Embryology at a Glance

Companion website

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

www.wiley.com/go/embryology

Embryology at a GlanceSamuel WebsterLecturer in Anatomy & EmbryologyCollege of MedicineSwansea UniversitySwansea, UK

Rhiannon de WreedeHonorary LecturerCollege of MedicineSwansea UniversitySwansea, UK

A John Wiley & Sons, Ltd., Publication

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

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Library of Congress Cataloging-in-Publication DataWebster, 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

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Cover image: © Joseph Mercier | Dreamstime.comCover design by Meaden Creative

Set in 9/11.5pt Times by Toppan Best-set Premedia Limited

1 2012

Contents  5

Contents

Preface  6 Acknowledgements  7List of abbreviations  8Timeline  9

Part 1  Early development   1  Embryology in medicine  10  2  Language of embryology  12  3  Introduction to development  14  4  Embyonic and foetal periods  16  5  Mitosis  18  6  Meiosis  20  7  Spermatogenesis  22  8  Oogenesis  24  9  Fertilisation  2610  From zygote to blastocyst  2811  Implantation  3012  Placenta  3213  Gastrulation  3414  Germ layers  3615  Neurulation  3816  Neural crest cells  4017  Body cavities (embryonic)  4218  Folding of the embryo  4419  Segmentation  4620  Somites  48

Part 2  Systems development 21  Skeletal system (ossification)  5022  Skeletal system  5223  Muscular system  54

24  Musculoskeletal system: limbs  5625  Circulatory system: heart tube  5826  Circulatory system: heart chambers  6027  Circulatory system: blood vessels  6228  Circulatory system: embryonic veins  6429  Circulatory system: changes at birth  6630  Respiratory system  6831  Digestive system: gastrointestinal tract  7032  Digestive system: associated organs  7233  Digestive system: congenital anomalies  7434  Urinary system  7635  Reproductive system: ducts and genitalia  7836  Reproductive system: gonads  8037  Endocrine system  8238  Head and neck: arch I  8439  Head and neck: arch II  8640  Head and neck: arch III  8841  Head and neck: arches IV–VI  9042  Central nervous system  9243  Peripheral nervous system  9444  The ear  9645  The eye  98

Part 3  Self-assessment MCQs  101MCQ answers  106EMQs  107EMQ answers  108Glossary  109

Index  114

Companion website 

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

www.wiley.com/go/embryology

6  Preface

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 text-

books 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 develop-ment with anatomy, to the ends of improved understanding and better patient care or scientific insight.

Acknowledgements  7

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.

8  List of abbreviations

List of abbreviations

AER ApicalectodermalridgeCAM CelladhesionmoleculeCN CranialnerveCSF CerebrospinalfluidECMO ExtracorporealmembraneoxygenationFGF FibroblastgrowthfactorFSH FolliclestimulatinghormoneGnRH GonadotrophinreleasinghormoneHbF FoetalhaemoglobinhCG HumanchorionicgonadotrophinhCS HumanchorionicsomatomammotrophinIUD Intrauterinedevice–contraceptivedeviceIUGR Intrauterinegrowthrestriction

IVC InferiorvenacavaIVD IntervertebraldiscIVF In vitrofertilisationLH LuteinisinghormoneLMP LastmenstrualperiodPDA PatentductusarteriosusPFO PatentforamenovalePTH ParathyroidhormonePZ ProliferatingzoneRh RhesusSVC SuperiorvenacavaTGF TransforminggrowthfactorZPA Zoneofpolarisingactivity

Time line  9

Timeline

Language of embryology (Chapter 2)

Introduction to development (Chapter 3)

Embryonic and foetal periods (Chapter 4)

Spermatogenesis (Chapter 7)

Oogenesis (Chapter 8)

Fertilisation (Chapter 9)

From zygote to blastocyst (Chapter 10)

Implantation (Chapter 11)

Placenta (Chapter 12)

Gastrulation (Chapter 13)

Formation of germ layers (Chapter 14)

Formation of the heart tube (Chapter 25)

Folding of the embryo (Chapter 18)

Neurulation (Chapter 15)

Segmentation (Chapter 19)

Formation of blood vessels (Chapter 27)

Somite development (Chapter 20)

Development of digestive system (Chapter 31)

Development of body cavities (Chapter 17)

Development of urinary system (Chapter 34)

Development of head and neck structures (Chapter 38–41)

Development of the eye (Chapter 45)

Migration of neural crest cells (Chapter 16)

Development of muscular system (Chapter 23)

Development of the ear (Chapter 44)

Development of central nervous system (Chapter 42)

Cranial neuropore closes (Chapter 15)

Development of endocrine system (Chapter 36)

Caudal neuropore closes (Chapter 15)

Heart tube divides into four chambers (Chapter 26)

Development of skeletal system (Chapter 22)

Development of peripheral nervous system (Chapter 43)

Development of musculoskeletal system (Chapter 24)

Development of respiratory system (Chapter 30)

Formation of the atrial septa (Chapter 26)

Ossification of skeletal system (Chapter 21)

Development of reproductive system (Chapter 35)

Foetus can hear external sounds (Chapter 44)

0 10 20 30Days Weeks

40 50 60 10 20 30 40Death

Menopause

Days Weeks Adult

PubertyAdult

0 10 20 30 40 50 60 10 20 30 40Death

Puberty Menopause

Embryology at a Glance, First Edition. Samuel Webster and Rhiannon de Wreede. 10  © 2012 John Wiley & Sons, Ltd. Published 2012 by John Wiley & Sons, Ltd.

1 Embryology in medicine

Figure 1.1The early embryo develops from a simple group of cells into complex shapes and structures in the early weeks

Figure 1.2Development continues beyond embryology and the foetus continues to grow and mature

Figure 1.3Development of biological structures andsystems continues through childhood,adolescence and into adulthood. Changescontinue to occur throughout life

Embryology in medicine Early development 11

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 organ-ism (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 for-mation 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 formatThis 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 descrip-tions 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 inven-tion 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 embryol-ogy 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 spe-cialised 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 proc-esses, just as they discover anatomical structures.

Embryology in modern medicineIf a student can build a good understanding of embryological and foetal development they will have a foundation for a better under-standing 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 determin-ing periods of susceptibility for the developing embryo to environ-mental 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 embry-ology 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 develop-ment be aroused by this book, we hope that you will seek out more detailed sources of information to consolidate your learning.