K31 as a novel marker for clear secretory cells in human eccrine sweat glands

K31 was previously considered as one of the hair keratins. During a study on differential markers between hair follicles and eccrine sweat glands, we observed that K31 was expressed in eccrine sweat gland cells in a scattered pattern, similar to the distribution of dark or clear secretory cells. To investigate the precise cell localization of K31 in human eccrine sweat glands and find new marker for eccrine sweat gland cells, human skin samples were fixed, paraffined and sectioned. The serial sections were stained for K31, dark secretory cell marker gross cystic disease fluid protein 15 (GCDFP15) and clear secretory cell marker carbonic anhydrase II (CAII). The exact cell localization of K31 was detected by double immunofluorescence staining of K31 and a serial of cell-specific markers, and further by dual stain using a combination of periodic acid-Schiff (PAS) and immunofluorescence for K31 and GCDFP15. The expression pattern of K31-positive cells was similar to that of CAII-positive cells but was different from that of GCDFP15-positive staining in serial sections. Double immunofluorescent staining showed that K31-positive cells co-expressed K7 and CAII, but not S100P, α-SMA or GCDFP15. Dual stain by combined PAS and immunofluorescence showed that K31-positive cells are negative for PAS staining. We conclude that K31 is a previously unreported eccrine clear cell marker that allows for distinction between clear and dark secretory cells, as well as between secretory coils and ducts of eccrine sweat glands in human eccrine sweat glands.