Hormone-responsive progenitors have a unique identity and exhibit high motility during mammary morphogenesis

Hormone-receptor-positive (HR⁺) luminal cells largely mediate the response to estrogen and progesterone during mammary gland morphogenesis. However, there remains a lack of consensus on the precise nature of the precursor cells that maintain this essential HR⁺ lineage. Here we refine the identification of HR⁺ progenitors and demonstrate their unique regenerative capacity compared to mature HR⁺ cells. HR⁺ progenitors proliferate but do not expand, suggesting rapid differentiation. Subcellular resolution, 3D intravital microscopy was performed on terminal end buds (TEBs) during puberty to dissect the contribution of each luminal lineage. Surprisingly, HR⁺ TEB progenitors were highly elongated and motile compared to columnar HR^- progenitors and static, conoid HR⁺ cells within ducts. This dynamic behavior was also observed in response to hormones. Development of an AI model for motility dynamics analysis highlighted stark behavioral changes in HR⁺ progenitors as they transitioned to mature cells. This work provides valuable insights into how progenitor behavior contributes to mammary morphogenesis.