Human breast epithelium in vitro: the re-expression of structural and functional cellular differentiation in long-term culture

Estrogen responsiveness and control of normal human breast proliferation

Our understanding of the hormonal control of the proliferation of normal human breast epithelium is still surprisingly meager. However, the results of a number of recent studies have confirmed that estrogen is the major steroid mitogen for the luminal epithelial cell population (the usual targets for neoplastic transformation). Estrogen seemingly exerts its effects on cell division indirectly as there is complete dissociation between the population of luminal epithelial cells expressing the estrogen receptor (ER)4 and those that proliferate. We suggest that the ER-negative proliferating cells represent a precursor or stem cell population that differentiates to ER-containing, nonproliferative cells. In turn, these ER-positive cells act as 'estrogen sensors' and transmit positive or negative paracrine growth signals to the precursor cells depending on the prevailing hormonal environment. As yet there is no direct evidence supporting this hypothesis but we suggest ways in which it may be obtained. The implication of these studies is that inhibition of luminal epithelial proliferation with tamoxifen or pure antiestrogens or by preventing ovarian steroid secretion should be an effective strategy for the prevention of breast cancer. In addition, we may be able to predict the risk of breast cancer in an individual by measuring the intrinsic estrogen sensitivity of her breast epithelium. Finally, study of the paracrine mechanisms of growth control in the normal human breast may provide new, more specific, therapeutic targets for breast cancer prevention.

Stem cells and the development of mammary cancers in experimental rats and in humans

Evidence based on immunocytochemical staining and ultrastructure suggests that morphological gradations between epithelial and myoepithelial cells, and possibly between epithelial cells and alveolar-like cells occur in terminal ductal structures of rat and human mammary glands. The benign carcinogen-induced rat and benign human mammary tumors can contain epithelial, myoepithelial-like and alveolar-like cells, whereas the malignant counterparts mainly contain only epithelial-like cells. Clonal epithelial cell lines from normal rat mammary glands, benign tumors, and SV40-transformed human mammary glands can differentiate to either myoepithelial-like or alveolar-like cells. In those of the rat, the differentiation processes occur in steps: intermediate cells along the myoepithelial-like pathway resemble intermediates in terminal ductal structures in vivo, and can also generate certain well-differentiated mesenchymal elements of the original tumours. Differentiation of the benign rat cells to alveolar-like cells with mammatrophic hormones and retinoids in vitro leads to a reduction in their tumor-forming ability in vivo. Cell lines from malignant rat mammary tumors of increasing metastatic potential and from human ductal carcinomas largely fail to yield myoepithelial-like or alveolar-like cells and are relatively slow-growing. Growth of the rat mammary epithelial cells in culture is stimulated by a pituitary-derived mammatrophic growth factor (PMGF), prostaglandin E2, and alpha-transforming growth factor; the response of the malignant cell lines to PMGF is reduced. It is suggested that stem cells exist in the rat and human glands that are capable of differentiating to the other major cell types of the mammary parenchyma, and that during the carcinogenic process they generate genetically unstable cells which lose their ability to differentiate and attempt to maximise their intrinsically slow growth rate.

Maintenance of normal human breast organoids within rat mammary fat pads in organ culture

Normal human breast organoids, derived by collagenase digestion of reduction mammaplasty tissue specimens, have been cultured in vitro for up to 28 days after injection into organ cultures of virgin rat mammary fat pads. The culture medium was serum-free Waymouth's MB 752/1 with hormonal additives. The rat mammary tissue responded well to growth-promoting and lactogenic stimuli in the culture medium, in agreement with previous investigations. Using immunohistochemistry casein was identified in rat epithelia exposed to lactogenic medium. Human organoids in culture remained viable but did not show hormone-responsiveness. Electron microscopy confirmed the presence of both luminal epithelial cells and myoepithelial cells. The serum-free culture of normal human breast organoids in a three-dimensional matrix provides a system in which to study factors controlling growth and differentiation.