Larsen's Human Embryology

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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…

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…

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…

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…

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…

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…

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.…

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…

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…