Mutational analysis has clearly demonstrated that the transcription factors encoded by hoxa-11 and hoxd-11 play essential, yet redundant, roles in kidney organogenesis(Nature 375:791, 1995). Whereas the single hoxa-11 or hoxd-11 mutants have normal kidneys, the double null mutants have renal hypoplasia with reduced numbers of normal appearing nephrons. In contrast to the nephrons, the collecting ducts of these double null mutant mice are dilated. Previous work detailing the expression pattern of hoxa-11 by in situ hybridization has not permitted us to propose a satisfactory mechanism by which gene disruption can cause both reduced nephron numbers and an abnormal collecting system. Therefore, we used immunohistochemistry to obtain a more precise developmental expression pattern of Hoxa-11 in mice.

Immunofluorescent staining was performed on serial sections of mice from embryonic (E) days 9.5, 10.5, 11.5, and 13.5 and from postnatal day 0 and 10. Staining was performed using a rabbit polyclonal antibody to mouse Hoxa-11. In addition, sections were stained for the markers, Pax-2, DBA, and E-cadherin.

Hoxa-11 was initially detected in the posterior intermediate mesoderm at E-10.5 prior to induction by the ureteric bud. Expression was not detected in the primitive mesonephric kidney. Hoxa-11 expression persisted in the nephrogenic zone at the periphery of the developing kidney throughout the period of nephron formation. Within this zone, expression was enhanced in the condensing mesenchyme immediately surrounding the branching ureteric bud (UB). Following the completion of nephrogenesis, Hoxa-11 was no longer detected in the periphery of the kidney. Interestingly, Hoxa-11 was detected in the mesenchymal cells surrounding the more mature and differentiated derivatives of the UB, the collecting ducts and the ureter.

Hoxa-11 is an early marker of the presumptive metanephric mesenchyme. Its initial expression pattern is not dependent on induction by the UB. As suggested for the Hox family of transcription factors, hoxa-11 may regulate proliferation of the expressing cells. Reduced proliferation of the presumptive metanephric mesenchyme would explain the reduced renal mass in the null mutant mice.

Later in organogenesis, Hoxa-11 expression persists in cells closely associated with differentiated derivatives of the UB. Absence of Hoxa-11 in these cells would explain the dilated collecting ducts in the null mice by either a change in signaling of the collecting duct by the mesenchyme or potentially by a change in mechanical support of the duct by the surrounding mesenchyme.