Differentiating neoplasms of hair germ.

Differentiating neoplasms of hair germ are benign epithelial-mesenchymal tumours of skin in which hair follicle development may be partly or completely recapitulated. The epithelial component is equivalent to the hair germ. The mesenchymal component is equivalent to the dermal papilla. Epithelial-mesenchymal interaction results in the morphogenesis of hair follicles. In neoplasms showing stromal induction, there is centrifugal organizations: hair bulbs are found at the periphery of tumour lobules and hairs are projected centrally to lie within small keratinizing cysts. Neoplasms of hair germ without advanced morpho-differentiation are termed ;trichoblastomas', and those neoplasms in which hair follicle development is advanced are called ;trichogenic trichoblastomas'.