Apocrine epithelium of the breast: does it result from metaplasia?

Benign and malignant breast lesions may show an apocrine epithelium considered to be the result of metaplasia. In an attempt to clarify the histogenesis of the breast apocrine epithelium we searched for the presence of apocrine cells or cells with apocrine differentiation during human breast development. We analysed 10 autopsy specimens of female breasts from fetuses between 28 and 40 weeks of gestational age and tissue from 6 normal breasts, obtained after breast reduction in nulliparous young women between 22 and 28 years of age. Formalin-fixed, paraffin-embedded sections were stained with haematoxylin-eosin, PAS-diastase and a monoclonal antibody (BRST-2) anti-GCDFP-15, which is a specific apocrine marker. A 40-week fetal breast was analysed by electron microscopy. No cells with histological and ultrastructural apocrine features were found in the ducts of fetal breasts. All fetal breasts showed some ducts with sparse GCDFP-15-immunoreactive cells; the number of these cells increased with gestational age. PAS-diastase was negative. No cells with apocrine morphology were found in ducts and lobules of normal adult breasts. Scattered GCDFP-15-positive luminal epithelial cells and rare PAS-diastase-positive cells were observed in some lobules of all adult breasts. Cells with biochemical characteristics (GCDFP-15 expression) of apocrine differentiation are evident during human fetal breast development and persist in adult mammary glands. Unknown stimuli may induce these cells to take on an apocrine morphology. Apocrine epithelium of the breast may be a normal process of differentiation rather than metaplasia. We suggest the term "apocrine differentiation precursor cells" for GCDFP-15-positive breast epithelial cells with no apocrine morphology.