Single-Cell Analysis in the Mouse and Human Mammary Gland.

The mammary gland is a complex organ, host to a rich array of different cell types. As the only organ to complete its development in adulthood, it delicately balances both cell intrinsic and external signalling from hormones, growth factors and other stimulants. The gland can undergo vast proliferation, restructuring and functional maturation during pregnancy and undo these gross changes to a nearly identical resting state during involution. The adaptive nature of the mammary gland underpins its function but also increases its susceptibility to cancer. While already characterised at a macro scale, understanding the complexities of mammary gland morphogenesis, development and tumorigenesis requires interrogation of cellular and molecular mechanisms. As outlined below, single-cell analysis is a key approach for this, allowing us to unbiasedly explore and characterise the functions and properties of individual cells from the genome to the proteome. Here, we introduce key single-cell analysis methods and give brief introductions to their respective workflows. We then discuss the structure, cell types and development of the mammary gland from birth, puberty and through pregnancy, as well as cancer formation. Additionally, we highlight the benefits and caveats of implementing single-cell methodologies and mouse models for studying critical time points of human development and disease. Finally, we highlight some limitations and future directions of single-cell techniques. This chapter provides a starting point for users hoping to further their understanding of mammary gland development and its link to cancer as explained by single-cell analysis studies.