The cell kinetics of the epidermis and follicular epithelium of the rat: variations with age and body site

Abstract

The skin from rats of differing age was used to quantify variations in the cell kinetics of the epidermis and the follicular epithelium of different body sites. Four parameters were assessed, namely the basal cell density (BCD), the labelling index (LI), the duration of DNA synthesis (ts) and the basal cell turnover time (tT). The BCDs of the epidermis of the dorsum and the upper surface of the foot were similar in rats of 7, 14 and 52 weeks of age, but there was an indication of a progressive decline with increasing age in the BCD of the epidermis of the ear and tail. There were no age‐related changes in the length of ts in any of the four body regions. The rate of cell proliferation, as indicated by the values of the LI and tT, was relatively rapid in the epidermis of the dorsum, foot and tail of rats aged 7 weeks (LI > 12%; tT < 80 h). In rats aged 14 weeks this rate of proliferation was maintaned in the epidermis of the dorsum. However, in the foot and tail the rate of cell proliferation was decreased (LI < 10%; tT > 85 h). A fall in the rate of proliferation of the epidermis of the dorsum was only seen in 52‐week‐old animals. In these animals the rates of proliferation in the foot and tail were similar to those at the age of 14 weeks. In the epidermis of the ear there was no appreciable change in the rate of cell proliferation with age. The values of the cell kinetic parameters varied in the different body sites. For example, in 52‐week‐old animals values for tT were relatively short in the epidermis of the tail and foot and appreciably longer in the epidermis of the dorsum and ear. Considered overall, values for the cell kinetic parameters of the epidermis were comparable with those for the follicular epithelium. The only major differences between the epidermis and the follicular epithelium were in the upper surface of the foot at 7 weeks of age, and in the tail at 7 and 14 weeks of age, where the LI was higher and the tT shorter in the epidermis than in the follicular epithelium. The relevance of the observed age‐ and body‐site‐related variations in the cell kinetics of the epidermis are discussed in relation to previously described differential changes in the radiosensitivity of the skin in this strain of rat.

Age‐related changes in the cell kinetics of rat foot epidermis

“…The present results are in keeping with earlier findings (Morris et al, 1989) where it was shown that there was a progressive decline in the rates of cell proliferation, in a variety of body sites, with age in the rat. The changes in the rates of cell proliferation appeared to be associated with age-related changes in the growth of those body sites (Morris eta]., 1989).…”

“…for the same region obtained using a double labelling technique (Morris et al, 1989) and than the values of 9-11 h suggested by the present single pulse LI data. The discrepancy in these estimates of Ts may be due to the effects of diurnal variation.…”

“…A similar distribution of labelled cells was reported in the epidermis of the mouse (Hegazy & Fowler, 1973a). This contrasts with pig, human and guinea-pig epidermis, where a significant proportion of labelled cells (20-35%) are situated suprabasally (Penneys et al, 1970 ;Yamaguchi & Tabachnick, 1972;Morris & Hopewell, 1985).…”

“…However, because the values of Ts derived from the The duration of the G I phase of the cell cycle was found to vary with age from 15-0 PLM curves appear to be overestimates, it is probable that these G F values were underestimated. If alternative values of T,, measured using the double labelling technique (Morris et al, 1989), were used to calculate the GF, the value for this parameter increased from 0.3 to 0.4 in 7-week-old rats, and from 0.2 to 0.6 in 52-week-old rats. The growth fraction of the epidermis has rarely been assessed with any degree of accuracy, however, for the dorsal skin of the mouse a value of 0.6 has been quoted (Potten et al, 1982).…”

“…For a given radiation dose the severity of damage was found to be greatest in 14-week-old rats and least in 52-week-old rats ; 7-week-old rats exhibited an intermediate level of damage. The changes in the LI and TT with age appear to be in keeping with this differential radiosensitivity (Morris et al, 1989). In order to investigate this suggestion more fully, a detailed cell kinetic study of the epidermis of the dorsal surface of the foot has been carried out using the percent labelled mitoses (PLM) technique.…”