POSTNATAL-DEVELOPMENT OF A SEXUALLY DIMORPHIC, HYPOTHALAMIC NUCLEUS IN GERBILS - A STEREOLOGICAL STUDY OF NEURONAL NUMBER AND APOPTOSIS

Steroid-sensitive, vocal courtship behavior is a function of a specifi c, hypothalamic nucleus, the sexually dimorphic area pars compacta (SD Apc) in the male adult gerbil. Gender-related differences in the numbe r of neurons in this nucleus are evident immediately after birth. By u sing unbiased stereological estimates of cell numbers in Nissl-stained , paraffin-wax sections of brain, we investigated the mechanisms diffe rentiating cell number between the sexes in the SDApc on postnatal day s 0, 3, 6, and 15. Cell death, identified by pyknosis, was greatest in the SDApc between days 0-3 in males, whereas in females, maximum valu es were reached between days 3-6. Similarly, the ratio of pyknotic to normal neurons peaked between days 0-3 in males and 3-6 in females but then declined in both sexes. Pyknotic cells were seldom seen in eithe r sex by day 15. Morphological characteristics of apoptosis including chromatin condensation, cell fragmentation, and ingestion of apoptic b odies by macrophages were all demonstrated by transmission electron mi croscopy. Macrophages showed specific morphological characteristics of microglia. Cell division (mitosis) was identified in the SDApc during postnatal days 0, 3, and 6 but the numbers of mitotic figures were lo w, negligible on day 15, and similar between the sexes. These results demonstrate that cell death and proliferation occur simultaneously in the neonatal gerbil brain. The stereological estimates of cell death i n the developing SDApc indicated a lower incidence of neuronal death o ccurring earlier in males than in females. (C) 1996 Wiley-Liss, Inc.