Ultrastructure of human yolk sac: Endoderm, mesenchyme, tubules and mesothelium

Abstract The ultrastructure of human yolk sacs obtained by hysterotomy or hysterectomy from the third to the eleventh week of pregnancy is described with reference to the endoderm, mesenchyme, endodermal tubules and mesothelium. During the fourth to the sixth week of pregnancy the endoderm proliferates to form cords of cells projecting into the mesenchyme. The endodermal cells contain abundant endoplasmic reticulum, glycogen and iron‐containing material. Various types of tubules in the endoderm are described. The mesenchymal cells vary in appearance; some are stellate with slender cytoplasmic processes, while others are round. A few show signs of phagocytic capacity. Both the endodermal and the coelomic epithelia possess microvilli and pinocytotic vesicles. A hypothesis relating to the function of the human vitelline endoderm is proposed.