Pediatr. Res. 15: 1248Res. 15: -1255 one can speculate that a decrease in glomerular vascular resistcatheterized fetal lambs between 106 and 140 days of gestation ance is a major determinant in the postnatal increase in glomerular (term, 145 days) and in six newborn lambs between 3 and 19 days perfusion rate and GFR. of age. The present study demonstrates for the first time in lambs that the nephrogenic zone disappears around 130 days of gestation and that the total glomerular counts per kidney in fetuses over Previous studies (5,13,14,16) have demonstrated that the 130 days (4682% + 41173 glomeruli per kidney) is not significantly cortical blood flow is distributed preferentially to the juxtameduldifferent than in newborn lambs (433704 + 21553). Glomerular lary area of the cortex during early postnatal life, and with density, determined in four cortical zones (zone I being the out-maturation, the development of cortical flow follows a centrifugal ermost portion of the cortex) did not show any significant changes pattern. Spitzer and Brandis (26) also demonstrated that the during fetal life; however, significant decreases in glomerular maturation of glomerular filtration rate (GFR) closely follows the density were observed in each cortical zone after birth. The relative developmental pattern of cortical blood flow, M~~~ recently, distribution of glomeruli during fetal life decreased in the outer ~~~~i~ and ~~~i~ (31, studying factors modulating the changes in portion (zones I and 11) and increased in the inner Portion (zones GFR during maturation, have suggested that glomerular perfusion 111 and IV) of the cortex as fetuses matured and approached term. rate (GPR) is an important determinant of the increase in GFR After birth, this difference became even more prominent; the outer during postnatal life, cortical fraction (zone I) decreased from 49.6 + 2.9% in fetuses of studies from our laboratory have demonstrated that during less than 120 days 37-8 * (P < Oeo5) in lambs, fetal life G F R increases at the same rate as the fetal body weight whereas the fraction found in zone 111 increased from 14.9 + 1.3% and kidney weight (18, 21). ~h~~~ observations are contrary to to 20.8 + 0.7% ( P < 0.05). Small but significant increases in what occurs after birth, where the increase in G F R is disproporglomerular filtration rate (GFR) ( P < 0.01) and total renal blood tionately higher than the concomitant rise in body weight and flow (P < 0.05) were observed during fetal life: GFR and total kidney weight (1 1, 20). Factors responsible for the difference in renal blood flow increased, respectively, from 1.84 2 0.11 and 37 the developmental patterns of fetal and newborn G F R have not f 2 ml/min in fetuses I30 days gestation. During the same period, filtration important factor responsible for the development of GFR in fraction (FF) did not increase significantly whereas a significant newborn rats (31, the pre...

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