Glomerular differentiation in metanephric culture.

Undifferentiated metanephric blastema, grown in transfilter culture with embryonic inductor, undergoes differentiation to nephrons, including glomeruli and tubules. As in natural nephrogenesis, mesenchymal cells aggregate and form a vesicle that elongates into an s-shaped loop; a bilaminar disc at the end of the s-shaped loop becomes the glomerulus. The glomerular tuft in culture is an arborizing structure of basement membrane covered with epithelium. The epithelial cells have foot processes separated by slit pores and bridged by slit diaphragms. The cells are coated with polyanionic material, demonstrable before the appearance of foot processes. The basement membrane of the tuft continues at a hilum with the basement membrane of Bowman's capsule. Laminae of basement membrane, lying between apposed layers of epithelium, enclose spaces that can be envisioned as vascular spaces for the ingrowth of glomerular capillaries in vivo. The glomerular structure creates, therefore, a sling for holding capillaries, as suggested by Zimmermann when he proposed the concept of the mesangium. There are, however, no blood vessels in the metanephric culture, and the glomerulus develops and differentiates without capillary ingrowth. The glomeruli lack mesangial cells and juxtaglomerular complex, suggesting that those are vascular derivatives. The development of the tuft does not depend upon invagination by sprouting blood vessels; it is inherent in the glomerular epithelium.