**1. Introduction**

256 Milk Production – An Up-to-Date Overview of Animal Nutrition, Management and Health

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## **1.1. Mammary development and function**

The development and function of the mammary gland occurs through a cyclical process that changes during the physiological states of pregnancy, lactation, and involution. During pregnancy, maternal hormones in the circulation are responsible for stimulation of mammary gland development and this ensures a sufficient number of mammary cells to produce milk during lactation. Immediately prior to parturition, lactogenic hormones stimulate differentiation of mammary cells, and they adopt a secretory phenotype. In addition to the influence of hormones, the mammary gland itself is thought to be involved in the regulation of mammary cell proliferation, differentiation, and response to hormones. Ultimately, milk yield is dependent on the number and metabolic activity of secretory cells; however, both of these factors are tightly regulated by the endocrine system and physiological state, and also by local factors including the uptake of nutrients by the mammary gland, the connective tissue surrounding the epithelium, and the frequency and degree of milk removal. The following sections will briefly discuss each of the above areas of regulation during lactogenesis and lactation, with emphasis on ruminant species.
