The high impact of breast cancer on women throughout the world and the need for better treatments requires a greater understanding of normal mammary gland development and how this goes awry in cancer. The mammary gland is a unique organ that undergoes most of its development after birth. During puberty, mammary ducts rapidly grow and branch to fill the mammary fat pad. This occurs through the directional proliferation and differentiation of stem and progenitor cells within terminal end buds (TEBs). TEBs are comprised of cap cells, which give rise to mature myoepithelial cells, and body cells, which include luminal progenitor cells and mature luminal cells. Lineage tracing, static imaging and mathematical modelling have provided clues into TEB function. Lineage tracing has shown that basal stem cells (cap cells) can give rise to both basal and luminal lineages, supposedly by entering the TEB body through asymmetric division or migration (Rios et al., 2014). However, there is also evidence that basal cells are unipotent (Scheele et al., 2017; Van Keymeulen et al., 2011) and recent studies have suggested that cap cells that enter the TEB body are predisposed to apoptosis (Paine et al., 2016; Sreekumar et al. 2017). TEB function is largely inferred from static time points and the resulting ductal tree, while the cell dynamics within live TEBs remains unclear. Cell migration within TEBs is not well characterised, asymmetric division of cap cells remains unconfirmed and the mechanics of duct formation, TEB growth and branching are largely unknown.
To address these issues, we sought to directly observe TEB cell behaviour by intravital imaging. To this end, we developed an intravital imaging technique that allows long-term, single cell resolution imaging of TEBs in vivo. To observe the behaviour of each TEB cell population, we have used doxycycline inducible, lineage-specific Cre recombinase to activate the multicoloured Confetti reporter. By imaging labelled TEBs in puberty and tracking single cells in 3D over many hours, we are now able to describe TEB cell dynamics in great detail. We are currently using this information to refine and strengthen current models of TEB function.