The amniotic ectoderm (green line) exists as an extension of the embryonic ectoderm, whilst the amniotic mesoderm lies in direct contact with the mesodermal layer of the visceral yolk sac (red dashed lines). Lateral edges of the amnion and yolk sac are joined to the embryo at the umbilical ring (boundary between the body wall and future attachment site of the umbilical cord, an allantois derivative). (A) 8–10 somite stage embryo at the onset of turning with hindgut and foregut pockets (dashed blue lines), external midgut region (solid blue line), and the ventral region, caudal to the heart, outside the amnion and yolk sac. Ventral folding morphogenesis encompasses multiple concurrent tissue rearrangements: 8-to-20 somites Abbreviations: AHP- anterior head process amn- amnion fg, foregut pocket hf-head folds PrCP-prechordal plate ys-visceral yolk sac. Arrows indicate direction of rostral-caudal (ii, iii) and lateral-to-ventral (iv) folding.
YORK YCAL NOMENCLATURE PLUS
The visceral yolk sac (extraembryonic visceral endoderm –yellow plus extraembryonic mesoderm –pink dashed line) undergoes ventral-to-lateral tissue movements to envelope the anterior region (head and heart) of the early-somite embryo. Concomitantly, lateral regions of gut endoderm and of the body wall (apposed somatic mesoderm and surface ectoderm layers) move ventrally toward the midline and fuse to close, respectively, the foregut tube and the body wall. Lateral-to-ventral folding of the bilateral heart primordia positions cardiac progenitors at the ventral line, where they merge to form the primitive heart tube. Concurrent rostral-caudal folding of neuroectoderm repositions the headfolds to reside anterior and dorsal to cardiac mesoderm. Rostral-caudal folding repositions endoderm cells along the midline as it transforms the sheet of anterior gut endoderm from a convex external layer into a concave internal foregut pocket protruding rostrally. The horizontal lines (labeled iv, v, vi) in respectively, panels i, ii, and iii indicate the level of the transverse section in the corresponding lower panel. (B) The tissue folding propelling VFM occurs along two axes, rostral-caudal (depicted in sagittal sections i–iii) and ventral-to-lateral (depicted in transverse sections, iv–vi), during the EHF (i, iv), 2 somites (ii, v), and 7 somites (iii, vi) stages. The anterior midline consists of three structures : the node, which will generate trunk and tail notochord anterior head process (AHP), axial mesoderm -derived from the early/mid gastrula organizer- that underlies the prospective midbrain and rostral hindbrain and Prechordal plate (PrCP), axial mesendoderm -derived from the early gastrula organizer- underlies the prospective forebrain. The heart primordia (red), which develop bilaterally from splanchnic mesoderm, reside just anterior to the nascent ectodermal headfolds. Before the onset of VFM, ectoderm (green) lines the amniotic cavity and encompasses the inner-most layer of the distal (embryonic) portion of the conceptus endoderm (yellow, blue) forms the surface layer and overlies the mesoderm (pink). (A) An oblique frontal view of an EHF mouse embryo planes indicate positions of sagittal and transverse sections shown in B. Ventral folding morphogenesis encompasses multiple concurrent tissue rearrangements: EHF, 0-to-8 somites
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