Evolution of the Granular Cells of the Juxtaglomerular Apparatus during Postnatal Development in the Albino Rat

The postnatal development of the juxtaglomerular apparatus was studied by fluorescent microscopy in a series of 275 albino Wistar rats aged from 2 to 90 days. After treatment of kidney sections with thioflavine T, the appearance and evolution of secretory function in the terminal segment of the afferent glomerular arteriole were assessed in terms of the granular cell index (GCI) and the juxtaglomerular cell granulation index (JGI). Variations in these indices were analyzed in function of age, body weight, renal weight and sex of the animals. It emerges quite clearly from these studies that granular cell differentiation and maturation takes place in four successive stages: (a) a growth and differentiation phase lasting from the 2nd day to the beginning of the 5th week of extra-uterine life; (b) a critical phase around day 30; (c) a declining phase from day 30 to day 45, and (d) a stabilization phase from day 45 onwards. Until day 30 this evolutionary pattern is paralleled by rising body weight in animals of both sexes; afterwards it becomes independent of the parameters studied, in the males as well as in the females. The pattern of kidney weight parallels that of growth in body size and weight and does not constitute a more specific variable than body weight in the study of the developing granules in the juxtaglomerular apparatus.