phenomena was described for follicle formation from isolated thyroid cells (Chambard et al., 1981). Using floating collagen gels as described by Emerman and Pitelka (1977) and radioactive carbon, Lee et al. (1984, 1985) showed that the milk that was produced and secreted indeed was produced endogenously. Evidence emerged that de novo synthesis of milk proteins was under the control of the ECM (Lee et al., 1984, 1985). The idea that the ECM influences gene expression was further supported by culturing mouse mammary cells on an EHS matrix where synthesis of β-casein was observed in more than 90% of the cells, together with the formation of glandular structures (Li et al., 1987; Barcellos-Hoff et al., 1989). Other cell types such as rat Sertoli cells cultured either on, or inside, an EHS matrix were shown to undergo striking changes in morphology and secretory activity (Hadley et al., 1985), and so were rat hepatocytes (Bissell et al., 1987; Schuetz et al., 1988), avian neural crest cell cultures (Maxwell and Forbes, 1987), and exocrine acinar epithelial cells (Oliver et al., 1987). Similar results were obtained when primary normal human prostate cells were cultured in EHS (Fong et al., 1991). The concept that certain components
