Dermal and epidermal cell functions in the growth and regeneration of hair follicles and other skin appendages

Abstract

Epithelial-mesenchymal interactions are central to the development of skin and skin appendages in vertebrates. These interactions continue throughout adult life and underpin the cyclic growth and loss of hair in mammals. While the molecular basis of such interactions are being gradually uncovered, at the cellular level many questions remain unanswered. For example, the localisation and role of hair follicle epithelial stem cells remains a subject of debate, as does the function of the dermal sheath component. In embryonic appendage development there is strong evidence for common signalling mechanisms, but the degree to which epithelial-mesenchymal communication diverges in different adult appendages remains as yet undiscovered. I have studied the replicative abilities of germinative epidermal (GE) cells of the rat vibrissa follicle by single and repeated plucking of fibres from individual follicles. In both cases, the cellular events following fibre removal were scrutinised at intervals up to 9 days using histology, and cell proliferation and cell death markers. Follicles that were repeatedly plucked had their growing hairs measured at regular intervals. By analysing cell proliferation patterns I found that the new regenerated epidermal matrix came from residual GE cells left in the follicle base - after both single and repeated depilations. Indeed plucking appeared to cause an initial inhibition of proliferative activity in the follicle upper outer root sheath, the other candidate region for supplying a new matrix. Length measurements of the regenerated hairs demonstrated that the repeatedly plucked follicles produced total cumulative lengths of fibre between 60 and 265% longer than expected, as determined by measuring the original club fibre lengths. In vivo amputation of plucked follicles demonstrated that the residual GE cells have the ability to regenerate a new matrix and new fibre without any contribution from cells from the upper region of the follicle. These studies, along with in vitro observations of prolonged replicative abilities of bulb cells suggest that the GE cells have a proliferative capacity which is beyond that of one cycle. This raises the possibility that GE cells are not transient amplifying cells with limited proliferative potential and strongly suggests that the duration of anagen cannot be attributed to the replicative limitations of the GE cells. The behaviour and interactive abilities of dermal cells isolated from three different skin appendages (rat vibrissa follicle, rat claw unit, and pigeon feather follicle) were characterised by cell culture, immunohistochemistry and dermal-epidermal recombinations. Dermal cells from all the above appendages demonstrated common aggregation properties in culture and all expressed a-smooth muscle actin. When recombined with epidermal cells and implanted onto host rats, dermal sheath cells from the lower part of vibrissa follicles produced a robust skin-like structure with a normal