The development of glomerular capillary tufts of the bullfrog kidney from a straight interstitial vessel to an anastomosed capillary network. A scanning electron microscopic study of vascular casts.

Blood vascular tufts of immature glomeruli of the post-metamorphous bullfrog kidney were reproduced by a resin casting medium and observed by scanning electron microscopy. The simplest glomerular tufts consisted of a single straight vessel continuous with the afferent and efferent vessels. In the next stage, the single vessel dilated and bent to form a U-shaped loop; this loop sprouted free-ending protrusions. Subsequently a conglomerated network of anastomosing capillaries developed. The network became larger, and increased in number of capillaries. Even during these later stages, the capillaries possessed luminal protrusions with free endings. Small holes and narrow grooves representing endothelial intrusions into the capillary lumens were noted only rarely. The findings supported the in-growth concept, in that an interstitial vessel invaded the cleft of the S-shaped renal vesicle to form a glomerular capillary network. Secondly, the glomerular capillaries multiplied by sprouting daughter capillaries rather than by luminal splitting.