Cellular outgrowth from isolated glomeruli. Origin and characterization

The incorporation of 3H-thymidine by isolated decapsulated glomeruli from pigs was studied up to 5 days after explantation by light and electron microscopic autoradiography. The visceral epithelial cells (VEC) were the only cell type to be labeled during the first 2.5 days, showing that this tracer can be applied as a specific marker for VEC in decapsulated glomeruli. From the third day, label was incorporated by another cell type, probably mesangial cells. Labeling of endothelial cells could not be observed. Isolated glomeruli were then incubated for 2.5 days with 3H-thymidine in order to label the VEC, and after 5 to 6 days the primary outgrowth of cells appeared. Light and electron microscopic autoradiography performed in situ showed that the majority of cells in the primary outgrowth consisted of labeled cells, thus showing their origin from the VEC. The VEC in culture appeared as very large, irregular cells with a slow proliferation rate. After 9 days of culture another cell type was seen in the outgrowth (type II cells), probably derived from the mesangial cells. This cell grew in multilayers and had a close similarity to smooth muscle cells in culture, with thick bundles of microfilaments in the periphery of the cell and an abundant intercellular matrix. A third cell type (type III) with a regular angulated, "epitheloid" shape was rarely seen in the outgrowths. Because 1 to 2% of the isolated glomeruli were encapsulated, the parietal epithelial cell is suggested as the mother cell for this cell type; this is also thought to be true because the parietal epithelial cell showed 3H-thymidine incorporation in autoradiographs of intact glomeruli.