Summary As covered in Chapter 4 , shortly after the three germ layers form during gastrulation, body folding forms the endodermal foregut at the cranial end of the embryo, thereby delineating the inner tube of the tube-within-a-tube body plan .…
Summary The nervous system consists of complex networks of neurons that carry information from the sensory receptors in the body to the central nervous system (CNS); integrate, process, and store it; and return motor impulses to various effector organs in…
Summary Even before neurulation begins, the primordia of the three primary brain vesicles — prosencephalon , mesencephalon , and rhombencephalon —are visible as broadenings of the neural plate. During the fifth week, the prosencephalon subdivides into the telencephalon and diencephalon…
Summary Development of bone and muscle occurs within mesenchymal regions of the embryo after the tube-within-a-tube body plan is established during the fourth week of gestation. Bone formation occurs in two ways. During endochondral ossification , a cartilage model first…
Summary The skin, or integument, consists of two layers: the epidermis and the dermis . The epidermis is formed mainly by the embryonic surface ectoderm, although it is also colonized by melanocytes (pigment cells), which are derived from neural crest…
Summary The gestation period of humans from fertilization to birth is usually 266 days, or 38 weeks. As covered in the Introduction, the embryonic period , during which most of the major organ systems are formed, ends at the end…
Summary Formation of the embryo and its parts involves morphogenesis , a form-shaping process controlled by fundamental cell behaviors that result in differential growth . Perturbation of differential growth due to a genetic mutation , teratogen exposure, or a combination…
Summary During the fourth week, the tissue layers laid down in the third week differentiate to form the primordia of most of the major organ systems of the body. Simultaneously, the embryonic disc undergoes a process of folding that creates…
Summary The first major event of the third week, gastrulation , commences with the formation of a longitudinal midline structure, the primitive streak , in the epiblast near the caudal end of the bilaminar embryonic disc. The cranial end of…
Summary As covered in the preceding chapter, the morula—formed by cleavage of the zygote—transforms during the first week into a blastocyst consisting of an inner cell mass, or embryoblast, and a trophoblast. At the beginning of the second week, the…