Figure 12

Parietal and visceral yolk sac at limb bud stage (A-D) and 70 days of gestation (E-G). (A) Overview to show attachment of the visceral yolk sac to the placental disk. Lateral to this the parietal yolk sac extends over the surface as a thin epithelial layer. The central part of the disk is covered with allantoic mesoderm. (B) Endoderm of the parietal yolk sac at the limb bud stage Note the apical microvilli and the desmosomes (arrowheads). TEM. (C) Visceral yolk sac; PAS. Beneath the endoderm is a layer of mesoderm carrying the vitelline vessels. (D) Visceral yolk sac; haematoxylin and eosin. (E) The yolk sac epithelium rests on Reichert's membrane, which separates it from the surface layer of cytotrophoblast; immunostained for cytokeratin. (F) Endoderm of the parietal yolk sac at 70 days. The cells rest on Reichert's membrane. (G) Visceral yolk sac at 70 days; immunostained for vimentin. Note the villous appearance of the yolk sac at this stage. Cyt = cytotrophoblasts, endo = endoderm, fm = allantoic or fetal mesoderm, pys = parietal yolk sac, Rm = Reichert's membrane, vys = visceral yolk sac, vv = vitelline vessels. Scale bars 500 μm (A), 5 μm (B), 40 μm (C-E), 10 μm (F) 100 μm (G).