Stromal fibroblasts influence human mammary epithelial cell morphogenesis.

T he inability to grow normal or premalignant human mammary epithelial cells as in vivo explants has thwarted research biologists for the past 30 years (1–4). Although human breast cancer cells can be passaged in vivo as a xenograft or in vitro as cell cultures, this capability does not extend to normal or premalignant human mammary cells, with a few rare exceptions (5). Previous studies have shown that normal and premalignant human mammary epithelial cells can be maintained as a xenograft, but expansive growth has been extremely rare (1–4). The future of preventing the onset and progression of breast cancer lies in elucidating the biology of premalignant development. This cannot be accomplished without appropriate in vivo models and the methodology to establish such models. Although rodent models are extremely valuable for understanding the salient biological and molecular changes critical for premalignant and malignant development, the best models still cannot replicate entirely the human situation. There are several reasons for this problem, but one of the most important is the fundamental difference in the microenvironment between the commonly used models, i.e., rodent and human. The mouse and rat mammary glands comprise primarily an adipose stroma with epithelial parenchyma; the human gland is composed of a fibrous–adipose stroma with epithelial parenchyma. In this issue of PNAS, Kuperwasser et al. (6) describe a method that allows the expansion of normal human breast epithelium as a xenograft in immunocompromised mice. The procedure is reproducible and, with a modest learning curve, is technically available to most any laboratory. The procedure is relatively inexpensive and theoretically applicable to premalignant breast epithelium, as well. The promise the method brings to the study of normal and premalignant breast disease is enormous. Researchers now have the means to develop transplantable cell lines of normal and premalignant human breast epithelium to test chemopreventive agents in an environment that resembles that from which human cancer arises. The success of the method rests on two fundamental pillars of mammary gland biology, one well established and one newly recognized. The well established pillar is the ability to transplant mammary epithelium into its orthopic site, the mammary fat pad. This procedure was established by DeOme et al. in 1959 (7) and has been used extensively in modern mammary studies (8). The second pillar is the recognition that the mammary stroma has an instructive role