What are mammary alveoli?

During the development of women, the mammary glands in their breasts will slowly grow and develop. Generally, when a woman's first menstrual period occurs, it indicates that the mammary glands have fully developed. In addition to development, women's breasts will also slowly atrophy over a certain period of time. The female mammary gland is composed of various mammary gland lobules, which are medically called mammary alveoli. Next, I will give you a detailed introduction to mammary alveoli.

The mammary gland is affected by nerves and hormones and undergoes obvious age and functional changes. The mammary glands have developed to their highest degree around the age of 20, begin to atrophy around the age of 40, and atrophy significantly after menopause. During the menstrual cycle, the size of the breasts changes slightly. During pregnancy and lactation. There are significant changes in the structure and function of the mammary gland. When adults are not pregnant, the mammary glands have no secretory activity, which is called quiescent mammary glands. Breast hyperplasia during pregnancy and vigorous secretion during the lactation period are called active breasts.

1. Quiescent breast:

The mammary gland has 15 to 25 lobes, each lobe is an independent gland, with the lactiferous duct opening at the top of the nipple. The lactiferous duct expands into sinuses under the areola, which are called lactiferous sinuses. The large ducts below the lactiferous sinus branch into small ducts, the ends of which are connected to the alveoli. The lobes of the mammary gland are separated by dense connective tissue and surrounded by adipose tissue. Connective tissue extends into the leaves and divides them into many leaflets. The glandular tissue of the quiescent breast is sparse, the lobes and lobules are not clearly distinguished, and groups of tubules can be seen scattered in a large amount of dense connective tissue. Abundant fat cells. The lobules have very few alveoli and narrow alveolar cavities.

During pregnancy, under the action of estrogen and progesterone, the small ducts and alveoli of the mammary gland proliferate rapidly, the alveoli become larger, the epithelium becomes a single layer of columnar or cuboidal cells, and the connective tissue and adipose tissue decrease accordingly. In the late stages of pregnancy, under the influence of prolactin secreted by the pituitary gland, the alveoli begin to secrete. The mammary gland is an apocrine gland, and its secretion contains lipid droplets, milk proteins, milk antibodies, etc., which is called colostrum. There are also macrophages in colostrum that eat fat, called colostrum bodies.

The structure of the breast during lactation is similar to that during pregnancy, but the gland is better developed and the alveolar cavity is enlarged. The alveoli are in different stages of secretion. Some alveoli are in the early stage of secretion, and the glandular cells are tall columnar; some alveoli are in the late stage of secretion, and the cells are cubic or flat. The glandular cavity is filled with milk, and the glandular cells are rich in rough endoplasmic reticulum and mitochondria. There are many secretory granules and lipid droplets in the secretory cells.

After weaning, the prolactin level decreases, the mammary gland stops secreting, the glandular tissue gradually atrophies, the connective tissue and adipose tissue increase, and the mammary gland enters a dormant period again. After menopause, the levels of estrogen and progesterone in the body decrease, breast tissue atrophies and degenerates, and fat also decreases.

The function of the mammary gland depends on complex neural and endocrine factors. The growth and development of the mammary gland is the result of the coordinated action of several hormones. Before puberty, breast development is slow. With the onset of puberty, estrogen levels increase, breast development accelerates, and breasts become larger. Under the stimulation of estrogen, the mammary duct branches proliferate. In adults, the breasts undergo slight cyclical changes as ovarian endocrine secretions change during the menstrual cycle. During pregnancy, progesterone stimulates the development of alveoli. The hormones synthesized by the placenta can replace or enhance the effects of ovarian steroid hormones and pituitary prolactin, causing the mammary gland to develop highly and the alveoli to proliferate significantly and be in a pre-secretory state. After delivery, prolactin stimulates milk secretion. The stimulation of sucking the nipple is transmitted to the paraventricular nucleus of the hypothalamus, prompting the neurosecretory cells of the paraventricular nucleus to produce oxytocin, which is released into the bloodstream through the posterior pituitary gland. Oxytocin stimulates the contraction of the myoepithelial cells of the mammary gland, causing milk to spurt out of the nipple. Due to the action of neurohormones, the milk in the alveolar cavity is emptied. The pituitary gland can continuously secrete prolactin, causing the gland cells to continuously secrete milk. After weaning, the stimulation of sucking disappears, the production of oxytocin stops, the milk in the gland cavity cannot be discharged, and lactation stops. The mammary gland gradually returns to its quiescent state.