APOPTOSIS AND ITS RELATION TO THE CELL-CYCLE IN THE DEVELOPING CEREBRAL-CORTEX

Large numbers of dying cells are found in proliferating tissues, sugge sting a link between cell death and cell division. We detected and qua ntified dying cells during pre- and early post-natal development of th e rat cerebral cortex using in situ end labeling of DNA fragmentation [terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end l abeling (TUNEL)] and electron microscopy. The proliferative zones that give rise to the neuronal and glial cell types of the cortex, the ven tricular and, to a larger extent, the subventricular zones showed high er incidence of cell death than other regions of the developing cortex during the period of neurogenesis. Gel electrophoresis of DNA isolate d from the subventricular zone of newborn animals showed a ladder patt ern that is characteristic of apoptosis. The number of apoptotic cells remained high in this zone for at least 2 weeks, during which period cells continued to divide. The correlation between cell division and c ell death was studied in the subventricular zone of newborn rats; cumu lative labeling with bromodeoxyuridine showed that 71% of TUNEL-labele d cells had taken up this S-phase marker before undergoing cell death. Using bromodeoxyuridine and [H-3]-thymidine in succession to identify a cohort of proliferating cells, we found that the clearance time of TUNEL-positive nuclei was 2 hr and 20 min. A comparison between the nu mber of mitotic figures and that of TUNEL-positive nuclei showed that cell death affects one in every 14 cells produced by dividing ventricu lar zone cells at embryonic day 16 and one in every 1.5 cells produced in the subventricular zone of newborn rats. In addition, we found tha t most of TUNEL-positive cells were in the G1 phase of their cell cycl e. We conclude that apoptosis is prominent in the proliferating neuroe pithelium of the developing rat cerebral cortex and that it is related to the progression of the cell cycle.