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Oklahoma Notes Basic-Sciences Review for Medical Licensure Developed at The University of Oklahoma, College of Medicine Suitable Reviews for: National Board of Medical Examiners (NBME), Part I Medical Sciences Knowledge Profile (MSKP) Foreign Medical Graduate Examination in the Medical Sciences (FMGEMS) Oklahoma Notes Embryology Robert E. Coalson Springer-Verlag New York Berlin Heidelberg London Paris Tokyo Robert E. Coalson, Ph.D. Department of Anatomical Sciences College of Medicine Health Sciences Center The University of Oklahoma Oklahoma City, OK 73190 U.S.A. Library of Congress Cataloging in Publication Data Coalson, Robert E. Embryology. (Oklahoma notes) 1. Embryology, Human-Outlines, syllabi, etc. 2. Embryology, Human-Examinations, questions, etc. 1. Title. II. Series. [DNIM: 1. Embryology examination questions. 2. Embryology-outlines. QS 18 C652ej QM601.C64 1987 612'.64'0076 86-29650 © 1987 by Springer-Verlag New York Inc. All rights reseIVed. This work may not be translated or copied in whole or in part without the written permission of the publisher (Springer Verlag, 175 Fifth Avenue, New York, New York 10010, USA), except for brief excerpts in connection with reviews or scholarly analysis. Use in connection with any form of information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed is forbidden. The use of general descriptive names, trade names, trademarks, etc. in this publication, even if the former are not especially identified, is not to be taken as a sign that such names, as understood by the Trade Marks and Merchandise Marks Act, may accordingly be used freely by anyone. While the advice and information in this book are believed to be true and accurate at the date of going to press, neither the authors nor the editors nor the publisher can accept any legal responsibility for any errors or omissions that may be made. The publisher makes no warranty, express or implied, with respect to the material contained herein. 9 8 7 6 5 4 (Fourth Printing. 1989) ISBN-13: 978-0-387-96334-1 e-ISBN-13: 978-1-4684-0284-1 DOl: 10.1007/978-1-4684-0284-1 Preface to the Oklahoma Notes In 1973, the University of Oklahoma College of Medicine instituted a requirement for passage of the Part I National Boards for promotion to the third year. To assist students in preparation for this examination, a two week review of the basic sciences was added to the curriculum in 1975. Ten review texts were written by the faculty: four in anatomical sciences and one each in the other six basic sciences. Self-instructional quizzes were also developed by each discipline and administered during tne review period. The first year the course was instituted the Total Score performance on National Boards Part I increased 60 points, with the relative standing of the school changing from 56th to 9th in the nation. The performance of the class has remained near the national candidate mean (500) since then, with a mean over the 12 years of 502 and a range of 467 to 537. This improvement in our own students' performance has been documented (Hyde et al: Performance on NBME Part I examination in relation to policies regarding use oftest. J. Med. Educ. 60:439-443, 1985). A questionnaire was administered to one of the classes after they had completed the boards; 82% rated the review books as the most beneficial part of the course. These texts have been recently updated and rewritten and are now available for use by all students of medicine who are preparing for comprehensive examinations in the Basic Medical Sciences. RICHARD M. HYDE, Ph.D. Executive Editor PREfACE This book was prepared to present an integrated of selected revi~w topics in Human Embryology. It is designed specifically for students who have completed standard courses in the various anatomical disciplines and who wish to review the developmental history of the major organ systems. This book will provide medical students with a highly suitable review for Part I of the National Boards (NBME, Part I). R. E. Coalson ACKNOWLEDGMENTS I wish to acknowledge the invaluable assistance provided by my colleagues at the University of Oklahoma Health Sciences Center during the preparation of this review. I am grateful for the advice and patience of the Medical Editorial Department of Springer-Verlag, New York, Inc., and for the artistic talents of Mr. Shawn Schlinke, who prepared all of the illustrations. I particularly thank Dr. Randall B. Grubb, who proofread and prepared the manuscript in final form. TABLE OF CONTENTS GAMETOGENESIS • • • 1 FEMALE REPRODUCTIVE CYCLE • 7 FERTILIZATION AND PREGNANCY • • 10 IMPLANTATION AND FORMATION OF THE DECIDUAE • • • 13 FORMATION OF THE PLACENTA • • • 16 FETAL MEMBRANES AND UMBILICAL CORD • • 20 EARLY DEVELOPMENT OF THE CONCEPTUS • 23 DEVELOPMENT OF GENERAL BODY FORM · 28 NERVOUS SYSTEM • 31 MUSCULOSKELETAL SYSTEM • • • 41 INTEGUMENTARY SYSTEM •• • 50 ORAL CAVITY AND DEVELOPMENT OF THE BRANCHIAL APPARATUS • • 54 DIFFERENTIATION OF THE BRANCHIAL APPARATUS • 58 FACE AND PALATE • • • • • • • • •• • • • • 64 DIGESTIVE SYSTEM AND MESENTERIES • 69 DIAPHRAGM AND BODY CAVITIES. • • 76 RESPIRATORY SYSTEM • 80 UROGENITAL SYSTEM •• • • • 84 CARDIOVASCULAR SYSTEM • .92 FETAL CIRCULATION AND CHANGES AT BIRTH 104 GAM[TOG[N[SIS CELLS AND CHROMOSOMES Diploid Cells: In humans, the cells comprising all renewing cell populations (including the precursors of germ cells) are diploid (2N) and possess a total of 46 chromosomes. When the chromosomes of dividing cells are karyotyped, it can be determined that, with one exception (in males), they occur in pairs which are morphologically identical, i.e., homologous. One member of each homologous pair is of maternal origin, the other, its homologue, is of paternal origin. The numbers one through 22 are used to designate the auto somal (non-sex) chromosome pairs; the 23rd pair is referred to simply as the sex chromosomes. In females (46,XX) the maternal and paternal sex chromosomes are morphologically identical and are homologous like the autosomal chromosome pairs. In males (46,XY), the maternal (X-chromosome) and paternal (Y-chromosome) sex chromosomes are not ident ical morpho log ically and are, therefore, nonhomologous. The nonhomologous condition of the sex chromosomes in males provides the only instance in which the parental origin of a particu lar chromosome can be determined readily; this distinction is possible because the V-chromosome is always derived from the father and the X-chromosome is always derived from the mother. diploid (2N) chromosome number = 46 autosomal chromosomes = 22 (homologous pairs) sex chromosomes = 2 females = XX (homologous) males = XY (nonhomologous) Haploid Cells (Gametes): The gametes (sperm and ova) are highly specialized reproductive cells containing only one-half (N or 23) the number of chromo somes found in renewing cell populations. The reduction in chromosome number is accomplished by two specialized cell cycles which are referred to as Meiosis I and Meiosis II. haploid (N) chromosome number = 23 autosomal chromosomes = 22 (one member from each pair) sex chromosomes = 1 females = X (always) males = X or Y When two haploid gametes fuse at the time of fertili HOMOLOGOUS CHROMOSOMES zation, the diploid number of chromosomes is restored PARTIAllY DUPliCATED in the zygote. ARM Chromosome Morphology: Structurally, all chromosomes CENTROMERE exhibit two characteristic areas or regions which are referred to as the centromere and the arms. The ARM position of the centromere, which determines the relative lengths of the arms, is remarkably constant for each chromosome and is one of the major character EARLY METAPHASE istics used in karyotyping. 2 CHROMATIDS PER CHROMOSOME 2 Chromosome Duplication: It is important to remember that when the chromosomes are duplicated in preparation for cell division, that duplication of the arms and centromeres occurs during different periods (and phases) of the cell cycle. Arm duplication (DNA replication) always occurs during the Intermitotic Period (S-phase). Centromeric duplication (splitting) always occurs during the Mitotic Period (late metaphase). During the early phases of the Mitotic Period, when chromosome condensation is well advanced, it can be determined easily that each chromosome is only partially duplicated and consists of a single centromere uniting the dupli cated arms. The two halves of each partially duplicated chromosome are called chromatids; chromatids become chromosomes when each possesses its own centro mere, i.e., during late metaphase. FACTORS REGULATING DIPLOID AND HAPLOID (GAMETE) PRODUCTION Diploid Cells: The two most significant events occurring during the regular cell cycles of diploid cells are DNA replication (S-phase) followed by centro meric replication (late metaphase) in the same cell cycle. When both events occur during the same cell cycle, the number of chromosomes remains constant, i.e., diploid, and the daughter cells will be genetically identical; this is the basic mechanism involved in maintaining the diploid chromosome number and genetic uniformity in the stem cells of all renewing cell populations. Haploid Cells (Gametes): During the two specialized cell cycles of gameto genesis (Meiosis I and Meiosis II), DNA replication and centromeric division occur in separate cell cycles. When DNA replication and centromeric division occur in separate cell cycles, the chromosome number is reduced by one-hal f, e., i. haploid, and the result ing daughter cells are not genet ically identical; this is the basic mechanism in reducing the chromosome number and providing for genetic variability in the highly specialized reproductive germ cells, i.e., gametes. During gametogenesis, DNA replication occurs during Meiosis I; centromeric splitting occurs during Meiosis II. MEIOSIS I Cells enter as 1° and leave as 2° gametocytes. Intermitotic Period: DNA replication by 1° gametocytes during S-phase Mitotic Period: centromere does NOT divide during metaphase As a consequence, the chromosome number is reduced by one-half (this is why the first meiotic division is called the reduction division) and the daughter cells (2° gametocytes) will not be identical because each receives only one member of each chromosome pair. MEIOSIS II Cells enter as 2° gametocytes and leave as 'tids'. Intermitotic Period: NO replication of DNA Mitotic Period: centromere divides during late metaphase 3 As a consequence the chromatids comprlslng each of the 23 chromosomes separate as complete chromosomes for distribution to the daughter 'tid' cells. (The daughter 'tid' cells will not be identical because of synapsis and crossing over in meiosis I.) Separation of DNA replication and centro meric division provides the basic mechanism for reducing the number of chromosomes and for genetic variability during the modified cell cycles of gametogenesis. Duration of Gametogenesis GAMETOGENESIS ® ® In males, gametogenesis xy xx begins at puberty and SPERMATOGONIW OOGONIW continues into advanced age. MEIOSIS I CD ® Thymidine labeling indicates Xy (DNA ~PUCATION) xx that the time required to produce a mature spermato 1° SPERMATOCYTE ,0 OOCYTE tziooonn aoftfe rD NtAh e blya sat rperpimlicaary ® ® MEIOSIS II ®2~3 "~ CD X spermatocyte is approxi mately 64 days. 2 0 SPERMA TOCYTES CCEHTJlJt,ERICOIVI~) 2° OOCYTE f'Ol..AR BOOY CD ®® ","'U"''''' 23 In females, gametogenesis SPERMATIOS X begins during late fetal ~ ~ life when all oogonia enter MATURE OVUM POlAR BODIES meiosis I and undergo their MA TUlE SPERM last replication of DNA. At birth the ovary contains only primary oocytes arrested in early prophase of meiosis I; meiosis I is completed many years later at the time of ovulation. The secondary oocyte completes meiosis II only at the time of fertilization. Consequently, the time required to produce a mature ovum fran an oogonium may be as long as 40-45 years; it is for this reason that abnormalities in chrom osome number are usually attributed to the female rather than the male gamete. CHROMOSOMAL ABNORMALITIES The most common event producing abnormalities in chromosome number is nondis junction. In nondisjunction, the chromosomes are unequally distributed to the daughter cells so that one cell receives an extra chromosome and the other cell is missing one chromosome. The underlying cause is thought to be delayed centromeric division at metaphase. NONDISJUNCTION CAN OCCUR ANYTIME A CELL DIVIDES (MITOTICALLY OR MEIOTICALLY) AND CAN AFFECT EITHER THE AUTOSOMAL OR SEX CHROMOSOMES. In the adult, nondisjunction in somatic cells is relatively unimportant but nondisjunction in germ cells, i.e., meiotic nondisjunction, is very important. Nondisjunction in Germ Cells produces two abnormal 'haploid gametes' (haploid plus one and haploid minus one chromosome) which at the time of fertilization produce a 'diploid zygote' which will also possess an extra chromosome (sex or autosomal) or be lacking one chromosome (sex or autosomal). Individuals developing from such abnormal zygotes will be trisomic or monosomic for the chromosome involved. Althouqh 22 pairs of autosomal chromosomes are present in human cells and any chromosome can be affected by nondisjunction (all have been reported in ab.orted materia 1), only a few survive to present as medica 1 problems.
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