Foxc1 gene null mutation causes ectopic budding and kidney hypoplasia but not dysplasia

Mice carrying the null-mutated Foxc1 gene frequently develop an anomalous double collecting system. These mice provide an ideal opportunity to specify the role of ectopic budding in the development of congenital anomalies of the kidney and urinary tract.

Tissue specimens were collected from Foxc1(ch/ch) mutants at several embryonic stages and at birth. The upper and lower pole kidneys were qualitatively and quantitatively examined by histology, in situ hybridization and immunohistochemistry.

Upper pole kidneys of newborn Foxc1(ch/ch) mice were significantly more hypoplastic and contained significantly fewer glomeruli than their lower pole counterparts. On embryonic day 14.5, the stage immediately before the formation of the first urine, the upper pole kidney was already smaller than the lower pole kidney. Neither histology nor immunostaining for kidney markers showed dysplastic regions in either kidney of newborn Foxc1(ch/ch) mice. Of note, expression of Foxc1 was restricted to maturing podocytes and was not detectable in any intermediate structure of nephron development in the nephrogenic zone.

Ectopic budding alone results only in kidney hypoplasia but not dysplasia. The development of dysplasticity in the maturing kidney involves gene(s) that function beyond the initial budding stage within the metanephros.