Lactating breast tissue. Light micrograph of a section through healthy breast (mammary gland) tissue during lactation. Glandular tissue in the breast has proliferated to produce spaces (alveoli, white) through which milk flows. The glands are arrange in lobules which make up 15 to 20 lobes. The milk drains from the lobes via the milk-bearing lactiferous ducts to the surface of the nipple.