The yolk sac endoderm (YSE) is a membrane that originates in the hypoblast layer of the inner cell mass and lines the inner side of the definitive yolk sac. The surface of the endoderm-derived epithelium has a dense microvillous border with numerous endocytic vesicles, indicating endocytic activity.
Critical differences in YSE location are seen between human and rodent yolk sac structures. The human yolk sac floats like a balloon within the exocoelomic cavity, whose interior is lined by the endodermal cells. In contrast, rodents feature an inverted yolk sac, where the endodermal layer faces outward and the yolk sac cavity is formed between the visceral and parietal endoderm-derived epithelial layers. Despite these differences, studies indicate that absorption of nutrients occurs in the endodermal cell layer of the yolk sac in both species.