Immunohistochemical localization of fibroblast growth factor 18 in hair follicles of healthy beagle dogs.

Increasing emphasis is being placed on the role of fibroblast growth factors (FGFs) in hair follicle cycling. In mice, expression of FGF18 mRNA peaks during the late telogen phase, leading to the hypothesis that FGF plays a role in anagen induction. There are no data on the presence of FGF18 in dogs. The main objective of this study was to identify and locate FGF18 in the canine hair follicle. The second objective was to assess potential differences in FGF18 concentration between biopsies taken in winter and summer, shoulder and flank regions, and between different sexes. Skin tissue from 10 healthy beagle dogs (three intact females, three spayed females and four intact males) was collected from the shoulder and flank. The biopsies were collected in February and August on day 0, after which the dogs were clipped and biopsies collected again from the shoulder and flank on days 1, 3, 7 and 17. Paraffin sections (4 μm thick) of the biopsies were stained with an anti-FGF18 antibody. The FGF18-positive cells were counted in the hair follicle epithelium from seven follicular units of each biopsy. Fibroblast growth factor 18 was detected as granular cytoplasmatic staining in follicles at the level of the inner root sheath, and rarely in the outer root sheath and dermal papilla. It was also detected in the apocrine glands, in arrector pili muscles and in vascular endothelial cells. There was no statistical difference in the number of FGF18-positive cells or follicles between sexes, different anatomical locations, seasons or the consecutive days of sampling.