Embryology at a Glance E-Book

Samuel Webster

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Serie: At a Glance
• Sprache: Englisch

Seitenzahl: 248

Veröffentlichungsjahr: 2012

Beschreibung

This brand new title provides a highly illustrated introduction to key embryological concepts, with concise, memorable descriptions of major embryological developments.

Embryology at a Glance introduces the basic principles of human development, from mitosis and meiosis, and walks you through the primary formation of each body system, with coverage of the continued development of the respiratory and vascular systems during the foetal and neonatal periods.

Fully geared towards the medical school curriculum, the coverage of major steps in human development allows a better understanding of adult anatomy, development-associated conditions, congenital abnormalities and their treatments.

Embryology at a Glance:

• Features full colour photographs and illustrations, including 3-dimensional illustrations where appropriate, and full labels
• Offers ‘one-stop’ coverage of the skeletal, muscular, circulatory, respiratory, nervous, reproductive, urinary, endocrine and digestive systems
• Highlights clinical correlations throughout
• Includes timelines so you won’t lose sight of the temporal aspect of embryology
• Includes Multiple Choice Questions (MCQs) and Extended Matching questions (EMQs) for revision and review

A companion website with links to the Dr Webster’s embryological and anatomical podcasts is available at: www.wiley.com/go/embryology

The clear, descriptive diagrams characteristic of the at a Glance series will help all medical students and health professionals develop an understanding of human development and its implications for clinical practice.

Leseprobe

Table of Contents

Cover

Companion website

Title page

Copyright page

Preface

Acknowledgements

List of abbreviations

Timeline

Part 1: Early development

1 Embryology in medicine

What is embryology?

Aims and format

Why study embryology?

Embryology in modern medicine

Why read this book?

2 Language of embryology

Time period: day 0–266

Introduction

Cranial–caudal

Dorsal–ventral

Medial–lateral

Proximal–distal

Sections

3 Introduction to development

Time period: day 0 to adult

Development

Growth

Differentiation

Signalling

Organisation

Morphogenesis

Clinical relevance

4 Embryonic and foetal periods

Time period: day 0 to birth

Embryonic period

Foetal period

Trimesters

Clinical and embryological timings

Clinical relevance

5 Mitosis

Time period: day 0 to adult

Cell division

Mitosis

Clinical relevance

6 Meiosis

Time period: day 0 to adult

Diversity

Human chromosomes

Meiosis I

Homologous recombination

Meiosis II

Clinical relevance

7 Spermatogenesis

Time period: puberty to death

Meiosis continued

Aims of spermatogenesis

Anatomy

Spermatocytogenesis

Spermiogenesis

Spermatozoa

Clinical relevance

8 Oogenesis

Time period: week 12 to menopause

Overview

Ovaries

Meiosis I

Puberty

Ovulation

Polar bodies

Meiosis II

Post-ovulation

Clinical relevance

9 Fertilisation

Time period: day 0

Fertilisation

Capacitation

Ovulation

Acrosome reaction

Cortical reaction

Meiosis II

Zygote

Mitosis and DNA

Chromosomes

Embryological and clinical timings

Clinical relevance

10 From zygote to blastocyst

Time period: days 0–5

Zygote

Cleavage

Morula

Blastocyst

Implantation

Twins

Clinical relevance

11 Implantation

Time period: days 5–13

Introduction

Implantation

The menstrual cycle (uterus)

Decidualization

The menstrual cycle (hormones)

Proliferative (follicular) phase

Secretory (luteal) phase

Implantation mechanism

Bilaminar germ disc

Clinical relevance

12 Placenta

Time period: day 7 to week 12

Introduction

Trophoblast

Structure

Function

Changes to the placenta

Clinical relevance

13 Gastrulation

Time period: day 14

Trilaminar disc

Primitive streak

Signalling

Clinical relevance

14 Germ layers

Time period: day 15

Trilaminar disc

Ectoderm

Mesoderm

Endoderm

Germ cells

Clinical relevance

15 Neurulation

Time period: days 18–28

Introduction

Notochord

Neural plate

Neural tube

Neural crest cells

Development of the central nervous system

Clinical relevance

16 Neural crest cells

Time period: from day 22

Neural crest cells

Migration and differentiation

Destinations

Clinical relevance

17 Body cavities (embryonic)

Time period: day 21 to week 8

Body cavities

Diaphragm

Clinical relevance

18 Folding of the embryo

Time period: days 17–30

Flat sheet

Longitudinal folding

Lateral folding

Tube within a tube

Clinical relevance

19 Segmentation

Time period: days 18–35

Introduction

Pair rule genes

Hox genes

Hox proteins

Segmentation clock

Vertebrates

Clinical relevance

20 Somites

Time period: days 20–35

Mesoderm

The somite

Sclerotome

Myotome

Dermotome

Skin

Innervation

Dermatomes

Clinical relevance

Part 2: Systems development

21 Skeletal system (ossification)

Time period: week 5 to adult

Introduction

Endochondral ossification

Intramembranous ossification

Joint formation

Clinical relevance

22 Skeletal system

Time period: day 27 to birth

Introduction

Cranium

Vertebrae

Axial bones

Appendicular bones

Clinical relevance

23 Muscular system

Time period: day 22 to week 9

Introduction

Skeletal muscle

Limbs

Head

Smooth muscle

Cardiac muscle

Clinical relevance

24 Musculoskeletal system: limbs

Time period: week 4 to adult

Introduction

Limb buds

Distal growth

Organisation

Digits

Dermatomes and myotomes

Clinical relevance

25 Circulatory system: heart tube

Time period: days 16–28

Formation of the heart tube

Looping and folding of the heart tube

Sinus venosus (right atrium)

Clinical relevance

26 Circulatory system: heart chambers

Time period: day 22

Dividing the heart into chambers

Atria

Ventricles

Valves

Neural crest cells

Clinical relevance

27 Circulatory system: blood vessels

Time period: day 18 to birth

Vasculogenesis

Angiogenesis

Primitive circulation

Aortic arches

Ductus arteriosus

Coronary arteries

Clinical relevance

28 Circulatory system: embryonic veins

Time period: day 18 to birth

Vitelline vessels

Umbilical vessels

Cardinal veins

Clinical relevance

29 Circulation system: changes at birth

Time period: birth (38 weeks)

Foetal blood circulation

Ductus venosus

Ductus arteriosus

Foramen ovale

Clinical relevance

30 Respiratory system

Time period: day 28 to childhood

Introduction

Lung bud

Respiratory tree

Alveoli

Circulation

Clinical relevance

31 Digestive system: gastrointestinal tract

Time period: days 21–50

Induction of the tube

Divisions of the gut tube

Blood supply

Lower foregut

Twists of the midgut

Story of the hindgut and the cloaca

Mesenteries

32 Digestive system: associated organs

Time period: day 21 to birth

Introduction

Lung bud

Spleen

Liver and gallbladder

Pancreas

33 Digestive system: congenital anomalies

Time period: birth

Facial abnormalities

Foregut abnormalities

Midgut abnormalities

Hindgut abnormalities

Associated organs

34 Urinary system

Time period: day 21 to birth

Introduction

Kidneys

Mesonephros

Metanephros

Blood supply

Bladder and urethra

Clinical relevance

35 Reproductive system: ducts and genitalia

Time period: day 35 to postnatal development

Introduction

Ducts

External genitalia

Sex determination

Clinical relevance

36 Reproductive system: gonads

Time period: day 30 to postnatal development

Introduction

Gonads

Blood supply

Clinical relevance

37 Endocrine system

Time period: day 24 to birth

Introduction

Pituitary gland

Hypothalamus

Pineal body

Adrenal glands

Thyroid gland

Parathyroid glands

Clinical relevance

38 Head and neck: arch I

Time period: day 21 onwards

Introduction

Arch I

Cleft I

Pouch I

Clinical relevance

39 Head and neck: arch II

Time period: day 21 onwards

Introduction

Arch II

Cleft II

Pouch II

Clinical relevance

40 Head and neck: arch III

Time period: day 28 onwards

Introduction

Arch III

Cleft III

Pouch III

Clinical relevance

41 Head and neck: arches IV–VI

Time period: day 28 onwards

Introduction

Cleft IV

Pouch IV

Clinical relevance

42 Central nervous system

Time period: day 22 to postnatal development

Introduction

Spinal cord

Brain

Neural crest cells

Meninges

Clinical relevance

43 Peripheral nervous system

Time period: day 27 to birth

Introduction

Spinal nerves

Dermatomes

Autonomic nervous system

Cranial nerves

44 The ear

Time period: 22 day to birth

Internal ear

Membranous labyrinth

Bony labyrinth

Middle ear

External ear

Clinical relevance

45 The eye

Time period: weeks 3–10

Introduction

Optic cup and lens

Retina

Optic nerve

Meninges

Cornea

Extraocular muscles

Clinical relevance

Part 3: Self-assessment

Self-assessment MCQs

Self-assessment MCQ answers

Self-assessment EMQs

Self-assessment EMQ answers

Glossary of medical conditions and terms

Index

Companion website

This book is accompanied by a website containing a link to Dr Webster’s website and podcasts:

www.wiley.com/go/embryology

This edition first published 2012 © 2012 by John Wiley & Sons, Ltd.

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Library of Congress Cataloging-in-Publication Data

Webster, Samuel, 1974-

 Embryology at a glance / Samuel Webster, Rhiannon de Wreede.

p. ; cm. – (At a glance series)

 Includes bibliographical references and index.

 ISBN 978-0-470-65453-8 (pbk. : alk. paper)

 I. De Wreede, Rhiannon. II. Title. III. Series: At a glance series (Oxford, England).

 [DNLM: 1. Embryonic Development. QS 604]

 612.6'4–dc23

2011049102

A catalogue record for this book is available from the British Library.

Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic books.

Cover image: © Joseph Mercier | Dreamstime.com

Cover design by Meaden Creative

Preface

We wrote this book for our students; those studying medicine with us, those listening to the podcasts wherever they may be, and those studying the other forms that biology takes on their paths to whatever goals they may have in life. We have introduced many students to the fascinating and often surprising processes of embryological development, and we hope to do the same in this book. It is written for anyone wondering, “where did I come from?”

The content of this book extends beyond the curricula of most medicine, health and bioscience teaching programmes in terms of breadth, but we have limited its depth. Many embryology textbooks cover development in detail, but students struggle to get started, and to get to grips with early concepts. Hopefully we have addressed these difficulties with this book.

We hope that you will use this book to begin your studies of embryology and development, but also that you will return to it when preparing for assessments or checking your understanding. You will find example assessment questions in Chapters 46 and 47, and a glossary in Chapter 48.

Let this be the start of your integration of embryonic development with anatomy, to the ends of improved understanding and better patient care or scientific insight.

Acknowledgements

Thank you to Kim and Robin for being so encouraging and putting up with the time demands of completing this book. We would also like to thank the editors at Wiley-Blackwell for leading us through this process and for their support and encouragement, and Jane Fallows for all her work with the illustrations.

List of Abbreviations

AER

Apical ectodermal ridge

CAM

Cell adhesion molecule

CN

Cranial nerve

CSF

Cerebrospinal fluid

ECMO

Extracorporeal membrane oxygenation

FGF

Fibroblast growth factor

FSH

Follicle stimulating hormone

GnRH

Gonadotrophin releasing hormone

HbF

Foetal haemoglobin

hCG

Human chorionic gonadotrophin

hCS

Human chorionic somatomammotrophin

IUD

Intrauterine device – contraceptive device

IUGR

Intrauterine growth restriction

IVC

Inferior vena cava

IVD

Intervertebral disc

IVF

In vitro

fertilisation

LH

Luteinising hormone

LMP

Last menstrual period

PDA

Patent ductus arteriosus

PFO

Patent foramen ovale

PTH

Parathyroid hormone

PZ

Proliferating zone

Rh

Rhesus

SVC

Superior vena cava

TGF

Transforming growth factor

ZPA

Zone of polarising activity

TimeLine

1

Embryology in Medicine

What is Embryology?

Animals begin life as a single cell. That cell must produce new cells and form increasingly complex structures in an organised and controlled manner to reliably and successfully build a new organism (Figures 1.1 and 1.2). As an adult human may be made up of around 100 trillion cells this must be an impressively well-choreographed compendium of processes.

Embryology is the branch of biology that studies the early formation and development of these organisms. Embryology begins with fertilisation, and we have included the processes that lead to fertilisation in this text. The human embryonic period is completed by week 8, but we follow development of many systems through the foetal stages, birth and, in some cases, describe how changes continue to occur into infancy, adolescence and adult life (Figure 1.3).

Aims and Format

This book aims to be concise but readable. We have provided a page of text accompanied by a page of illustrations in each chapter. Be aware that the concise manner of the text means that the topic is not necessarily comprehensive. We aim to be clear in our descriptions and explanations but this book should prepare you to move on to more comprehensive and detailed texts and sources.

Why Study Embryology?

Our biological development is a fascinating subject deserving study for interest’s sake alone. An understanding of embryological development also helps us answer questions about our adult anatomy, why congenital abnormalities sometimes occur and gives us insights into where we come from. In medicine the importance of an understanding of normal development quickly becomes clear as a student begins to make the same links between embryology, anatomy, physiology and neonatal medicine.

The study of embryology has been documented as far back as the sixth century BC when the chicken egg was noted as a perfect way of studying development. Aristotle (384–322 BC) compared preformationism and epigenetic theories of development. Do animals begin in a preformed way, merely becoming larger, or do they form from something much simpler, developing the structures and systems of the adult in time? From studies of chickens’ eggs of different days of incubation and comparisons with the embryos of other animals Aristotle favoured epigenetic theory, noting similarities between the embryos of humans and other animals in very early stages. In a chicken’s egg, a beating heart can be observed with the naked eye before much else of the chicken has formed.

Aristotle’s views directed the field of embryology until the invention of the light microscope in the late 1500s. From then onwards embryology as a field of study was developed.

A common problem that students face when studying embryology is the apparent complexity of the topic. Cells change names, the vocabulary seems vast, shapes form, are named and renamed, and not only are there structures to be concerned with but also the changes to those structures with time. In anatomy, structures acquire new names as they move to a new place or pass another structure (e.g. the external iliac artery passes deep to the inguinal ligament and becomes the femoral artery). In embryology, cells acquire new names when they differentiate to become more specialised or group together in a new place; structures have new names when they move, change shape or new structures form around them. With time and study students discover these processes, just as they discover anatomical structures.

Embryology in Modern Medicine

If a student can build a good understanding of embryological and foetal development they will have a foundation for a better understanding of anatomy, physiology and developmental anomalies. For a medical student it is not difficult to see why these subjects are essential. If a baby is born with ‘a hole in the heart’, what does this mean? Is there just one kind of hole? Or more than one? Where is the hole? What are the physiological implications? How would you repair this? If that part of the heart did not form properly what else might have not formed properly? How can you explain to the parents why this happened, and what the implications are for the baby and future children? A knowledge of the timings at which organs and structures develop is also important in determining periods of susceptibility for the developing embryo to environmental factors and teratogens.

Why Read This Book?

We appreciate that the subject of embryology still induces concern and despair in students. However, if it helps you in your profession you should want to dig deep into the wealth of understanding it can give you. We also appreciate that you have enough to learn already and so this book hopes to represent embryology in an accessible format, as our podcasts try to do.

One thing that has not changed with the development of embryology as a subject is that the more information that is gathered, the more numerous are the questions left unanswered. For example, we barely mention the molecular aspects of development here. Should your interest in embryology and mechanisms of development be aroused by this book, we hope that you will seek out more detailed sources of information to consolidate your learning.

2

Language of Embryology

Time Period: Day 0–266

Introduction

The language used to describe the embryo and the developmental processes that mould it is necessarily descriptive. It is similar to anatomical terminology, but there are some common differences that the reader should be aware of.

The embryo does not, and for most of its existence cannot, take on the anatomical position. The embryo is more curved and folded than the erect adult. The adult anatomical position is described as the body being erect with the arms at the sides, palms forward and thumbs away from the body (Figure 2.1). The anatomical relationships of structures are described as if in this position, so for the embryo we need to rethink this a little.

Cranial–Caudal

Anatomically speaking, you may interchangeably use cranial or superior, and caudal or inferior. Cranial clearly refers to the head end of the embryo and caudal (from the Latin word cauda, meaning ‘tail’) refers to the tail end (Figure 2.2). If you imagine the early sheet of the embryo with the primitive streak (see Chapter 13) showing us the cranial and caudal ends, you can imagine that it can be clearer to use these terms rather than superior and inferior.

The term ‘rostral’ may also be used in place of cranial. Rostral is derived from the Latin word rostrum, meaning ‘beak’.

Dorsal–Ventral

The dorsal surface of the embryo and the adult is the back (Figure 2.2). Dorsal also refers to the surface of the foot opposite to the plantar surface, the surface of the tongue covered with papillae, and the superior surface of the brain, so some care is needed.

The ventral surface of the embryo is the front or anterior of the embryo, opposite the dorsal surface.

Medial–Lateral

As with adult anatomy, structures nearer to the midline sagittal plane are more medial, and structures further from the midline are more lateral (Figure 2.3). This also helps us describe the left–right axis of the embryo.

Proximal–Distal

Proximal and distal are a little different from medial and lateral, but similarly describe structures near to the centre of the body (proximal) and further from the centre (distal) (Figure 2.1). These terms are typically used to describe limb structures. The hand is distal to the elbow, for example.

Sections

Often, to show the parts of the embryo being described, illustrations must be of a section of the embryo or a structure. These sections may be transverse, median, coronal or oblique. You can see these planes of sections in the illustrations on the opposite page (Figures 2.4–2.6).

3

Introduction to Development

Time Period: Day 0 to Adult

Development

Development, in this book, describes our journey from a single cell to a complex multicellular organism. Development does not end at birth, but continues with childhood and puberty to early adulthood.

We must describe how a cell from the father and a cell from the mother combine to form a new genetic individual, and how this new cell forms others, how they become organised to form new shapes, specialised interlinked structures, and grow. With this knowledge we become able to understand how these processes can be interfered with, and how abnormalities arise.

Growth

Growth may be described as the process of increasing in physical size, or as development from a lower or simpler form to a higher or more complex form.

In embryology, growth with respect to a change in size may occur through an increase in cell number, an increase in cell size or an increase in extracellular material (Figure 3.1).

Increasing cell number occurs by cells dividing to produce daughter cells by proliferation. Proliferation is a core mechanism of increasing the size of a tissue or organism, and is also found in adult tissues in repair or where there is an expected continual loss of cells such as in the skin or gastrointestinal tract. Stem cells are particularly good at proliferating.

An increase in cell size occurs by hypertrophy. In adults, muscle cells respond to weight training by hypertrophy, and this is one way in which muscles become larger. During development, hypertrophy of cartilage cells during endochondral ossification is an important part of the growth of long bones. Be aware that the term hypertrophy can also be used to describe a structure that is larger than normal.

Cells may surround themselves with an extracellular matrix, particularly in connective tissues such as bone and cartilage. By accretion these cells increase the size of the tissue by increasing the amount of extracellular matrix, either as part of development or in response to mechanical loading.

Cells may also die by programmed cell death, or apoptosis. This might be considered an opposite to growth, and in development is an important method of forming certain structures like the fingers and toes.

Differentiation

During development, cells become specialised as they move from a multipotent stem cell type towards a cell type with a particular task, such as a muscle cell, a bone cell, a neuron or an epithelial cell. When the cell becomes more specialised it is considered to have differentiated into a mature cell type. If that cell divides, its daughter cells will also be of that mature cell type.

In humans, a mature cell is unlikely to dedifferentiate back into a stem cell, but the process by which this can occur is being exploited in the laboratory with the aim of producing stem cells from adult tissues. These stem cells could then be pushed to differentiate into the cell type needed to grow new tissue or treat a disease.

Signalling

A signal from one group of cells influences the development of another (adjacent, nearby or distant) group of cells. Hormones act as signals, for example. For a cell to be affected by a signal it must possess an appropriate receptor.

In the embryo the signalling of a vast array of different proteins by different groups of cells allows those cells to gain information about their current and future tasks, be that migration, proliferation, differentiation or something else.

Organisation

Early in development the ball of cells or simple sheets of the embryo do not give much clue about which cells will form which structures. It is difficult to determine which part will become the head and which will become the tail. However, the cells are aware of their position and the roles that they will have and we can see this by looking at the signalling proteins and connections between cells.

For example, the upper limb begins to develop as a simple bud of cells. The cells in that bud must be organised to produce the structures of the arm, the forearm and the hand. The ulna bone must form in the right place relative to the radius, and the thumb must form appropriately in relation to the fingers. This may occur partly because a group of cells on the caudal aspect of the limb bud produces a morphogen that diffuses across the early limb bud (Figure 3.2). Cells near the site of morphogen production experience a high concentration, and cells further away on the cranial side of the bud experience a lower concentration. Development of these cells progresses differently as a result. If experimentally you transplant some of the morphogen-producing cells to the cranial part of the limb bud, duplicate digital structures form. See Chapter 23 for more about limb development.

This is one example of how cells organise themselves and others during development. With organisation, structure follows.

Morphogenesis

The formation of shape during development is morphogenesis. Cells are able to change the ways in which they adhere to one another, they can extend processes and pull themselves along, migrating to new locations, and they can change their own shapes. In a tissue there may be a change in cell number, cell size or accretion of extracellular material. In these ways a tissue gains and changes shape.

An early example of morphogenesis in embryonic development occurs with the change from simple flat sheets of cells to the rolled up tubes of the embryo and gastrointestinal tract (Figure 3.3). A simple structure has become more complex. Chapter 13 covers this in more detail.

Clinical Relevance

Interruptions of signalling, proliferation, differentiation, migration, and so on, cause congenital abnormalities. Teratogens that affect development during key periods may have significant effects. For example, if the drug thalidomide is taken during early limb development it can cause phocomelia (hands and feet attached to abnormally shortened limbs). Other environmental factors and genetic mutations can cause abnormal development. The embryo is most sensitive during weeks 3–8.

Dysmorphogenesis is a term used for the abnormal development of body structures. It may occur because of malformation or deformation. If the processes required to normally form a structure fail to occur the result is a malformation. If the neural tube fails to close, for example, the resulting neural tube defect is a malformation. A deformation occurs if external mechanical forces affect development. For example, damage to the amniotic sac can cause amniotic bands that may wrap around developing limbs and cause amputation of limbs or digits.

4

Embryonic and Foetal Periods

Time Period: Day 0 to Birth

Embryonic Period

The embryonic period is considered to be the period from fertilisation to the end of the eighth week. The period from fertilisation to implantation of the blastocyst into the uterus (2 weeks) is sometimes called the period of the egg.

During the period of the egg the early zygote rapidly proliferates to produce a ball of cells that makes its way along the uterine tube towards the uterus. The complexity of the blastocyst increases as it progresses towards the site of implantation.

During the embryonic period the major structures of the embryo are formed, and by 8 weeks most organs and systems are established and functioning to some extent, but many are at an immature stage of development. At the end of the eighth week the external features of the embryo are recognisable; the eyes, ears and mouth are visible, the fingers and toes are formed, and limbs have elbow and knee joints.

Foetal Period

From the ninth week to birth the foetus matures during the foetal period. The foetus grows rapidly in size, mass and complexity, and its proportions change (for example, head to trunk, and limbs). The foetus’ weight increases considerably in the latter stages of the foetal period. Organs and systems continue in their functional development, and some systems change considerably at birth (for example, the respiratory and circulatory systems).

Birth in humans normally occurs between 37 and 42 weeks after fertilisation.

Trimesters

The nine calendar month gestation period is split into 3-month periods called trimesters. During the first trimester the embryonic and early foetal periods occur. In the second trimester the uterus becomes much larger as the foetus grows considerably, and symptoms of morning sickness tend to subside. A foetus in the third trimester turns and the head drops into the pelvic cavity (engagement) in preparation for birth. Babies born prematurely during the third trimester may survive, particularly with specialised intensive care treatment.

Clinical and Embryological Timings

Embryologists use timings from the date of fertilisation, and all the timings in this book will relate to that time. Embryologists studying the embryos of animals often have an advantage in being able to fairly accurately note when fertilisation occurred. Clinically, the date of fertilisation is more difficult to determine.

A woman’s menstrual cycle will take around 28 days to complete, starting with the first day of the menstrual period (bleed) and returning to the same point (Figure 4.1). Menstruation occurs for 3–6 days, followed by the proliferative phase for 10–12 days. Ovulation occurs around 14 days before the start of the next menstrual period. If the released ovum is fertilised menstruation will not occur. Fertilisation must occur within 1 day of ovulation.

The event of the last menstrual period can be used to date the period of gestation clinically, although the date on which fertilisation took place will be uncertain because of variability in the length of the cycle between the start of menstruation and ovulation.

Clinically, gestational timings are around 2 weeks longer than an embryologist’s timing (Figure 4.2). If the embryonic period is complete at the end of week 8, a clinician would record this as the end of week 10 (Figure 4.3).

Clinical Relevance

If you are a medical, nursing or health sciences student then you must be aware of the 2-week difference between embryologists’ and clinicans’ gestation timings.

A gestation period of 40 weeks is equal to 10 lunar months. A period of 10 lunar months is, on average, 7 days longer than any 9 calendar months. Using the mother’s date of the start of her last menstrual period you can quickly calculate an estimated date of delivery by adding 9 calendar months and 7 days.

An awareness of the period of the egg, the embryonic period and the trimesters helps understand the periods of susceptibility of the embryo and the foetus. For example, after the period of the egg and during the embryonic period the embryo is particularly vulnerable to the effects of teratogens and environmental insults. The respiratory system develops significantly during the third trimester, so linking the timing of a premature birth to the potential requirements of the baby are important.

5

Mitosis

Time Period: Day 0 to Adult

Cell Division

Cell division normally occurs in eukaryotic organisms through the process of mitosis, in which the maternal cell divides to form two genetically identical daughter cells (Figure 5.1). This allows growth, repair, replacement of lost cells and so on. A key process during mitosis is the duplication of DNA to give two identical sets of chromosomes, which are then pulled apart and new cells are formed around each set. The new cells may be considered to be clones of the maternal cell.

Mitosis