Fig. 1

Development of pronephric glomerular filtration slits. (A) Diagram of the vertebrate glomerular filter. Glomerular capillaries (cap) are lined with fenestrated endothelial cells (endo) that rest on a basement membrane (bm). Apposed on the opposite face of the basement membrane, podocyte foot processes (fp) form filtration slits that contain the slit-diaphragm as a specialized cell–cell junction. Arrows represent the path of blood fluid filtration. (B) At 84 hpf, the glomerulus (gl), the pronephric tubule (pt) and the pronephric ducts (pd) are fully formed between the gut (g), liver (liv) and the notochord (nc). Bowman′s space (bs) is clearly evident around the glomerulus (gl) and is drained by the pronephric tubules. The glomerular capillary tuft originates from the dorsal aorta (da). (C) Electron micrograph of 72 hpf podoctye foot processes (fp) reveals broad spreading cell processes as opposed to the fine interdigitations present at later stages of development. Some slit-diaphragms (sd) are present. (D) Electron micrograph at 96 hpf shows a capillary lumen (cap) and endothelial cells (endo). Podocytes (pod) and their foot processes (fp) surround the capillary along the basement membrane (bm). Slit-diaphragms (sd, arrowheads) are clearly visible between the individual foot processes. On a tangential section of a capillary wall (inset), interdigitating foot processes (fp) can be seen (arrows).