BRS Embryology 6th Edition PDF

Embryology Embryology Ronald W. Dudek, Ph.D. Professor Department of Anatomy and Cell Biology Brody School of Medicine East Carolina University Greenville, North Carolina Acquisitions Editor: Crystal Taylor Product Manager: Catherine Noonan Marketing Manager: Joy Fisher-Williams Designer: Holl...

Embryology Embryology Ronald W. Dudek, Ph.D. Professor Department of Anatomy and Cell Biology Brody School of Medicine East Carolina University Greenville, North Carolina Acquisitions Editor: Crystal Taylor Product Manager: Catherine Noonan Marketing Manager: Joy Fisher-Williams Designer: Holly Reid McLaughlin Compositor: S4Carlisle Publishing Servises Sixth Edition Copyright © 2014, 2011, 2008, 2005, 1998, 1994 Lippincott Williams & Wilkins, a Wolters Kluwer business. 351 West Camden Street Two Commerce Square Baltimore, MD 21201 2001 Market Street Philadelphia, PA 19103 All rights reserved. This book is protected by copyright. No part of this book may be reproduced or transmitted in any form or by any means, including as photocopies or scanned-in or other electronic copies, or utilized by any information storage and retrieval system without written permission from the copyright owner, except for brief quotations embodied in critical articles and reviews. Materials appearing in this book prepared by individuals as part of their official duties as U.S. government employees are not covered by the above-mentioned copyright. To request permission, please contact Lippincott Williams & Wilkins at 2001 Market Street, Philadelphia, PA 19103, via e-mail at [email protected], or via website at lww.com (products and services). Printed in China Library of Congress Cataloging-in-Publication Data ISBN 978-1-4511-9038-0 Cataloging-in-Publication data available on request from the Publisher. DISCLAIMER Care has been taken to confirm the accuracy of the information present and to describe generally accepted practices. However, the authors, editors, and publisher are not responsible for errors or omissions or for any consequences from application of the information in this book and make no warranty, expressed or implied, with respect to the currency, completeness, or accuracy of the contents of the publication. Application of this informa- tion in a particular situation remains the professional responsibility of the practitioner; the clinical treatments described and recommended may not be considered absolute and universal recommendations. The authors, editors, and publisher have exerted every effort to ensure that drug selection and dosage set forth in this text are in accordance with the current recommendations and practice at the time of publication. However, in view of ongoing research, changes in government regulations, and the constant flow of information relating to drug therapy and drug reactions, the reader is urged to check the package insert for each drug for any change in indications and dosage and for added warnings and precautions. This is particularly important when the recommended agent is a new or infrequently employed drug. Some drugs and medical devices presented in this publication have Food and Drug Administration (FDA) clearance for limited use in restricted research settings. It is the responsibility of the health care provider to ascer- tain the FDA status of each drug or device planned for use in their clinical practice. To purchase additional copies of this book, call our customer service department at (800) 638-3030 or fax orders to (301) 223-2320. International customers should call (301) 223-2300. Visit Lippincott Williams & Wilkins on the Internet: http://www.lww.com. Lippincott Williams & Wilkins customer service representatives are available from 8:30 am to 6:00 pm, EST. 987654321  Preface The sixth edition of BRS Embryology includes improvements based on suggestions and comments from the many medical students who have used this book in preparation for the USMLE Step 1 examination and those students who have reviewed the book. I pay close attention to these suggestions and comments in order to improve the quality of this book. The goal of BRS Embryology is to provide an accurate and quick review of important clinical aspects of embryology for the future physician. In addition, we have added color to the diagrams. In this regard, I have used the following color scheme. The ectoderm/ neuroectoderm and derivatives are colored blue. The neural crest cells and derivatives are colored purple. The mesoderm and derivatives are colored red. When multiple mesoder- mal structures are involved (e.g., reproductive systems), I used light red and dark red. The endoderm and derivatives are colored yellow. Many times in the history of science, certain biological concepts become entrenched and accepted as dogma even though recent evidence comes to light to challenge these con- cepts. One of these concepts is the process of twinning. Recent evidence calls into question the standard figures used in textbooks on how the process of twinning occurs. In particular, it is becoming increasingly difficult to ignore the fact that dizygotic twins are sometimes monochorionic. Although we by far do not know or attempt to explain exactly how twinning occurs, it seems that the interesting cell and molecular events involved in twinning occur in the first few cell divisions during first three or four days after fertilization. You are not a twin because the inner cell mass splits. The inner cell mass splits because you are a twin. This evidence warrants a new twinning figure (Figure 6.6) that does not comport with the standard figures but tries to embrace recent evidence, although many may call it controver- sial. Progress in our scientific understanding of twinning will never occur if our concept of the twinning process is overly simplistic and reinforced by standard figures repeated over and over in textbooks. Some published references that speak to this twinning issue include Boklage,1,2 Yoon et al.,3 Williams et al.,4 and Hoekstra et al.5 I understand that BRS Embryology is a review book designed for a USMLE Step 1 review and that you will not be faced with a question regarding this twinning concept, but I know my readers are sophisticated enough to appreciate the scientific and clinical value of being challenged to question traditional concepts as “grist for the mill” in discussions with your colleagues. I would appreciate receiving your comments and/or suggestions concerning BRS Em- bryology sixth edition, especially after you have taken the USMLE Step 1 examination. Your suggestions will find their way into the seventh edition. You may contact me at dudekr@ ecu.edu. Ronald W. Dudek, PhD v vi Preface REFERENCES 1. Boklage CE. Traces of embryogenesis are the same in monozygotic and dizygotic twins: not compatible with double ovulation. Hum Reprod. 2009;24(6):1255–1266. 2. Boklage CE. How New Humans Are Made: Cells and Embryos, Twins and Chimeras, Left and Right, Mind/Self/Soul, Sex, and Schizophrenia. Hackensack, NJ: World Scientific Publishing; 2010. 3. Yoon G, Beischel LS, Johnson JP, et al. Dizygotic twin pregnancy conceived with assisted repro- ductive technology associated with chromosomal anomaly, imprinting disorder, and monocho- rionic placentation. J Pediatr. 2005;146:565–567. 4. Williams CA, Wallace MR, Drury KC, et al. Blood lymphocyte chimerism associated with IVF and monochorionic dizygous twinning: case report. Hum Reprod. 2004;19(12):2816–2821. 5. Hoekstra C, Zhao ZZ, Lambalk CB, et al. Dizygotic twinning. Hum Reprod Update. 2008;14(1):37–47.  Contents Preface v 1. PREFERTILIZATION EVENTS 1 I.Sexual Reproduction 1 II.Chromosomes 1 III.Meiosis 2 IV. Oogenesis: Female Gametogenesis 2 V. Spermatogenesis: Male Gametogenesis 4 VI. Clinical Considerations 4 Study Questions for Chapter 1 8 Answers and Explanations 10 2. WEEK 1 OF HUMAN DEVELOPMENT (DAYS 1–7) 12 I.Fertilization 12 II.Cleavage and Blastocyst Formation 13 III.Implantation 14 IV. Clinical Considerations 14 Study Questions for Chapter 2 15 Answers and Explanations 17 3. WEEK 2 OF HUMAN DEVELOPMENT (DAYS 8–14) 18 I.Further Development of the Embryoblast 18 II.Further Development of the Trophoblast 18 III.Development of Extraembryonic Mesoderm 20 IV. Clinical Considerations 20 Study Questions for Chapter 3 22 Answers and Explanations 24 4. EMBRYONIC PERIOD (WEEKS 3–8) 26 I. General Considerations 26 II. Further Development of the Embryoblast 26 vii viii Contents III. Vasculogenesis (De Novo Blood Vessel Formation) 28 IV. Hematopoiesis (Blood Cell Formation) 31 V. Clinical Considerations 31 Study Questions for Chapter 4 33 Answers and Explanations 35 5. CARDIOVASCULAR SYSTEM 37 I.Formation of Heart Tube 37 II.Primitive Heart Tube Dilations 37 III.The Aorticopulmonary Septum 39 IV. The Atrial Septum 41 V. The Atrioventricular Septum 43 VI. The Interventricular Septum 45 VII. The Conduction System of the Heart 46 VIII. Coronary Arteries 47 IX. Development of the Arterial System 47 X. Development of the Venous System 49 Study Questions for Chapter 5 50 Answers and Explanations 53 6. PLACENTA AND AMNIOTIC FLUID 55 I. Formation of the Placenta 55 II. Placental Components: Decidua Basalis and Villous Chorion 55 III.Placental Membrane 58 IV. The Placenta as an Endocrine Organ 59 V. The Umbilical Cord 60 VI. Circulatory System of the Fetus 60 VII. Amniotic Fluid 62 VIII. Twinning 62 IX. Clinical Considerations 65 Study Questions for Chapter 6 68 Answers and Explanations 70 7. NERVOUS SYSTEM 71 I. Overview 71 II. Development of the Neural Tube 71 III. Neural Crest Cells 73 IV. Placodes 75 V. Vesicle Development of the Neural Tube 75 VI. Histogenesis of the Neural Tube 76 VII. Layers of the Early Neural Tube 78  Contents ix VIII. Development of the Spinal Cord 78 IX. Development of the Myelencephalon 79 X. Development of the Metencephalon 80 XI. Development of the Mesencephalon 81 XII. Development of the Diencephalon, Optic Structures, and Hypophysis 82 XIII. Development of the Telencephalon 83 XIV. Development of the Sympathetic Nervous System 85 XV. Development of the Parasympathetic Nervous System 85 XVI. Development of the Cranial Nerves 86 XVII. Development of the Choroid Plexus 86 XVIII. Congenital Malformations of the Central Nervous System 87 Study Questions for Chapter 7 93 Answers and Explanations 96 8. EAR 98 I. Overview 98 II. The Internal Ear 98 III. The Membranous and Bony Labyrinths 100 IV. Middle Ear 100 V. External Ear 101 VI. Congenital Malformations of the Ear 101 Study Questions for Chapter 8 104 Answers and Explanations 105 9. EYE 106 I. Development of the Optic Vesicle 106 II. Development of other Eye Structures 109 III. Congenital Malformations of the Eye 110 Study Questions for Chapter 9 113 Answers and Explanations 114 10. DIGESTIVE SYSTEM 115 I.Overview 115 II.Derivatives of the Foregut 115 III.Derivatives of the Midgut 123 IV. Derivatives of the Hindgut 127 V. Anal Canal 130 VI. Mesenteries 130 Study Questions for Chapter 10 131 Answers and Explanations 133 x Contents 11. RESPIRATORY SYSTEM 134 I. Upper Respiratory System 134 II. Lower Respiratory System 134 Study Questions for Chapter 11 142 Answers and Explanations 143 12. HEAD AND NECK 144 I. Pharyngeal Apparatus 144 II. Development of the Thyroid Gland 144 III. Development of the Tongue 146 IV. Development of the Face 147 V. Development of the Palate 148 VI. Development of the Mouth 149 VII. Development of the Nasal Cavities 149 VIII. Clinical Considerations 150 Study Questions for Chapter 12 153 Answers and Explanations 154 13. URINARY SYSTEM 155 I.Overview 155 II.Development of the Metanephros 155 III.Relative Ascent of the Kidneys 156 IV. Blood Supply of the Kidneys 157 V. Development of the Urinary Bladder 158 VI. Development of the Female Urethra 159 VII. Development of the Male Urethra 160 VIII.Clinical Considerations 160 IX. Development of the Suprarenal Gland 164 Study Questions for Chapter 13 167 Answers and Explanations 168 14. FEMALE REPRODUCTIVE SYSTEM 169 I.The Indifferent Embryo 169 II.Development of the Gonads 169 III.Development of the Genital Ducts 171 IV. Development of the Primordia of External Genitalia 173 V. Tanner Stages of Female Sexual Development 174 VI. Clinical Considerations 174 Study Questions for Chapter 14 178 Answers and Explanations 179  Contents xi 15. MALE REPRODUCTIVE SYSTEM 180 I.The Indifferent Embryo 180 II.Development of the Gonads 180 III.Development of the Genital Ducts 182 IV. Development of the Primordia of External Genitalia 184 V. Tanner Stages of Male Sexual Development 184 VI. Clinical Considerations 184 VII. Summary 189 Study Questions for Chapter 15 190 Answers and Explanations 191 16. INTEGUMENTARY SYSTEM 192 I.Skin 192 II.Hair and Nails 196 III.Mammary, Sweat, and Sebaceous Glands 199 IV. Teeth 201 Study Questions for Chapter 16 203 Answers and Explanations 204 17. SKELETAL SYSTEM 205 I.Skull 205 II.Vertebral Column 209 III.Ribs 214 IV. Sternum 214 V. Bones of the Limbs and Limb Girdles 214 VI. Osteogenesis 215 VII. General Skeletal Abnormalities 215 Study Questions for Chapter 17 218 Answers and Explanations 219 18. MUSCULAR SYSTEM 220 I.Skeletal Muscle 220 II.Smooth Muscle 221 III.Cardiac Muscle 222 IV. Clinical Considerations 222 Study Questions for Chapter 18 224 Answers and Explanations 225 xii Contents 19. UPPER LIMB 226 I.Overview of Development 226 II.Vasculature 226 III.Musculature 228 IV. Nerves: The Brachial Plexus 228 V. Rotation of the Upper Limb 229 VI. Skeletal 229 Study Questions for Chapter 19 231 Answers and Explanations 232 20. LOWER LIMB 233 I.Overview of Development 233 II.Vasculature 233 III.Musculature 235 IV. Nerves: The Lumbosacral Plexus 235 V. Rotation of the Lower Limb 236 VI. Skeletal 237 Study Questions for Chapter 20 239 Answers and Explanations 240 21. BODY CAVITIES 241 I.Formation of the Intraembryonic Coelom 241 II.Partitioning of the Intraembryonic Coelom 241 III.Positional Changes of the Diaphragm 242 IV. Clinical Considerations 243 Study Questions for Chapter 21 244 Answers and Explanations 245 22. PREGNANCY 246 I.Endocrinology of Pregnancy 246 II.Pregnancy Dating 247 III.Pregnancy Milestones 247 IV. Prenatal Diagnostic Procedures 248 V. Fetal Distress During Labor (Intrapartum) 249 VI. The Apgar Score 249 VII. Puerperium 250 VIII. Lactation 250 IX. Small-for-Gestational Age (SGA) Infant 250 X. Collection and Storage of Umbilical Cord Blood (UCB) 251 Study Questions for Chapter 22 252 Answers and Explanations 253  Contents xiii 23. TERATOLOGY 254 I. Introduction 254 II. Infectious Agents 254 III. Torch Infections 256 IV. Childhood Vaccinations 258 V. Category X Drugs (Absolute Contraindication In Pregnancy) 258 VI. Category D Drugs (Definite Evidence of Risk to Fetus) 259 VII. Chemical Agents 260 VIII. Recreational Drugs 261 IX. Ionizing Radiation 261 Study Questions for Chapter 23 262 Answers and Explanations 263 Comprehensive Examination 264 Answers and Explanations 272 Credits 279 Index 287 chapter 1 Prefertilization Events I. SEXUAL REPRODUCTION Sexual reproduction occurs when female and male gametes (oocyte and spermatozoon, respectively) unite at fertilization. Gametes are direct descendants of primordial germ cells, which are first observed in the wall of the yolk sac at week 4 of embryonic development and subsequently migrate into the future gonad region. Gametes are produced by gametogenesis (called oogenesis in the female and spermatogenesis in the male). Gametogenesis employs a specialized process of cell divi- sion, meiosis, which uniquely distributes chromosomes among gametes. II. CHROMOSOMES (FIGURE 1.1) A single chromosome consists of two characteristic regions called arms (p arm = short arm; q arm = long arm), which are separated by a centromere. During meiosis I, single chromosomes undergo DNA replication, which duplicates the arms. This forms duplicated chromosomes, which consist of two sister chromatids attached at the centromere. A. Ploidy and N number. Ploidy refers to the number of chromosomes in a cell. The N number refers to the amount of DNA in a cell. 1. Normal somatic cells and primordial germ cells contain 46 single chromosomes and 2N amount of DNA. The chromosomes occur in 23 homologous pairs; one member (homologue) of each pair is of maternal origin, and the other is of paternal origin. The term “diploid” is classically used to refer to a cell containing 46 single chromosomes. Chromosome pairs 1–22 are autosomal (nonsex) pairs. Chromosome pair 23 consists of the sex chromosomes (XX for a female and XY for a male). 2. Gametes contain 23 single chromosomes (22 autosomes and 1 sex chromosome) and 1N amount of DNA. The term “haploid” is classically used to refer to a cell containing 23 single chromosomes. Female gametes contain only the X sex chromosome. Male gametes contain either the X or Y sex chromosome; therefore, the male gamete determines the genetic sex of the individual. B. The X chromosome. A normal female somatic cell contains two X chromosomes (XX). The female cell permanently inactivates one of the X chromosomes during week 1 of embryonic development. The choice of which X chromosome (maternal or paternal) is inactivated is random. The 1 2 BRS Embryology Chromatid 1 Chromatid 2 FIGURE 1.1. A schematic diagram of chromosome 18 showing it in its “single- p arm chromosome” state and in the “dupli- cated-chromosome” state that is formed DNA replication by DNA replication during meiosis I. It is important to understand that both the Meiosis I “single-chromosome” state and the Centromere Centromere “duplicated-chromosome” state will be counted as one chromosome 18. As long q arm as the additional DNA in the “duplicated chromosome” is bound at the centro- mere, the structure will be counted as Chromosome 18 Chromosome 18 one chromosome 18 even though it has “single chromosome” “duplicated chromosome” twice the amount of DNA. inactivated X chromosome (called the Barr body) can be observed by light microscopy near the nuclear membrane. C. The Y chromosome. A normal male somatic cell contains one X chromosome and one Y chromosome (XY). III. MEIOSIS Meiosis is a specialized process of cell division that occurs only during the production of gametes within the female ovary or male testes. Meiosis consists of two divisions (meiosis I and II), which result in the formation of four gametes, each containing half the number of chromosomes (23 single chromosomes) and half the amount of DNA (1N) found in normal somatic cells (46 single chromo- somes, 2N). A. Meiosis I. Events that occur during meiosis I include the following: 1. Synapsis: pairing of 46 homologous duplicated chromosomes. 2. Crossing over: exchange of large segments of DNA. 3. Alignment: alignment of 46 homologous duplicated chromosomes at the metaphase plate. 4. Disjunction: separation of 46 homologous duplicated chromosomes from each other; centromeres do not split. 5. Cell division: formation of two secondary gametocytes (23 duplicated chromosomes, 2N). B. Meiosis II. Events that occur during meiosis II include the following: 1. Synapsis: absent. 2. Crossing over: absent. 3. Alignment: alignment of 23 duplicated chromosomes at the metaphase plate. 4. Disjunction: separation of 23 duplicated chromosomes to form 23 single chromosomes; centromeres split. 5. Cell division: formation of four gametes (23 single chromosomes, 1N). IV. OOGENESIS: FEMALE GAMETOGENESIS (FIGURE 1.2) A. Primordial germ cells (46, 2N) from the wall of the yolk sac arrive in the ovary at week 6 and differentiate into oogonia (46, 2N), which populate the ovary through mitotic division. B. Oogonia enter meiosis I and undergo DNA replication to form primary oocytes (46, 4N). All primary oocytes are formed by month 5 of fetal life. No oogonia are present at birth.  Chapter 1 Prefertilization Events 3 Oogonia (46 single chromosomes, 2N) DNA Replication Meiosis I Dormant in dictyotene Primary oocyte of meiosis I until puberty (46 duplicated chromosomes, 4N) Synapsis Crossing over Chiasma Alignment and disjunction Centromeres do not split Secondary oocyte (23 duplicated chromosomes, 2N) 1st polar body Meiosis II Alignment and disjunction Arrested in metaphase Centromeres split of meiosis II Cell division Mature oocyte Fertilization (23 single chromosomes, 1N) 2nd polar body FIGURE 1.2. Oogenesis: female gametogenesis. Note that only one pair of homologous chromosomes is shown (red, maternal origin; blue, paternal origin). Synapsis is the process of pairing of homologous chromosomes. The point at which the DNA molecule crosses over is called the chiasma and is where exchange of small segments of maternal and paternal DNA occurs. Note that synapsis and crossing over occur only during meiosis I. The polar bodies are storage bodies for DNA unnecessary for the further function of the cell and probably degenerate. There is no evidence that polar bodies divide or undergo any other activity. C. Primary oocytes remain dormant in prophase (dictyotene) of meiosis I from month 5 of fetal life until puberty. After puberty, 5 to 15 primary oocytes begin maturation with each ovarian cycle, with usually only 1 reaching full maturity in each cycle. D. During the ovarian cycle and triggered by the luteinizing hormone (LH) surge, a primary oocyte completes meiosis I to form two daughter cells: the secondary oocyte (23, 2N) and the first polar body, which degenerates. E. The secondary oocyte promptly begins meiosis II but is arrested in metaphase of meiosis II about 3 hours before ovulation. The secondary oocyte remains arrested in metaphase of meiosis II until fertilization occurs. 4 BRS Embryology F. At fertilization, the secondary oocyte completes meiosis II to form a mature oocyte (23, 1N) and a second polar body. G. Approximate number of oocytes 1. Primary oocytes: At month 5 of fetal life, 7 million primary oocytes are present. At birth, 2 million are present (5 million have degenerated). At puberty, 40,000 are present (1.96 million more have degenerated). 2. Secondary oocytes: Twelve secondary oocytes are ovulated per year, up to 480 over the entire reproductive life of the woman (40 years × 12 secondary oocytes per year = 480). This number (480) is obviously overly simplified since it is reduced in women who take birth control pills (which prevent ovulation), in women who become pregnant (ovulation stops during pregnancy), and in women who may have anovulatory cycles. V. SPERMATOGENESIS: MALE GAMETOGENESIS (FIGURE 1.3) Spermatogenesis is classically divided into three phases: A. Spermatocytogenesis 1. Primordial germ cells (46, 2N) from the wall of the yolk sac arrive in the testes at week 6 and remain dormant until puberty. At puberty, primordial germ cells differentiate into type A spermatogonia (46, 2N). 2. Type A spermatogonia undergo mitosis to provide a continuous supply of stem cells throughout the reproductive life of the male. Some type A spermatogonia differentiate into type B spermatogonia (46, 2N). B. Meiosis 1. Type B spermatogonia enter meiosis I and undergo DNA replication to form primary spermatocytes (46, 4N). 2. Primary spermatocytes complete meiosis I to form secondary spermatocytes (23, 2N). 3. Secondary spermatocytes complete meiosis II to form four spermatids (23, 1N). C. Spermiogenesis 1. Spermatids undergo a postmeiotic series of morphological changes to form sperm (23, 1N). These changes include the (a) formation of the acrosome, (b) condensation of the nucleus, and (c) formation of head, neck, and tail. The total time of sperm formation (from spermatogonia to spermatozoa) is about 64 days. 2. Newly ejaculated sperm are incapable of fertilization until they undergo capacitation, which occurs in the female reproductive tract and involves the unmasking of sperm glycosyltransferases and the removal of adherent plasma proteins coating the surface of the sperm. VI. CLINICAL CONSIDERATIONS A. Offspring of older women 1. Prolonged dormancy of primary oocytes may be the reason for the high incidence of chromosomal abnormalities in the offspring of older women. Since all primary oocytes are formed by month 5 of fetal life, a female infant is born with her entire supply of gametes. Primary oocytes remain dormant until ovulation; those ovulated late in the woman’s reproductive life may have been dormant for as long as 40 years. 2. The incidence of trisomy 21 (Down syndrome) increases with advanced age of the mother. The primary cause of Down syndrome is maternal meiotic nondisjunction. Clinical findings  Chapter 1 Prefertilization Events 5 Dormant until Primordial germ cells puberty Type A spermatogonia Spermatocytogenesis Type B spermatogonia (46 single chromosomes, 2N) DNA Replication Meiosis I Primary spermatocyte (46 duplicated chromosomes, 4N) Synapsis Crossing over Chiasma Alignment and disjunction Centromeres do not split Secondary spermatocyte (23 duplicated chromosomes, 2N) Meiosis II Alignment and disjunction Centromeres split Cell division Cell division Spermatids (23 single chromosomes, 1N) Spermiogenesis Sperm FIGURE 1.3. Spermatogenesis: male gametogenesis. Note that only one pair of homologous chromosomes is shown (red, maternal origin; blue, paternal origin). Synapsis is the process of pairing of homologous chromosomes. The point at which the DNA molecule crosses over is called the chiasma and is where exchange of small segments of maternal and paternal DNA occurs. Note that synapsis and crossing over occur only during meiosis I. 6 BRS Embryology include moderate mental retardation, microcephaly, microphthalmia, colobomata, cataracts and glaucoma, flat nasal bridge, epicanthal folds, protruding tongue, Brushfield spots, simian crease in the hand, increased nuchal skin folds, congenital heart defects, and an association with a decrease in α-fetoprotein. B. Offspring of older men An increased incidence of achondroplasia (a congenital skeletal anomaly characterized by retarded bone growth) and Marfan syndrome are associated with advanced paternal age. C. Male infertility 1. Sperm number and motility: Infertile males produce less than 10 million sperm/mL of semen. Fertile males produce from 20 to more than 100 million sperm/mL of semen. Normally, up to 10% of sperm in an ejaculate may be grossly deformed (two heads or two tails), but these sperm probably do not fertilize an oocyte because of their lack of motility. 2. Hypogonadotropic hypogonadism is a condition where the hypothalamus produces reduced levels of gonadotropin-releasing factor (GnRF), leading to reduced levels of follicle-stimulating hormone (FSH) and LH, and finally, reduced levels of testosterone. Kallmann syndrome is a genetic disorder characterized by hypogonadotropic hypogonadism and anosmia (loss of smell). 3. Drugs: cancer chemotherapy, anabolic steroids, cimetidine (histamine H2-receptor antagonist that inhibits stomach HCl production), spironolactone (a K+-sparing diuretic), phenytoin (an antiepileptic drug), sulfasalazine (a sulfa drug used to treat ulcerative colitis, Crohn disease, rheumatoid arthritis, and psoriatic arthritis), and nitrofurantoin (an antibiotic used to treat urinary tract infections). 4. Other factors: Klinefelter syndrome (XXY), seminoma, cryptochordism, varicocele, hydrocele, mumps, prostatitis, epididymitis, hypospadias, ductus deferens obstruction, and impotence. D. Female infertility 1. Anovulation is the absence of ovulation in some women due to inadequate secretion of FSH and LH and is often treated with clomiphene citrate (a fertility drug). Clomiphene citrate competes with estrogen for binding sites in the adenohypophysis, thereby suppressing the normal negative feedback loop of estrogen on the adenohypophysis. This stimulates FSH and LH secretions and induces ovulation. 2. Premature ovarian failure (primary ovarian insufficiency) is the loss of function of the ovaries before age 40, resulting in infertility. The cause is generally idiopathic, but cases have been attributed to autoimmune disorders, Turner syndrome, Fragile X syndrome, chemotherapy, or radiation treatment. The onset can be seen in early teenage years, but varies widely. If a girl never begins menstruation, the condition is called primary ovarian failure. Clinical findings include amenorrhea, low estrogen levels, high FSH levels, and small ovaries without follicles (seen by ultrasound). 3. Pelvic inflammatory disease (PID) refers to the infection of the uterus, uterine tubes, and/ or ovaries leading to inflammation and scar formation. The cause is generally a sexually transmitted infection (STI), usually by Neisseria gonorrhea or Chlamydia trachomatis; however, many other reasons are possible (lymphatic spread, hematogenous spread, postpartum infections, postabortal [miscarriage or abortion] infections, or intrauterine device infections). Clinical findings include fever, tenderness of the cervix, lower abdominal pain, discharge, painful intercourse, or irregular menstrual bleeding; some cases are asymptomatic. 4. Polycystic ovarian syndrome is a complex female endocrine disorder defined by oligo- ovulation (infrequent, irregular ovulations), androgen excess, multiple ovarian cysts (by ultrasound). The cause is uncertain, but a strong genetic component exists. Clinical findings include anovulation, irregular menstruation, amenorrhea, ovulation-related infertility, high androgen levels or activity resulting in acne and hirsutism, insulin resistance associated with obesity, and type 2 diabetes. 5. Endometriosis is the appearance of foci of endometrial tissue in abnormal locations outside the uterus (e.g., ovary, uterine ligaments, pelvic peritoneum). The ectopic endometrial  Chapter 1 Prefertilization Events 7 t a b l e 1.1 Chance of Pregnancy in Days Near Ovulation Time Chance of Pregnancy (%) 5 days before ovulation 10 4 days before ovulation 16 3 days before ovulation 14 2 days before ovulation 27 1 day before ovulation 31 Day of ovulation 33 Day after ovulation 0 tissue shows cyclic hormonal changes synchronous with the cyclic hormonal changes of the endometrium in the uterus. Clinical findings include infertility, dysmenorrhea, pelvic pain (most pronounced at the time of menstruation), dysuria, painful sex, and throbbing pain in the legs. E. The estimated chance of pregnancy (fertility) in the days surrounding ovulation is shown in Table 1.1. 8 BRS Embryology Study Questions for Chapter 1 1. Which of the following is a major character- (D) A crossover chromosome istic of meiosis I? (E) A homologous pair (A) Splitting of the centromere (B) Pairing of homologous chromosomes 6. All primary oocytes are formed by (C) Reducing the amount of DNA to 1N (A) week 4 of embryonic life (D) Achieving the diploid number of (B) month 5 of fetal life chromosomes (C) birth (E) Producing primordial germ cells (D) month 5 of infancy (E) puberty 2. A normal somatic cell contains a total of 46 chromosomes. What is the normal complement 7. When does formation of primary spermato- of chromosomes found in a sperm? cytes begin? (A) 22 autosomes plus a sex chromosome (A) During week 4 of embryonic life (B) 23 autosomes plus a sex chromosome (B) During month 5 of fetal life (C) 22 autosomes (C) At birth (D) 23 autosomes (D) During month 5 of infancy (E) 23 paired autosomes (E) At puberty 3. Which of the following describes the num- 8. In the production of female gametes, which ber of chromosomes and amount of DNA in a of the following cells can remain dormant for 12 gamete? to 40 years? (A) 46 chromosomes, 1N (A) Primordial germ cell (B) 46 chromosomes, 2N (B) Primary oocyte (C) 23 chromosomes, 1N (C) Secondary oocyte (D) 23 chromosomes, 2N (D) First polar body (E) 23 chromosomes, 4N (E) Second polar body 4. Which of the following chromosome com- 9. In the production of male gametes, which positions in a sperm normally results in the of the following cells remains dormant for 12 production of a genetic female if fertilization years? occurs? (A) Primordial germ cell (A) 23 homologous pairs of chromosomes (B) Primary spermatocyte (B) 22 homologous pairs of chromosomes (C) Secondary spermatocyte (C) 23 autosomes plus an X chromosome (D) Spermatid (D) 22 autosomes plus a Y chromosome (E) Sperm (E) 22 autosomes plus an X chromosome 10. Approximately how many sperm will be 5. In the process of meiosis, DNA replication of ejaculated by a normal fertile male during each chromosome occurs, forming a structure sexual intercourse? consisting of two sister chromatids attached to (A) 10 million a single centromere. What is this structure? (B) 20 million (A) A duplicated chromosome (C) 35 million (B) Two chromosomes (D) 100 million (C) A synapsed chromosome (E) 350 million 8  Chapter 1 Prefertilization Events 9 11. A young woman enters puberty with (C) Alignment approximately 40,000 primary oocytes in (D) Crossing over her ovary. About how many of these pri- (E) Disjunction mary oocytes will be ovulated over the entire reproductive life of the woman? 15. During ovulation, the secondary oocyte (A) 40,000 resides at what specific stage of meiosis? (B) 35,000 (A) Prophase of meiosis I (C) 480 (B) Prophase of meiosis II (D) 48 (C) Metaphase of meiosis I (E) 12 (D) Metaphase of meiosis II (E) Meiosis is completed at the time of 12. Fetal sex can be diagnosed by noting the ovulation presence or absence of the Barr body in cells obtained from the amniotic fluid. What is the 16. Concerning maturation of the female gam- etiology of the Barr body? ete (oogenesis), when do the oogonia enter (A) Inactivation of both X chromosomes meiosis I and undergo DNA replication to form (B) Inactivation of homologous chromosomes primary oocytes? (C) Inactivation of one Y chromosome (A) During fetal life (D) Inactivation of one X chromosome (B) At birth (E) Inactivation of one chromatid (C) At puberty (D) With each ovarian cycle 13. How much DNA does a primary spermato- (E) Following fertilization cyte contain? (A) 1N 17. Where do primordial germ cells initially (B) 2N develop? (C) 4N (A) In the gonads at week 4 of embryonic (D) 6N development (E) 8N (B) In the yolk sac at week 4 of embryonic development 14. During meiosis, pairing of homologous (C) In the gonads at month 5 of embryonic chromosomes occurs, which permits large seg- development ments of DNA to be exchanged. What is this (D) In the yolk sac at month 5 of embryonic process called? development (A) Synapsis (E) In the gonads at puberty (B) Nondisjunction  Answers and Explanations 1. B. Pairing of homologous chromosomes (synapsis) is a unique event that occurs only dur- ing meiosis I in the production of gametes. Synapsis is necessary so that crossing over can occur. 2. A. A normal gamete (sperm in this case) contains 23 single chromosomes. These 23 chromo- somes consist of 22 autosomes plus 1 sex chromosome. 3. C. Gametes contain 23 chromosomes and 1N amount of DNA, so that when two gametes fuse at fertilization, a zygote containing 46 chromosomes and 2N amount of DNA is formed. 4. E. A sperm contains 22 autosomes and 1 sex chromosome. The sex chromosome in sperm may be either the X chromosome or the Y chromosome. The sex chromosome in a secondary oocyte is only the X chromosome. If an X-bearing sperm fertilizes a secondary oocyte, a genetic female (XX) is produced. Therefore, sperm is the arbiter of sex determination. 5. A. The structure formed is a duplicated chromosome. DNA replication occurs, so that the amount of DNA is doubled (2 × 2N = 4N). However, the chromatids remain attached to the cen- tromere, forming a duplicated chromosome. 6. B. During early fetal life, oogonia undergo mitotic divisions to populate the developing ovary. All the oogonia subsequently give rise to primary oocytes by month 5 of fetal life; at birth, no oogonia are present in the ovary. At birth, a female has her entire supply of primary oocytes to carry her through reproductive life. 7. E. At birth, a male has primordial germ cells in the testes that remain dormant until puberty, at which time they differentiate into type A spermatogonia. At puberty, some type A spermatogo- nia differentiate into type B spermatogonia and give rise to primary spermatocytes by undergo- ing DNA replication. 8. B. Primary oocytes are formed by month 5 of fetal life and remain dormant until puberty, when hormonal changes in the young woman stimulate the ovarian and menstrual cycles. From 5 to 15 oocytes will then begin maturation with each ovarian cycle throughout the woman’s repro- ductive life. 9. A. Primordial germ cells migrate from the wall of the yolk sac during week 4 of embryonic life and enter the gonad of a genetic male, where they remain dormant until puberty (about age 12 years), when hormonal changes in the young man stimulate the production of sperm. 10. E. A normal fertile male will ejaculate about 3.5 mL of semen containing about 100 million sperm/mL (3.5 mL × 100 million = 350 million). 11. C. Over her reproductive life, a woman will ovulate approximately 480 oocytes. A woman will ovulate 12 primary oocytes per year, provided that she is not using oral contraceptives, does not become pregnant, or does not have any anovulatory cycles. Assuming a 40-year reproductive period gives 40 × 12 = 480. 12. D. The Barr body is formed from inactivation of one X chromosome in a female. All somatic cells of a normal female will contain two X chromosomes. The female has evolved a mechanism for permanent inactivation of one of the X chromosomes presumably because a double dose of X chromosome products would be lethal. 13. C. Type B spermatogonia give rise to primary spermatocytes by undergoing DNA replication, thereby doubling the amount of DNA (2 × 2N = 4N) within the cell. 10  Chapter 1 Prefertilization Events 11 14. D. Synapsis (pairing of homologous chromosomes) is a unique event that occurs only during meiosis I in the production of gametes. Synapsis is necessary so that crossing over, whereby large segments of DNA are exchanged, can occur. 15. D. The secondary oocyte is arrested in metaphase of meiosis II about 3 hours before ovulation, and it remains in this meiotic stage until fertilization. 16. A. All primary oocytes are formed by month 5 of fetal life, so no oogonia are present at birth. 17. B. Primordial germ cells, the predecessors to gametes, are first seen in the wall of the yolk sac at week 4 of embryonic development, and they migrate into the gonads at week 6.  Week 1 of Human chapter 2 Development (Days 1–7)* I. FERTILIZATION Fertilization occurs in the ampulla of the uterine tube and includes three phases. A. Phase 1: Sperm penetration of corona radiata involves the action of both sperm and uterine tube mucosal enzymes. B. Phase 2: Sperm binding and penetration of the zona pellucida 1. Sperm binding occurs through the interaction of sperm glycosyltransferases and ZP3 receptors located on the zona pellucida. Sperm binding triggers the acrosome reaction, which entails the fusion of the outer acrosomal membrane and sperm cell membrane, resulting in the release of acrosomal enzymes. 2. Penetration of the zona pellucida requires acrosomal enzymes, specifically acrosin. Sperm contact with the cell membrane of a secondary oocyte triggers the cortical reaction, which entails the release of cortical granules (lysosomes) from the oocyte cytoplasm. This reaction changes the secondary oocyte cell membrane potential and inactivates sperm receptors on the zona pellucida. These changes are called the polyspermy block, which renders the secondary oocyte cell membrane impermeable to other sperm. However, efficiency of the polyspermy block remains questionable since diandric triploidy (an embryo with three sets of chromosomes, two of which come from the father) is quite common. C. Phase 3: Fusion of sperm and oocyte cell membranes occurs with subsequent breakdown of both membranes at the fusion area. 1. The entire sperm (except the cell membrane) enters the cytoplasm of the secondary oocyte arrested in metaphase of meiosis II. The sperm nuclear contents and the centriole pair persist, but the sperm mitochondria and tail degenerate. The sperm nucleus becomes the male pronucleus. Since all sperm mitochondria degenerate, all mitochondria within the zygote are of maternal origin (i.e., all mitochondrial DNA is of maternal origin). The oocyte loses its centriole pair during meiosis so that the establishment of a functional zygote depends upon the sperm centriole pair (a cardinal feature of human embryogenesis) to produce a microtubule- organizing center (MTOC). *The age of a developing conceptus can be measured either from the estimated day of fertilization (fertilization age) or from the day of the last normal menstrual period (LNMP age). In this book, age is presented as the fertilization age. 12  Chapter 2 Week 1 of Human Development (Days 1–7) 13 2. The secondary oocyte completes meiosis II, forming a mature ovum and a second polar body. The nucleus of the mature ovum is now called the female pronucleus. 3. Male and female pronuclei fuse, forming a zygote (a new cell whose genotype is a combination of maternal and paternal chromosomes). 4. Syngamy is a term that describes the successful completion of fertilization, that is, the formation of a zygote. Syngamy occurs when the male and female pronuclei fuse and the cytoplasmic machinery for proper cell division exists. 5. The life span of a zygote is only a few hours because its existence terminates when the first cleavage division occurs. II. CLEAVAGE AND BLASTOCYST FORMATION (FIGURE 2.1) A. Cleavage is a series of mitotic divisions of the zygote where the plane of the first mitotic division passes through the area of the cell membrane where the polar bodies were previously extruded. 1. Cleavage in humans is holoblastic, which means the cells divide completely through their cytoplasm. Cleavage in humans is asymmetrical, which means the daughter cells are unequal in size (i.e., one cell gets more cytoplasm than the other) at least during the first few cell divisions. Cleavage in humans is asynchronous, which means only one cell will divide at a time; generally, the largest daughter cell will divide next at least during the first few cell divisions. 2. The process of cleavage eventually forms a blastula consisting of cells called blastomeres. A 2-Cell 4-Cell Morula Blasotocyst blastula blastula Blastomere Zygote Fertilization Zona pellucida Secondary oocyte arrested in metaphase Day 1 Day 7 B Syncytiotrophoblast Embryoblast Cytotrophoblast Blastocyst cavity FIGURE 2.1. A. The stages of human development during week 1. B. A day 7 blastocyst. 14 BRS Embryology 3. A cluster of blastomeres (16–32 blastomeres) forms a morula. 4. Blastomeres are totipotent up to the eight-cell stage (i.e., each blastomere can form a complete embryo by itself ). Totipotency refers to a stem cell that can differentiate into every cell within the organism, including extraembryonic tissues. B. Blastocyst formation involves fluid secreted within the morula that forms the blastocyst cavity. The conceptus is now called a blastocyst. 1. The inner cell mass of the blastocyst is called the embryoblast (becomes the embryo). The embryoblast cells are pluripotent. Pluripotency refers to a stem cell that can differentiate into ectoderm, mesoderm, and endoderm. 2. The outer cell mass of the blastocyst is called the trophoblast (becomes the fetal portion of the placenta). C. Zona pellucida degeneration occurs by day 4 after conception. The zona pellucida must degenerate for implantation to occur. III. IMPLANTATION (FIGURE 2.1) The blastocyst usually implants within the posterior superior wall of the uterus by day 7 after fertil- ization. Implantation occurs in the functional layer of the endometrium during the progestational (secretory) phase of the menstrual cycle. The trophoblast proliferates and differentiates into the cy- totrophoblast and syncytiotrophoblast. Failure of implantation may involve immune rejection (graft- versus-host reaction) of the antigenic conceptus by the mother. IV. CLINICAL CONSIDERATIONS A. Ectopic tubal pregnancy (ETP) 1. ETP occurs when the blastocyst implants within the uterine tube due to delayed transport. 2. The ampulla of the uterine tube is the most common site of an ectopic pregnancy. The rectouterine pouch (pouch of Douglas) is a common site for an ectopic abdominal pregnancy. 3. ETP is most commonly seen in women with endometriosis or pelvic inflammatory disease. 4. ETP leads to uterine tube rupture and hemorrhage if surgical intervention (i.e., salpingectomy) is not performed. 5. ETP presents with abnormal uterine bleeding, unilateral pelvic pain, increased levels of human chorionic gonadotropin (hCG) (but lower than originally expected with uterine implantation pregnancy), and a massive first-trimester bleed. 6. ETP must be differentially diagnosed from appendicitis, an aborting intrauterine pregnancy, or a bleeding corpus luteum of a normal intrauterine pregnancy. B. Testicular teratocarcinoma (TTC) 1. TTC is a germ cell neoplasm. In its early histologic stages, a TTC resembles a blastocyst with three primary germ layers and may be loosely referred to as “male pregnancy.” 2. TTC contains well-differentiated cells and structures from each of the three primary germ layers: for example, colon glandular tissue (endoderm), cartilage (mesoderm), and squamous epithelium (ectoderm). 3. TTC also contains undifferentiated pluripotent stem cells called embryonic carcinoma (EC) cells. 4. TTC is associated with elevated α-fetoprotein levels. 5. TTC can be experimentally produced by implanting a blastocyst in an extrauterine site. The ability of blastocysts to form TTC suggests a relationship between the inner cell mass and EC cells. This relationship has been confirmed by isolation of cell lines from blastocysts called embryonic stem (ES) cells, which have biochemical characteristics remarkably similar to those of EC cells.  Study Questions for Chapter 2 1. A 20-year-old woman presents at the emer- 5. Which of the following events is involved gency department with severe abdominal in the cleavage of the zygote during week 1 of pain on the right side with signs of internal development? bleeding. She indicated that she has been (A) A series of meiotic divisions forming sexually active without contraception and blastomeres missed her last menstrual period. Based on (B) Production of highly differentiated this information, which of the following dis- blastomeres orders must be included as an option in the (C) An increased cytoplasmic content of diagnosis? blastomeres (A) Ovarian cancer (D) An increase in size of blastomeres (B) Appendicitis (E) A decrease in size of blastomeres (C) Normal pregnancy (D) Ectopic tubal pregnancy 6. Which of the following structures must (E) Toxemia of pregnancy degenerate for blastocyst implantation to occur? 2. When does a secondary oocyte complete its (A) Endometrium in progestational phase second meiotic division to become a mature (B) Zona pellucida ovum? (C) Syncytiotrophoblast (A) At ovulation (D) Cytotrophoblast (B) Before ovulation (E) Functional layer of the endometrium (C) At fertilization (D) At puberty 7. Which of the following is the origin of the (E) Before birth mitochondrial DNA of all human adult cells? (A) Paternal only 3. How soon after fertilization occurs within (B) Maternal only the uterine tube does the blastocyst begin (C) A combination of paternal and maternal implantation? (D) Either paternal or maternal (A) Within minutes (E) Unknown origin (B) By 12 hours (C) By day 1 8. Individual blastomeres were isolated from a (D) By day 2 blastula at the 4-cell stage. Each blastomere was (E) By day 7 cultured in vitro to the blastocyst stage and in- dividually implanted into four pseudopregnant 4. Where does the blastocyst normally foster mothers. Which of the following would implant? you expect to observe 9 months later? (A) Functional layer of the cervix (A) Birth of one baby (B) Functional layer of the endometrium (B) Birth of four genetically different babies (C) Basal layer of the endometrium (C) Birth of four genetically identical babies (D) Myometrium (D) Birth of four grotesquely deformed babies (E) Perimetrium (E) No births 15 16 BRS Embryology 9. Embryonic carcinoma (EC) cells were isolated (D) A yellow- and black-coated offspring from a yellow-coated mouse with a teratocarci- (E) A yellow- and white-coated offspring noma. The EC cells were then microinjected into the inner cell mass of a blastocyst isolated from 10. In oogenesis, which of the following events a black-coated mouse. The blastocyst was sub- occurs immediately following the completions sequently implanted into the uterus of a white- of meiosis II? coated foster mouse. Which of the following (A) Degeneration of the zona pellucida would be observed after full-term pregnancy? (B) Sperm penetration of the corona radiata (A) A yellow-coated offspring (C) Formation of a female pronucleus (B) A black-coated offspring (D) Appearance of the blastocyst (C) A white-coated offspring (E) Completion of cleavage  Answers and Explanations 1. D. Ectopic tubal pregnancy must always be an option in the diagnosis when a woman in her reproductive years presents with such symptoms. Ninety percent of ectopic implantations occur in the uterine tube. Ectopic tubal pregnancies result in rupture of the uterine tube and internal hemorrhage, which presents a major threat to the woman’s life. The uterine tube and embryo must be surgically removed. The symptoms may sometimes be confused with appendicitis. 2. C. At ovulation, a secondary oocyte begins meiosis II, but this division is arrested at metaphase. The secondary oocyte will remain arrested in metaphase until a sperm penetrates it at fertiliza- tion. Therefore, the term “mature ovum’” is somewhat of a misnomer because it is a secondary oocyte that is fertilized, and, once fertilized, the new diploid cell is known as a zygote. If fertil- ization does not occur, the secondary oocyte degenerates. 3. E. The blastocyst begins implantation by day 7 after fertilization. 4. B. The blastocyst implants in the functional layer of the uterine endometrium. The uterus is composed of the perimetrium, myometrium, and endometrium. Two layers are identified within the endometrium: (1) the functional layer, which is sloughed off at menstruation, and (2) the basal layer, which is retained at menstruation and serves as the source of regeneration of the functional layer. During the progestational phase of the menstrual cycle, the functional layer undergoes dramatic changes; uterine glands enlarge and vascularity increases in preparation for blastocyst implantation. 5. E. Cleavage is a series of mitotic divisions by which the large amount of zygote cytoplasm is successively partitioned among the newly formed blastomeres. Although the number of blas- tomeres increases during cleavage, the size of individual blastomeres decreases until they re- semble adult cells in size. 6. B. The zona pellucida must degenerate for implantation to occur. Early cleavage states of the blastula are surrounded by a zona pellucida, which prevents implantation in the uterine tube. 7. B. The mitochondrial DNA of all human adult cells is of maternal origin only. In human fertil- ization, the entire sperm enters the secondary oocyte cytoplasm. However, sperm mitochondria degenerate along with the sperm’s tail. Therefore, only mitochondria present within the second- ary oocyte (maternal) remain in the fertilized zygote. 8. C. This scenario would result in four genetically identical children. Blastomeres at the 4- to 8-cell stage are totipotent, that is, capable of forming an entire embryo. Since blastomeres arise by mitosis of the same cell (zygote), they are genetically identical. This phenomenon is impor- tant in explaining monozygotic (identical) twins. About 30% of monozygotic twins arise by early separation of blastomeres. The remaining 70% originate at the end of week 1 of development by a splitting of the inner cell mass. 9. D. This scenario would result in a yellow- and black-coated offspring. Because EC cells and in- ner cell mass cells have very similar biochemical characteristics, they readily mix with each other, and development proceeds unencumbered. Because the mixture contains cells with yellow-coat genotype and black-coat genotype, offspring with coats of two colors (yellow and black) will be produced. The offspring are known as mosaic mice. 10. C. The secondary oocyte is arrested in metaphase of meiosis II, and it will remain in this meiotic stage until fertilization occurs. Following fertilization, the secondary oocyte completes meio- sis II, forming a mature ovum and a polar body. The nucleus of the mature ovum is called the female pronucleus, which fuses with the male pronucleus to form a zygote. 17 chapter 3 Week 2 of Human Development (Days 8–14) I. FURTHER DEVELOPMENT OF THE EMBRYOBLAST (FIGURE 3.1) During this period, the embryoblast differentiates into two distinct cellular layers: the dorsal epiblast layer (columnar cells) and the ventral hypoblast layer (cuboidal cells). The epiblast and hypoblast together form a flat, ovoid-shaped disk known as the bilaminar embryonic disk. Within the epiblast, clefts develop and eventually coalesce to form the amniotic cavity. Hypoblast cells migrate and line the inner surface of the cytotrophoblast to form the exocoelomic membrane, which delimits a space called the exocoelomic cavity (or primitive yolk sac). The primitive yolk sac is later called the defini- tive yolk sac when a portion of the exocoelomic cavity pinches off as an exocoelomic cyst. At the future site of the mouth, hypoblast cells become columnar shaped and fuse with epiblast cells to form a circular, midline thickening called the prochordal plate. II. FURTHER DEVELOPMENT OF THE TROPHOBLAST (FIGURE 3.1) A. Syncytiotrophoblast. The syncytiotrophoblast is the outer multinucleated zone of the trophoblast where no mitosis occurs (i.e., it arises from the cytotrophoblast). During this period, the syncytiotrophoblast continues its invasion of the endometrium, thereby eroding endometrial blood vessels and endometrial glands. Lacunae form within the syncytiotrophoblast and become filled with maternal blood and glandular secretions. In addition, endometrial stromal cells (decidual cells) at the site of implantation become filled with glycogen and lipids and also supply nutrients to the embryoblast. The isolated lacunae fuse to form a lacunar network through which maternal blood flows, thus establishing early uteroplacental circulation. Although a primitive circulation is established between the uterus and future placenta, the embryoblast receives its nutrition via diffusion only at this time. B. Cytotrophoblast. The cytotrophoblast is mitotically active as new cytotrophoblastic cells migrate into the syncytiotrophoblast, thereby fueling the growth of the syncytiotrophoblast. In addition, cytotrophoblastic cells also produce local mounds called primary chorionic villi that bulge into the surrounding syncytiotrophoblast. 18  A B C D E FIGURE 3.1. A. A day 8 blastocyst is shown partially implanted into the endometrium. Extraembryonic mesoderm (EEM) has not formed yet. B. A day 12 blastocyst is shown completely implanted within the endometrium, and epithelium has regenerated. This type of implantation is known as interstitial implantation. EEM begins to form. C. A day 13 blastocyst. A lacunar network forms, establishing an early uteroplacental circulation. An exocoelomic cyst begins to pinch off (small arrows). D. A day 14 blastocyst. The embryoblast can be described as two balloons (amniotic cavity and yolk sac) pressed together at the bilaminar embryonic disk. The curved open arrow indicates that the embryoblast receives mater- nal nutrients via diffusion. E. A sonogram at about week 3 shows a hyperechoic rim representing the chorion (thick arrow) surrounding the chorionic cavity (or gestational sac). Within the chorionic cavity, two tiny cystic areas (i.e., the amnion and yolk sac) separated by a thin echogenic line (i.e., embryonic disk) can be observed. Note the hyperechoic base of the endometrium (long arrows) and two endometrial cysts (short arrows). 19 20 BRS Embryology III. DEVELOPMENT OF EXTRAEMBRYONIC MESODERM (FIGURE 3.1) The extraembryonic mesoderm develops from the epiblast and consists of loosely arranged cells that fill the space between the exocoelomic membrane and the cytotrophoblast. Large spaces develop in the extraembryonic mesoderm and coalesce to form the extraembryonic coelom. The extraembry- onic coelom divides the extraembryonic mesoderm into the extraembryonic somatic mesoderm and extraembryonic visceral mesoderm. The extraembryonic somatic mesoderm lines the trophoblast, forms the connecting stalk, and covers the amnion. The extraembryonic visceral mesoderm covers the yolk sac. As soon as the extra- embryonic somatic mesoderm and extraembryonic visceral mesoderm form, one can delineate the chorion, which consists of the extraembryonic somatic mesoderm, cytotrophoblast, and syncytiotro- phoblast. As the chorion is delineated, the extraembryonic coelom is now called the chorionic cavity. The conceptus is suspended by the connecting stalk within the chorionic cavity. IV. CLINICAL CONSIDERATIONS A. Human chorionic gonadotropin (hCG) is a glycoprotein produced by the syncytiotrophoblast, which stimulates the production of progesterone by the corpus luteum (i.e., maintains corpus luteum function). This is clinically significant because progesterone produced by the corpus luteum is essential for the maintenance of pregnancy until week 8. The placenta then takes over progesterone production. hCG can be A assayed in maternal blood at day 8 or maternal urine at day 10 and is the basis of pregnancy testing. hCG is detectable throughout a pregnancy. Low hCG values may predict a spontaneous abortion or indicate an ectopic pregnancy. Elevated hCG values may indicate a multiple pregnancy, hydatidiform mole, or gestational trophoblastic neoplasia. B. RU-486 (mifepristone; Mifeprex) initiates menstrua- tion when taken within 8–10 weeks of the start of the last menstrual period. If implantation of a conceptus has occurred, the conceptus will be sloughed along with the endometrium. RU-486 is a progesterone- receptor antagonist (blocker) used in conjunction with misoprostol (Cytotec; a prostaglandin E1 [PGE1] analogue) and is 96% effective at terminating B pregnancy. C. Hydatidiform mole (complete or partial; Figure 3.2) represents an abnormal placenta characterized by marked enlargement of chorionic villi. A complete mole is distinguished from a partial mole by the amount of chorionic villous involvement. The hallmarks of a complete mole include gross, generalized edema of chorionic villi forming grape-like, transparent vesicles, hyperplastic proliferation of surrounding trophoblastic cells, and absence of an embryo/fetus. Clinical signs diagnostic of a mole include preeclampsia during FIGURE 3.2. Hydatidiform mole.  Chapter 3 Week 2 of Human Development (Days 8–14) 21 the first trimester, elevated hCG levels (.100,000 A mIU/mL), and an enlarged uterus with bleeding. About 3% to 5% of moles develop into gestational trophoblastic neoplasia, so follow-up visits after a mole is detected are essential. The photograph (Figure 3.2A) shows gross edema of the chorionic villi forming grape-like vesicles. The light micrograph (Figure 3.2B) shows edema of the chorionic villi (cv) surrounded by hyperplastic trophoblastic cells (tc). D. Gestational trophoblastic neoplasia (GTN; choriocar- cinoma; Figure 3.3) is a malignant tumor of the trophoblast that may occur following a normal or ectopic pregnancy, abortion, or hydatidiform mole. With a high degree of suspicion, elevated hCG levels are diagnostic. Nonmetastatic GTN (i.e., confined to the uterus) is the most common form of the neoplasia, and treatment is highly successful. However, the prognosis of metastatic GTN is poor if it spreads to the liver or brain. The photograph (Figure 3.3A) shows hemorrhagic nodules metastatic to the liver. This is due to the B rapid proliferation of trophoblastic cells combined with marked propensity to invade blood vessels. The central portion of the lesion is hemorrhagic and necrotic, with only a thin rim of trophoblastic cells at the periphery. The light micrograph (Figure 3.3B) shows the distinctive alternating arrangement of mononuclear cytotrophoblastic cells (cy) and multinucleated syncytiotrophoblastic cells (sy). E. Oncofetal antigens (Table 3.1) are cell surface antigens that normally appear only on embryonic cells but for unknown reasons are re-expressed in human malignant cells. Monoclonal antibodies directed against specific oncofetal antigens provide an avenue for cancer therapy. FIGURE 3.3. Gestational trophoblastic neoplasia. t a b l e 3.1 Oncofetal Antigens and Tumor Markers Antigen Associated Tumor α-Fetoprotein (AFP) Hepatocellular carcinoma, germ cell neoplasms, yolk sac or endodermal sinus tumors of the testicle or ovary α-1-Antitrypsin (AAT) Hepatocellular carcinoma, yolk sac or endodermal sinus tumors of the testicle or ovary Carcinoembryonic antigen (CEA) Colorectal cancer, pancreatic cancer, breast cancer, and small cell cancer of the lung; bad prognostic sign if elevated preoperatively β2-Microglobulin Multiple myeloma (excellent prognostic factor), light chains in urine (Bence Jones protein) CA 125 Surface-derived ovarian cancer CA 15-3 Breast cancer CA 19-9 Pancreatic cancer (excellent marker) Neuron-specific enolase (NSE) Small cell carcinoma of the lung, seminoma, neuroblastoma Prostate-specific antigen (PSA) Prostate cancer Human chorionic gonadotropin (hCG) Trophoblastic tumors, hydatidiform mole (benign), choriocarcinoma (malignant) Bombesin Small cell carcinoma of the lung, neuroblastoma Lactate dehydrogenase (LDH) Hodgkin disease CA, cancer antigen. 22 BRS Embryology Study Questions for Chapter 3 1. Which of the following components plays 6. A 16-year-old girl presents on May 10 in the most active role in invading the endome- obvious emotional distress. On questioning, she trium during blastocyst implantation? relates that on May 1 she experienced sexual (A) Epiblast intercourse for the first time, without using (B) Syncytiotrophoblast any means of birth control. Most of her anxiety (C) Hypoblast stems from her fear of pregnancy. What should (D) Extraembryonic somatic mesoderm the physician do to alleviate her fear? (E) Extraembryonic visceral mesoderm (A) Prescribe diazepam and wait to see if she misses her next menstrual period 2. Between which two layers is the extraembry- (B) Use ultrasonography to document onic mesoderm located? pregnancy (A) Epiblast and hypoblast (C) Order a laboratory assay for serum hCG (B) Syncytiotrophoblast and cytotrophoblast (D) Order a laboratory assay for serum (C) Syncytiotrophoblast and endometrium progesterone (D) Exocoelomic membrane and (E) Prescribe diethylstilbestrol (“morning- syncytiotrophoblast after pill”) (E) Exocoelomic membrane and cytotrophoblast 7. Carcinoembryonic antigen (CEA) is an on- cofetal antigen that is generally associated with which one of the following tumors? 3. During week 2 of development, the embryo- blast receives its nutrients via (A) Hepatoma (B) Germ cell tumor (A) diffusion (C) Squamous cell carcinoma (B) osmosis (D) Colorectal carcinoma (C) reverse osmosis (E) Teratocarcinoma (D) fetal capillaries (E) yolk sac nourishment For each of Questions 8–13 concerning a 14-day-old blastocyst, select the most appropri- 4. The prochordal plate marks the site of the ate structure in the accompanying diagram. future (A) umbilical cord (B) heart (C) mouth (D) anus (E) nose 5. Which of the following are components of the definitive chorion? (A) Extraembryonic somatic mesoderm and epiblast (B) Extraembryonic somatic mesoderm and cytotrophoblast (C) Extraembryonic somatic mesoderm and syncytiotrophoblast (D) Extraembryonic somatic mesoderm, cytotrophoblast, and syncytiotrophoblast 8. Future site of the mouth (E) Extraembryonic visceral mesoderm, cytotrophoblast, and syncytiotrophoblast 9. Forms definitive structures found in the adult 22  Chapter 3 Week 2 of Human Development (Days 8–14) 23 10. Chorion 15. At what location does the amniotic cavity develop? 11. Chorionic cavity (A) Between the cytotrophoblast and syncytiotrophoblast 12. Primary chorionic villi (B) Within the extraembryonic mesoderm (C) Between the endoderm and mesoderm 13. Connecting stalk (D) Within the hypoblast (E) Within the epiblast 14. A 42-year-old woman presents with com- plaints of severe headaches, blurred vision, 16. At the end of week 2 of development slurred speech, and loss of muscle coordina- (day 14), what is the composition of the tion. Her last pregnancy 5 years ago resulted in embryonic disk? a hydatidiform mole. Laboratory results show a (A) Epiblast only high hCG level. Which of the following condi- (B) Epiblast and hypoblast tions is a probable diagnosis? (C) Ectoderm and endoderm (A) Vasa previa (D) Ectoderm, mesoderm, and endoderm (B) Placenta previa (E) Epiblast, mesoderm, and hypoblast (C) Succenturiate placenta (D) Choriocarcinoma (E) Membranous placenta  Answers and Explanations 1. B. The syncytiotrophoblast plays the most active role in invading the endometrium of the mother’s uterus. During the invasion, endometrial blood vessels and endometrial glands are eroded and a lacunar network is formed. 2. E. The extraembryonic mesoderm is derived from the epiblast and is located between the exocoelomic membrane and the cytotrophoblast. The overall effect is to completely separate the embryoblast from the trophoblast, with the extraembryonic mesoderm serving as a conduit (connection) between them. 3. A. During week 2 of development, the embryoblast receives its nutrients from endometrial blood vessels, endometrial glands, and decidual cells via diffusion. Diffusion of nutrients does not pose a problem, given the small size of the blastocyst during week 2. Although the begin- nings of a uteroplacental circulation are established during week 2, no blood vessels have yet formed in the extraembryonic mesoderm to carry nutrients directly to the embryoblast (this occurs in week 3). 4. C. The prochordal plate is a circular, midline thickening of hypoblast cells that are firmly attached to the overlying epiblast cells. The plate will eventually develop into a membrane called the oropharyngeal membrane at the site of the future mouth. It is interesting to note that at this early stage of development the cranial versus caudal region of the embryo is established by the prochordal plate, and since the prochordal plate is located in the midline, bilateral symmetry is also established. 5. D. The definitive chorion consists of three components: extraembryonic somatic mesoderm, cytotrophoblast, and syncytiotrophoblast. The chorion defines the chorionic cavity in which the embryoblast is suspended and is vital in the formation of the placenta. 6. C. Human chorionic gonadotropin (hCG) can be assayed in maternal serum at day 8 of devel- opment and in urine at day 10. If this teenager is pregnant, the blastocyst would be in week 2 of development (day 10). Laboratory assay of hCG in either the serum or urine can be completed; however, serum hCG might be more reliable. It is important to note that if she is pregnant, she will not miss a menstrual period until May 15, at which time the embryo will be entering week 3 of development. 7. D. Oncofetal antigens are normally expressed during embryonic development, remain unex- pressed in normal adult cells, but are re-expressed on transformation to malignant neoplastic tissue. CEA is associated with colorectal carcinoma. 8. E. The prochordal plate indicates the site of the future mouth. At this early stage of develop- ment, the orientation of the embryo in the cranial versus caudal direction is established. The prochordal plate is a thickening of hypoblast cells that are firmly attached to the epiblast cells. 9. C. The bilaminar embryonic disk develops definitive adult structures after gastrulation occurs, as contrasted with the trophoblast, which is involved in placental formation. 10. D. The chorion consists of three layers—extraembryonic somatic mesoderm, cytotrophoblast, and syncytiotrophoblast. The chorion is vital in the formation of placenta. 11. G. The chorion forms the walls of the chorionic cavity in which the conceptus is suspended by the connecting stalk. Note that the inner lining of the chorionic cavity is extraembryonic mesoderm. 12. A. The cytotrophoblast is mitotically active, so that local mounds of cells (primary chorionic villi) form that bulge into the surrounding syncytiotrophoblast. As development continues, 24  Chapter 3 Week 2 of Human Development (Days 8–14) 25 primary chorionic villi form secondary chorionic villi and finally tertiary chorionic villi as part of placental formation. 13. B. The extraembryonic mesoderm can be thought of as initially forming in a continuous layer and then splitting as isolated cavities begin to appear everywhere except dorsally near the am- niotic cavity and epiblast. When the isolated cavities coalesce, the extraembryonic coelom (or chorion cavity) and connecting stalk are formed. 14. D. After a hydatidiform mole, it is very important to assure that all the invasive trophoblastic tis- sue is removed. High levels of hCG are a good indicator of retained trophoblastic tissue because such tissue produces this hormone. In this case, the trophoblastic tissue has developed into a malignant choriocarcinoma and metastasized to the brain, causing her symptoms of headache, blurred vision, and so on. 15. E. The amniotic cavity develops within the epiblast, and it is a cavity that contains the embryo and amniotic fluid. 16. B. The embryoblast consists of the two distinct cell layers (epiblast and hypoblast) at the end of development week 2 (day 14) and forms the bilaminar embryonic disk. chapter 4 Embryonic Period (Weeks 3–8) I. GENERAL CONSIDERATIONS A. By the end of the embryonic period, all major organ systems begin development, although functionality may be minimal. B. During the embryonic period, the uteroplacental circulation cannot satisfy the increasing nutritional needs of the rapidly developing embryo, so development of the cardiovascular system is essential. C. During the embryonic period, folding of the embryo occurs in two distinct planes. Craniocaudal folding progresses due to the growth of the central nervous system (CNS) and the amnion. Lateral folding progresses due to the growth of the somites, amnion, and other components of the lateral body wall. D. Both the craniocaudal folding and lateral folding change the shape of the embryo from a two- dimensional disk to a three-dimensional cylinder. E. By the end of week 8, the embryo has a distinct human appearance. F. During the embryonic period, the basic segmentation of the human embryo in the craniocaudal direction is controlled by the Hox (homeobox) complex of genes. G. The development of each individual organ system will be reviewed in forthcoming chapters. However, it is important to realize that all organ systems develop simultaneously during the embryonic period. In addition, embryogenesis proceeds at a slower pace in the female embryo compared with the male embryo due to the presence of the paternally imprinted X chromosome. II. FURTHER DEVELOPMENT OF THE EMBRYOBLAST A. Gastrulation (Figure 4.1) 1. Gastrulation is the process that establishes the three definitive germ layers of the embryo (ectoderm, intraembryonic mesoderm, and endoderm), forming a trilaminar embryonic disk by day 21 of development. These three germ layers give rise to all the tissues and organs of the adult. 26  Chapter 4 Embryonic Period (Weeks 3–8) 27 Level and view of sections A and B A Primitive pit B Primitive groove Cranial Caudal end end Cardiogenic area Prochordal Cloacal plate (future Primitive node membrane Level of mouth) (future anus) section C Notochord C Caudal Primitive groove end Epiblast (ectoderm) Mesoderm Endoderm Hypoblast FIGURE 4.1. Schematic representation of gastrulation. Embryoblast at the upper left is for orientation. A. Dorsal view of the epiblast (blue). B. Dotted arrows (red) show the migration of cells through the primitive streak during gastrulation. C. Cross section showing the migration of cells that will form the intraembryonic mesoderm (red) and displace the hypo- blast (yellow) to form the endoderm. Epiblast cells migrate to the primitive streak and invaginate into a space between the epiblast and hypoblast. Some of these migrating epiblast cells displace the hypoblast to form the definitive endoderm. The remainder of the epiblast cells migrates laterally, cranially, and along the midline to form the definitive intraembryonic mesoderm (e.g., cardiogenic area, notochord). After the formation of the endoderm and intraembryonic mesoderm, the epiblast is called the definitive ectoderm. 2. Gastrulation is heralded by the formation of the primitive streak and is caused by a proliferation of epiblast cells. The primitive streak consists of the primitive groove, primitive node, and primitive pit. 3. As early as the bilaminar and trilaminar stages of embryogenesis, the left side/right side (L/R) axis determination begins with the asymmetric activity of sonic hedgehog protein (Shh) only on the future left side since Shh activity is suppressed on the future right side by activin. In addition, the neurotransmitter serotonin (5HT) plays an important role in L/R axis determination. 4. The cloacal membrane is the future site of the anus where the epiblast and hypoblase cells fuse. The cloacal membrane is located caudal to the primitive streak. 5. The ectoderm, intraembryonic mesoderm, and endoderm of the trilaminar embryonic disk are all derived from the epiblast. The term intraembryonic mesoderm describes the germ layer that forms during week 3 (gastrulation), in contrast to the extraembryonic mesoderm, which forms during week 2. 28 BRS Embryology 6. Intraembryonic mesoderm forms various tissues and organs found in the adult, whereas extraembryonic mesoderm is involved in placenta formation. In this regard, later chapters do not use the term “intraembryonic mesoderm” when discussing tissue and organ development of the adult, but instead shorten the term to “mesoderm.” B. Changes involving intraembryonic mesoderm (Figure 4.2) 1. Paraxial mesoderm is a thick plate of mesoderm located on each side of the midline. Paraxial mesoderm becomes organized into segments known as somitomeres, which form in a craniocaudal sequence. Somitomeres 1–7 do not form somites but contribute mesoderm to the pharyngeal arches. The remaining somitomeres further condense in a craniocaudal sequence to form 42–44 pairs of somites. The first pair of somites forms on day 20, and new somites appear at a rate of 3 per day. The caudal-most somites eventually disappear to give a final count of approximately 35 pairs of somites. The number of somites is one of the criteria for determining the age of the embryo. Somites further differentiate into the following components: a. Sclerotome forms the cartilage and bone components of the vertebral column. b. Myotome forms epimeric and hypomeric muscles. c. Dermatome forms dermis and subcutaneous area of skin. 2. Intermediate mesoderm is a longitudinal dorsal ridge of mesoderm located between the paraxial mesoderm and lateral mesoderm. This ridge develops into the urogenital ridge, which forms the future kidneys and gonads. 3. Lateral mesoderm is a thin plate of mesoderm located along the lateral sides of the embryo. Large spaces develop in the lateral mesoderm and coalesce to form the intraembryonic coelom. The intraembryonic coelom divides the lateral mesoderm into two layers: a. Intraembryonic somatic mesoderm (also called somatopleure) b. Intraembryonic visceral mesoderm (also called visceropleure or splanchnopleure) 4. Notochord is a solid cylinder of mesoderm extending in the midline of the trilaminar embryonic disk from the primitive node to the prochordal plate. It has a number of important functions, which include the following: a. The notochord induces the overlying ectoderm to differentiate into neuroectoderm to form the neural plate. b. The notochord induces the formation of the vertebral body of each of the vertebrae. c. The notochord forms the nucleus pulposus of each intervertebral disk. 5. Cardiogenic region is a horseshoe-shaped region of mesoderm located at the cranial end of the trilaminar embryonic disk rostral to the prochordal plate. This region forms the future heart. 6. Specific derivatives of mesoderm are indicated in Table 4.1. C. Changes involving ectoderm. The major change involving a specific portion of ectoderm is its induction by the underlying notochord to differentiate into neuroectoderm and neural crest cells, thereby forming the future nervous system. Specific derivatives of ectoderm are indicated in Table 4.1. D. Changes involving endoderm. Specific derivatives of endoderm are indicated in Table 4.1. III. VASCULOGENESIS (DE NOVO BLOOD VESSEL FORMATION) Vasculogenesis occurs in two general locations as follows. A. In extraembryonic mesoderm: 1. Angiogenesis occurs first within extraembryonic visceral mesoderm around the yolk sac on day 17. 2. By day 21, angiogenesis extends into extraembryonic somatic mesoderm located around the connecting stalk to form the umbilical vessels and in secondary villi to form tertiary chorionic villi.  Chapter 4 Embryonic Period (Weeks 3–8) 29 Anterior Somite A Paraxial Neural folds Lateral folds neuropore Posterior mesoderm of amnion neuropore Heart Connection Level of sections A, B, C with yolk sac Endoderm Notochord Start of intraembryonic coelom Neural tube B Somite Intermediate mesoderm Surface Lateral ectoderm mesoderm Intraembryonic somatic mesodern Endoderm Intraembryonic Intraembryonic coelom visceral mesoderm C Dermatome Neural tube Myotome Notochord Sclerotome FIGURE 4.2. Schematic representation showing changes involving intraembryonic mesoderm. Picture in the upper right is for orientation. A. Cross section at day 19. B. Cross section at day 21, with arrows indicating lateral folding of the embryo. C. Cross section showing differentiation of the somite. Ectoderm and neuroectoderm, blue; mesoderm, red; endoderm, yellow. 30 BRS Embryology t a b l e 4.1 Germ Layer Derivatives Ectoderm Mesoderm Endoderm Epidermis, hair, nails, sweat and Muscle (smooth, cardiac, skeletal) Hepatocytes sebaceous glands Extraocular muscles, ciliary muscle of Principal and oxyphil cells of Utricle, semicircular ducts, vestibular eye, iris stroma, ciliary body stroma parathyroid ganglion of CN VIII Substantia propria of cornea, corneal Thyroid follicular cells thymus Saccule, cochlear duct (organ of endothelium, sclera, choroid Epithelial reticular cells of thymus Corti), spiral ganglion of CN VIII Muscles of tongue (occipital somites) Acinar and islet cells of pancreas Olfactory placode, CN I Pharyngeal arch muscles Acinar cells of submandibular and Ameloblasts (enamel of teeth) Laryngeal cartilages sublingual glands Adenohypophysis Connective tissue Epithelial lining of: Lens of eye Dermis and subcutaneous layer of skin Gastrointestinal tract Anterior epithelium of cornea Bone and cartilage Trachea, bronchii, lungs Acinar cells of parotid gland Dura mater Biliary apparatus Acinar cells of mammary gland Endothelium of blood and lymph vessels Urinary bladder, female urethra, Red blood cells, white blood cells, most of male urethra Epithelial lining of: microglia, and Kupffer cells Inferior 2/3 of vagina Lower anal canal Spleen Auditory tube, middle ear cavity Distal part of male urethra Kidney Crypts of palatine tonsils External auditory meatus Adrenal cortex Testes, epididymis, ductus deferens, seminal vesicle, ejaculatory duct Ovary, uterus, uterine tubes, superior 1/3 of vagina Derivatives Neuroectoderm All neurons within brain and spinal cord Retina, i