Advances in Assisted Reproduction Technologies E-Book
69,68 €
Mehr erfahren.
- Herausgeber: Bentham Science Publishers
- Kategorie: Wissenschaft und neue Technologien
- Serie: Recent Advances in Biotechnology
- Sprache: Englisch
More than 4 decades have passed since the birth of the first in vitro fertilized baby in 1978. The use of assisted reproductive technology (ART) to overcome infertility has increased steadily with the simultaneous increase in the number of fertility centers in every part of the world. Access to infertility clinics is playing an important role in the treatment of different forms of infertility (like tubal disease, ovarian aging, or ovarian dysfunction).
This book captures the state of current and recent advances in assisted reproduction technology in humans and livestock in an easy and comprehensive way for non-experts and learners. 10 chapters cover the biology of reproduction, and male ART methods (sperm retrieval and freezing) and female ART methods (oocyte activation, and cryopreservation), and finally embryo ARTs (assisted hatching and cloning techniques) with simple definitions and explanations. Tips to overcome problems are also presented where appropriate along with references for further reading.
This book is a simple primer for students who are involved in courses in embryology or reproductive technologies as part of programs in biology, biotechnology, medicine and physiology.
Sie lesen das E-Book in den Legimi-Apps auf:
Seitenzahl: 458
Veröffentlichungsjahr: 2002
Ähnliche
BENTHAM SCIENCE PUBLISHERS LTD.
End User License Agreement (for non-institutional, personal use)
This is an agreement between you and Bentham Science Publishers Ltd. Please read this License Agreement carefully before using the ebook/echapter/ejournal (“Work”). Your use of the Work constitutes your agreement to the terms and conditions set forth in this License Agreement. If you do not agree to these terms and conditions then you should not use the Work.
Bentham Science Publishers agrees to grant you a non-exclusive, non-transferable limited license to use the Work subject to and in accordance with the following terms and conditions. This License Agreement is for non-library, personal use only. For a library / institutional / multi user license in respect of the Work, please contact: [email protected].
Usage Rules:
All rights reserved: The Work is the subject of copyright and Bentham Science Publishers either owns the Work (and the copyright in it) or is licensed to distribute the Work. You shall not copy, reproduce, modify, remove, delete, augment, add to, publish, transmit, sell, resell, create derivative works from, or in any way exploit the Work or make the Work available for others to do any of the same, in any form or by any means, in whole or in part, in each case without the prior written permission of Bentham Science Publishers, unless stated otherwise in this License Agreement.You may download a copy of the Work on one occasion to one personal computer (including tablet, laptop, desktop, or other such devices). You may make one back-up copy of the Work to avoid losing it.The unauthorised use or distribution of copyrighted or other proprietary content is illegal and could subject you to liability for substantial money damages. You will be liable for any damage resulting from your misuse of the Work or any violation of this License Agreement, including any infringement by you of copyrights or proprietary rights.Disclaimer:
Bentham Science Publishers does not guarantee that the information in the Work is error-free, or warrant that it will meet your requirements or that access to the Work will be uninterrupted or error-free. The Work is provided "as is" without warranty of any kind, either express or implied or statutory, including, without limitation, implied warranties of merchantability and fitness for a particular purpose. The entire risk as to the results and performance of the Work is assumed by you. No responsibility is assumed by Bentham Science Publishers, its staff, editors and/or authors for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products instruction, advertisements or ideas contained in the Work.
Limitation of Liability:
In no event will Bentham Science Publishers, its staff, editors and/or authors, be liable for any damages, including, without limitation, special, incidental and/or consequential damages and/or damages for lost data and/or profits arising out of (whether directly or indirectly) the use or inability to use the Work. The entire liability of Bentham Science Publishers shall be limited to the amount actually paid by you for the Work.
General:
Any dispute or claim arising out of or in connection with this License Agreement or the Work (including non-contractual disputes or claims) will be governed by and construed in accordance with the laws of Singapore. Each party agrees that the courts of the state of Singapore shall have exclusive jurisdiction to settle any dispute or claim arising out of or in connection with this License Agreement or the Work (including non-contractual disputes or claims).Your rights under this License Agreement will automatically terminate without notice and without the need for a court order if at any point you breach any terms of this License Agreement. In no event will any delay or failure by Bentham Science Publishers in enforcing your compliance with this License Agreement constitute a waiver of any of its rights.You acknowledge that you have read this License Agreement, and agree to be bound by its terms and conditions. To the extent that any other terms and conditions presented on any website of Bentham Science Publishers conflict with, or are inconsistent with, the terms and conditions set out in this License Agreement, you acknowledge that the terms and conditions set out in this License Agreement shall prevail.Bentham Science Publishers Pte. Ltd. 80 Robinson Road #02-00 Singapore 068898 Singapore Email: [email protected]
PREFACE
More than 4 decades have passed since the birth of the first in vitro fertilized baby in 1978. The use of assisted reproductive technology (ART) to overcome infertility has increased exponentially with the simultaneous increase in the number of fertility centers in every part of the world. The use of ART continues to increase around the world, due to ever-increasing global access to infertility clinics in the treatment of different forms of infertility (like tubal disease, ovarian aging, or ovarian dysfunction).
This book captures the current and recent advances in assisted reproduction technology in humans and livestock in an easy and comprehensive way for the non-expert and the junior embryologists through simplifying each ART tool by providing definition and explanation, and how the methods are practiced and how to overcome troubleshooting, and showing the ARTs uses and significance.
This book will be an ideal reference for junior embryologists as it provides take-home messages for the current and recent ARTs. It contains ten chapters with a detailed explanation of normal reproductive physiology, male ARTs such as sperm retrieval and freezing, female ARTs such as oocyte activation, and cryopreservation, and finally embryo ARTs that include assisted hatching and cloning techniques.
List of Contributors
Physiology of the Reproductive System
Mohamed M. Z. Hamada1,Islam M. Saadeldin1,2,3,*Abstract
The reproductive system of the living organism is the biological system made up of all the anatomical organs involved in sexual reproduction. This system involves the interaction of several fluids and hormones to regulate the functions of the reproductive system. The ultimate goal of the reproductive system is to successfully produce gametes (sperms and oocytes) to attain a combination of genetic material between two individuals, which allows for the possibility of greater genetic fitness of the offspring. In this chapter, we introduce the physiological process of gonadal development, male, and female reproductive system, embryo formation, and development to give the reader the basic concepts for application in the field of assisted reproductive techniques.
GONADAL DEVELOPMENT AND SEX DETERMINATION
Gonadal Development
The gonads represent a unique embryological situation in that: the rudiments of all body organs except the gonads can normally differentiate into only one type of organ. For example, a lung rudiment can become only a lung, and a liver rudiment can develop only into a liver. On the other hand, the gonadal rudiment has two normal options. When it differentiates, it can develop into either an ovary or a testis. The path of differentiation taken by this rudiment determines the future sexual development of the organism. Before this decision is made, the mammalian gonad first develops through a bi-potential (indifferent) stage, during which time
it has neither female nor male characteristics (Figs. 1 and 2). The indifferent gonads consist of several components:
1. Coelomic epithelium, which is the precursor of Sertoli cells in males and granulosa cells in females.
2. Mesenchymal stromal cells, which are the precursor of Leydig cells in males and theca cells in females.
3. Germ cells that have migrated there from the yolk sac endoderm.
This assembly is organized into the indifferent gonads into two layers, cortex and medulla, and proceeds as follows:
Fig. (1)) Differentiation of the indifferent gonad components to their analog in both testes and ovary.In a Male Fetus
Spermatogenic tubules begin to be formed at 6 weeks. This is followed by differentiation of the Sertoli cells at 7 weeks and Leydig cells at 8 to 9 weeks. At this point, the testes are structurally recognizable, and testosterone secretion begins.
Fig. (2)) Development of the gonads and their ducts in mammals. The upper figure represents the undifferentiated gonads and the presence of both male and female ducts. In the lower figures the male and female development is due to gonadal differentiation.The germ cells become enclosed within the medulla, whereas the cortex is regressed. No known hormonal influences are required for the differentiation of the indifferent gonad into a testis till that stage. The urogenital groove (sinus) is the progenitor of the external genitalia. The Wolffian duct differentiates into the epididymis and the vas deferens.
In a Female Fetus
Differentiation of the indifferent gonad into an ovary does not start until 9 weeks gestation. At this time, the activity of both X chromosomes within the germ cells is essential. The germ cells begin to undergo mitosis, giving rise to daughter cells called oogonia, which continue to proliferate. Shortly thereafter, meiosis is initiated in some oogonia, and each is surrounded by differentiating granulosa cells and precursor theca cells to form follicles.
The germ cells, now known as primary oocytes remain in the first stage, or prophase of meiosis until they are activated hormonally at puberty.
In contrast to the male arrangement of gonadal zones, the cortex, which contains the follicle, predominates whereas the medulla regresses. The primitive ovary begins to synthesize estrogen which contributes to the latter ovarian differentiation by blocking androgen actions.
Sex Determination
The genital system is generally composed of primary sex organs including the gonads, and secondary sex organs, which include the rest of the sex organs like internal and external sex organs.
Primary Sex Determination
The determination of the gonads, in mammals primary sex determination is strictly genetic and is not influenced by the environment. In most cases, the female is XX and the male is XY, every individual must have at least one X chromosome. Since the female is XX, each of her eggs has a single X chromosome. The male, being XY, can generate two types of sperms: half bear the X chromosome, half the Y. If the egg receives another X chromosome from the sperm, the resulting individual is XX, the ovary is formed, and is female; if the egg receives a Y chromosome from the sperm, the individual is XY, testes are formed, and becomes male. Thus, Y chromosome is an important factor for determining sex in mammals.
Mechanisms of Primary Sex Determination
Several genes have been found essential for normal sex differentiation these are:
Sex Determining Region of the Y Chromosome (SRY gene)
The organization of indifferent gonad into the characteristic spermatogenic tubules of the male is directed by a segment on the short arm of the Y chromosome known as the sex-determining region of the Y chromosome (SRY gene). SRY has different effects on converting the bipotential gonads into the testis, as follows:
a) It works directly to convert the coelomic epithelium into male-specific Sertoli cells.
b) SRY encodes another gene called testes determining factor (TDF). This TDF stimulates the development of primary sex cords to a seminiferous tubule.
Steroidogenic Factor I (SFI)
SFI is another protein activated by SRY, it is essential in masculinizing both the Leydig and Sertoli cells:
a) In Sertoli cells, SFI works in collaboration with Sox9 to elevate the levels of antimullerian hormone (AMH) transcription.
b) In Leydig cells, SFI activates the genes encoding the enzymes that facilitate the synthesis of testosterone.
Sox9 gene (Autosomal Sex Reversal)
It is one of the autosomal genes involved in sex determination. It is essential for testis formation since it is expressed only in the male genital ridge.
Gene Encoding H-Y antigen
It is identical to or closely linked to the SRY or TDF genes. H-Y antigen is a glycoprotein present on all male cells except diploid germ cells. This H-Y antigen causes virilization of the cells of the indifferent gonad.
Autosomal Genes for Androgen Receptors
These genes are responsible for the formation of androgen receptors in target organs for sensitizing the genital ducts and external genitalia to the masculinizing effect of testosterone and dihydrotestosterone.
DaxI (Ovary-determining Gene on X Chromosome)
It is a potential ovary-determining gene present on the X chromosome.
It is expressed in the genital ridges of the mouse embryos, shortly after SRY expression. Indeed, in XY mice, SRY and DaxI are expressed in the same cells; DaxI appears to antagonize the function of SRY, and it down-regulates SFI expression. Thus DaxI is probably a gene involved in ovary determination.
WNT4 (Ovary-determining Gene on an Autosome)
The WNT4 gene is another gene that may be critical in ovary determination. This gene is expressed in the mouse genital ridge while it is still in its bipotential stage. WNT4 expression then becomes undetectable in XY gonads, whereas it is maintained in XX gonads as they begin to form ovaries (Fig. 3).
Fig. (3)) Summary of mammalian sex determination. Dashed arrows mean inhibition.Secondary sex determination in mammals involves the development of the female and male phenotypes in response to hormones secreted by the gonads. Both female and male secondary sex determination has two major temporal phases, the first occurs within the embryo during organogenesis and the second occurs during adolescence.
Male Phenotype Determination
The formation of the male phenotype involves the secretion of two testicular hormones; AMH and androgens (testosterone and dihydrotestosterone).
Antimullerian Hormone (AMH)
It is a 560-amino acid glycoprotein secreted from the Sertoli cells and causes degradation of the Mullerian duct. AMH bind to the mesenchyme cells surrounding the Mullerian duct causing these cells to secrete a paracrine factor that induces apoptosis in the Mullerian duct epithelium.
Testosterone and Dihydrotestosterone (DHT)
Testosterone secreted from fetal Leydig cells is responsible for the differentiation of the Wolffian duct into the epididymis, vas deferens, and seminal vesicles. DHT is produced in some target organs like the urogenital sinus and swellings by reduction of testosterone under the effect of 5 α-reductase enzymes. DHT is responsible for the differentiation of urogenital sinus and swellings into the male external genitalia including the scrotum and penis. In absence of this enzyme, the male external genitalia will be a female one.
Female Phenotype Determination
The absence of TDF, testosterone, and AMH allows the following changes:
a) Indifferent gonads differentiate into the ovary.
b) Wolffian ducts degenerate.
c) Mullerian ducts develop into the oviducts, uterus, and cervix.
d) Tissue around the urogenital sinus becomes the clitoris, labia, and vagina.
Thus, the development of the female phenotype depends on the absence of androgens during early development.
MALE REPRODUCTIVE SYSTEM
Introduction
The male reproductive system is made up of several individual organs acting in concert to produce spermatozoa and deliver them to the reproductive tract of the female. This concerted effort involves both the neuro-endocrine (hypothalamus and pituitary gland) and the genital system.
The gonad consists of two testes, each suspended within the scrotum by a spermatic cord and external cremaster muscle. During embryogenesis, the testes develop retro-peritoneally on the posterior wall of the abdominal cavity. As they descend into the scrotum, they carry with them a portion of the peritoneum. This peritoneal out-pouching, the tunica vaginalis, forms a serous cavity that partially surrounds the anterolateral aspect of each testis, permitting it some degree of mobility within its compartment in the scrotum [1].
Spermatozoa produced by the seminiferous tubules of testes enter a duct system including short straight ducts, tubuli recti, which connect the opened end of each seminiferous tubule to the rete testis, a system of labyrinthine spaces housed within the mediastinum. The spermatozoa leave the rete testis through 10-20 short tubules, vasa efferentia, which eventually fuse with the epididymis, which is connected with ductus deferens. In addition, there are accessory sex glands that play an important role, for instance providing the seminal fluid. These glands include paired ampullae, paired seminal vesicle, a prostate gland, and paired bulbourethral glands (Cowper’s gland) [2].
The Male Gonad (Testes)
Each testis is surrounded by a capsule of dense, irregular collagenous connective tissue known as tunica albuginea. Immediately deep into this layer is a highly vascularized loose connective tissue, the tunica vasculosa, which forms the vascular capsule of the testis.
From the tunica albuginea, connective tissue septa radiate to subdivide each testis into intercommunicating compartments called testis lobules.
In all domestic animals except stallions, these septa units near the center of the testis form a fibrous cord called the mediastinum testis. Each testis lobule has one to four blindly ending seminiferous tubules, which are surrounded by a richly innervated and highly vascularized loose connective tissue derived from tunica vasculosa. Dispersed throughout this connective tissue are small conglomerations of endocrine cells, the interstitial cells of Leydig, which are responsible for testosterone production [3].
Seminiferous Tubules
Seminiferous tubules are highly convoluted hollow tubules, 30 to 70 cm long and 150 to 250 μm in diameter, it is surrounded by extensive capillary beds. About 1000 seminiferous tubules are present in the testes, for a total length of nearly 0.5 km, dedicated to the production of spermatozoa (Fig. 4).
Fig. (4)) Cross-section of Mammalian Testis.The seminiferous epithelium is composed of two types of cells.
1. Spermatogenic cells produce spermatozoa a process called spermatogenesis.
2. Sertoli cell which is has many functions either in the male reproductive system or during spermatogenesis.
Spermatogenic Process
Spermatogenesis is the production of sperms from the primordial germ cells (PGCs) lining the seminiferous tubules. It is divided into:
(1) Spermatocytogenesis. (2) Meiosis. (3) Spermiogenesis (Spermateliosis).
Spermatocytogenesis
Definition: It is the process in which the spermatogonia lining the seminiferous tubules differentiate into primary spermatocytes.
Mechanism: The spermatogonia lie on the basal lamina of the seminiferous tubules and, after puberty, become influenced by testosterone to enter the cell cycle to produce sperms.
Meiotic Division
Definition: Meiotic Division is composed of the first and second divisions. In the first meiotic division, the diploid number of chromosomes in each primary spermatocyte is reduced to haploid number forming two secondary spermatocytes. In the second meiotic division, each secondary spermatocytes divide into two spermatids of the same haploid number of chromosomes. Thus each primary spermatocyte (diploid number, 2n) gives rise by meiotic division to four spermatids (haploid number, n) as shown in Fig. (5).
Spermiogenesis
Definition: It is the process by which, the immotile spermatids are differentiated into motile sperms without division.
Mechanism: The newly formed spermatid has a central nucleus, well-developed Golgi body, endoplasmic reticulum, mitochondria, and a pair of centrioles. The sequence of spermatid transformation to sperm is as follows:
Fig. (5)) Different divisions during the stages of spermatogenesis.1. Hydrolytic enzymes are formed on the rough endoplasmic reticulum, modified in the Golgi apparatus, and packed as small, membrane-bound vesicles. These small vesicles fuse, forming an acrosomal vesicle, which enlarges to its final size known as the acrosome or acrosomal cap.
2. The nucleus becomes condensed and flattened and most of the spermatid cytoplasm is removed.
3. One centriole becomes attached to the nucleus and the other will elongate to form the tail of the spermatozoon.
4. Mitochondria location is shifted and becomes associated with the proximal portion of the developing flagellum. They form the mitochondrial sheath, which constitutes the middle piece of the mature spermatozoon (Fig. 6).
Fig. (6)) Schematic representation of the sperm.In mice, the entire spermatogenesis process takes 34.5 days, the spermatogonial stages last 8 days, meiosis lasts 13 days, and spermiogenesis takes up another 13.5 days. In most domestic animals, this process takes about 60-65 days.
The differentiation of mammalian sperm is not completed in the testes. After being expelled into the lumen of the seminiferous tubules, the sperm are stored in the epididymis, where they acquire the ability to move. Motility is achieved through changes in the ATP-generating system as well as changes in the plasma membrane that make it more fluid.
The sperm released during ejaculation can move, yet they cannot yet bind to and fertilize an egg. The final sages of sperm maturation (called capacitation).
Sperm Capacitation
Definition: It is the set of changes that allow the sperm to be able to fertilize the ova. This process does not occur until the sperm has been subjected to the fluid of the female reproductive tract for a certain period. The process involves the removal of a protective substance termed glycerophosphocholine (GPC), present on the acrosome of the sperm that has a protective purpose and thus interferes with the fertilizing efficiency of the sperm. The acrosome reaction involves the release of a series of hydrolytic enzymes from the acrosomal cap like hyaluronidase that digests the hyaluronic acid that binds the granulosa cells covering the oocyte. Another enzyme called acrosine has also been released that digest the cellular coating around the oocyte. Detachment of the acrosomal cap is triggered by the rapid influx of calcium ions that induce disruption of the acrosome and release of its hydrolytic enzymes. Detachment of the acrosome exposes a special protein on the head of sperm called fertilin that is important for fusing the sperm head with the denuded zona pellucida. The acrosome reaction also results in vigorous flagellar tail movement of the sperm that allow penetration of the oocyte. Once sperm is attached to the oocyte its membrane is depolarized preventing entry of other sperms inside (polyspermy) [4].
Sertoli Cells
The lateral cell membranes of adjacent Sertoli cells form occluding (tight) junctions with each other, thus subdividing the lumen of the seminiferous tubules into two isolated compartments:
1. The basal (outer) compartment, located basal to the tight junctions and contains the spermatogonia only.
2. The adluminal (inner) compartment: it is wider and begins from the tight junction till the lumen of the seminiferous tubules. It contains the primary and secondary spermatocytes, spermatids, and spermatozoa.
Thus the tight junctions of these cells establish a blood-testis barrier that isolates the adluminal compartment from the interstitial tissues, thereby protecting the developing gametes from the immune system.
Because spermatogenesis begins after puberty, the newly differentiating germ cells, which have different chromosome numbers as well as express different surface membrane receptors and molecules, would be considered foreign cells by the immune system. If there is no tight junction of Sertoli cells for the isolation of germ cells from the interstitial tissue surroundings, an immune response would be mounted against them.
Functions
1. They give physical and mechanical support for germ cells.
2. They secrete a fructose-rich medium that nourishes the sperms and facilitates the transport of the sperm to the genital ducts.
3. They establish a blood-testis barrier which has very important functions:
It protects the spermatocytes, spermatids, and sperms from damaging substances present in the bloodstream.It prevents the entry of anti-sperm immunoglobulin molecules into the lumen of seminiferous tubulesIt prevents the passage of antigens, produced during the differentiation of germ cells, from returning back and mixed with blood. Thus it prevents the autoimmune reactions leading to the death of sperms.Its secretion is rich in K+, HCO3- and ABP, this secretion, in the inner compartment of the seminiferous tubules, provides a driving force to expel the sperm to the duct system.Its secretion is rich in K+, HCO3- and ABP, this secretion, in the inner compartment of the seminiferous tubules, provides a driving force to expel the sperm to the duct system.4. They synthesize and release very important hormones and other proteins:
Androgen-binding protein (ABP): It is a macromolecule that facilitates an increase in the concentration of testosterone in the seminiferous tubule by binding to it and preventing it from leaving the tubule.It has the ability to convert the androgens to estrogens by the aromatase enzyme, in addition to its ability to synthesize estrogen.Inhibin hormone: It is a hormone that inhibits the release of follicle-stimulating hormone (FSH) by the anterior pituitary.During embryogenesis Sertoli cells synthesize and release the anti-Mullerian duct hormone (AMH), which suppresses the formation of the Mullerian duct (the precursor of the female reproductive tract) and thus establishes the maleness of the developing embryo.Leydig Cells
They are small collections of endocrine cells dispersed in a richly vascularized loose connective tissue, the tunica vasculosa, which fill the spaces between the seminiferous tubules. These cells secrete the male sex hormone (androgen).
Androgen
Androgens are steroid hormones that exert masculinizing effects, and they promote protein anabolism and growth. Testosterone from the testes is the most active androgen, and adrenal androgens have less than 20% of its activity [5].
Secretion of the adrenal androgens is controlled by ACTH, but not by gonadotrophins (FSH and LH). Gonadotrophins are important regulators of steroidogenesis in the gonads. In the testes, LH acts on the Leydig cells whilst FSH acts on the Sertoli cells. About 98% of testosterone in plasma is bound to proteins: 65% to a β-globulin called gonadal steroid-binding globulin (GBG) or sex steroid-binding globulin, and 33% bound to albumin. A small amount of circulating testosterone is converted to estradiol, but most of it is converted into 17-ketosteroids, principally androsterone.
The secreted androgens are metabolized in the liver by oxidation and reduction. The reduced metabolites are conjugated to glucuronides and sulfates and are excreted in the urine. About two-thirds of the urinary 17-ketosteroids are of adrenal origin, and one-third are of testicular origin [6].
Functions
Testosterone diffuses to the target cells, wherein many sites, it is reduced to dihydrotestosterone (DHT) by the 5α-reductase enzyme. Testosterone receptor binds have a greater affinity for DHT than for testosterone. Thus, DHT formation is a way of amplifying the action of testosterone in target tissues.
I. During embryogenesis
1. Testosterone-receptor complexes are responsible for the maturation of Wolffian duct structures and consequently for the formation of male internal genitalia during development.
2. DHT-receptor complexes are needed to form the male external genitalia from the urogenital sinus and structures.
II. At Puberty
a) Metabolic effects
Androgens are anabolic hormone it increases the synthesis and decreases the breakdown of protein so has a positive nitrogen balance. It stimulates bone growth during puberty, but it ultimately halts linear growth by closing the epiphyseal growth centers. It causes enlargement of the muscle mass of males by increasing the size of muscle fibers. It also increases red blood cell mass by stimulating erythropoietin synthesis and by directly affecting the maturation of erythroid precursors.
b) On secondary sex characters
It enlarges the larynx, thickens the vocal cords, and thereby deepens the voice.It is required for the growth of secondary sex organs including the penis, scrotum, and prostate, and stimulates prostatic secretions.It is responsible for the typical body conformation of males (broad shoulders, horn shape); male aggressive behaviors; and expression of sexual desire (libido).It is essential for maintaining the spermatogenesis process.It stimulates the hair follicles to produce the typical masculine hair distribution. It stimulates the growth of sebaceous glands and their production of sebum.CONTROL OF TESTIS FUNCTIONS
Neuroendocrine Control
Regulation of the reproductive axis begins at the level of the hypothalamus, where neurosecretory cells synthesize and release GnRH in a pulsatile fashion into the hypothalamic-hypophysial-portal circulation.
In response, gonadotropes in the anterior pituitary synthesize and release the gonadotropins (FSH and LH), that control the gonadal function. This is known as the Hypothalamic-Pituitary-Gonadal Axis (Fig. 7).
Fig. (7)) Neuro-Endocrine Control of Testes Function (Hypothalamo-Hypophysial-Testicular Axis). Dashed arrows mean inhibition.The pulsatility of GnRH and gonadotropins (FSH/LH) actions on their target organs and the production of high concentrations of testosterone are essential components in spermatogenesis. FSH, LH, and testosterone coordinate with local estradiol, inhibin, and activin as well as prolactin and GH in the regulation of spermatogenesis.
An adult male is unlike a female in that FSH and LH act on different cell types whose secretions have separate negative feedback effects on the secretion of both gonadotropins.
GnRH stimulates the gonadotropes to secrete FSH/LH. Because GnRH is secreted in a pulsatile manner, FSH and LH also are secreted in a pulsatile manner.FSH stimulates Sertoli cells to produce inhibin, which in turn inhibits FSH secretion; LH stimulates Leydig cell to produce testosterone, which in turn inhibits LH secretion.Inhibin has a negative feedback effect on the pituitary gonadotropes, whereas testosterone exerts a negative feedback effect primarily in the hypothalamus.When plasma testosterone drops; the frequency of pulsatile GnRH secretion increases. Thus plasma FSH and LH are maintained in a dynamic equilibrium with plasma inhibin and testosterone respectively.Pituitary Gonadotropins
We mentioned before, that FSH acts exclusively on Sertoli cells, whilst LH acts exclusively on Leydig cells. Now, we can discuss, in some detail, the effect of each one on its target cell.
Effects of FSH on Sertoli Cells
2. FSH stimulates the Sertoli cell’s production of activin, which modulates mitochondrial changes that occur as spermatogonia enter meiosis and become primary spermatocytes. Thus FSH, acting at least in part via Sertoli cells, enhances the early stages of sperm production.
3. FSH stimulates Sertoli cells to synthesize ABP, inhibins and stimulates estrogen synthesis from testosterone provided by Leydig cells.
4. Other products of FSH actions on Sertoli cells are providing energy sources, such as lactic acid, to the germ cells and facilitating the expulsion of spermatozoa into the lumen of the tubule. The release of FSH is inhibited by the inhibin hormone produced by Sertoli cells.
Effect of LH on Leydig Cells
The physiologic role of LH is to maintain testosterone secretion by Leydig cells. In addition, LH produces a locally high concentration of androgen in the testes, and this maintains spermatogenesis. The release of LH is inhibited by increased levels of testosterone and DHT.
Androgen
High intracellular testosterone levels are essential for normal spermatogenesis. Testosterone is concentrated in the tubule by ABP. The stages from spermatogonia to spermatids appear to be androgen-independent. The maturation from spermatids to spermatozoa depends on androgen acting on Sertoli cells in which the spermatozoa are embedded. FSH acts, also on the Sertoli cells to facilitate the last stages of spermatid maturation.
Inhibin and Activin
Inhibins and activins are disulfide-linked dimeric glycoproteins. Inhibin is dimers of α subunit linked to either a βA or βB subunit to generate inhibin A (α βA) and inhibin B (α βB). Dimerization of β subunits alone gives rise to three forms of activins referred as activin A (βA βA), activin B (βB βB), and activin AB or activin C (βA βB).
Inhibins and activins were isolated from gonads mainly from Sertoli cells in males and granulosa cells of ovarian follicles in females. They can modulate the pituitary FSH secretion as activin stimulates FSH whereas inhibin inhibits FSH secretion through negative feedback.
Testicular Temperature
In domestic animals, normal testicular function, especially normal spermatogenesis, is temperature-dependent and requires an environmental temperature 3C to 5C lower than core body temperature.
Hence, in normal domestic males, the testes are located outside the abdominal cavity, in the scrotum. Failure of one or both of the testes to descend into the scrotum is known as cryptorchidism.
Mechanism Of Testicular Temperature Regulation
Role of Testicular Venous Pampiniform Plexus
The vascular supply of each testis is derived from the testicular artery, which descends with the testis into the scrotum accompanying the ductus deferens. The testicular artery forms several branches before it pierces the capsule of the testis to form the intra-testicular vascular elements.
The capillary beds of the testes are collected into several veins forming the pampiniform plexus of veins, which are wrapped around the testicular artery.
Blood in the pampiniform plexus of veins, which is cooler than that of the testicular artery, acts to reduce the temperature of the arterial blood, thus forming a countercurrent heat exchange system.
In this fashion, it helps keep the testis’s temperature a few degrees lower than that of body temperature. At this cooler temperature, spermatozoa develop normally; at body temperature, spermatozoa that develop will be sterile.
Role of the Scrotum
Each testis is suspended within the scrotum by a spermatic cord and external cremaster muscle. The scrotum, along with the cremaster muscle and the vascular anatomy of the testicular arteries and veins, protects and thermo-regulates the testes in the following way:
The scrotum is essentially a hairless skin pouch with a subcutaneous fibroelastic and muscular layer called tunica dartos, and without subcutaneous fat.Contraction and relaxation of the tunica dartos and cremaster muscles occur with changes in ambient temperature as well as in response to other tactile stimuli. Contraction of these muscles pulls the testes toward the abdomen to decrease the loss of temperature (e.g. during cold weather), while relaxation keeps the testes hanging away from the abdomen to increase the heat loss by radiation (e.g. during hot weather).The scrotum of some species such as the horse contains numerous sweat and a sebaceous gland that further contribute to the thermoregulatory mechanism.Cryptorchidism
Definition: It is the failure of one or both of the testes to descend into the scrotum. Although the cryptorchid testis is still capable of producing androgens, it is incapable of producing normal spermatozoa. Consequently bilaterally cryptorchid males would be sterile. The cryptorchid testis is more prone to torsion of the spermatic cord and 10 times more likely to be neoplastic. Cryptorchidism appears to be genetic, although the exact mechanism is not completely understood and may vary between species. It is most common in boars, dogs, and stallions and least common in bulls, rams, and bucks.
DUCT SYSTEM
The genital ducts can be subdivided into two categories:
The Intra-testicular ducts connect the seminiferous tubules to the epididymis. They include tubuli recti and rete testis and extra-testicular ducts, which are associated with each testis, including ductuli efferentes (vasa efferentia), the epididymis, ductus (vas) deferens, and the ejaculatory duct.
Intra-testicular Ducts
Tubuli Recti: They are short, straight tubes continuous with the seminiferous tubules and deliver the spermatozoa formed by the seminiferous tubules into the rete testis. These short tubules are lined by Sertoli cells in their first half, near the seminiferous tubule, and by the cuboidal cell in their second-half near the rete testis. The presence of Sertoli cells in the tubuli recti is an essential role to get rid of the remnant sperm cytoplasm.Rete Testis: They are multiple narrow tubules forming anastomosing channels continuous with the tubuli recti. They are lined with cuboidal cells that have short microvilli with a single flagellum to deliver the immotile sperm to the ductuli efferentes.Extra-testicular Ducts
Ductuli efferentes (vasa efferentia); composed of 10-20 short tubules attached by vascular connective tissue. They are lined with two types of cells: High columnar ciliated cells, which are responsible for the movements of the immotile sperms to the epididymis.Non-ciliated low columnar cells, which are responsible for reabsorption of luminal fluid secreted by the Sertoli cells.Thus, one of the functions of the efferent duct (vasa efferentia) cells is to absorb about 90% of the water from the lumen of the rete testis. This concentrates the sperm, giving them a longer life span and providing more sperm per ejaculate. This absorption of water is regulated by estrogen secreted by Sertoli cells.
If estrogen or its receptor is absent in mice, this water is not absorbed, and the mouse will be sterile. While blood levels of estrogen are higher in females than males, the levels of estrogen in the rete testis are even higher than that in female blood.
Epididymis
It is a single, thin, long highly convoluted tubule of considerable length (from 2 m in the cat to 80 m in the stallion). Anatomically, it is divided into three segments: head, or caput; body, or corpus; and tail, or cauda. The epididymis is not only a conduit for spermatozoa, but also provides a special environment in which spermatozoa are concentrated, undergo maturation, and acquire a fertilizing capacity.
Spermatozoa entering the caput from the rete testis are immotile and incapable of fertilization. Only after they undergo migration and maturation (through the caput and corpus), both motility and the fertilization capacity are achieved in the cauda epididymis.
The cauda epididymis serves as a storage depot for mature sperm. The media of cauda epididymis is slightly acidic to suppress the sperm activities, so the sperm can live for months in the epididymis. However, the lumen of the epididymis is lined by two types of cells: basal cells that function as stem cells, regenerating themselves, as well as the principal cells which have different functions including:
Reabsorb the luminal fluid.Phagocytose is the remnants of sperm cytoplasm that were not removed by Sertoli cells.Manufacturing glycerophosphocholine (GPC), which inhibits sperm capacitation until the sperm enters the female genital tract.Ductus Deferens (Vasa Deferentia)
Each ductus deferens is a thick-walled muscular tube that conveys the spermatozoa from the cauda epididymis to the ejaculatory duct, which in turn deliver the sperms and seminal fluid into the prostatic urethra. The ductus deferens passes through the inguinal rings into the abdomen and connects the cauda epididymis with the pelvic urethra.
In most species, the terminal portion of the ductus deferens enlarges to form prominent ampullae such as those found in the bull and stallions. In other species, the ampullae are either absent or are anatomically indifferent from the ductus deferens. The ampullae serve as an additional storage depot for sperms, and in some species, such as the bull, stallion, and dog, the ampullary glands add to the ejaculate. In stallion, the secretion of ampullary glands is rich in ergothioneine, which acts as an anti-oxidizing agent protecting susceptible chemicals in semen from oxidation.
Accessory Genital Glands
Grossly, the main components of semen are the spermatozoa produced by the seminiferous tubules; and the seminal fluid produced by the accessory genital ducts. The seminal fluid (seminal plasma) produced by the accessory genital glands provides:
Substrates for conveying the sperms to the female genital tract and ensuring the maturation of the sperm.It supplies an energy source (fructose) and protects from changes in osmotic pressure especially in stallion (citrate), and oxidizing agents (ergothioneine).It also contains clotting factors, in the form of fibrinogen or a gel that is present in seminal vesicle fluid in the stallion, that forms a partial clot in semen, helping to minimize the escape of semen from the female genitalia.Seminal Vesicle
Lies laterally to the terminal parts of each ductus deferens. In ruminants, it is a compact lobulated gland, in the stallion and boar, it is a large pyriform glandular sacs. In dog and cat, a seminal vesicle is absent. The duct of the seminal vesicles and the ductus deferens share a common ejaculatory duct that opens into the urethra. The secretions of the seminal vesicles are the terminal portions of the ejaculate.
These secretions are rich in fructose, which is considered an important oxidative substrate for the spermatozoa; as well as large quantities of prostaglandins and fibrinogen.Prostaglandins are thought to play a role in fertilization by making the cervical mucous more receptive to the sperm movement; and by enhancing the antiperistaltic contractions of the uterus and fallopian tubes, helping the movement of sperms toward the ovary.The clotting enzymes present in the prostatic secretion act on the fibrinogen of seminal vesicle secretions to form a weak fibrin coagulum, which holds up the semen in the vagina.Moreover, the seminal vesicle secretions contain Ca2+, zinc, LH, FSH, prolactin, testosterone, estradiol, inhibin, oxytocin, endorphin, and a variety of enzymes. The exact source and the role of these components remain obscure.Prostate Gland
Present in all animals, It is intimately associated with the pelvic urethra, but among different species, it varies in size and appearance. Just before ejaculation (during emission), secretions of the prostate are discharged into the urethra meeting the sperms coming from ductus deferens. The motility of spermatozoa is suppressed in the epididymis due to its acidic medium, being rich in citric acid and metabolic end-products of spermatozoa, in addition, the vaginal secretions are also acidic in nature.
The prostatic secretion is slightly alkaline, containing Ca2+, citrate (in the stallion, the prostatic secretion is rich in citric acid), clotting enzymes, and a profibrinolysin. This alkalinity of prostatic secretion plays a role in neutralizing the acidic medium of the seminal fluids, and the vagina and thus enhances the motility of the sperms.Fibrinolysin formed from prostatic profibrinolysin, dissolves the coagulum, which is formed by the action of prostatic clotting enzymes on the seminal vesicle fibrinogen, and thus sperms can be released.In stallion, prostatic secretion is rich in citric acid, which is known to act as an osmotic pressure buffering system. It maintains the osmotic pressure in the seminal fluid within the sperm heads, thus preventing the potential bursting of the sperm heads due to an influx of water from the surrounding fluids.Bulbourethral Gland (Cowper’s Gland)
These glands are dorsal to the urethra near the termination of its pelvic portion. They are present in all animals except dogs.
The secretions of these glands are the first secretions produced during sexual excitation. These secretions are clear, serous, and slightly alkaline. These secretions help to clean out the urine and bacteria collected in the urethra prior to ejaculation. The secretions also act as lubricants easing the passage of sperm.
PHYSIOLOGY OF MALE SEXUAL ACT
Sexual intercourse, mating, copulation, or coitus are all synonymous with male and female interaction to facilitate the deposition of male gametes into the female reproductive tract for fertilization. The sexual responses are mediated by the coordinated activity of the autonomic (Parasympathetic and sympathetic) and somatic innervation. Although these reflexes differ in detail in males and females, basic similarities allow the two sexes to be considered together, not only in humans but also in mammals generally. The relevant autonomic effects include:
Mediation of the vascular dilation which causes a penile or clitoral erection.Stimulation of prostatic or vaginal secretionsSmooth muscle contraction of the vas deferens during ejaculation or rhythmic vaginal contraction during orgasm in both sexes.Parasympathetic Activity
The reproductive organs of both males and females receive preganglionic parasympathetic innervation from the sacral segments (S2-S4) of the spinal cord and reach the target organs via the pelvic nerves. The activity of postganglionic neurons causes dilation of penile or clitoral arteries, and a corresponding relaxation of the smooth muscles of venous (cavernous) sinusoids, which leads to expansion of sinusoidal spaces. As a result, the amount of blood in the tissue is increased, leading to a sharp rise in the pressure and an expansion of cavernous spaces, that is to say, erection.
The parasympathetic activity also provides excitatory input to the vas deferens, seminal vesicles, and prostate in males and vaginal glands in females. The chemical Mediators of Parasympathetic in Penile Tissue:
Acetylcholine and Vasoactive intestinal peptide (VIP) as contransmitters since parasympathetic fibers supplying penile tissue contain both acetylcholine and VIP. Thus these fibers release both acetylcholine and VIP as chemical transmitters when stimulated.Nitric Oxide (NO): Some parasympathetic nerve fibers supplying deep cavernous tissues of the penis secretes NO synthase, the enzyme that catalyzes the formation of NO from arginine. NO activates guanylyl cyclase resulting in increased production of cGMP, and cGMP is a potent vasodilator. Thus it seems clear that NO plays a prominent role in the production of erection.Sympathetic Activity
In contrast, the sympathetic activity causes vasoconstriction and sub-subsequently loss of erection. Emission means the movement (first part of ejaculation) of semen into the urethra. Emission is a sympathetic response as a result of afferent pathways from touch receptors in the gland penis and reaches the spinal cord through the internal pudendal nerves.
The preganglionic sympathetic innervation originates from the thoracolumbar segments (T11-L2) and reaches the target organs via the corresponding sympathetic chain ganglia and the inferior mesenteric and pelvic ganglia. The response of sympathetic activity leads to contraction of smooth muscles in vas deferens and seminal vesicles, and subsequently propulsion of semen into the urethra.
Somatic Activity
The somatic component of reflex sexual function arises from α-motor neurons in the lumbar and sacral spinal cord segments. These neurons provide excitatory innervation to the bulbocavernosus and ischiocavernosus muscles, which are active during ejaculation proper in males and mediate the contractions of the perineal (pelvic floor) muscles that accompany orgasm in both males and females.
Ejaculation proper means the propulsion of semen outside the urethra. Thus the sexual function can be summarized into three events:
Erection as a response to parasympathetic activity.Ejaculation: which can be divided into: Emission as a result of sympathetic activity.Ejaculation proper as a result of parasympathetic and somatic activities.FEMALE REPRODUCTION
Physiology of the female reproductive system includes the following items.
Ovarian Cycle
The ovarian cycle includes three main phases:
Follicular phase during which two main events affecting each other take place in parallel: Oogenesis process by which mature oocyte is produced.Folliculogenesis process by which mature follicles are produced and secrete estrogen, the female sex hormone.Ovulation, is a phase in which the dominant follicle (Graafian follicle) ovulates releasing mature secondary oocyte and the remaining cellular part of the Graafian follicle changes to corpus luteum.Luteal phase
