Fig. 5: Functions of the primate secondary yolk sac.

a Vasculogenesis of the yolk sac forms an intricate vascular plexus that envelopes the yolk sac from Carnegie stage (CS) 6A. Haematopoiesis ensues in preparation for the onset of embryonic circulation, mediated by the developing heart. Reciprocal crosstalk signalling between the endoderm and mesoderm regionalises foci of condensed mesoderm, which form primitive blood islands. b PGCs are specified in the embryonic region and migrate via the yolk sac into the hindgut and up through the dorsal mesentery towards the genital ridges. Schematic drawn after ref. ¹⁶⁴. The original images were published in Medical Physiology E-Book¹⁶⁴, Copyright Elsevier (Health Sciences, ISBN 1455733288, 9781455733286). c The mesothelium exhibits hallmarks of absorption, degradation and re-synthesis, evident by high concentration of LRP2–CUBN–AMN endovesicular complexes in the plasma membrane. Lysosomes contain hydrolytic enzymes, such as the Cathepsins, to mediate the degradation of maternal proteins and other complex molecules in the nutrient-rich exocoelomic cavity. Nutrients are either transported or re-synthesised and exocytosed either directly into surrounding blood vessels or transported through the extracellular environment towards the endodermal tubules and the yolk sac cavity. d The endoderm contains high concentrations of rER, sER, glycogen vesicles and exocytotic vesicles. Yolk sac endoderm cells synthesise and exocytose key carrier proteins such at AFP, TTR, ALB and TF into the yolk sac cavity, which are absorbed by primitive gut endoderm. Nu nucleus, rER rough endoplasmic reticulum, sER smooth endoplasmic reticulum, Mt Mitochondrion, SLC solute carrier family, Ly lysosome, CTSE Cathepsin family member E, LRP2 LDL-related receptor 2, CUBN Cubulin, ApoA apolipoprotein A, TTR transthyretin, TF transferrin, AFP alpha-fetoprotein, AMN amnionless.