Fig. 1. Characterization of hypospadiac and proliferative defects in Hoxa13^GFP-homozygous mutants. (A,B) Hoxa13^GFP is expressed in both the GT mesenchyme (M) as well as the urethral plate epithelium (UPE) in E12.5 littermates. Arrows denote the distal UPE (dUPE) which is thickened in homozygous mutants. (C,D) Proliferation in the GT mesenchyme of E12.5 male embryos. Proliferation was measured in the lateral shelf mesenchyme (LSM) by counting phosphohistone H3 positive cells in a 2.5x2.5 cm region (white square). Note that proliferation is maintained in the cells immediately adjacent to the UPE in heterozygous embryos (C, arrowheads), whereas mutant embryos exhibit a dramatic decrease immediately adjacent to the UPE (D, arrowheads). (E) Closure of the distal urethra and glans by proliferating mesenchyme in an E15.5 heterozygous male embryo. Arrow denotes the initial site of closure. Arrowhead indicates future site of the rectal mesenchyme condensation. (F) Hypospadia in homozygous-mutant littermates. Arrow denotes the loss of urethral closure as well as the persistence of the epidermal layer, which is not covered by the proliferating mesenchyme. Arrowhead indicates precocious condensation of rectal mesenchyme. Note that formation of the rectum (R) is precocious and more rostral in homozygous mutants (F). Scale bars: 50 µm.

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