Human development is a continuous process that begins when an oocyte (ovum) from a female is fertilized by a sperm (spermatozoon) from a male to form a single-celled zygote ( Fig. 1.1 ). Cell division, cell migration, programmed cell death (apoptosis), differentiation, growth, and cell rearrangement transform the fertilized oocyte, a highly specialized, totipotent cell, the zygote, into a multicellular human being. Most developmental changes occur…

Chapter 1 1 Health-care professionals are expected to give intelligent answers to the questions people ask, such as, “When does the baby's heart start to beat?” “When does it move its limbs?” “When is the embryo most at risk for effects from alcohol?” For prenatal diagnosis and any medical treatment before birth, physicians—especially family doctors, obstetricians, and pediatricians—need to know how the embryo and fetus develop,…

Birth defects (anomalies) are developmental disorders present at birth. As a global problem, it has been estimated that annually almost 8 million children worldwide are born with a serious birth defect. Birth defects are the leading cause of infant mortality and may be structural, functional, metabolic, or behavioral. A birth defect is a structural abnormality of any type. There are four clinically significant types of birth…

The integumentary system consists of the skin and its appendages: sweat glands, nails, hairs, sebaceous glands, and arrector muscles of hairs. The system also includes the mammary glands and teeth. Development of Skin and Appendages The skin, the outer protective covering of the body, is a complex organ system and is the body's largest organ. The skin consists of two layers (epidermis and dermis) that are…

Development of Eyes and Related Structures 17 The eyes are derived from four sources: Neuroectoderm of brain Surface ectoderm of head Mesoderm between the above layers Neural crest cells of the prosencephalon and mesencephalon Early eye development results from a series of inductive signals and is first evident at the beginning of the fourth week, when optic grooves (sulci) appear in the cranial neural folds (…

The nervous system consists of three main regions: The central nervous system (CNS), which includes the brain and spinal cord and is protected by the cranium and vertebral column The peripheral nervous system (PNS), which includes the neurons outside the CNS; cranial nerves and ganglia and spinal nerves and ganglia, which connect the brain and spinal cord with peripheral structures The autonomic nervous system (ANS), which…

Skeletal System As the notochord and neural tube form in the third week, the intraembryonic mesoderm lateral to these structures thickens to form two longitudinal columns of paraxial mesoderm ( Fig. 15.1 A and B ). Toward the end of the third week, these columns, located in the trunk, become compacted and segmented into blocks of mesoderm— somites (see Fig. 15.1 C ). Externally the somites…

The cardiovascular system is the first major system to function in the embryo. The primordial heart and vascular system appear in the middle of the third week ( Fig. 14.1 ). The heart begins to beat at approximately day 22 ( Fig. 14.2 ). This precocious development is necessary because the rapidly growing embryo can no longer satisfy its nutritional and oxygen requirements by diffusion alone.…

The urogenital system is divided functionally into the urinary system and genital system . Both systems are closely associated, especially during their early stages of development. The urogenital system develops from the intermediate mesenchyme (embryonic connective tissue in the mesoderm) derived from the dorsal body wall of the embryo ( Fig. 13.1 A and B ). During folding of the embryo in the horizontal plane (see…

The alimentary system (digestive system) extends from the mouth to the anus with all its associative glands and organs. The primordial gut (earliest stage of development) forms during the fourth week as the head, caudal eminence (tail), and lateral folds incorporate the dorsal part of the umbilical vesicle (yolk sac) (see Chapter 6 , Fig. 6.1 ). The primordial gut is initially closed at its cranial…

The lower respiratory organs (larynx, trachea, bronchi, and lungs) begin to form during the fourth week of development. The respiratory system starts as a median outgrowth—the laryngotracheal groove —that appears in the floor of the caudal end of the primordial pharynx ( Fig. 11.1 A and B ). This primordium of the tracheobronchial tree develops caudal to the fourth pair of pharyngeal pouches. The endodermal lining…

The pharyngeal apparatus consists of pharyngeal arches, pharyngeal pouches, pharyngeal grooves, and pharyngeal membranes ( Fig. 10.1 ). These embryonic structures contribute to the formation of the face and neck. Open full size image Fig. 10.1 Illustrations of the pharyngeal apparatus. A, Lateral view showing the development of four pharyngeal arches. B and C, Ventral (facial) views showing the relationship of the pharyngeal arches to the…

Early in the fourth week of development, the intraembryonic coelom —the primordium of the body cavities—appears as a horseshoe-shaped cavity ( Fig. 9.1 A ). The bend in this cavity at the cranial end of the embryo represents the future pericardial cavity, and its limbs indicate the future pleural and peritoneal cavities. The distal part of each limb of the intraembryonic coelom is continuous with the…

The fetal part of the placenta and fetal membranes separate the embryo or fetus from the endometrium —the inner layer of the uterine wall. The chorion, amnion, umbilical vesicle, and allantois constitute the fetal membranes. An interchange of substances (e.g., nutrients and oxygen) occurs between the maternal and fetal blood through the placenta. The vessels in the umbilical cord connect the placental circulation with the fetal…

Development during the fetal period is concerned primarily with body growth and differentiation of tissues, organs, and systems. Rudimentary organ systems were formed during the embryonic period. The rate of body growth during the fetal period is rapid, and fetal weight gain is phenomenal during the terminal weeks ( Table 7.1 ). Ultrasonographic measurements of the crown – rump length (CRL) can be used to determine…

All major external and internal structures are established during the fourth to eighth weeks. By the end of this period, the main organ systems have begun to develop. Exposure of embryos to teratogens (e.g., drugs and viruses) during this period may cause major birth defects (see Chapter 19 ). As the tissues and organs form, the shape of the embryo changes so that, by the eighth…

Rapid development of the embryo from the trilaminar embryonic disc during the third week is characterized by: Appearance of the primitive streak Development of the notochord Differentiation of three germ layers The third week of development occurs during the week of the first missed menstrual period, that is, 5 weeks after the first day of the last normal menstrual period. Cessation of menstruation is often the…

Implantation of the blastocyst is completed during the second week of development. As this process takes place, changes occur, producing a bilaminar embryonic disc composed of two layers, the epiblast and hypoblast ( Fig. 4.1 A ). The embryonic disc gives rise to germ layers that form all the tissues and organs of the embryo. Extraembryonic structures forming during the second week include the amniotic cavity,…

Development begins at fertilization when a sperm penetrates an oocyte to form a zygote. A zygote is a highly specialized, totipotent cell, which has the ability to differentiate into any type of cell. It contains chromosomes and genes derived from the mother and father. The zygote divides many times and is progressively transformed into a multicellular human being through cell division, migration, growth, and differentiation (see…