The development, maturation, and differentiation of mammary gland is regulated by hormones and growth factors acting on the stromal and epithelial cells.

The mammary gland is a hormonal y responsive, exocrine gland with a milk microbiome and functional, structural, and molecular changes that evolve  over a persons lifetime.

Estradiol and progesterone levels increase at puberty and initiate breast development.

The breast consists of glandular tissue, surrounded by a network of blood vessels and lymphatic channels, suspended in stroma of fat, elastin, and collagen.

Development results in 5-10 primary milk ducts originating at the nipple, 20-40 segmental ducts and 10-100 subsegmental ducts that end in terminal-duct lobular units.

glandular tissue consists of progressively smaller branching ducts terminating in alveoli, which function at the site of milk secretion.

Cyclic changes during the menstrual cycle increase the rate of cell proliferation during the luteal phase.

During the luteal phase of the menstrual cycle the breast may increase in size by up to 15%.

Estrogens promote proliferation of mammary ducts.

Progesterone promote growth of breast lobules and alveoli.

Studies show that there is no clear anatomic tissue plane that separates breast tissue from subcutaneous fat and residual breast tissue is present after mastectomy in 21-94% of patients (Robertson SA).

Breast tissue can at times be extended into the axilla and below the inframmamary fold.

Swelling, tenderness, and pain during the 10 days preceding menstruation related to distension of the ducts, hyperemia and edema of interstitial tissues.

All changes noted above regress during menstruation

The total number of breast lobules diminishes during menopause.

The proliferative epithelial cells of the developing breast is more susceptible to radiation injury than is the tissue of older patients.

Median breast weight double in pregnancy from 200 gm to 400 gm.

Fibrous changes and cyst formation, fibrocystic changes, occur in up to 50-60% of patients and entail no increased risk of breast cancer.

Contains 2 types of epithelial cells: Luminal cells and basal cells.

Luminal cells important in lactation, lining the milk ducts and alveoli where they secrete milk.

A luminal progenitor cell may be the precursor of breast cancers.

Basal cells function during lactation as a contractile elements circulating middle to the nipple.

Basal cell activity probably unrelated to the development of cancer cells.

However, there is basal to luminal cell signaling via the gene p63, a master regulator of basal cell growth.

Without p63 gene the luminal progenitor cells fail to differentiate as normal during pregnancy with no producing luminal cells.

During pregnancy and lactation, the glandular portion of the breast proliferates, and the epithelial cells undergo a transformation into a secretary phenotype.

With nipple stimulation, oxytocin and prolactin levels rise; oxytocin causes milk ejection for immediate feeding, and prolactin stimulates milk secretion for maintenance of lactation.

Changes in breast tissue occur throughout a woman’s life, with expansion and development of the memory gland during the pubertal years, proliferation and involution during the menstrual years, glandular and ductal changes during lactation, and post menopausal fatty deposition and involution after menopause.

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