Embryology at a Glance

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.

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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

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