Structure and development of mammary tissue

Milk constituents are synthesized from components obtained from the blood consequently, the mammary gland has a plentiful blood supply and also an elaborate nervous system to regulate excretion. 

The substrates for milk synthesis enter the secretory cell across the basal membrane (outside), are utilized, converted and interchanged as they pass inwards through the cell and the finished milk constituents are excreted into the lumen across the lumenal or apical membrane. Myoepithelial cells (spindle shaped) form a basket around each alveolus and are capable of contracting on receiving an electrical, hormonally mediated, stimulus, thereby causing ejection of milk from the lumen into the ducts. 

---