An Ultrastructural Study of the Intrauterine Development of Rabbit Metanephric Glomeruli.

The electron microscopic observations correlated with the light microscopic findings were carried out on the intrauterine development of rabbit metanephric glomeruli. The capillaries of the glomerulus develop from a stromal capillary of the metanephric blastema when this capillary localizes within the cleftal space between the middle and lower limbs of the S-shaped metanephric vesicle. The development of the glomerular capillaries is brought about by elongation and repeated branching of the original intracleftal capillary, resulting in the invagination of the hemispheric lower limb. On the other hand, the mesangial cells originate from mesenchymal cells with which the intracleftal capillarly is invariably associated in its ingrowth into the lower cleft. These observations raise an objection to the still widely accepted concept on glomerular differentiation that the glomerular capillaries develop independently from the general circulation of the metanephric kidney, with which they only later unite.