K15 & Id3 expression in intact and regenerating adult vibrissae rodent hair follicles

Abstract

The hair follicle can renew itself through an intrinsic stem cell population and has the ability to undergo hair follicle growth cycle; its regenerative activities under normal and experimental conditions together with a feature of easy access make it an ideal model system to investigate adult stem cell activities. The different stages of the rat vibrissae hair follicle cycle have been well documented and described; recent studies have provided an insight into the molecular control of hair follicle cycling and development. However, current understanding of stem cells activity during different stages of hair follicle cycle is limited and controversial. Keratin 15 is proposed as an epithelial stem cell marker, during follicle cycle. Keratin 15 is found predominately to cover the lower hair follicle, including the hair follicle matrix. Its overall expression varies depending on the different stages, these findings are consistent with the "traffic light model", and i.e. epithelial stem cells migrate during follicle stages. However, we have proposed some corrections to this model. Id3 is a transcriptional regulator protein, expressed in both epithelial and dermal cells during hair follicle cycle, and the cellular translocation phenomenon especially in stage of Anagen were observed, and this may suggest the cells exchange the activities between proliferation and differentiation. The regenerative properties of vibrissa follicle are well established but many questions were left especially in the cellular and molecular level. For example, the sources of regenerative dermal papillae together with epithelial stem cells activity during regeneration of hair follicles remain unclear. Both keratin and ID proteins have been previously reported to have roles in the skin wound healing process, but in the hair follicle regenerating process their roles remains unclear. The number of K15 positive epithelial cells significantly increased at the initial stages of regenerating follicles, this confirmed previous results, which showed the outside epithelial cells filling in at this stage. K15 was more specifically expressed in the outer root sheath cells along the glassy membrane in the later stages of regenerating follicle, suggesting that the epithelial stem cells had migrated down to interact with dermal sheath cells to reform dermal papillae. In contrast, the response of Id3 positive cells to the injured hair follicle is relatively late; epithelial cells gradually increased their expression of Id3 in mid-regenerating stage. Id3 highlighted dermal cell accumulation below amputation site in the later regenerating stages. This result suggests that ID3 positive dermal cells accumulation may be mimicking the dermal condensation environment in the hair follicle development. Finally, the explant end bulb of hair follicle and isolated dermal papillae differentiate into osteoclasts in vitro suggesting the stem cell multi-potential features. This work demonstrated that sufficient stem cell niche signalling conditions could mimic stem cell niche for cell differentiation.