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)
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
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.