Spatially coordinated cell cycle activity and motility govern bifurcation of mammary branches

Branching morphogenesis is an evolutionary solution to maximize epithelial function in a compact organ. It involves successive rounds of branch elongation and branch point formation to generate a tubular network. In all organs, branch points can form by tip splitting, but it is unclear how tip cells coordinate elongation and branching. Here, we addressed these questions in the embryonic mammary gland. Live imaging revealed that tips advance by directional cell migration and elongation relies upon differential cell motility that feeds a retrograde flow of lagging cells into the trailing duct, supported by tip proliferation. Tip bifurcation involved localized repression of cell cycle and cell motility at the branch point. Cells in the nascent daughter tips remained proliferative but changed their direction to elongate new branches. We also report the fundamental importance of epithelial cell contractility for mammary branching morphogenesis. The co-localization of cell motility, non-muscle myosin II, and ERK activities at the tip front suggests coordination/cooperation between these functions. This study shows that mammary gland branching morphogenesis is orchestrated by spatiotemporally coordinated changes in tip cell motility and proliferation to alternate between branch elongation and branch point formation.

Automated summary

Branching morphogenesis is an evolutionary strategy for enhancing epithelial function in compact organs. However, the coordination between cell elongation and branching remains unclear. In the embryonic mammary gland, live imaging revealed that tip cells advance through directional migration while elongation depends on differential cell motility, with lagging cells being fed into the trailing duct and supported by tip proliferation. Branching involves localized repression of cell cycle and motility, and daughter cells change direction to elongate new branches while remaining proliferative. Additionally, epithelial cell contractility is crucial for mammary branching morphogenesis.