The breast is a modified sweat gland, organised as lobular clusters of bilayered acini, draining into bilayered ducts, distributed within a fibro-adipose stroma. The ducts increase in calibre as the nipple is approached, allowing abnormalities of the ductal epithelium (particularly ductal carcinoma in-situ) to grow preferentially in the direction of the nipple. The largest draining ducts of the breast are the sub-areolar ducts.

Normal breast labelled Normal lobule
Normal anatomical structure of the mammary gland. Patrick. J. Lynch 2006.

To understand breast pathology, in particular the processes of hyperplasia and neoplasia, a knowlege of the normal histological structure of the duct/lobular units is required. Both mammary ducts and lobules have a similar structure, with a lining of single epithelial cells, supported by a myoepithelial layer.

Normal lobule LP
The picture shows a normal lobule, comprising clustered bilayered breast acini, draining into a small breast duct. Note the oval, circumscribed profile of the lobule. Benign proliferations retain this circumscribed profile, termed 'lobulocentricity', a feature not shared by invasive tumours.

Each acinus and duct has an inner epithelial lining, which expresses low molecular weight cytokeratin eg. cytokeratin 7,8 and 18 and which also expresses oestrogen receptors to varying degrees. The supporting myoepithelial or basal layer expresses high molecular weight cytokeratin eg. cytokeratin 5. It also co-expresses myoid markers eg. smooth muscle actin, smooth muscle myosin and calponin. Myoepithelial cells also show nuclear expression of p63. They are hormone receptor negative.

Normal acinus HPMyoepithelium

CK5 in normal duct