Distinction between differentiation and senescence and the absence of increased apoptosis in human keratinocytes undergoing cellular aging in vitro

We have examined the processes of differentiation and apoptosis in relation to cellular aging by using a culture system based on serial passaging of monolayer cultures of human keratinocytes in a serum-free low calcium medium. Differentiation was analyzed by cellular morphology and by the expression of keratinocyte transglutaminase. Keratinocyte cultures at all passages could be induced to differentiate into mature keratinocytes by raising the calcium concentration in the medium. Differentiation could also be induced in senescent cultures, but the process was significantly slower. Apoptosis was analyzed by cellular morphology and by the expression of tissue transglutaminase. Apoptotic cells constituted a very small proportion of the culture and no detectable change in the incidence of apoptosis occurred during serial passaging.