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.
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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.
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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.
Development
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:
- Growth of breast ducts
- Development of hormonally-responsive specialised periductal stroma
- Development of lobules - a prolonged process that is accelerated by pregnancy
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
- Expansion ++ of TDLUs
- Epithelium accumulates fat droplets
- Luminal portion of cytoplasm vacuolated
- Non-specialised stroma apparently diminished - TDLUs crowded
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- Epithelial cells vacuolated
- Small cytoplasmic blebs on luminal surfaces
- Cytoplasm generally diminished in comparison with late pregnancy - release of secretions
- Secretion is apocrine with residual nuclei post-secretion protruding into lumen like hobnails
- Three months post lactation required for return to 'normal'
- See also Benign calcification (3)
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.
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