Interdigitation index -: a parameter for differentiating between young and older skin specimens

Background/aims: Quantitative morphometry developed rapidly during the last decade due to advances is computers and software. We wish to establish a simple baseline for the morphometric differences due to intrinsic ageing between young and old cohorts: the interdigitation index. It is an expression of the shape of the border between the epidermis and dermis. Methods: We used volar forearm biopsies of women, since the volar forearm is usually not photodamaged. The biopsies were fixed in buffered formalin, embedded in paraffin and sectioned. Separate sections were stained by hematoxylin-eosin, by orcein and by dimethyl-methylene blue. We had seven female volunteers in each group; the young cohort had a mean age of 26.6 years, the older cohort 50.9 years. We chose a cohort that was just about postmenopausal, since in the future we wish to evaluate the effect of externally-applied agents on postmenopausal female skin and the earlier it is applied the better its chance of being effective. Results: We found no difference between the young and older cohort with regard to epidermal thickness. We found a decrease of glycosaminoglycen (GAG) as measured by dimethyl-methylene blue staining. The results of the elastic staining by orcein, although in line with the reports in the literature, are not useful for evaluating the intrinsic ageing process, at least not by the simple percentage of area stained procedure. We introduced a new parameter: the interdigitation index. It is a simple measurement of the interdigitation in the epidermal-dermal junction, known to be diminished by age. This index was diminished by about 20% between the young and older cohort. Conclusions: Quantitative morphometry using simple epidermal and dermal measurements on biopsies of the volar forearm of women is suitable for following intrinsic ageing of the skin and offers a simple objective method for following the ageing process of the skin.