Comparative Studies of the Juxtaglomerular Apparatus

Publisher Summary Specific components of the mammalian juxtaglomerular apparatus (JGA) are juxtaglomerular (JG) cells containing renin-forming granules, the macula densa (MD), and the extraglomerular niesangiuni (EGM) JG granules are present in tetrapods, glomerular and aglomerular teleosts, and holocephalians. Some sarcoptrrygians and primitive bony fishes have also JG granules. JG cells appear to have approached closer to the glomeruli during tetrapod evolution. The MD and the EGM are present only in mammals. Avian kidneys have a transitional MD, but no EGM. The MD and the EGM are absent in reptiles, amphibians, and fishes. Renin has been found in the kidneys of tetrapods and bony fishes, although some bony fishes lack JG granules. The renin–angiotensin system (RAS) appeared first in primitive forms during the early evolution of bony fishes. A variety of possible physiological roles of the RAS have been proposed. These include blood pressure regulation, control of aldosterone secretion, control of glomerular filtration rate by constricting afferent or efferent glomerular arterioles, stimulation of intestinal transfer of sodium and water, and stimulation of a dipsogenic mechanism in the central nervous system. The physiological roles of the RAS in nonmanimalian vertebrates are unknown.