Retinoids induce lumen morphogenesis

The development of most glandular organs begins with the invagination of an existing epithelial sheet into the underlying mesenchyme. The primary bud thus generated subsequently undergoes a series of morphogenetic events that culminate in the formation of branching tubes (excretory ducts) and hollow spheres (follicles, alveoli or acini). A crucial step in the development of glandular units is lumen formation, that is, the creation of a central cavity within an initially solid epithelial primordium (Hogg et al., 1983; Gumbiner, 1996; Coucouvanis and Martin, 1995; Hogan and Kolodziej, 2002). Over the past decade, it has become increasingly clear that the processes involved in the generation of epithelial architecture are orchestrated by diffusible cues. Thus, a number of studies have led to the identification of polypeptide growth factors that promote the elongation and branching of epithelial tubes (e.g. Montesano et al., 1991b; Vega et al., 1996; Bellusci et al., 1997; Sakurai et al., 2001). However, with the exception of a few recent papers (Soriano et al., 1995; Lipschutz et al., 2000; Hirai et al., 2001; O’Brien et al., 2002), there is a paucity of information about the molecular signals specifically responsible for lumen formation. This stems in part from the limited availability of in vitro systems suited for analyzing this morphogenetic process (Petersen et al., 1992; Soriano et al., 1995; Yap et al., 1995; Hirai et al., 1998; Blatchford et al., 1999). The isolation of clonal murine mammary epithelial cell lines endowed with the ability to form tubulocystic structures in collagen gels (Montesano et al., 1998) has provided an additional convenient model to investigate the molecular mechanisms of lumen formation. However, the identification of morphogenetic factors in serum-supplemented cultures is potentially hampered by the presence of numerous undefined components, including polypeptide growth factors, hormones and other biologically active molecules, which may mask or modify the effects of exogenously added agents. To circumvent this drawback, and to define the minimal requirements for lumen formation, we have developed a serum-free culture system in which mammary epithelial cells form solid multicellular colonies in collagen gels (R.M., unpublished). The goal of the present study was to use this experimental system as a bioassay to identify the molecular signals involved in lumen morphogenesis. 4419