their differentiation functions after one or more days in culture (Bissell and Tilles, 1971). Michalopoulos and Pitot (1975) determined in 1975 that it was possible to trigger differentiation of specific epithelial cells, such as hepatocytes, by modifying the behavior of the substratum to which they were attached; in particular, these researchers observed that, when cells exerted pressure on the thick collagen gels on which they were growing, the gels detached and floated on the top of the medium. Thus, floatation itself created a more permissive environment for differentiation, although they did not report a mechanism. When these researchers imitated the phenomena by rimming the gels to allow them to float, adult liver cells expressed tissue-specific markers of differentiation. Emerman and Pitelka (1977) performed the same experimental condition with dissociated normal mammary epithelial cells derived from pregnant mice in the presence of lactogenic hormones and showed that when gels were made to float, cells maintained some milk protein expression for a month in culture, a phenomenon that was not observed on plastic, glass, or attached collagen gels (Emerman and Pitelka, 1977).
