ONTOGENY OF THE SEXUALLY DIMORPHIC AREA OF THE GERBIL HYPOTHALAMUS

The sexually dimorphic area (SDA) of the gerbil hypothalamus is a set of cell groups in the medial preoptic area that is essential for mascu line sexual behavior and implicated in the hormonal control of scent m arking and ultrasound production. The adult SDA shrinks after gonadect omy unless the gerbils receive testosterone. So does the SDA pars comp acta, a small cell group in the SDA of males that is seldom seen in fe males. Here, development of the SDA and SDApc, and of a second, small, compact cell group, the cmSDApc, that lies caudal and medial to the S DApc, is described. Development of the SDApc and cmSDApc was studied q uantitatively by assessing their incidence and volume in both sexes fr om birth (PND 1) to adulthood (PND 150). The volume of the entire SDA was studied from PND 45 to 150. In male gerbils, puberty begins around PND 40 and is complete by PND 90-120. The male SDA enlarged relative to the cross-sectional area of the hypothalamus as puberty began, but the female SDA did not. The SDApc was present in virtually all gerbils at birth and was the same size in both sexes. Over the next two weeks , the SDApcs of females disappeared while those of males persisted and doubled in size. Like the SDApc, the cmSDApc was larger and more comm on in males than in females, but it became smaller and less prevalent in both sexes during the first two weeks after birth.