Properties of Vibrissa follicle cells during follicle development and regeneration, and their interactions with embryonic stem cells
Follicular dermal cells possess properties that regulate induction of follicle formation, hair growth, and follicle end bulb regeneration after amputation. While the molecular basis for these developmental interactions are being uncovered, it is evident that innervation, pigmentation and vascular! sation of the follicle, together with its role as a stem cell repository, make interpretations of molecular function within the follicle, complex. This thesis examined aspects of vibrissa follicle development using immunohistochemistry, and demonstrated that segregation of the follicle dermis and epithelial differentiation were defined by lamin-A antibody. Versican, a proteoglycan recently implicated in dermal papilla induction, was absent from early dermal condensations, but its expression correlated with follicle innervation during development and the adult hair growth cycle. When lower follicle regeneration was studied with in situ hybridisation and immunohistochemistry, versican showed two distinct expression domains. These were the dermal components of the end bulb and the follicle neck. BrdU labelling of cell division showed regeneration of the epithelial component to be consistent with stem cell location. Paucity of dermal cell proliferation left the precise origin of the new dermal papilla unresolved, but a-smooth muscle actin expression showed that dermal sheath cells moved through the glassy membrane. Sonic hedgehog expression indicated that epithelial-mesenchymal interactions, evident in follicle development, were mirrored in regeneration. A co-culture model investigated the capacity of follicular dermal cells to induce embryonic stem (ES) cell differentiation. Unexpectedly, follicle cells were seen to maintain ES cells in an undifferentiated condition. Differentiation assays demonstrated that ES cells remained pluripotent after co-culture, lnterleukin-6 family cytokines, known to maintain ES cell pluripotency, were detected by RT-PCR in cultured cells and vibrissa follicles. Thus, since the follicle dermis produces these cytokines, they may be acting to inhibit the differentiation of follicular epithelial stem cells and/or maintain multipotent stem cell populations in the follicle.