The pig yolk sac I. Fine structure of the posthaematopoietic organ.

Yolk sacs from pig embryos ranging between 18 mm and 55 mm in length were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and histochemistry. The organ was no longer present in embryos of 70 mm length. The endoderm proliferates in embryos of about 20 mm length with gland-like endodermal cell columns and finally becomes stratified, representing over 90% of the yolk sac mass. The endodermal cells show a high activity of oxidoreductases and lysosomal enzymes; their luminal surface bears few absorptive specializations. The mesothelium is inert, as judged from its surface ultrastructure, organelle composition and enzyme content. TEM reveals the endodermal cells to be polarized even in stratified areas. They resemble liver parenchymal cells with respect to their basal villi, which are exposed to capillaries with discontinuous or fenestrated endothelium. Giant mitochondria with crystalline inclusions in the mature endodermal cytoplasm are outnumbered by large stacks of the rough ER, which can amount to 60% of the cytoplasm. This conspicuous RER is suspected to be the production site of serum proteins which are discharged into the vascular bed. Close to the time of the organ's regression, an unusual storage of material in terminal buds of the ER was found. Intercellular canaliculi and the endocytic apparatus of the endoderm are thought to serve regression.