The role of epimorphin as a extracellular morphogen was originally identified in studies of lung branching morphogenesis. The same molecule was subsequently identified as syntaxin-2, a member of the syntaxin family of vesicle fusion proteins. Lung organ cultures, which contain both epithelial and mesenchymal cells, can branch in 3D collagen if recombinant growth factors are provided, but this activity is completely blocked by the addition of anti-epimorphin antibodies.

Since that initial discovery, epimorphin has been found to play a role in morphogenesis of many tissues, although it is the mammary gland in which the action of epimorphin has been most clearly defined. In the mammary gland, epimorphin expression reflects its role in mesenchymal-to-epithelial signaling: in tissue sections of postnatal mouse mammary glands, epimorphin is expressed within the stroma and around the epithelial ducts, while staining of isolated primary cells showed epimorphin expression in fibroblasts and myoepithelial cells, but not in luminal epithelial. During lactation, epimorphin can also be found around luminal epithelial cells, in the ductal lumina, and in the milk. As with lung organ cultures, branching morphogenesis can be induced in “tissue organoids” isolated from the mammary gland, if the organoids are cultured in 3D collagen and treated with EGF, HGF, FGF-1, or KGF; also as with the lung organ cultures, growth-factor induced branching could be inhibited by the addition of anti-epimorphin antibodies.
Evaluation of a number of mammary epithelial cell lines showed that a majority expressed epimorphin, while no epimorphin expression was detectable in others. However, for both types of cell lines, epimorphin activity was required for morphogenesis. Growth factor-induced branching of epimorphin-expressing cell lines could be inhibited with antibodies against epimorphin and the particular exogenous growth factor used, but not with antibodies against any other growth factor (i.e., HGF-induced branching could be inhibited by anti-HGF antibodies, but EGF-induced branching was not blocked by anti-HGF antibodies). These results demonstrated that epimorphin was not acting by inducing the expression of alternative growth factors. For non-epimorphin expressing luminal epithelial cell lines, branching required the addition of both exogenous epimorphin and any one of several different growth factors. Experiments using recombinant epimorphin also revealed its potential to mediate dramatically different morphogenic processes depending upon orientation of presentation. Presented in a polar fashion to the basal surface of the cell clusters, epimorphin stimulated branching morphogenesis; presented in an apolar fashion around all surfaces of the cells in the clusters, epimorphin instead stimulated the formation of cystic structures with large central lumen (Figure). For both types of morphogenesis, the morphogenic signal was provided by epimorphin, while the extent of growth (branch length elongation or increase in luminal diameter) was proportional to the proliferative activity induced by the particular growth factor.