International Board of Certified Lactation Consultant (IBCLC) Practice Exam

The hormone that causes the contraction of myoepithelial cells surrounding the alveoli is oxytocin. Myoepithelial cells play a crucial role in lactation as they help to expel milk from the alveoli into the ducts during breastfeeding. When the infant suckles, nerve signals stimulate the release of oxytocin from the posterior pituitary gland. This hormone acts on the myoepithelial cells, causing them to contract, which in turn pushes the milk through the ducts and towards the nipple for the baby to feed. Understanding the roles of the other hormones is important for contextual clarity. Estrogen and progesterone primarily function in regulating the development of breast tissue and preparing it for lactation during pregnancy, but they do not directly cause the contraction of myoepithelial cells. Prolactin is essential for stimulating milk production, but it does not initiate the ejection of milk from the alveoli. Therefore, while all these hormones play significant roles in lactation, oxytocin is specifically responsible for the action of myoepithelial cell contraction, making it the correct answer.