mammary gland(redirected from milk gland)
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mammary gland,organ of the female mammal that produces and secretes milk for the nourishment of the young. A mammal may have from 1 to 11 pairs of mammary glands, depending on the species. Generally, those mammals that bear larger litters have more glands. The mammary gland of the cow and of some other mammals is known as the udder.
In humans, there is one pair of mammary glands, also known as mammae, or breasts. They are rudimentary in both sexes until the age of puberty when, in response to ovarian hormones, they begin to develop in the female. During pregnancy, they distend still further in preparation for nursing the infant. Pregnant women are prevented from lactating (producing milk) by the presence in the blood of high levels of estrogenestrogen
, any one of a group of hormones synthesized by the reproductive organs and adrenal glands in females and, in lesser quantities, in males. The estrogens cause the thickening of the lining of the uterus and vagina in the early phase of the ovulatory, or menstrual, cycle
..... Click the link for more information. and progesteroneprogesterone
, female sex hormone that induces secretory changes in the lining of the uterus essential for successful implantation of a fertilized egg. A steroid, progesterone is secreted chiefly by the corpus luteum, a group of cells formed in the ovary after the follicle
..... Click the link for more information. , secreted by the placenta until birth occurs.
After birth, response to prolactin, the milk-stimulating hormone, is no longer inhibited by placental hormones, and lactation begins. Mammary tissue contains between 15 and 20 compartments called lobes, each of which is divided into smaller compartments called lobules. The lobes and lobules are connected by a network of tubes whose cells manufacture the liquid and fatty substances that form milk. The tubes of each lobe connect with a duct, and all ducts lead to the nipple, where the milk is secreted when the nipple is sucked by the young. The letdown of milk during the nursing process is aided by oxytocinoxytocin
, hormone released from the posterior lobe of the pituitary gland that facilitates uterine contractions and the milk-ejection reflex. The structure of oxytocin, a cyclic peptide consisting of nine amino acids, was determined in 1953 and in the same year it was
..... Click the link for more information. , a hormone secreted by the pituitary. The physical force of an infant's sucking on the breast is a major stimulus to milk production. Disorders of the mammary gland include mastitismastitis
, inflammation of the breast. Mastitis most commonly occurs in nursing mothers between the first and third weeks after childbirth, usually of the first child. It is an infection that results when bacteria enter through cracked nipples; the organisms may already be in
..... Click the link for more information. and breast cancerbreast cancer,
cancer that originates in the breast. Breast cancer is the second leading cause of cancer death in women (following lung cancer). Although the vast majority of the cases occur in women, some men also get breast cancer.
..... Click the link for more information. .
A unique anatomical structure of mammals that secretes milk for the nourishment of the newborn. The mammary gland contains thousands of milk-producing units called alveoli, each of which consists of a unicellular layer of epithelial cells arranged in a spheroid structure. The alveolar epithelial cells take up a variety of nutrients from the blood that perfuses the outer surface of the alveolar structures. Some of the nutrients are then secreted directly into the alveolar lumen; other nutrients are used to synthesize the unique constituents of milk which are then secreted. Each alveolus is connected to a duct through which milk flows. The ducts from many alveoli are connected via a converging ductal system which opens externally by way of the lactiferous pore.
Surrounding each alveolus and its associated small ducts are smooth muscle cells called myoepithelial cells. These cells contract in response to the posterior pituitary hormone oxytocin; milk is thus forced out of the alveoli, through the ductal system, and out the lactiferous pore for the nourishment of the newborn. The release of oxytocin is a neuroendocrine reflex triggered by the stimulation of sensory receptors by the suckling of the newborn. See Endocrine mechanisms
Mammary glands are basically highly modified and specialized sebaceous glands which derive from ectoderm. In the embryo, mammary lines, formed on both sides of the midventral line, mark the location of future mammary glands. Along the mammary lines discrete ectodermal ingrowths, called mammary buds, produce a rudimentary branched system of ducts at birth. In all species (except the monotremes) a nipple or teat develops in concert with the mammary buds. In the most primitive mammal (the duckbill or platypus), which lacks nipples or teats, milk simply oozes out of the two mammary gland areas and is lapped up by the young.
From birth to sexual maturity the mammary gland consists of a nipple and a rudimentary ductal system in both males and females. At the onset of puberty in the female, the enhanced secretion of estrogen causes a further development of the mammary ductal system and an accumulation of lipids in fat cells. After puberty in women, the mammary gland consists of about 85% fat cells and a partially developed ductal system. See Estrogen
During pregnancy the mammary gland comes under the influence of estrogen and progesterone which are derived from both the ovary and placenta. These hormones cause a further branching of the ductal system and the development of milk-secreting structures, the alveoli. In humans, approximately 200 alveoli are surrounded by a connective tissue sheath forming a structure called a lobule. About 26 lobules are packaged via another connective tissue sheath into a larger structure called a lobe. Each of 15–20 lobes is exteriorized into the nipple via separate lactiferous pores. See Progesterone
A complement of hormones maximizes the development of the ductal and lobuloalveolar elements in the mammary gland. Optimal ductal growth is attained with estrogen, a glucocorticoid, prolactin, and insulin. Maximal lobuloalveolar growth is obtained with estrogen, progesterone, growth hormone, prolactin, a glucocorticoid, and insulin. During pregnancy estrogen and progesterone stimulate mammary development but inhibit milk production.
During the final third of pregnancy, the alveolar epithelial cells begin secreting a fluid called colostrum. This fluid fills the alveoli and causes a gradual enlargement of the breast or udder. At parturition, the inhibitory influence of estrogen and progesterone is removed, and the gland can secrete milk under the influence of a further complement of hormones including prolactin, a glucocorticoid, insulin, and the thyroid hormones. See Gland, Lactation, Mammalia, Milk, Pregnancy