EVIDENCE THAT INITIATED KERATINOCYTES CLONALLY EXPAND INTO MULTIPLE EXISTING HAIR-FOLLICLES DURING PAPILLOMA HISTOGENESIS IN SENCAR MOUSE SKIN

We have previously shown that the precursors of cutaneous papillomas i n SENCAR mice initiated with 7,12-dimethylbenz[a]anthracene and promot ed with 12-O-tetradecanoylphorbol-13-acetate are focal hyperplastic le sions that we refer to as squamous cell hyperplastic foci (SCHF). Ha-r as gene codon 61 mutations were frequently found in SCHF, providing ev idence that these lesions represent clones of initiated cells. We repo rt here the pathogenesis of multiple hair follicle involvement in more advanced SCHF and describe the role of the hair follicle in papilloma histogenesis. Detailed histological evaluation of 83 SCHF and 25 earl y papillomas revealed a morphological continuum from the least develop ed SCHF, involving only one hair follicle, to advanced SCHF and early papillomas, which involved more than 10 hair follicles. These results provide evidence of the recruitment of additional hair follicles as SC HF progress. In advanced SCHF and early papillomas the bulk of the epi thelial component in all cases consisted of several markedly hyperplas tic adjacent hair follicles, whereas the involved interfollicular epid ermis (IFE) was generally less hyperplastic. All of the hair follicles involved in SCHF appeared to have been preexisting, based on their pa ttern of spacing, that they were consistently normal appearing below t he level of the sebaceous glands, and that they were in the same phase of the hair cycle as surrounding, uninvolved hair follicles. Also, no evidence of follicular neogenesis was observed in serially sectioned SCHF, and coalescence of smaller lesions was rare. To investigate whet her the involvement of multiple hair follicles in SCHF was due to expa nsion of initiated cells into existing hair follicles or, possibly, to a paracrine mechanism, we analyzed different levels of three serially sectioned SCHF and one early papilloma for Ha-ras mutations. These an alyses revealed cells with Ha-ras gene codon 61 mutations at multi pie levels that involved different hair follicles. Overall, our results p rovide evidence that as initiated cells clonally expand, they spread a cross the IFE and populate the upper permanent portions of existing ha ir follicles. The abnormal proliferation of the infundibula of the hai r follicles involved in SCHF appears to give rise to most of the epith elial component of papillomas. (C) 1997 Wiley-Liss, Inc.