A human organoid system that self-organizes to recapitulate growth and differentiation of a benign mammary tumor

Significance Organoid culture allows the study of tissue function and development in vitro. Here, we show that mammary cells can autonomously reassemble in vitro to form a common benign mammary lesion [usual ductal hyperplasia (UDH)]. Currently, very little is known about how UDH develops. In vitro, we observe a coordinated process reminiscent of embryonic development of the mammary gland that recapitulates typical morphological hallmarks of UDH at all stages, suggesting that, in vivo, such lesions also form through the same sequence of events. Thus, we speculate that UDH lesions form through activation of embryonic-like developmental programs in adult mammary cells. The culture protocols introduced here will be useful to study supracellular coordination of mammary cell behavior.