Changes in adhesive properties of epithelial cells during early morphogenesis of the mammary gland.

Previous studies have demonstrated that cell adhesion systems are downregulated in epithelial buds at the earliest stages of submandibular gland and hair follicle development, but are restored at subsequent stages. Here it is shown that epithelial cell adhesion systems are also remodeled during early mammary gland development. Immunofluorescence and electron microscopy of the mouse mammary bud demonstrated that cell-cell adhesion systems were hardly detectable, with significant downregulation of expression of desmosomal molecules, but not of E-cadherin and beta-catenin. Hemidesmosomal structures were also rarely found, although their component molecules were expressed. Differences in cell adhesivity between cells of the mammary bud and those of the overlying epidermis were shown by the finding that the mammary cells formed smaller aggregates than the epidermal cells and were not randomly mixed with the epidermal cells. At subsequent stages, the mammary epithelium restored cell-cell adhesion systems along with de novo expression of tight junction molecules. These data, together with previous findings, indicate that remodeling of epithelial cell adhesion systems is a general feature underlying the early development of several ectoderm-derived organs and support the idea that segregation and rearrangements of cells are involved in early epithelial morphogenesis.