Mammary gland (IMAGE) University of California - Santa Cruz Caption The breast forms a tree-like ductal structure that grows from the nipple into a tissue rich in adipocytes (fat cells), blood vessels, and immune cells. During pregnancy, alveoli form at the tips of the ducts. They have an outer layer of myoepithelial cells (green) that contract to squeeze milk from the inner layer of alveolar cells (pink). Credit Image by Lindsay Hinck, UC Santa Cruz Usage Restrictions Credit must accompany any reuse of this image. License Original content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.