with an oestradiol surge. (a) Increase in plasma oestradiol concentrations following the oestradiol surge. (b) Changes in plasma LH concentrations, which follow the oestradiol surge. (c) Plasma prolactin concentrations in the females. (d) Sexual interaction with males indicating mounts without ejaculation (red line) and mounts with ejaculations (green line). (e) Level of aggressive behaviour received by the females. Note that the dominant female receives high levels of sexual behaviour and low levels of aggression, has low plasma levels of prolactin and shows a surge of LH in response to an oestradiol surge. The converse is true of the subordinate female.

Chapter 07

Figure 7.1 Cross‐section through part of an adult testis to show the four compartments: vascular (V); interstitial (I), including the lymphatic vessels and containing the Leydig cells; basal (B); and adluminal (A). The latter two compartments lie within the seminiferous tubules. The interstitial and basal compartments are separated by an acellular basement membrane surrounded externally by myoid cells and invested with a loose coat of interstitial fibrocytes (see lower insert box). Myoid cells are linked to each other by punctate junctions. No blood vessels, lymphatic vessels or nerves traverse this boundary into the seminiferous tubule. Within the tubule, the basal and adluminal compartments are separated by rows of zonular tight, adherens and gap junctional complexes (see upper insert box), linking together adjacent Sertoli cells round their complete circumference and forming the

blood–testis barrier

. Intracellular to these junctional complexes are bundles of actin filaments running parallel to the surface around the ‘waist’ of the Sertoli cells, and internal to the filaments are cisternae of rough endoplasmic reticulum. Within the basal compartment are the spermatogonia, whilst spermatocytes, round and elongating spermatids and spermatozoa are in the adluminal compartment, in intimate contact with the Sertoli cells with which they form special junctions.

Figure 7.2 Relative concentrations of substances in the venous plasma (vascular compartment), lymph (interstitial compartment) and testicular fluid leaving the seminiferous tubules (adluminal compartment). Note that seminiferous tubule fluid differs markedly from plasma and lymph.

Figure 7.3 Section through immature testis. Note that each tubule is surrounded by a basement membrane (BM), and within the tubule there is no lumen (T) and a relatively homogeneous‐looking set of cells comprising a very few prospermatogonial stem cells and mostly Sertoli cells.

Figure 7.4 Cells in the mitotic proliferative phase of spermatogenesis in the rat (present in the basal intratubular compartment). From the population of proliferating spermatogonial stem cells, the so‐called transient amplifying progenitor cells or TAPCs, arise type A1 spermatogonia, which have large, ovoid, pale nuclei with a dusty, homogeneous chromatin. The intermediate spermatogonia have a crusty or scalloped chromatin pattern on their nuclear membranes, and this feature is heavily emphasized in type B spermatogonia, in which the nuclei are also smaller and rounded.

Figure 7.5 Progress of a rat primary spermatocyte through meiosis in the adluminal intratubular compartment. DNA synthesis is completed in the resting primary spermatocyte, although limited ‘repair DNA’ associated with crossing over occurs in late zygotene and early pachytene. In leptotene, the chromatin becomes filamentous as it condenses. In zygotene, homologous chromosomes thicken and come together in pairs (synapsis) attached to the nuclear membrane at their extremities, thus forming loops or ‘bouquets’. In pachytene, the pairs of chromosomes (bivalents) shorten and condense, and nuclear and cytoplasmic volume increases. It is at this stage that autosomal crossing over takes place (the two sex chromosomes are paired in the

sex vesicle, SV

). The synapses (S) can be seen at light‐microscopic level as chiasmata during diplotene and diakinesis, as the chromosomes start to pull apart and condense further. The nuclear membrane then breaks down, followed by spindle formation, and the first meiotic division is completed to yield two secondary spermatocytes each containing a single set of chromosomes. These are very short‐lived and rapidly enter the second meiotic division, the chromatids separating at the centromere to yield four haploid round spermatids.

Figure 7.6 Progress of a rat round spermatid through the packaging phase of the spermatogenic lineage. The Golgi apparatus (G) of the newly formed round spermatid (1) gives rise to glycoprotein‐rich lysosomal‐like granules, which coalesce to a single acrosomal vesicle (A) that grows over the nuclear surface to form a cap‐like structure (2). Between the acrosome and the nucleus, a subacrosomal cytoskeletal element, the perforatorium, forms in many species. The nuclear membrane at this site loses its nuclear pores. The two centrioles (C) lie against the opposite pole of the nuclear membrane, and a typical flagellum (F) (9 + 2 microtubules) grows outwards from the more distal centriole (2), while from the proximal centriole the neck or connecting piece forms, linking the tail to the nucleus. The nucleus moves with its attached acrosomal cap towards the cytoplasmic membrane and elongation begins (3 and 4). Chromatin condensation commences (Box 7.3) beneath the acrosomal cap, generating a nuclear shape, which is characteristic for the species (3–6), and superfluous nuclear membrane and nucleoplasm are lost. The Golgi apparatus detaches from the now completed acrosomal cap and moves posteriorly as the acrosome starts to change its shape. Nine coarse fibres form along the axis of the developing tail, each aligned with an outer microtubule doublet of the flagellum (see Figure 7.7 for details). In the final phase, the mitochondria migrate to the anterior part of the flagellum, and condense around it as a series of rods forming a spiral (see Figure 7.7). The superfluous cytoplasm appears ‘squeezed’ down the spermatid and is shed as the spermatozoa are released (hatched) (6). The mature spermatozoon has remarkably little cytoplasm left.

Figure 7.7 (a) Diagram of a primate spermatozoon (50 mm long) showing on the left, the main structural regions; and on the right, the boundaries between them. The surface membrane structure within each region is highly characteristic, having a unique lipid, sugar, surface charge and protein composition that differs from those in adjacent regions. These differences are probably maintained both by the inter‐regional boundary structures and by underlying molecular attachments to cytoskeletal elements. The differences are important functionally (see Chapters 11 and 12). (b) Sagittal section of the head, neck and top of the mid‐piece. Note the elongated (green) nucleus with highly compact chromatin and the acrosomal sac. The posterior end of the nuclear membrane is the only part to retain nuclear pores and forms the implantation fossa, which is connected to the capitulum by fine filaments. The capitulum, in turn, is connected to the outer dense fibres by two major and five minor segmented columns and is also the site of termination of the two central microtubules of the flagellum. The more distal of the two centrioles degenerates late in spermiogenesis. In a few species both centrioles are lost; they are not essential for sperm motility, only for the initial formation of the axoneme during spermiogenesis. (c) Sketch of the mid‐piece (surface membrane removed). Note the sheath of spiral mitochondria, and the axoneme of the tail comprising nine circumferential doublets of microtubules and two central microtubules; peripheral to each outer doublet is a dense fibre. (d) Section and sketch of the principal piece (surface membrane removed). The mitochondria are replaced by a fibrous sheath, comprising two longitudinal columns interconnected by ribs. The two fibrous sheath columns connect to underlying outer dense fibres 3 and 8. The outer dense fibres terminate towards the end of the principal piece, and the fibrous sheath then attaches directly to outer microtubules 3 and 8 before itself fading away in the end piece.

Figure 7.8 The top panel illustrates the passage of one rat spermatogonium through the spermatogenic process. The length of the block illustrating each cellular stage is proportional to the amount of time spent in that stage. When a new cell type arises by division, vertical solid grey lines separate adjacent blocks (M, mitosis). During meiotic prophase and spermiogenesis, however, cells change morphology by progressive differentiation, not quantal jumps. This continuum of change is indicated by the use of vertical broken lines to delineate blocks. A, In and B, spermatogonia; R, L, Z, P and Di, resting, leptotene, zygotene, pachytene and diplotene primary spermatocytes respectively; II, secondary spermatocytes. Each of the lower panels shows the history of other spermatozoa, which commenced development by cyclic generation of new type A spermatogonia from the stem cell population at progressively later time intervals. The interval between each of these events is about 12 days. Note that four such events occur before the upper spermatozoon (and its siblings in the family) have completed development and been released. As cells progress through spermatogenesis, they move progressively from the basal tubule to the luminal centre. Thus, several different cell types will be present in one cross‐section of a tubule at the same time, although at different points on the radial axis through the tubule. This feature is illustrated more compactly in Figure 7.9, in which the sections indicated by the dashed lines numbered 1–8 are summarized.

Figure 7.9 The sections indicated in Figure 7.8 are summarized here. Read from left to right. A vertical grey line between two cells in the sequence indicates a cell division; otherwise the changes are not quantal but occur by progressive differentiation. Abbreviations as for Figure 7.8. RPS, resting primary spermatocyte. (

Note

: because the spermatogenic process is continuous, it can be sub‐divided more or less finely. The sub‐division pattern used here is a basic one. A commonly used, more finely divided and thus more complex pattern uses 16 rather than eight sub‐divisions numbered I to XVI. A rough equivalence is stage 1: IX–XI, stage 2: XII–XIII, stage 3: XIV–XVI, stage 4: I, stage 5: II–IV, stage 6: V–VI, stage 7: VII, and stage 8; VIII.)

Figure 7.10 Cross‐sections through rat seminiferous tubules. (a–d) Four adjacent tubules from the same adult testis. Note that, within each tubule, the sets of cell associations along all radial axes are the same. However, each tubule has a different set of cell associations from its neighbour. Thus, tubule (a) is at stage 8/1 in Figures 7.8 and 7.9, tubule (b) is at stage 2, tubule (c) is at stage 5, and tubule (d) is at stage 7. Tubule (e) is from an adult rat testis 4 weeks after hypophysectomy. Note that spermatogenesis fails during the early meiotic stage with no cells more mature than a primary spermatocyte present. Note also the lack of a tubular lumen, indicating cessation of fluid secretion. Panel (f) shows staining of the intertubular region of an adult intact testis for the Leydig cells. Note their reddish foamy cytoplasm indicative of steroidogenesis.

Figure 7.11 Dissected seminiferous tubule from a rat testis. Note that whole segments of the tubule are at the same stage (numbered) of the cycle of the seminiferous epithelium, and that adjacent segments tend to be either just advanced or just retarded.

Chapter 08

Figure 8.1 Summary of the steroidogenic pathways in the human testis. The principal (Δ5) path for testosterone synthesis in the Leydig cells of the human is indicated by heavy lines, but the Δ4 pathway is also used and may be more important in other species. In addition to testosterone, some of the intermediates in the pathway are released into the blood: androstenedione at 10% and dehydroepiandrosterone at about 6% of testosterone levels in man. Some testosterone and androstenedione enter Sertoli cells. Here they may bind to androgen receptors directly or after conversion to the more potent dihydrotestosterone. Androgens may also be converted to oestrogens. In humans prepubertally, this occurs predominantly in the Sertoli cells, but postpubertally in the Leydig cells. Oestrogen also enters the seminiferous tubule fluid (see Chapter 11).

Figure 8.2 The biosynthesis of inhibins and activins occurs from three genes producing one

α‐preproprotein

(specific for inhibin) and

two β‐proproteins

(which can form part of either activin or inhibin). In each case, the N‐terminus (dark blue) is cleaved off and the subunit peptides are then linked in different combinations. Activins take three forms depending on the β chain composition:

activin A