The relationship between apoptosis and keratinization in human epidermis

Abstract

The relationship between keratinization in human epidermis and apoptosis is studied using the technique of in situ end labeling of DNA strand breaks combined with transmission electron microscopy. Moreover, quantitative analysis of free 3'-OH DNA ends in individual keratinocytes was performed on a computer using the NIH Image program. The number of free 3'-OH DNA ends in keratinocyte nuclei was significantly higher in cells from the lower spinous layer than in the basal layer, although chromatin condensation was not detected in the keratinocytes of the lower spinous layer. As chromatin condensation progressed from the upper spinous cells to the granular cells, the number of 3'-OH DNA ends increased and was highest in granular cell nuclei. However, no other characteristic of apoptosis, such as decreased cell volume, membrane blebbing or apoptotic bodies was observed in keratinocytes in any layer of the epidermis. Moreover, horny cells lost their nuclei, together with the abrupt decrease in the number of 3'-OH ends. Therefore, keratinization does not thoroughly satisfy all the criteria of apoptosis, although cells undergoing keratinization show both chromatin condensation and DNA fragmentation. It is noteworthy that DNA fragmentation has already occurred in keratinocytes of the lower spinous layer which show no apoptotic changes.