[Dynseli]

The breast is composed of glandular, connective, and adipose tissues. This glandular tissue is made up of numerous milk producing lobules and ducts. The breast originates over the span of the pectoralis major muscle, while most of it lies over the "second to the sixth ribs."

The endocrine system, including the pituitary gland, regulates mammary glands. Areolar and mammary glands are exocrine glands.

Breasts contain 'estrogen receptor alpha' (ER-α), 'progesterone receptor' (PR), and 'prolactin receptor' (PrLR) proteins. Estradiol, progesterone and prolactin normally activate the respective receptors that cause breast growth. Estrogens, and progestogens are different categories of sex hormones. Prolactin is another type of hormone that reduces estradiol.

Estrogens cause elongation or forward growth by stimulating (lactiferous) mammary duct cells, through activation of ER-α. PR activation, believed to be specific to 'Progesterone Receptor B' (PRB), causes growth of milk producing cells or sidebranching. Progestogens also cause stimulation of connective breast tissue (cooper's ligaments). Density, Areolar gland development, and gland lactation development are caused by prolactin receptor activation.

Estrogens, and progestogens are different categories of hormones, and prolactin is another type of hormone. Progesterone and prolactin reinforce each other's presence, while lowering estradiol levels. Estradiol, in turn, reduces prolactin and progesterone levels.

Most breast development occurs during the luteal phase, usually days 14 to 28, which is associated with high levels of progesterone and a lesser but significant amount of estrogen. The luteal phase is signaled to start by luteinizing hormone (LH).

During the follicular phase, days 1 through 14, IGF-1 and prolactin are positively correlated with breast size. 'Insulin-like growth factor 1' (IGF-1) causes breast epithelial cell proliferation, and it may allow for breast size maintenance, and growth. Some hormones may be necessary for keeping gains made during the luteal phase.

Estrogen is normally high during ovulation (between menstruation and the luteal phase) and causes breast growth. Breast swelling during luteal phase is also growth, and perhaps antagonists undo this action that is associated with water weight. Progestins may possibly be ER-α antagonists, and estrogens might be PRB antagonists. Certain receptor antagonisms might be necessary to lessen desensitization, due to up or down regulation. However, an overload of antagonisms may damage receptors, desensitizing them. Lower androgen levels are associated with larger breast sizes.

References:
* gland activation - Hormone Action in the Mammary Gland
* mod - Botanical modulation of menopausal symptoms: Mechanisms of action? Botanical modulation of menopausal symptoms: Mechanisms of action?
* endogenus - Breast size in relation to endogenous hormone levels, body constitution, and oral contraceptive use in healthy nulligravid women aged 19-25 years
* mlpswelling - Breast:premenstrual tenderness and swelling
* yj - Yen & Jaffe's Reproductive Endocrinology (6th Ed.): The breast

See also:
* Thread - Herbs shown to scientifically influence healthy breast tissue
* Thread - Hormones and fertiliy (ovaries, FSH, mint, clover, ZEN, etc) - about infertility
* Thread - Hormones and receptors; definitions and abbreviations