Effects of sex steroids on the development of sexual dimorphism in mouse innominate bone.

The sexual dimorphism of the innominate bone was examined in 14 strains of mice. In female mice of all strains, the pubis was significantly longer and thinner than that in the strain-matched males. In 13 of 14 strains, the ischium in the male was longer and thicker than in the female. In the testicular-feminized male (Tfm) mouse, the ischium was longer and thinner than that in the wild-type male, resembling that of the wild-type female. The pubis of the Tfm mouse was longer than in the wild-type males. The pubis width in the Tfm mouse was between those of the wild-type male and female. Gonadectomy at ages of 5, 10, 20, 30, and 60 days in both sexes showed that the ischium develops as the female type when sex hormones are absent. In contrast, postnatal testicular androgen induces the male-type ischium. Gonadectomy at 60 days had a slight effect on the pubis, indicating that sexual dimorphism of the pubis was determined before 60 days of age. Estrogen receptors (ER) were immunohistochemically demonstrated in bone cells of 0- to 60-day-old mice. ER was found exclusively in the periosteum of the pubis at the day of birth; however, it appeared in bone cells of all parts of pelvis at 10-60 days. These results indicate that sexual dimorphism of the pubis is consistent for the 14 mouse strains examined, and that the shape of the pubis is determined by sex steroids before 60 days of age. Since ER exist in the bone cells, morphogenesis of the pelvis may be regulated by these sex steroids.