Cell death in the development of the posterior cortex in male and female rats

Work. from our laboratory has shown that adult male rats have 19% more neur ons than female rats in the binocular region and 18% more in the monocular region of the primary visual cortex (Reid and Juraska [1992] J Comp Neurol 321:448-455; Nunez et al., [1999] Soc Neurosci Abstr 25:229). In the curren t experiment, we investigated whether cell death in male and female rats (p ostnatal days 2-35) contributes to the formation of these differences. Usin g stereological techniques, we investigated neuron density along with pykno tic and apoptotic (TdT-mediated deoxyuridine triphosphate nick end-labeled) cell density in the developing posterior cortex (future primary visual cor tex). Although no sex differences in neuronal density were found in early d evelopment, we observed a differential time course of cell death between th e sexes. Consistent with earlier reports, males displayed a rapid rise in c ell death, with a peak on day 7 followed by a sharp decline to negligible l evels by day 15. Females, however, displayed moderate peaks of cell death o n days 7 and II, with the persistence of low-to-modest levels until day 25. Similar patterns were obtained from both pyknotic and apoptotic cell quant ification. Also, a formula was developed to estimate the percentage of cell s that die during development and the amount of time a dying cell is visibl e. This study demonstrates that there is a prolonged period of cell death i n the posterior cortex of developing female rats that appears to result in more cell death in females than males. This may be an important mechanism b y which the sex difference in adult neuron number is created. J. Comp. Neur ol. 436:32-41, 2001. (C) 2001 Wiley-Liss, Inc.