IN-SITU LABELING OF APOPTOTIC CELL-DEATH IN THE CEREBRAL-CORTEX AND THALAMUS OF RATS DURING DEVELOPMENT

Apoptosis is a form of naturally occurring cell death that plays a fun damental role during development and is characterized by internucleoso mal DNA fragmentation. In this study we used specific in situ labeling of DNA breaks (Gavrieli et al. [1992] J. Cell. Biol. 119:493-501) to analyze the distribution of apoptotic cells in rat cerebral cortex and thalamus at different developmental stages from embryonic day 16 to a dulthood. Control experiments and electron microscopy confirmed that t he reaction product was confined to the nucleus of selected cells. Plo tting and counting of labeled nuclei in counterstained paraffin sectio ns showed that apoptosis occurred mainly during the first postnatal we ek and was absent in embryonic and adult samples. In the cortex, the n umber of apoptotic cells progressively increased from birth to the fir st postnatal week, with a peak between postnatal (P) day 5 and P8, and subsequently decreased. At the time of maximal expression of apoptosi s, labeled nuclei were present mainly in layer VIb and underlying whit e matter and at the border between cortical plate and layer I. Only a few apoptotic cells were found scattered in the thalamus, without a pa rticular concentration in selected areas, but with a peak at P5. Diffe rences in the number of apoptotic cells between cortex and thalamus su ggest that apoptotic cell death may have a different functional signif icance in the two brain areas. (C) 1995 Wiley-Liss, Inc.