Elongating mammary duct. Green: myoepithelial cells; Red: luminal epithelial cells.

Image courtesy of Dr. Andrew J. Ewald, University of California, San Francisco, USA.

The formation of epithelial tissues relies largely upon collective cell movement, but how these tissues maintain epithelial features such as cell-cell contact during reorganization remains poorly understood. Zena Werb and colleagues now report that mammary branching morphogenesis results from the transition of a bilayered epithelium into a multilayered epithelium, followed by a mechanism of collective migration that lacks leading protrusions.

Mammary branching morphogenesis occurs predominantly during puberty; however, the mammary epithelium is also extensively remodelled during pregnancy and lactation. Quiescent mammary ducts have two cell layers, whereby a myoepithelial (ME) layer surrounds a luminal epithelial (LE) layer. At the end of each primary duct, multilayered terminal end buds (TEBs) form transiently and rearrange to generate bilayered ducts.

The authors imaged 3D cultures of mammary epithelium fragments and showed that new ducts formed from complex cysts — multilayered highly proliferative structures that resemble TEBs in vivo. Complex cysts have a single basal ME layer and multiple LE layers. New ducts initiated at gaps in the ME layer. Luminal epithelial cells advanced collectively past ME cells, which was followed by ME migration to restore a surrounding ME layer. Thus, branching morphogenesis results from the interplay between ME and LE motility, with ME cells appearing to restrain the duct elongation.

During duct elongation cells retained an epithelial identity; E-cadherin and -catenin both localized to cell-cell contacts. Furthermore, the authors observed that although the multilayered epithelium was polarized at the tissue level, individual cells lacked distinct apical and basolateral domains. The authors propose that this low-polarity multilayered organization is a "morphogenetically active epithelial state" that is specialized for growth and remodelling.

But what are the molecular pathways that regulate ductal initiation and morphogenesis? Inhibition of the small GTPase Rac1 and myosin light chain kinase (MLCK) prevented ductal initiation, whereas inhibition of Rho kinase (ROCK) prevented the transition from a multilayered epithelium to a single luminal layer. Together these results show that duct initiation depends on a combination of proliferation and the presence of Rac and MLCK, whereas repolarization to form a bilayer requires the presence of ROCK.

Cells at the tip of extending ducts continuously exchange position. Interestingly, the front cells of advancing mammary ducts lacked cellular extensions or actin-rich protrusions such as the filopodia and lamellipodia, which are typically found at the leading edge of migrating cells. This work shows that ductal branching morphogenesis occurs via a different type of collective epithelial migration that lacks leading cell protrusions. As protrusive activity functions to guide cells, the mechanisms that promote directional movement of elongating mammary ducts remain to be identified.

Author: Kim Baumann