**4.4. Vascular endothelial cells**

Mammary gland development, occurring during pre- and postnatal life of female mammals, serves to create a highly branched network of ducts and alveoli made of secretory epithelium that actively synthesizes and secretes milk at the time of lactation. To fulfill its function, the mammary gland also requires an expanded network of vascular endothelium. Currently, it is thought that the vasculature not only provides nutrients to the developing and functionally active mammary parenchyma, but also it is important for maintaining homoeostasis of the mammary epithelium.
