Ebook Veterinary embryology: Part 1

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Part 1 book "Veterinary embryology" includes content: Division, growth and differentiation of cells; gametogenesis, fertilisation, cleavage, aspects of cell signalling and gene functioning during development, establishment of the basic body plan, coelomic cavities, foetal membranes, forms of implantation and placentation, cardiovascular system.

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VETERINARY EMBRYOLOGY
VETERINARY EMBRYOLOGY T. A. McGeady, MVB, MS, MSc, MRCVS Former Senior Lecturer in Veterinary Anatomy, Histology and Embryology, Department of Veterinary Anatomy, Faculty of Veterinary Medicine, University College Dublin P. J. Quinn, MVB, PhD, MRCVS Professor Emeritus, Former Professor of Veterinary Microbiology and Parasitology, Faculty of Veterinary Medicine, University College Dublin E. S. FitzPatrick, FIBMS Department of Veterinary Anatomy, Faculty of Veterinary Medicine, University College Dublin M. T. Ryan, MSc, PhD Molecular Biology Laboratory, Faculty of Veterinary Medicine, University College Dublin Illustrations by S. Cahalan, MVB Faculty of Veterinary Medicine, University College Dublin
© 2006 TA McGeady, PJ Quinn, ES FitzPatrick, MT Ryan and S Cahalan Editorial Ofﬁces: Blackwell Publishing Ltd, 9600 Garsington Road, Oxford OX4 2DQ, UK Tel: +44 (0)1865 776868 Blackwell Publishing Professional, 2121 State Avenue, Ames, Iowa 50014-8300, USA Tel: +1 515 292 0140 Blackwell Publishing Asia, 550 Swanston Street, Carlton, Victoria 3053, Australia Tel: +61 (0)3 8359 1011 The right of the Author to be identiﬁed as the Author of this Work has been asserted in accordance with the Copyright, Designs and Patents Act 1988. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by the UK Copyright, Designs and Patents Act 1988, without the prior permission of the publisher. First published 2006 by Blackwell Publishing Ltd ISBN-10: 1-4051-1147-X ISBN-13: 978-1-4051-1147-8 Library of Congress Cataloging-in-Publication Data Veterinary embryology / T.A. McGeady . . . [et al.].; illustrations by S. Cahalan.— 1st ed. p. cm. Includes bibliographical references and index. ISBN-13: 978-1-4051-1147-8 (pbk. : alk. paper) ISBN-10: 1-4051-1147-X (pbk. : alk. paper) 1. Veterinary embryology. I. McGeady, T. A. (Thomas A.) SF767.5.V48 2006 636.089′264—dc22 2005022781 A catalogue record for this title is available from the British Library For further information on Blackwell Publishing, visit our website: www.blackwellpublishing.com
Contents Preface ix Morphogens 49 Acknowledgements xi Differentiation 49 Gene structure and organisation 49 1 Division, growth and differentiation of cells 1 X-chromosome inactivation 49 The cell cycle 1 DNA methylation and parental Mitosis 1 imprinting in mammals 49 Meiosis 5 Promoters, enhancers and silencers 50 Transcription factors 50 2 Gametogenesis 10 Gene systems essential for development 50 Spermatogenesis 10 Experimental measurement of gene Oogenesis 13 expression 53 Experimental evaluation of gene function 53 3 Fertilisation 17 Concluding comments 53 Capacitation 18 Cellular events in the process of fertilisation 18 7 Establishment of the basic body plan 54 Barriers to polyspermy 18 Ovum activation 19 8 Coelomic cavities 59 In vitro fertilisation 21 Pleural and pericardial cavities 59 Comparative fertilisation rates 21 Diaphragm 61 Sex determination 22 Peritoneal cavity 64 Parthenogenesis 22 Omenta 65 Sex ratio 23 Chromosomes of domestic animals 23 9 Foetal membranes 66 Development of the foetal membranes 68 4 Cleavage 25 Birds 68 Cleavage in primitive chordates, Mammals 70 amphibians, avian species and mammals 25 Stem cells 30 10 Forms of implantation and placentation 78 Implantation 78 5 Gastrulation 34 Placentation in mammals 81 Primitive chordates 34 Functional aspects of the placenta 101 Amphibians 34 Avian species 34 11 Cardiovascular system 105 Mammals 36 Development of the cardiac tubes 106 Establishment of left–right symmetry Molecular aspects of cardiac development 112 in vertebrates 38 Formation of the cardiac chambers 112 Twinning 38 Conducting system of the heart 117 Developmental anomalies of the 6 Aspects of cell signalling and gene cardiovascular system 130 functioning during development 42 Cellular messengers and receptors 42 12 Embryological and post-natal features of Types of signalling 43 haematopoiesis 136 Induction and competence 44 Embryological aspects of haematopoiesis 136 Paracrine signalling during development 44 Cell differentiation and maturation during Apoptosis 48 haematopoiesis 139
vi CONTENTS Stem cells in human adults and mature 17 Urinary system 233 animals 146 Kidney 233 Immunodeﬁciency 147 Molecular basis of metanephros development 235 Inherited defects in natural immunity 151 Unilobar kidneys 238 Multilobar kidneys 240 13 Nervous system 153 Bladder 240 Dorsal–ventral patterning of the neural tube 153 Developmental anomalies of the urinary Neural crest 154 system 240 Differentiation of the cellular components of the neural tube 155 18 Male and female reproductive systems 244 Spinal nerves 157 Primordial germ cells 244 Myelination of peripheral nerve ﬁbres 161 Undifferentiated stage of gonad formation 245 Changes in the relative positions of the Differentiation and maturation of the testes 245 spinal cord and the developing Differentiation and maturation of the ovaries 245 vertebral column 161 Features of equine gonadal development 248 Anomalies of the spinal cord 161 Genital ducts 249 Differentiation of the brain sub-divisions 163 Formation of the genital fold 251 Ventricular system of the brain and External genitalia 252 cerebrospinal ﬂuid circulation 174 Factors which inﬂuence sexual Molecular aspects of brain development 175 differentiation in mammals 253 Brain anomalies 176 Sex determination 254 Brain stem and spinal cord 177 Molecular aspects of gonadogenesis 254 Cranial nerves 178 Inﬂuence of hormones on development Peripheral nervous system 178 of genital ducts and external genitalia 255 Autonomic nervous system 178 Sexual differentiation, associated brain Enteric nervous system 181 function and subsequent sexual Meninges 182 behaviour at puberty 257 Anomalies of sexual development 257 14 Muscular and skeletal systems 184 Descent of the testes 259 Differentiation of somites 184 Ovarian migration 262 Muscular system 184 Cryptorchidism 262 Skeletal muscle 184 Development of the mammary gland 263 Cytodifferentiation of muscle 186 Comparative features of mammary gland Skeletal system 187 development in domestic animals 265 Skeletal anomalies 203 19 Structures in the head and neck 268 15 Digestive system 205 Pharyngeal region 268 Molecular regulation of alimentary tract Derivatives of the pharyngeal apparatus 269 development 207 Face 270 Oesophagus 209 Nasal cavities 272 Stomach 209 Oral cavity 277 Liver 213 Tongue 277 Pancreas 213 Salivary glands 278 Spleen 216 Teeth 279 Development and rotation of the intestines Comparative aspects of dentition 281 in domestic animals 217 Molecular aspects of tooth development 282 Comparative features of the intestines 217 Development of the skull 282 Hindgut 220 Congenital malformations of face and Developmental anomalies of the oral cavity 283 alimentary tract 221 20 Endocrine system 286 16 Respiratory system 225 Pituitary gland 286 Formation of the larynx 225 Pineal gland 289 Trachea, bronchi and lungs 225 Adrenal glands 289 Molecular aspects of respiratory development 231 Thyroid gland 291 Anomalies of the respiratory system 231 Parathyroid glands 291
CONTENTS vii Thymus 293 23 Age determination of the embryo and foetus 331 Pancreatic islets 293 24 Genetic, chromosomal and environmental 21 Eye and ear 295 factors which adversely affect pre-natal Eye 295 development 337 Ear 304 Mutations 337 Chromosomal abnormalities 338 22 Integumentary system 313 Teratogens 339 Epidermis 313 Therapeutic drugs and chemicals 339 Dermis 314 Cytotoxic drugs used for treating Hypodermis 315 neoplastic diseases 348 Hair 315 Poisonous plants 348 Mammalian skin glands 318 Infectious agents 349 Avian skin 320 Assessing the aetiology of congenital disease 353 Congenital and inherited defects of the skin 322 Hooves and claws 322 Glossary 355 Horns and related structures 328 Index 364
Preface An understanding of the origin, development and matu- reviewed. Age determination and aspects of mutagenesis ration of cells in the developing embryo and, later, in the and teratogenesis are brieﬂy reviewed in ﬁnal chapters. foetus provides veterinary students with information relevant to organ primordia and development of body Although this book is intended primarily as a textbook systems. A study of embryology offers the student an for undergraduate veterinary students, it may be of value understanding of the development, structure, ﬁnal form to colleagues engaged in teaching embryology, either as and relationships of tissues and organs. Developmental part of a veterinary curriculum or in courses relating defects and the clinical conditions to which they give rise to animal science or developmental biology. Research can be more completely understood through a knowledge workers engaged in projects on animal reproduction of the factors which control developmental processes and allied topics may ﬁnd particular chapters relevant and the deleterious affects of environmental teratogens to their ﬁelds of investigation. on normal embryological development. Throughout the book, emphasis is placed on the origin This book is primarily concerned with developmental and differentiation of tissues and organs and their aspects of cells, tissues, organs and body systems of relationships to each other. This approach provides a animals. Where feasible, comparative aspects of human logical foundation for acquiring an understanding of embryology are included. Drawings of cells, tissues and the form and relationships of cells, tissues, organs and organs, along with ﬂow diagrams and tables, are used to structures in deﬁned regions of the body. Such know- provide a clear understanding of information contained ledge is a fundamental requirement for the appreciation in the text. of topographical anatomy, a cornerstone in the acquisi- tion of clinical skills, interpretation of diagnostic imag- There are 24 chapters in this book, each dealing with ing and the implementation of surgical procedures. topics which are fundamental to an understanding Molecular aspects of embryology provide an introduc- of the sequential stages of embryological and foetal tion to genes and the transcription factors which promote development. Cell division, gametogenesis, fertilisation, or regulate orderly development of the embryo and cleavage and gastrulation are presented in sequential foetus. Developmental defects of clinical signiﬁcance chapters. Succeeding chapters are concerned with cell are also included. The classiﬁcation used throughout the signalling, establishment of a body plan, formation of book generally conforms to the Nomina Embryologica foetal membranes and placentation. Body systems are Veterinaria system proposed in 1994. Selected review considered in separate chapters and the embryological articles and textbooks are listed in each chapter as aspects of structures associated with special senses are sources of additional information.
Acknowledgements We wish to acknowledge the constructive comments and Veterinary Microbiology and Parasitology by and advice of colleagues who reviewed chapters and Professors S. Carrington and G. Mulcahy and by proofread sections of the book or who offered technical Dr B. Markey. support and guidance during the completion and assem- bly of the text: We wish to thank the library staff of the Faculty of Veterinary Medicine, especially Ms G. Ryan, for the H. Bassett, S. Baynes, J. Cassidy, W.J.C. Donnelly, help and facilities provided. M. Dore, M. Gleeson, O. Golden, T. Grimes, P.J. Hartigan, S. Hogan, D. Hogg, E. Hughes, J. Irwin, Through her careful reading of the manuscript, Ms A. Kelly, D. Kilroy, F. LeMatti, D. Maguire, G. Mary Sayers, copy editor, improved the accuracy of the McCarthy, T. McElligot, T. Miceli, E. Murphy, text and the clarity of the illustrations. Ms Samantha J. O’Donovan, K. O’Driscoll, E. O’Neill, P. O’Neill, Jackson and Ms Sally Rawlings and their colleagues at J. Quinn, M. Quinn, C. Reid, J. Roche, M. Scanlon and Blackwell Publishing provided advice on layout and T. Sweeney. style and offered encouragement as the book approached the end of its uncertain gestational period. We are appreciative of the space and facilities made available in the Departments of Veterinary Anatomy Dublin, September 2005
1 Division, Growth and Differentiation of Cells The mammalian body is composed of an array of phases, the cell is metabolically active, fulﬁlling its spe- organs, tissues and individual cells which function in a cialised function preparatory to the next phase of the specialised and highly coordinated manner. Although cycle, but DNA replication does not take place. During these cells, tissues and organs exhibit considerable the S phase, DNA synthesis occurs prior to chromo- diversity in both structure and function, they all derive somal replication. This is followed by mitosis which from a single cell, a fertilised ovum. The fertilised ovum occurs during the M phase. Collectively, the G1, S and is a product of the fusion of two specialised reproductive G2 phases constitute the interphase (Fig. 1.1). Cells cells, gametes, of male and female origin. Following which enter a G0 state may remain transiently or perma- fertilisation, the ovum undergoes a series of divisions nently in that state. Certain fully differentiated cells, which ultimately lead to the formation of pluripotent such as neurons, do not divide, and continue to function stem cells, from which all cells, tissues and organs of the permanently in a G0 state. Other cell types, such as body arise. The study of this process of growth and dif- epithelial cells and hepatocytes, can re-enter the cell ferentiation, beginning with the fertilisation of an ovum cycle from G0 and proceed to mitotic division in and progressing to a fully formed individual animal, is response to appropriate stimuli. termed embryology. A number of stimuli such as growth factors, mitogens Cells associated with tissue formation and regeneration and signals from other cells and from the extra-cellular are described as somatic cells. Specialised reproductive matrix can induce cells in a G0 state to re-enter the cells, referred to as germ cells, include gametes of male cell cycle near the end of the G1 phase. Growth factors and female origin and their precursors. which bind to cell surface receptors activate intra- cellular signalling pathways. In most mammalian cells, Coordinated and regulated cell division is essential for the activation of genes encoding cyclins and cyclin- embryological development. Somatic cell division con- dependent kinases (Cdks) speciﬁc to the G1 phase regu- sists of nuclear division, mitosis, followed by cytoplas- late the cell cycle and commit the cell to enter the S mic division, cytokinesis. In mitotic division of somatic phase. This process is initiated at the restriction point, a cells, the daughter cells produced are genetically iden- stage at which mammalian cells become committed to tical. A form of cell division distinctly different from entering the S phase and are then capable of completing mitosis occurs in germ cells. In this form of cell division, the cell cycle independent of extra-cellular inﬂuences. referred to as meiosis, the cells produced contain half the number of chromosomes of the progenitor germ cell The rate of cell division varies in different cell types and and are not genetically identical. Somatic cell division at different stages of differentiation. Variations in cell combined with other cellular processes such as pro- cycle length are largely attributed to differences in the gressive differentiation, migration, adhesion, hypertro- length of the G1 phase, which can range from six hours phy and apoptosis are prerequisites for embryological to several days. Early embryonic development is charac- development. terised by rapid cell division, but as cells become more differentiated during organ development, the rate of cell division generally decreases. The cell cycle Somatic cells undergo a series of molecular and morpho- Mitosis logical changes as part of the cell cycle. These changes occur in four sequential phases, namely G1, S, G2 and The nuclei of somatic cells of each mammalian species M, and also a quiescent phase, termed G0 (Fig. 1.1). The have a deﬁned number of chromosomes (Table 1.1). G1 and G2 phases are termed resting phases. In these A somatic cell with a full complement of chromosomes
2 DIVISION, GROWTH AND DIFFERENTIATION OF CELLS 1 Figure 1.1 Stages in somatic cell division indicating the major phases of the cell cycle.
1 DIVISION, GROWTH AND DIFFERENTIATION OF CELLS 3 Table 1.1 The number of chromosomes in diploid human and microtubule spindles or asters. The spindles are respon- animal cells. sible for the movement of the centrosomes to opposite poles of the dividing cell. Species Number of chromosomes (2n) Microtubules, an essential part of the mitotic apparatus, are visible microscopically only during the M phase. Indi- Humans 46 vidual microtubules are cylindrical structures, composed Cats 38 of 13 parallel protoﬁlaments consisting of alternating Cattle 60 α-tubulin and β-tubulin subunits. An individual micro- tubule may grow or shrink by a process of polymerisa- Chickens 78 tion of α-tubulin and β-tubulin. A growing microtubule Dogs 78 has a structure referred to as a guanidine-triphosphate (GTP) cap. The β-subunit of a microtubule contains GTP Donkeys 62 capable of being hydrolysed to guanidine-diphosphate Goats 60 (GDP). This, in turn, alters the conformation of the sub- units, resulting in shrinking of the microtubules. If GTP Horses 64 hydrolysis occurs more rapidly than subunit addition, Pigs 38 the cap is lost and the microtubule shrinks. Shrinking and growing are a dynamic process and these changes Rabbits 44 enable the microtubules to actively orientate and move Rats 42 chromosomes during mitosis and meiosis. Sheep 54 Metaphase Events during the metaphase stage of mitosis can be is referred to as diploid and given the designation 2n. divided into two phases, pro-metaphase and metaphase. The term mitosis is used to describe nuclear division Disintegration of the nuclear envelope marks the of somatic cells, a process which usually results in the beginning of pro-metaphase. A kinetochore, a protein production of two cells, with the same chromosome complex which forms on the centromeres during late complement as the progenitor cell from which they prophase, acts as a platform for attachment to micro- derived. Mitosis is essential for embryonic growth and tubules. Chromosomes attach to the microtubules via development and for repair and replacement of tissue their kinetochores and the combination of these two throughout life. The stages of mitosis occur as a distinct latter structures is termed a kinetochore microtubule. sequence of cytological events, which are part of the cell The formation of the kinetochore microtubule enables cycle. the movement of chromosomes to take place. During metaphase, the chromosomes are positioned midway between the poles of the cell at a region termed the Stages of mitosis metaphase plate. Each sister chromatid is attached to the Preparatory to mitosis, the chromosomes are replicated centrosome by its kinetochore microtubule (Fig. 1.2C). in the S phase of the cell cycle forming sister chromatids. Within the nuclear envelope sister chromatids remain Anaphase attached at a constricted region of the chromosome called a centromere. Following the G2 phase, mitosis, which During the anaphase stage, the pairs of conjoined sister can be divided into four stages, prophase (Fig. 1.2B), chromatids synchronously separate as the centromeres metaphase (Fig. 1.2C), anaphase (Fig. 1.2D) and ﬁnally split and the attached kinetochore microtubules shorten. telophase (Fig. 1.2E), begins. The stages of mitosis are The newly separated chromatid sets are drawn towards usually followed by cytoplasmic division or cytokinesis opposite poles of the cell (Fig. 1.2D). (Fig. 1.2F). Telophase Prophase The two groups of identical chromosomes (former chro- The ﬁrst stage of mitosis is prophase (Fig. 1.2B). Dur- matids) clustered at their respective poles, de-condense ing this period, the chromosomes, consisting of closely and a nuclear envelope forms around each set. The for- associated sister chromatids, condense. Outside the mation of nuclear envelopes marks the end of mitosis, a nucleus, the centrosomes, composed of paired centrioles process which results in equal and symmetrical division previously replicated during interphase, begin to form of the nucleus (Fig. 1.2E).
4 DIVISION, GROWTH AND DIFFERENTIATION OF CELLS 1 Figure 1.2 An outline of the sequential stages in mitosis (A to G). After the G2 phase, prophase commences followed by metaphase, anaphase, telophase and cytokinesis, leading to the formation of two daughter cells.
1 DIVISION, GROWTH AND DIFFERENTIATION OF CELLS 5 within a single cell in the bone marrow or in peripheral Cytokinesis lymphoid tissue. With the accumulation of large popu- Following the formation of the nuclear envelope, a con- lations of abnormal cells, clinical effects of neoplasia tractile ring of actin and myosin pinches the cell wall become evident. and divides the cytoplasm, resulting in the formation of two daughter cells (Figs. 1.2F and G). This latter pro- Meiosis cess, termed cytokinesis, typically results in the forma- tion of two equally-sized daughter cells. Occasionally, This process of cell division occurs only during gameto- unequal amounts of cytoplasm or organelles may be genesis. Meiosis differs from mitosis in several respects: distributed to the daughter cells during cytokinesis. In (1) The resulting gametes are haploid and are given the some instances mitosis may occur without subsequent designation ‘n’. cytokinesis, resulting in the formation of binucleate or, (2) There is a reciprocal exchange of genetic material occasionally, multinucleate cells. between non-sister chromatids (Fig. 1.3). (3) The resulting gametes are a product of the random In lower organisms such as amphibians, the cytokinesis segregation of maternally-derived and paternally- which occurs early in development can generate daugh- derived chromatids. ter cells in which the factors which direct the fate of the cells may not be uniformly distributed. This unequal Meiosis is divided into two stages, meiosis I and II. division of fate determinants results in differing devel- opmental potential in individual daughter cells. In The ﬁrst meiotic division mammals, experimental evidence suggests that cell divisions which give rise to totipotential cells occur early Meiosis I consists of prophase I (Figs. 1.4B and C), in development. This suggests that, in mammals, cyto- metaphase I (Fig. 1.4D), anaphase I (Fig. 1.4E) and plasmic determinants are shared uniformly between telophase I (Fig. 1.4F). The amount of DNA in a cell daughter cells and that the initial stages of differentia- entering prophase I doubles. tion arise as a result of cell communication and micro- environmental factors. Prophase I During prophase I, many crucial intracellular events Regulation of mitosis occur (Figs. 1.4B and C). This process can be further The enzyme M-cyclin-dependent kinase (M-Cdk) has subdivided into ﬁve substages: leptotene, zygotene, a central role in the initiation of mitosis following the pachytene, diplotene, and diakinesis. At the diakinesis G2 phase of the cell cycle. This heterodimeric protein, stage, the chromosomes become short and thick, the which is a complex of Cdk1 and M-cyclin, is activated centrosomes are positioned at the poles and the nuclear by the removal of inhibitory phosphate groups in the membrane begins to disintegrate. late G2 phase. The M-Cdk protein induces events essen- tial for mitosis, including phosphorylation of the pro- During prophase I, segments of chromosome are ex- teins which control microtubule dynamics, chromatin changed between homologous but non-sister chromatids condensation, rearrangement of both the cytoskeleton (Fig. 1.4C). This process is referred to as crossover. At this and organelles and, ﬁnally, dissolution of the nuclear stage, duplicated homologous chromosomes assemble envelope. Although the mitotic cell cycle is normally side by side and assume a tetrad conﬁguration. highly regulated, undesirable alterations in the func- Chromatid arms within the tetrad may then overlap to tioning of the genes known as proto-oncogenes or form a chiasma, which allows crossover to take place tumour suppressor genes, responsible for the control between paternally-derived and maternally-derived of cell proliferation or differentiation, may lead to chromatids (Fig. 1.3). As a consequence of crossover, malignant transformation of normal tissue. Typically, recombinant chromatids acquire an allocation of changes in two or more of these regulatory genes genetic material derived from both paternal and mater- appear to be required for cells to undergo malignant nal chromatids. The crossover events which occur transformation. during meiosis extend the genetic variation beyond that which is possible from the random segregation Mitotic division in successive generations of cells derived of maternal and paternal chromatids. It is generally from a neoplastic cell continues to give rise to abnormal accepted that the variability arising from the recom- cells which are not subject to normal regulatory pro- bination confers evolutionary advantage on animal cesses. Neoplastic conditions such as leukaemia, lym- populations in accordance with the principles of natural phoma and myeloma can arise from gene alteration selection.
6 DIVISION, GROWTH AND DIFFERENTIATION OF CELLS 1 cell. During metaphase, the homologous chromosome pairs are positioned at the metaphase plate by the kine- tochore microtubules (Fig. 1.4D). Anaphase I During anaphase I, the tetrad splits into two dyads (half a tetrad), which move to opposite poles of the cell. Unlike the anaphase stage of mitosis, splitting of the centromeres does not occur because in this instance only one kinetochore forms on each dyad. The distribution of paternally derived and maternally derived homologous chromosomes at this point is ran- dom, and it is this variable arrangement which under- lies the Mendelian principle of random assortment (Fig. 1.4E). Telophase I In telophase I, nuclear envelopes develop around the separate chromosome sets and cytokinesis follows (Figs. 1.4F and G). In the formation of primary sperma- tocytes, progenitors of male gametes, the cytoplasm is divided equally between the two cells. However, during the formation of oocytes, female gametes, one of the two resulting cells retains the greater portion of cyto- plasm. The smaller of the two cells is termed a polar body. A short resting phase, termed interkinesis, follows telophase I and replication of DNA does not occur dur- ing this phase. The second meiotic division Prophase II The events of prophase II are similar to prophase I. The nucleus contains a set of dyads each composed of a pair of chromatids connected by a shared centromere (Fig. 1.5A). Metaphase II The phase termed metaphase II is similar to metaphase I in that the chromosomes are positioned at the met- aphase plate by the kinetochore microtubules. In this instance, however, kinetochores form on each of the Figure 1.3 Chiasma formation and reciprocal exchange of individual chromatids. This allows the microtubules to genetic material between non-sister homologous chromatids attach separately to each chromatid (Fig. 1.5B). during meiosis I. Anaphase II Metaphase I During anaphase II, the dyads are separated into indi- As in mitosis, homologous chromosome pairs attach via vidual chromatids by the kinetochore microtubules and their kinetochores to the microtubules arising from the the sets of chromatids are drawn towards opposite poles centrosomes which are located at opposite poles of the of the dividing cell (Fig. 1.5C).