Postnatal maturation of the rabbit cortical collecting duct

The mature, fully differentiated cortical collecting duct plays a major role in the final renal regulation of Na+, K+ and H+ transport. To characterize the growth of this segment, we measured the outer diameter and the dry weight of cortical collecting ducts isolated from newborn, 1-month-old, and adult rabbits. During the 1st month of life no significant changes were observed; however, there was a 60% increase in both parameters after the 4th week of life. Growth-related accretion of K+ was demonstrated by showing tubular K+ content to increase by 60% with maturation. Concomitant with the increase in tubular size, total cell number per millimeter of tubular length rose by 30%. Approximately 50% of the observed increment in tubular size could be accounted for by cell hyperplasia, with the remaining increase resulting from cell hypertrophy. Hypertrophy of principal cells was confirmed by scanning electron microscopy, which demonstrated a doubling of the circumferential width without any change in longitudinal length. Hyperplasia was confirmed, using a fluorescent chromatin stain, by our finding of a mitotic frequency of 3/1000 cells in the neonatal mid-cortical collecting duct; the observed number of mitoses was 10-fold higher at the most cortical end (ampulla). The number of intercalated cells per millimeter of tubule length, identified by bright green fluorescence after cortical collecting ducts were stained with 6-carboxyfluorescein diacetate, was found to double during maturation, the increase being significant only after the 4th postnatal week. We conclude that maturation of the mid-cortical collecting duct results from both cellular hyperplasia and hypertrophy. It is unlikely that this segment plays a major role in regulating Na+, K+, and H+ transport in the neonatal kidney.