Summary The upper limb buds appear on day 24 as small bulges on the lateral body wall at about the level of C5 to T1. By the end of the fourth week, the upper limb buds have grown to form pronounced structures protruding from the body wall, and the lower limb buds first appear, forming at about the level of L1 to S1. Limb morphogenesis takes…

Summary The eyes first appear early in the fourth week in the form of a pair of lateral grooves, the optic sulci , which evaginate from the forebrain neural groove to form the optic vesicles . As soon as the distal tip of the optic vesicle reaches the surface ectoderm, it invaginates, transforming the optic vesicle into a goblet-shaped optic cup that is attached to the…

Summary The ear is a composite structure with multiple embryonic origins. The external and middle ears arise from the first and second pharyngeal arches and the intervening pharyngeal cleft, membrane, and pouch. The inner ear, in contrast, develops from an ectodermal otic placode that forms on either side of the neural tube at the level of the future caudal hindbrain. At the end of the third…

Summary The skeleton of the head and pharynx is made up of the neurocranium —the bones that support and protect the brain and sensory organs (olfactory organs, eyes, and inner ears)—and the viscerocranium —the bones of the face and pharyngeal arches. The neurocranium can be subdivided into cranial base (the bones underlying the brain), cranial vault (the bones covering the brain), and sensory capsules (the bones…

Summary The development of the reproductive system is closely integrated with the primitive urinary organs in both males and females, as they share similar common tubular structures enabling both uresis and gamete transport. In addition to the nephric structures, the intermediate mesoderm on both sides of the dorsal body wall gives rise to a gonadal ridge. By the sixth week, the germ cells migrating from the…

Summary The urinary system maintains the electrolyte and water balance of the body fluids that bathe the tissues in a salty, aqueous environment. The development of this system involves the transient formation and subsequent regression or remodeling of vestigial primitive systems, thereby providing a glimpse of evolutionary history (another glimpse is provided by the development of the pharyngeal apparatus, covered in Chapter 17) . The development…

Summary The endodermal gut tube created by body folding during the fourth week (covered in Chapter 4 ) consists of a blind-ended cranial foregut, a blind-ended caudal hindgut, and a midgut open to the yolk sac through the vitelline duct. The gut endoderm forms the lining (epithelium) of the gastrointestinal tract, and it recruits the adjacent splanchnic mesoderm to form the other layers and major components…

Summary Starting on day 17, vessels begin to arise in the splanchnic mesoderm of the yolk sac wall from aggregations of cells called hemangioblasts . From these aggregates, two cell lineages arise: primitive hematopoietic progenitor cells and endothelial precursor cells . Vasculogenesis (de novo blood vessel formation) commences in the splanchnic mesoderm of the embryonic disc and continues later in the paraxial mesoderm. In the embryonic…

Summary In response to inductive and permissive signals emanating from the endoderm, ectoderm, and midline mesoderm, cardiogenic precursors form a cardiac primordium within the splanchnic mesoderm at the cranial end of the embryonic disc called the cardiac crescent , or first heart field . In response to signals from the underlying endoderm, a subpopulation of cells within the first heart field form a pair of lateral…

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 . On day 22, the foregut produces a ventral evagination called the respiratory diverticulum or lung bud , which is the primordium of the lungs. As the…

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 the body. The development of the CNS is covered in Chapter 9; this chapter covers the development of the peripheral nervous system (PNS). The PNS and…

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 , and the rhombencephalon subdivides into the metencephalon and myelencephalon . Thus, along with the mesencephalon, there are five secondary brain vesicles . During this period,…

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 forms and is eventually replaced with bone. This type of ossification underlies formation of the axial skeleton (vertebral column, ribs, and sternum), cranial base, and appendicular…

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 cells, and by Langerhans cells , which are immune cells of bone marrow origin. The dermis of the trunk is a mesodermal tissue. The ventral dermis…

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 of the eighth week of gestation. The remainder of gestation constitutes the fetal period , which is devoted mainly to the maturation of organ systems and…

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 of the two processes results in dysmorphogenesis and the formation of structural birth defects . Structural birth defects consist of both malformations —involving perturbation of developmental…

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 the basic vertebrate body form, called the tube-within-a-tube body plan . A major force responsible for embryonic folding is the differential growth of different portions of…

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 the primitive streak is expanded as the primitive node ; it contains a circular depression called the primitive pit , which is continuous caudally down the…

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 embryoblast splits into two layers: the epiblast and the hypoblast , or primitive endoderm . A cavity, called the amniotic cavity , develops at the embryonic…

Summary A textbook of human embryology could begin at any of several points in the human life cycle. This textbook starts with a discussion of the origin of specialized cells called primordial germ cells (PGCs) . PGCs can be first identified within the wall of the yolk sac , one of the extraembryonic membranes, during the fourth to sixth weeks of gestation. These PGCs will give…