The anatomy and the physiology of the breast should be familiar to you.
The areolar tissue is pigmented, with smooth muscle and elastic fibers. Montgomery's areolar sebaceous glands (which prevent chapping) undergo hyperplasia during pregnancy; they are the little bumps. The breast is composed of a system of branching ducts draining into 6-12 lactiferous ducts. The systems are extensively intertwined, and I would urge you to ignore talk about "separate lobes". Elastic fibers surround the lactiferous ducts and their branches. The lactiferous duct widens to become the lactiferous sinus underneath the nipple.
Little groups of terminal ducts / acini are surrounded by a solid fibrous stroma. The stroma between these units is fibrofatty. As a woman gets older, there is usually more fat relative to stroma in the breast. This shows cancers (which are non-fatty) to advantage on mammography.
The duct system branches several times and ends in the collecting ducts. Terminal ductules branch off the collecting duct. One collecting duct and its terminal ductules, plus the accompanying stroma, is called a "lobule". You can recognize lobules in normal breast because the stroma is looser and contains less fat and no elastin, and the little ductules are clustered together.
During pregnancy, true secretory units sprout from each terminal duct, coming to dominate the breast histology. After delivery, milk production begins. You may see secretory units in a woman whose breasts have become tender during the first few cycles on the oral contraceptive pill, or "for no reason".
It's not clear to me that the male breast "is relatively insensitive to hormonal influences" (as Big Robbins tells us); men simply have little estrogen and less progesterone on board. Hence the breast tissue does not develop except under unusual circumstances.
Hormones to remember:
Estrogen: Develops the big ducts
Progesterone: Develops the lobules and ductules ("acini")
Prolactin: Develops the secretory units and causes milk production
Oxytocin: Makes the myoepithelial cells contract and express milk
During pregnancy, estrogen and progesterone prevent the milk from being produced. When the pregnancy ends, lactation begins soon. Stimulation of the nipple causes production of both prolactin (which keeps lactation going) and oxytocin (which makes the milk come down).
At all levels of the duct-and-acinar system, there is a single layer of myoepithelium. You can stain it for smooth-muscle-actin or S100 or high-MW keratin or smooth-muscle myosin heavy-chain (probably best); a pathologist may use a special type of anti-keratin stain too. This contracts in response to oxytocin to let the milk come down. During the second half of the monthly cycle, progesterone causes some proliferation of ducts and stroma in the lobules. When the cycle ends, these changes regress. After menopause, the lobules may vanish, leaving only the larger ducts.
Mother's hormones may produce some breast development in the newborn baby girl, and there may even be a bit of secretion ("witch's milk").
You remember that some breast parenchyma extends toward the axilla as the "tail of Spence". Generally, the upper outer quadrant of the breast is the most massive anyway, which probably explains why most breast diseases are most common here.
You should remember the duct cells, lobules, and myoepithelial cells.
You remember the anatomic "milk line" (check on most non-human female mammals). Supernumerary nipples and supernumerary breasts (polythelia and polymastia) arise here, and are very common. Accessory breasts may or may not have nipples, but undergo the same changes during menstruation, pregnancy, lactation, and carcinogenesis as normal breast does.
Inverted nipples are common, especially in larger breasts, and may make nursing more difficult. If a previously-normal nipple inverts, you have a problem, i.e., something has retracted underneath, and it's the stroma of a cancer until proven otherwise.
Virginal hypertrophy: very large breast(s) developing around puberty. Really hyperplasia, of course. The etiology is unknown, and occurrence is sporadic. (Nowell's law at work, probably.)
Hypomastia: almost complete failure of breast development. (* Around half of these women have mitral-valve prolapse. See NEJM 309: 1230, 1984.)
By contrast, very large breasts are likely to cause serious low-back problems.
INFLAMMATIONS: Not common.
Acute mastitis and breast abscess: Usually occurs during early lactation, less often in patients with dermatitis. The bug is usually staph aureus (abscess-maker), less often streptococcus (spreading cellulitis).
Fat necrosis: A solid mass, often in a fat breast, caused by a blow or other injury. Necrotic fat cells surrounded by a mixed inflammatory infiltrate, later with calcification, foreign body reaction, scarring. Before there was much notice paid to domestic violence, the etiology was "mysterious".
Periductal mastitis ("recurrent subareolar abscess"): A hyperkeratinizing squamous metaplasia going too far down a lactiferous duct. This gets inflamed and needs to be cleaned up by a surgeon. Almost all these people are smokers, and both men and women are affected.
Duct ectasia: An uncommon cause of a breast mass, usually in older women, usually tender and with nipple retraction. Chronic inflammation and fibrosis around ducts are typical. The ducts are loaded with a lipid-and-macrophage rich material. The underlying cause is unknown; many of these women turn out to have pituitary prolactinomas.
*"Plasma cell mastitis", an old diagnosis, is probably just duct ectasia with a lot of plasma cells.
* Lymphocytic mastopathy is evidently an autoimmune disease. It runs with Hashimoto's thyroiditis and type I diabetes. These lesions are patchy but may produce masses.
* Granulomatous lobular mastitis is confined to the breast lobules. All these women have been pregnant. Probably there is some autoimmune reaction against the secretory units.
Obviously you'll want to rule out TB, sarcoidosis, and reaction to a ruptured implant.
Galactocele: One or more ducts became plugged during lactation.
* "Mondor's disease" is thrombophlebitis of the breast, a minor mystery.
Rupture of an implant is commonplace, and in fact the fibrous capsules which ordinarily form around implants helps contain this.
Author: Ed Friedlander, M.D., Pathologist, Pathguy.com