Renal growth in infancy and childhood--experimental studies of regulatory mechanisms.

During the peri- and early postnatal period, nephrogenesis is completed and kidney growth is accomplished both by cellular proliferation and enlargement. The number of nephrons in a given species is predetermined, whereas cellular growth can be influenced by environmental factors in an age-dependent manner. Unilateral nephrectomy or a high-protein diet stimulates renal growth more in the young than in the adult. Conversely, pyelonephritis inhibits renal growth in infancy but not in adulthood. The relative importance of hyperplasia and hypertrophy for renal growth also changes with renal maturation. The mechanisms behind these developmental changes in regulation of renal growth are largely unknown, but age-dependent changes in the expression of several proto-oncogene products have been demonstrated. These include growth factor receptors as well as components of the intracellular system that transfers the signal from an activated growth factor receptor to the cell nucleus. Studies on rat proximal tubule cells in primary culture might be of great value in expanding our knowledge of growth regulation in the developing kidney. Such studies have already shown that under identical environmental conditions the basal proliferative rate is age dependent, that the proliferative response to growth stimulation changes postnatally, and that this is associated with changes of both the response of the Na+/H(+)-exchanger and the expression of the c-fos proto-oncogene.