Langerhans cells (LCs) are dendritic cells present in many epithelia. They are of mesenchymal origin, as shown by the absence of desmosomes and tonofilaments and the presence of cytoplasmic vimentin [3]. It has been shown that LCs assume a key role in the cutaneous immune system [14] and, in humans, they constitute about 2% to 4% of the total epidermal cell pool [9]. The mechanism which regulates the number of these cells in the epidermis is still a matter of conjecture. We have used two techniques, 3H-thymidine incorporation as revealed by autoradiography and S100 protein immunoperoxidase staining, to evaluate the LC labeling index in human epidermis.
Skin biopsy specimens were obtained from five normal individuals, 21 to 45 years old, from different regions of the body (foreskin, forearm, and abdomen). They were immediately incubated, under agitation, for ] h at 37 ~ C, in Eagles' basal medium containing 3H-thymidine (specific activity 40 Ci/mM; Amersham International) at a final concentration of 5 gCi/ml. After fixing in Bouin's solution for 24 h, the tissues were dehydrated in alcohol and xylene and embedded in paraffin; 4-gm sections were fixed to gelatin-coated glass slides.
Tissue sections were deparaffinized and hydrated in xylene, followed by graded alcohol baths and finally distilled water. After quickly rinsing in PBS, the peroxide methanol procedure to decrease endogenous peroxidase labeling was performed. For 45 min at 37~ tissue sections were incubated in, successively, rabbit anti-Sl00 protein immunoglobulins (1:50 in PBS, Dako, Denmark), diluted biotinylated antibody solution, and avidin biotin complexes (Vectastain