Cell renewal, cell differentiation and programmed cell death (apoptosis) in pilomatrixoma

Pilomatrixoma is a benign tumour of the cutaneous adnexa. Histologically, pilomatrixoma comprises masses of immature basophilic cells, small numbers of polygonal squamoid cells, few transitional cells, and clusters of 'shadow cells'. The mechanism leading to the formation of shadow cells is still unknown. Skin biopsy specimens of pilomatrixoma (n = 15) were studied histologically, immunohistologically, and by applying the in situ end-labelling technique. The basal layer of the basophilic cells induced most of the proliferating cells with high expression of bcl-2 and cytokeratin 19. The overlying basophilic cells showed a negligible mitotic activity, a high significant accumulation of p53 protein, and a heterogeneous, but progressive loss of bcl-2 and cytokeratin 19. They developed either into squamoid cells or into transitional cells. The squamoid cells were characterized as differentiated cells resembling mature keratinocytes of stratified mucosa. The transitional cells could be shown to represent apoptotic cells proceeding to shadow cells. The data suggest that apoptosis is the main mechanism leading to the development of the dead shadow cells and is most probably responsible for the banal biological behaviour of pilomatrixoma. Apart from that, pilomatrixoma represents a suitable biological model to study apoptosis in humans.