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Trilaminar Germ Disc Contents part one General Embryology ........................................... 1 chapter 1 Gametogenesis: Conversion of Germ Cells Into Male and Female Gametes ..... ..................................................... 3 chapter 2 First Week of Development: Ovulation to Implantation ............ ....... 31 chapter 3 Second Week of Development: Bilaminar Germ Disc ............ .......... 51 chapter 4 Third Week of Development: Trilaminar Germ Disc ............. .......... 65 chapter 5 Third to Eighth Week: The Embryonic Period ............... ................ 87 chapter 6 Third Month to Birth: The Fetus and Placenta ................... ............ 117 chapter 7 Birth Defects and Prenatal Diagnosis ................... ..................... 149 part two Special Embryology ............................................ 169 chapter 8 Skeletal System ............................. ..................................... 171 ix x Contents chapter 9 Muscular System ........................... ..................................... 199 chapter 10 Body Cavities ............................ ........................................ 211 chapter 11 Cardiovascular System ......................... ................................ 223 chapter 12 Respiratory System ........................... .................................. 275 chapter 13 Digestive System ........................... ..................................... 285 chapter 14 Urogenital System .......................... .................................... 321 chapter 15 Head and Neck ............................ ...................................... 363 chapter 16 Ear ................................. ................................................ 403 chapter 17 Eye ................................ ................................................ 415 chapter 18 Integumentary System .......................... ................................ 427 chapter 19 Central Nervous System ......................... ............................... 433 part three Appendix......................................................... 483 Answers to Problems .......................... ................................. 485 Figure Credits .......................... ......................................... 499 Index ................................. ............................................. 507 Preface The ninth edition of Langman’s Medical Embryology adheres to the tradition established by the original publication—it provides a concise but thorough de- scription of embryology and its clinical significance, an awareness of which is essential in the diagnosis and prevention of birth defects. Recent advances in ge- netics, developmental biology, maternal-fetal medicine, and public health have significantly increased our knowledge of embryology and its relevance. Because birth defects are the leading cause of infant mortality and a major contributor to disabilities, and because new prevention strategies have been developed, under- standing the principles of embryology is important for health care professionals. To accomplish its goal, Langman’s Medical Embryology retains its unique ap- proach of combining an economy of text with excellent diagrams and scanning electron micrographs. It reinforces basic embryologic concepts by providing numerous clinical examples that result from abnormalities in developmental processes. The following pedagogic features and updates in the ninth edition help facilitate student learning: Organization of Material: Langman’s Medical Embryology is organized into two parts. The first provides an overview of early development from gametogenesis through the embryonic period; also included in this section are chapters on placental and fetal development and prenatal diagnosis and birth defects. The second part of the text provides a description of the fundamental processes of embryogenesis for each organ system. Molecular Biology: New information is provided about the molecular basis of normal and abnormal development. Extensive Art Program: This edition features almost 400 illustrations, includ- ing new 4-color line drawings, scanning electron micrographs, and ultrasound images. Clinical Correlates: In addition to describing normal events, each chapter con- tains clinical correlates that appear in highlighted boxes. This material is de- signed to provide information about birth defects and other clinical entities that are directly related to embryologic concepts. vii viii Preface Summary: At the end of each chapter is a summary that serves as a concise review of the key points described in detail throughout the chapter. Problems to Solve: These problems test a student’s ability to apply the infor- mation covered in a particular chapter. Detailed answers are provided in an appendix in the back of the book. Simbryo: New to this edition, Simbryo, located in the back of the book, is an interactive CD-ROM that demonstrates normal embryologic events and the origins of some birth defects. This unique educational tool offers six original vector art animation modules to illustrate the complex, three-dimensional as- pects of embryology. Modules include normal early development as well as head and neck, cardiovascular, gastrointestinal, genitourinary, and pulmonary system development. Connection Web Site: This student and instructor site (http://connection. LWW.com/go/sadler) provides updates on new advances in the field and a syl- labus designed for use with the book. The syllabus contains objectives and definitions of key terms organized by chapters and the “bottom line,” which provides a synopsis of the most basic information that students should have mastered from their studies. I hope you find this edition of Langman’s Medical Embryology to be an excellent resource. Together, the textbook, CD, and connection site provide a user-friendly and innovative approach to learning embryology and its clinical relevance. T. W. Sadler Twin Bridges, Montana part one General Embryology 1 chapter 1 Gametogenesis: Conversion of Germ Cells Into Male and Female Gametes Primordial Germ Cells Development begins with fertilization, the pro- cess by which the male gamete, the sperm, and the female gamete, the oocyte, unite to give rise to a zygote. Gametes are derived from primordial germ cells (PGCs) that are formed in the epiblast during the second week and that move to the wall of the yolk sac (Fig. 1.1). During the fourth week these cells begin to migrate from the yolk sac toward the developing gonads, where they arrive by the end of the fifth week. Mitotic divisions increase their number during their migration and also when they arrive in the gonad. In preparation for fertilization, germ cells undergo gametogenesis, which includes meiosis, to reduce the number of chromosomes and cytodifferentiation to complete their maturation. CLINICAL CORRELATE Primordial Germ Cells (PGCs) and Teratomas Teratomas are tumors of disputed origin that often contain a variety of tissues, such as bone, hair, muscle, gut epithelia, and others. It is thought that these tumors arise from a pluripotent stem cell that can differentiate into any of the three germ layers or their derivatives. 3 4 Part One: General Embryology Figure 1.1 An embryo at the end of the third week, showing the position of primordial germ cells in the wall of the yolk sac, close to the attachment of the future umbilical cord. From this location, these cells migrate to the developing gonad. Some evidence suggests that PGCs that have strayed from their normal mi- gratory paths could be responsible for some of these tumors. Another source is epiblast cells migrating through the primitive streak during gastrulation (see page 80). The Chromosome Theory of Inheritance Traits of a new individual are determined by specific genes on chromosomes inherited from the father and the mother. Humans have approximately 35,000 genes on 46 chromosomes. Genes on the same chromosome tend to be inher- ited together and so are known as linked genes. In somatic cells, chromosomes appear as 23 homologous pairs to form the diploid number of 46. There are 22 pairs of matching chromosomes, the autosomes, and one pair of sex chro- mosomes. If the sex pair is XX, the individual is genetically female; if the pair is XY, the individual is genetically male. One chromosome of each pair is derived from the maternal gamete, the oocyte, and one from the paternal gamete, the Chapter 1: Gametogenesis: Conversion of Germ Cells Into Male and Female Gametes 5 sperm. Thus each gamete contains a haploid number of 23 chromosomes, and the union of the gametes at fertilization restores the diploid number of 46. MITOSIS Mitosis is the process whereby one cell divides, giving rise to two daughter cells that are genetically identical to the parent cell (Fig. 1.2). Each daughter cell receives the complete complement of 46 chromosomes. Before a cell enters mitosis, each chromosome replicates its deoxyribonucleic acid (DNA). During this replication phase the chromosomes are extremely long, they are spread diffusely through the nucleus, and they cannot be recognized with the light mi- croscope. With the onset of mitosis the chromosomes begin to coil, contract, and condense; these events mark the beginning of prophase. Each chromo- some now consists of two parallel subunits, chromatids, that are joined at a narrow region common to both called the centromere. Throughout prophase the chromosomes continue to condense, shorten, and thicken (Fig. 1.2A), but only at prometaphase do the chromatids become distinguishable
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