Prenatal development of subepithelial vasculature related to appearance of ruminal papillae in the bovine rumen.

The mucosal surface of the fetal bovine rumen was studied with ruminal papillae standing at an interval of 400 microns on an average with a slight increase in distance from each other during gestation. Nevertheless, the overall area of the inner surface of the rumen was greatly increased with fetal age, suggesting a successive development of new ruminal papillae appearing at the sites among older ones. In the 3rd month of gestation, the subepithelial microvasculature consisted into 2 layers (adluminal and submucosal) of capillary networks with anastomoses between them. Junctional sites of the adluminal capillary and the anastomosing ramus were gradually elevated by extension of the ramus to form hairpin-like, and later papilla-like, capillary projections in the lamina propria. These projections further pushed the surface epithelium up and formed the cores of the ruminal papillae. The number of capillary projections, regardless of their sizes, in the lamina propria always exceeded that of ruminal papillae at any time of gestational period. Therefore, it seems that the successive formation of new ruminal papillae is preceded by the formation of subepithelial capillary projections in the lamina propria.