Breast – Tutorial
Please read Unit 13 – Introduction to Female Reproductive System Tissues prior to completing the activities in this chapter.
Introduction to Breast Tissue
The breasts (mammary glands) are modified apocrine glands adapted for lactation (milk production) located under the skin and superficial to the pectoral muscles in the anterior chest. Each breast is a complex gland consisting of 15-20 lobes each with its own lactiferous duct (milk duct). Each breast has an associated nipple where lactiferous ducts from each of the underlying secretory lobes open onto the surface of the skin. The spaces between glandular tissue of the breast are occupied by adipose and dense connective tissue which provides physical support for the breast.
Lactating mammary glands are classified as tubulo-alveolar glands which are comprised of elongated tube-like glands with enlarged sac-like alveoli at their ends. The lumens of tubulo-alveolar glands are lined with a cuboidal epithelium which secretes milk into the lumen of the gland. The lumens of lactiferous ducts which transport milk toward the nipple are also lined with cuboidal epithelium. Sandwiched between cuboidal cells and the basement membrane of both the secretory and ductal portions of mammary glands are smooth muscle like epithelial cells known as myoepithelial cells. Myoepithelial cells contract when a baby is nursing to help push milk out of the nipple in a process commonly known as milk let-down (see bottom paragraph for more detail). In nonlactating (resting) breast tissue, the glandular tissue is greatly reduced in size, but the ductal system remains in place although the lactiferous ducts are reduced in diameter compared to times of lactation.
After the onset of puberty in a female, estrogen stimulates the development of glandular tissue and increased fat deposition within adipose tissue of the breast and progesterone stimulates increased development of the rudimentary lactiferous duct system that was present throughout childhood. These post-puberty changes in breast tissue prepare the breast for potential lactation should pregnancy occur. During pregnancy, hormonal changes occur that cause breast tissue to undergo structural and functional changes which include further increasing the size of milk secreting glandular tissue and preparing the glands for milk production. The hormones that stimulate these changes include human placental prolactin, estrogens, progesterone, and prolactin from the anterior pituitary gland. By the end of the second trimester of pregnancy the mammary glands are fully developed and ready to begin producing milk.
After a baby is delivered, ejection of milk from the breast is stimulated when an infant suckles at the breast in a process known as the milk ejection reflex (commonly known as milk let-down). During this reflex, the nursing infant stimulates tactile receptors in the nipple which then stimulates secretory neurons in the hypothalamus of the mother’s brain. The secretory neurons originating in the hypothalamus then release oxytocin into the blood from the posterior pituitary gland. When oxytocin reaches breast tissue it causes contraction of special smooth muscle like epithelial cells known as myoepithelial cells which line the lactiferous ducts resulting in milk let-down.
Tutorial: Use the image slider below to learn more about the structure and characteristics of breast tissue.
Microscopy: Use the image slider below to learn how to use a microscope to study breast tissue on a microscope slide.
Tutorial: Use the hotspot image below to learn more about the structure and function of lactating breast tissue.
