Epithelia and mesenchymes are two distinct types of tissues found in virtually every organ: epithelia are composed of closely associated, largely immobile cells; in contrast, mesenchyme contains more mobile cells that form loosely associated agglomerations. During normal development, transitions of epi thelia to mesenchyme can occur, and mesenchyme can differentiate into new epithelia. Such transitions are not confined to development. In partic ular, the loss of epithelial character in malignant carcinomas, which results in the appearance of invasive, motile cells, is of major importance in tumor progression (Birchmeier et al 1993).

In the early mammalian embryo, all cells are uniform and have the same developmental potential. At the blastula stage, only a single cell type resembling epithelia exists. During gastrulation, the three germinal cell layers, ectoderm, endoderm, and mesoderm, are formed. Whereas ectoderm and endoderm remain largely epithelial in character, the newly induced mesoderm is the first example in ontogenesis of conversion of epithelial cells to mesenchyme. Mesoderm formation has been studied extensively and the inductive signals have been elucidated in Xenopus laevis. In this review, we do not discuss this or other examples of the developmentally regulated conversion of epithelia into mesenchyme since the subject has been reviewed elsewhere (Gurdon 1987; Valles et al 1991; Green & Smith 1991; Moon & Christian 1992; Stem 1992; Jessell & Melton 1992). During further embryogenesis, organs develop and often cells derived from more than one germinal layer contribute to their formation. In general, two major cell types are found in developing organs: mesenchyme, which derives from mesoderm or neural ectoderm, and epithelia, which can originate from all three germinal cell layers. An example of morphogenic interaction between epithelia and mesenchyme is found during development of the lung: outgrowth of epithelial cells from the endoderm into mesen chyme, the splanchnic mesoderm, generates the anlagen of the lung. All epithelia develop by further growth, branching and differentiating from the first endodermal buds. In parallel, the mesenchymal cell compartment also expands and differentiates (Spooner & Wessells 1970). This principle, ie epithelia that grow and branch in response to mesenchymal signals, is a theme found in the development of many other organs such as salivary gland, pancreas, prostate, pituitary, kidney, and breast (Grobstein 1953; Wessells & Cohen 1976; Lasnitzki & Mizuno 1980; Kusakabe et al 1985; Saxen 1987; Sakakura 1991). In this review we discuss the molecular nature of signals given by the mesenchyme and received by epithelia, which then respond by morphogenesis, differentiation, and growth during organ devel opment.