Chapter 10: Cell Populations at Gastrulation

Initially, the conceptus at stage 6 features a bilaminar disc composed of the epithelial epiblast and the visceral hypoblast, situated where the amniotic cavity and yolk sac meet. The older classification of three definitive germ layers is considered incorrect for these early stages, as the epiblast and hypoblast contain mixed cell populations that first establish extraembryonic tissues, with the three earliest cell lineages being trophoblast, hypoblast, and epiblast. The beginning of this profound rearrangement period is marked by the appearance of the primitive streak in the caudal region of the elongated embryonic disc, which confers the future cranio-caudal axis. Formation of the primitive streak, which is induced by the underlying visceral hypoblast, is analogous to the blastopore in lower vertebrates. Epiblast cells intensely proliferate at the streak and undergo ingression, an example of epithelial-to-mesenchymal transformation (EMT), as they migrate away from the epiblast and deep into the space between the epiblast and hypoblast. The developmental fate of these cells is determined by the specific time and position of their passage through the primitive streak or its most rostral part, the primitive node (or Hensen’s node). Cells ingressing through the node produce the axial cell populations, including the prechordal plate, notochord, embryonic endoderm, and the medial halves of the somites. The notochord arises from the medial primitive node and is essential for inducing motor neurons and the maintenance of the neural floor plate. Concurrently, the epithelial cells remaining on the dorsal surface become the embryonic ectoderm, which includes both surface ectoderm and the medial neural ectoderm (neural plate). The migrating cells form the non-polarized mesoblast (termed mesenchyme once they reach their destination), which separates the epiblast and endoderm layers, except at the rostral buccopharyngeal membrane and the caudal cloacal membrane. The intraembryonic mesoblast eventually differentiates into the paraxial mesenchyme, lateral plate mesenchyme, and cardiogenic mesoblast. Following the formation of the trilaminar disc, embryonic folding (head, tail, and lateral) repositions structures, constricting the yolk sac into the foregut, midgut, and hindgut. Simultaneously, the intraembryonic coelom forms from confluent vesicles within the rostral lateral plate mesenchyme via mesenchymal-to-epithelial transformation. This horseshoe-shaped tube compartmentalizes into the precursors of the pericardial cavity, pleural cavities (pericardioperitoneal canals), and peritoneal cavity, and is crucial for providing a primitive fluid circulation for nutrient delivery to deeper tissues until the blood vascular system fully develops around stage 13. All tissue development relies on the dynamic interaction between specialized epithelia (polarized, forming sheets with basal laminae) and mesenchyme (non-polarized, separated by the three-dimensional extracellular matrix, or ECM), where complex matrix molecules like fibronectin and hyaluronic acid structure the space and propagate developmental instructions.