An in vitro morphological study of the mouse visceral yolk sac and possible yolk sac immunocyte precursors.

Mouse visceral yolk sac has been organ cultured from 9 days of gestation, a time prior to the thymus being lymphoid, until 12 days of gestation, a time after which the thymus is lymphoid. During the culture period the endodermal epithelial cells survived well, erythropoiesis diminished, endothelial-lined cavities formed in the mesodermal mass, and cells developed which have been classified as large, medium and small immunocyte precursors. The cytoplasm of the immunocyte precursors contains polysomes, spherical mitochondria, a few profiles of rough endoplasmic reticulum, occasional granules and a large Golgi complex. This study offers morphological support for the yolk sac origin of immunocyte precursors in the mouse which may seed the thymus and liver.