Collagen Fibrils within the Renal Glomerulus

The fine structure of the frog glomerulus has been described in detail by Pak Poy (2); the present observations confirm his findings as far as its principal structure is concerned. He pointed out that the main morphological difference between the frog glomerulus and those of mammals is in the basement membrane, that is, the structure between endothelial and epithelial (podo-cyte) cells. Beneath the epithelial lining, a homogeneous , vaguely defined, dense membrane about 200 m# thick can be seen which seems to be homologous in structure with the lamina densa of the basement membrane in the mouse glomeru-lus (3). It is noteworthy that in the frog glomeru-lus, no special differentiation of the ground substance is evident along the endothelial cell lining. The space between the lamina densa and the endothelial cells is occupied by a fine reticular or fibrillar ground substance which shows variable width in section. It is in this space that a cell (called the pericapillary cell by Pak Poy) and the profiles of its processes are encountered. For the purposes of this study, the kidney of the frog, Rana pipiens Schreber, was fixed with os-mium-sucrose mixture reported by Caulfield (6) and embedded in methacrylate. The sections were stained with 2 per cent uranyl acetate solution according to the method described by Watson (1), and subsequently blanketed between formvar and carbon films. Even under low magnification (Fig. 1), one can recognize the presence of dense fibers running apparently at random in the ground substance between the lamina densa and the endothelial * Investigation supported in part by a grant (No. cell. In higher magnification, these fibers are about 55 m# in diameter and show the characteristic fine cross-striation of collagen fibrils, namely, 640 A period and six intraperiod bands (Fig. 2 and insert). It is to be noted that the relationship between these fibrils and the surface of the peri-capillary cell is often quite intimate. These fibrils are recognized in the axial portion of the capillary loop as well as the peripheral, although their relative localization to the vascular pole of the glomer-ulus is not clear. Collagen fibrils have not heretofore been observed by electron microscopy within the glomer-ulus, although it is generally assumed that the basement membrane is part of the connective tissue in nature, and the presence of reticular or argyrophilic fibers has been reported from light microscope studies (4, 5). It is, however, clear that …