Lactogenesis. The transition from pregnancy to lactation.

The most important factors in initiation of the cascade of changes in the mammary epithelium that constitute lactogenesis stage II seem to be a prepared mammary epithelium, progesterone withdrawal, maintained plasma prolactin (in most species), and removal of milk from the breast within an undefined interval after birth. Although the molecular mechanisms by which prolactin regulates milk protein synthesis are the subject of intense and productive studies, the specific mechanisms by which progesterone and milk removal interact with the mammary epithelial cell at parturition have not been studied, perhaps because no in vitro model system exists that mimics lactogenesis stage II, or because of the complexity of the changes that must be coordinated during this process, or because of a lack of general understanding of the complex progression of changes in the function of the breast as it goes from the quiescent state of pregnancy to the active secretory state of lactation. With new technologies designed to investigate the biology of complex systems arising from the growing knowledge of the genome of human and animal species and the growing availability of animal and tissue culture models for these processes, physicians can expect a rapid increase in the molecular understanding of lactogenesis in the near future. These fundamental studies must be coupled with good prospective clinical studies if physicians are to obtain a useful, comprehensive understanding of lactogenesis in women.