Learning Objectives

Identify the two-layer architecture of the normal breast acinus. Recognize the histological features of apocrine secretion (“snouting”). Understand the role of myoepithelial cells and their response to oxytocin. Distinguish between the luminal epithelial layer and the basal myoepithelial layer.

Histology of the Breast Acinus

At high magnification, the functional secretory unit of the breast—the acinus—reveals a highly specialized structure designed for milk production and ejection.

• Luminal Epithelial Cells: These cells line the inner lumen of the acinus. They demonstrate apocrine secretion, characterized by “snouting” or cytoplasmic extrusions into the lumen as they release secretory products.
• Myoepithelial Cells: Located just outside the epithelial layer, these cells may appear slightly vacuolated. They are contractile cells that express smooth muscle markers.
• Hormonal Control: The process of suckling triggers the release of oxytocin from the posterior pituitary, which induces the myoepithelial cells to contract, squeezing milk into the ductal system (the “let-down” reflex).

Key Cellular Identification

Activity