The relationship between apoptosis and keratinization in human epidermis

The relationship between keratinization in human epidermis and apoptosis is studied using the technique of in situ end labeling of DNA strand breaks c ombined with transmission electron microscopy. Moreover, quantitative analy sis of free 3'-OH DNA ends in individual keratinocytes was performed on a c omputer using the NIH Image program. The number of free 3'-OH DNA ends in k eratinocyte nuclei was significantly higher in cells from the lower spinous layer than in the basal layer, although chromatin condensation was not det ected in the keratinocytes of the lower spinous layer. As chromatin condens ation progressed from the upper spinous cells to the granular cells, the nu mber of 3'-OH DNA ends increased and was highest in granular cell nuclei. H owever, no other characteristic of apoptosis, such as decreased cell volume , membrane blebbing or apoptotic bodies was observed in keratinocytes in an y layer of the epidermis. Moreover, horny cells lost their nuclei, together with the abrupt decrease in the number of 3'-OH ends. Therefore, keratiniz ation does not thoroughly satisfy all the criteria of apoptosis, although c ells undergoing keratinization show both chromatin condensation and DNA fra gmentation. It is noteworthy that DNA fragmentation has already occurred in keratinocytes of the lower spinous layer which show no apoptotic changes.