Mixed tumours of the skin: a histopathological, enzyme-histochemical and immunohistochemical study

Twenty-eight cases of mixed tumour of the skin were studied and subclassified into three types: eccrine (1 case), indeterminate (7), and apocrine type (20). The indeterminate type was defined as mixed tumours having tubulo-alveolar patterns with two layers of epithelium, but without apocrine secretion or pilosebaceous differentiation. Enzyme-histochemical studies were performed on four cases (one indeterminate, three apocrine): in the indeterminate type the tubular epithelial cells showed eccrine differentiation while in the apocrine type tubules were found showing the direction of differentiation to be toward the apocrine gland, but tubules with eccrine differentiation were intermingled in all three. Immunohistochemically, no differences were observed between the indeterminate and the apocrine type: hints of eccrine features were observed in both groups. Thus, the indeterminate type could be an eccrine tumour and the apocrine type showed direction of differentiation toward both eccrine and apocrine glands. It is concluded that mixed tumours of the skin are fundamentally eccrine neoplasmas, and that the apocrine features may represent apocrine metaplasia.