A comparison of hypothalami of rats and mice: lack of gross sexual dimorphism in the mouse.

Anterior hypothalami from 10 male and 10 female CBA/j type mice were sectioned serially and examined histologically for possible sexual dimorphisms in appearance and size of neuronal aggregations. Measurements of volumes of suprachiasmatic nuclei and an adjacent cell aggregation, designated as the medial portion of the anterior hypothalamic nucleus (AHm) and similar in appearance and position to the sexually dimorphic nucleus of the rat, were made. In contrast to known sexual dimophism present in the rat, no sex differences in volumes of mouse hypothalamic structures were found. This lack of gross sexual dimorphism in the mouse could be explained by (a) insensitivity of dividing and developing AHm neurons to androgens, or by (b) a possible influence of androgens upon overall AHm morphology in both sexes, due to transfer of androgens from males to females in utero.