Normal Structure

In simple terms the breast comprises a branching system of ducts leading down from the nipple ending in glands (acini aggregated into lobules) which have the potential to secrete milk. Approximately 12 large ducts emerge from the breast at the nipple as lactiferous ducts. Their subdivisions form rather complicated breast segments that are discrete physiologically but probably interdigitate with each other at their boundaries. The ducts branch repeatedly ultimately giving rise to the terminal duct lobular unit (TDLU) see below.

Terminal duct lobular unit (TDLU)
Low power view showing floret type arrangement with central duct and surrounding lobules Medium power view showing terminal duct and related acini
Medium power view of terminal duct lobular unit- note basal myoepithelial layer Terminal duct - basal myoepithelial cells marked with arrows

The tissue between the ducts and glands (which makes up the bulk of the volume of the adult breast) is composed of fat and fibrous tissue in varying proportions. This tissue is given the generic name "stroma" and specialised (closely related to and supporting lobular units) and non-specialised types are recognizable.

Specialised and non-specialised stroma within TDLU and between TDLUs respectively
Loose, specialised stroma within lobule and denser less cellular non-specialised stroma outside lobule area Dense, non-specialised stroma in between two lobules

The amount of glandular tissue in the breast is dependent on hormonal activity and fluctuates with the menstrual cycle. The ducts and glands are lined by two layers of epithelium which in turn is supported by a basement membrane. Most diseases of the breast affect ducts and glands and carcinomas (cancers) arise from the epithlial cells lining these branching structures.


The breast develops from the breast bud which is formed in the first six weeks of life in utero. Further normal breast development does not occur until puberty:

The adult premenopausal breast

During the proliferative phase of the menstrual cycle lobules are small, mitoses are infrequent and specialised stroma is condensed. In the secretory phase of the cycle the TDLUs increase in size with a more loose oedematous stroma. They show epithelial mitotic activity and the cytoplasm is vacuolated. In the perimenstrual phase ther may be some sloughing of epithelium into the TDLU lumen evident as debris and a mild lymphocytic intraepithelial infiltrate can be seen. These variations do not give rise to diagnostic problems in routine practice.

Pregnancy and lactation

Normal (late) pregnant breast
Crowded hyperplastic lobular units with luminal secretion Acini expanded with secretion and evident cytoplasmic vacuolation

The postmenopausal breast

The postmenopausal breast shows a reduction in the amount of glandular tissue and relatively increased stroma. Glandular units show fewer acini and specialised stroma is diminished in amount. Also the epithelial and myoepithelial cells are smaller. There may be areas of dense fibrosis and benign vascular calcification may be seen. The entire spectrum of premenopausal lobular appearances can be seen in quite elderly post menopausal women.

Involuted lobule set in dense non-specialised stroma in the post-menopausal breast
Terminal ductule with small number of atrophic acini set in dense, non-specialised stroma

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