Sample Chapter Before We are Born 8e by Moore To Order Call Sms at 91 8527622422
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Sample Chapter Before We are Born 8e by Moore To Order Call Sms at 91 8527622422






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Sample Chapter Before We are Born 8e by Moore To Order Call Sms at 91 8527622422 Sample Chapter Before We are Born 8e by Moore To Order Call Sms at 91 8527622422 Document Transcript

  • C H A P T E R 1 Introduction to Human Development Importance of and Advances in Embryology 1 Descriptive Terms 5 Clinically Oriented Questions 5 uman development begins when an oocyte (ovum) from a female is fertilized by a sperm (spermatozoon) from a male. Development involves many changes that transform a single cell, the zygote, into a multicellular human being. The term conceptus refers to the entire products of conception, which includes the embryo from fertilization onward and its membranes (e.g., placenta). Embryology is concerned with the origin and development of a human being from a zygote to birth. The stages of development before birth are shown in Figures 1-1 and 7-3. H IMPORTANCE OF AND ADVANCES IN EMBRYOLOGY The study of prenatal stages and the mechanisms of human development help us understand the normal relationships of adult body structures and the causes of congenital anomalies. Much of the modern practice of obstetrics involves applied or clinical embryology. Because some children have birth defects, such as spina bifida or congenital heart disease, the significance of embryology is readily apparent to pediatricians. Advances in surgery, especially in procedures involving the prenatal and pediatric age groups, have made knowledge of human development more clinically significant. Rapid advances in molecular biology have led to the use of sophisticated techniques (e.g., recombinant DNA technology, chimeric models, transgenics, and stem cell manipulation) in research laboratories to explore such diverse issues as the genetic regulation of morphogenesis, the temporal and regional expression of specific genes, and the mechanisms by which cells are committed to form the various parts of the embryo. For the first time, researchers are beginning to understand how, when, and where selected genes are activated and expressed in the embryo during normal and abnormal development. 1
  • TIMETABLE OF HUMAN PRENATAL DEVELOPMENT 1 TO 6 WEEKS Oocyte Primary follicles EARLY DEVELOPMENT OF OVARIAN FOLLICLE MENSTRUAL PHASE PROLIFERATIVE PHASE Day 1 of last normal menstrual cycle Mature follicle Antrum Oocyte Ovulation COMPLETION OF DEVELOPMENT OF FOLLICLE Ovary Oocyte Oocyte CONTINUATION OF PROLIFERATIVE PHASE OF MENSTRUAL CYCLE AGE (weeks) 1 Stage 1 Zona pellucida 2 Stage 2 begins 3 4 Stage 3 begins 5 Trophoblast 6 Stage 4 7 Stage 5 begins Implantation begins 1 Fertilization Zygote divides Morula Early blastocyst Late blastocyst Embryoblast SECRETORY PHASE OF MENSTRUAL CYCLE 8 Lacunae appear in syncytiotrophoblast 10 Cytotrophoblast 12 11 Maternal blood Lacunar Amnion Eroded network gland Primary umbilical vesicle Closing plug 9 Amniotic cavity Stage 6 begins Extraembryonic 13 mesoderm Primary villi 14 Connecting stalk Amnion 2 Bilaminar embryonic disc 15 First missed menstrual period 16 Stage 7 begins 17 Primary umbilical vesicle Embryonic disc Trilaminar embryo 18 Amnion Stage 8 begins 22 Stage 10 begins 23 Migration of cells from primitive streak Rostral neuropore 24 Stage 11 begins 20 Neural plate 3 Arrows indicate migration of mesenchymal cells 19 Brain Neural groove Neural groove Somite Somite 4 Primordia of eye and ear present. Neural folds fusing 29 31 26 25 Stage 12 begins Nasal pit CRL: 5.0 mm 36 Lens pits, optic cups, nasal pits forming 37 Stage 16 begins Upper limb bud 3 pairs of pharyngeal arches Stage 14 begins Eye Site of otic (ear) pit 28 Stage 13 begins Large head 33 CRL = crownrump length Stage 15 begins Upper limb bud 34 Cerebral vesicles distinct Handplate Lower limb bud 39 Pharyngeal arches Indicates actual size Heart Primordial mouth 38 27 Forebrain 32 Developing eye 5 Primitive streak Otic pit 2 pairs of pharyngeal arches 30 First pairs of somites Thyroid gland begins to develop Primitive streak Rostral neuropore closes Caudal neuropore 21 Neural groove Primitive node Length: 1.5 mm Heart bulge Heart begins to beat Stage 9 begins Neural plate Primitive streak Primitive streak Embryonic disc Prechordal plate Coelom Footplate present CRL:7.0 mm 40 Ear External acoustic meatus 41 Stage 17 begins CRL: 4.0 mm 35 Eye Cord CRL: 8.0 mm 42 Ear Eye 6 Digital rays Eye Oral and nasal cavities confluent Footplate CRL:9.0 mm Eye Digital rays Upper lip and nose formed CRL:10.0 mm Foot plate Ventral view Figure 1–1 Early stages of human development. Development of an ovarian follicle containing an oocyte, ovulation, and the phases of the menstrual cycle are shown. CRL: 13.0 mm
  • CHAPTER 1 I N T R OD U C T I ON T O H U MAN D E V E L OP M ENT TIMETABLE OF HUMAN PRENATAL DEVELOPMENT AGE (weeks) 43 Actual size 44 Stage 18 begins 45 46 Head large but chin poorly formed. Grooves between digital rays indicate fingers. 7 CRL: 16 mm 50 Stage 20 begins Upper limbs longer and bent at elbows. Eyelids forming 51 Eye 52 Stage 21 begins Fingers distinct but webbed. Smooth chorion 53 9 54 Stage 22 begins 60 Placenta 10 55 56 Genitalia Stage 23 Ear Elbow 61 Toes 62 CRL: 30 mm Genitalia 63 Phallus Urogenital fold Urogenital fold Labioscrotal fold Labioscrotal fold Perineum Elbow CRL: 45 mm 66 67 Clitoris Labium minus Face has more developed profile. Note growth of chin compared to day 44. CRL: 18 mm Phallus Ear Knee 65 Wrist, fingers fused Knee Perineum 64 External ear Wrist or Large forehead 59 49 Eyelid Eye Urethral groove Anus Wrist Toes Stage 19 begins Genital tubercle Fingers Eye Beginning of fetal period or External genitalia have begun to differentiate. 58 48 Actual size Anal membrane Ear Toes 57 Genital tubercle Urogenital membrane Uterine cavity Nose 8 47 Amniotic sac Wall of uterus Urogenital groove Ears still lower than normal. Labium majus Figure 1–1, cont’d 68 CRL: 50 mm 69 70 Glans of penis Genitalia have or characteristics but still not fully formed. Urethral groove Scrotum CRL: 61 mm 3
  • 4 BEFORE W E A R E B OR N E SSE N T I AL S OF E MBR Y OL OG Y AN D BI RT H D E F EC T S Superior Cranial Anterior Dorsal Posterior Ventral Caudal Inferior A B Sagittal plane Lateral C D Median section Transverse section E Frontal (coronal) section Figure 1–2 Illustrations of descriptive terms of position, direction, and planes of the body. A, Lateral view of an adult in the anatomical position. B, Lateral view of a 5-week embryo. C and D, Ventral views of a 6-week embryo. The median plane is an imaginary vertical plane of section that passes longitudinally through the body, dividing it into right and left halves. A sagittal plane refers to any plane parallel to the median plane. A transverse plane refers to any plane that is at right angles to both the median and the frontal planes. E, Lateral view of a 7-week embryo. A frontal (coronal) plane is any vertical plane that intersects the median plane at a right angle and divides the body into front (anterior, or ventral) and back (posterior, or dorsal) parts. In describing development, it is necessary to use words denoting the position of one part to another or to the body as a whole. For example, the vertebral column develops in the dorsal part of the embryo and the sternum in the ventral part of the embryo.
  • CHAPTER 1 The critical role of genes, signaling molecules, receptors, and other molecular factors in regulating early embryonic development is rapidly being delineated. In 1995, Edward B. Lewis, Christiane Nüsslein-Volhard, and Eric F. Wieschaus were awarded the Nobel Prize in Physiology or Medicine for their discovery of genes that control embryonic development. Such discoveries are contributing to a better understanding of the causes of spontaneous abortion and congenital anomalies. In 1997, Ian Wilmut and colleagues were the first to produce a mammal (a sheep dubbed Dolly) by cloning using the technique of somatic cell nuclear transfer. Since then, other animals have been cloned successfully from cultured differentiated adult cells. Interest in human cloning has generated considerable debate because of social, ethical, and legal implications. Moreover, there is concern that cloning may result in an increase in the number of infants born with birth defects and serious diseases. Human embryonic stem cells are pluripotential and capable of developing into diverse cell types. The isolation and culture of human embryonic and other stem cells may hold great promise for the development of molecular therapies. DESCRIPTIVE TERMS In anatomy and embryology, special terms of position, direction, and various planes of the body are used. Descriptions of the adult are based on the anatomical I N T R OD U C T I ON T O H U MAN D E V E L OP M ENT 5 position, the position in which the body is erect, the upper limbs are at the sides, and the palms are directed anteriorly (see Fig. 1-2A). The descriptive terms of position, direction, and planes used for embryos are shown in Figure 1-2B to E. CLINICALLY ORIENTED QUESTIONS 1. 2. 3. What is the difference between the terms conceptus and embryo? What are the products of conception? Why do we study human embryology? Does it have any practical value in medicine and other health sciences? Physicians date a pregnancy from the first day of the last normal menstrual period, but the embryo does not start to develop until approximately 2 weeks later (see Fig. 1-1). Why do physicians use this terminology? The answers to these questions are at the back of the book.