Breast Physiology PDF

MAMMOGENESIS AND LACTATION PRESENTED BY: DR. MAWADA AHMED ELNOUR UNIVERSITY OF GEZIRA FACULTY OF MEDICINE DEPARTMENT OF PHYSIOLOGY Development of the Breasts (mammogenesis):  The breasts begin to develop at puberty (stimulated by the estrogens of the m...

MAMMOGENESIS AND LACTATION PRESENTED BY: DR. MAWADA AHMED ELNOUR UNIVERSITY OF GEZIRA FACULTY OF MEDICINE DEPARTMENT OF PHYSIOLOGY Development of the Breasts (mammogenesis):  The breasts begin to develop at puberty (stimulated by the estrogens of the monthly female menstrual cycle).  Estrogens stimulate:  Growth of the breasts’ mammary glands.  The deposition of fat to give the breasts mass.  The greater growth occurs during pregnancy ( due to the high estrogen level from the placenta) and only then the glandular tissue become completely developed for the production of milk.  The breasts in non pregnant females consist mostly of adipose tissue and duct system.  The size of the breast has no relationship with how much milk a mother will make.  Each breast is composed of about 20 lobules, each with an excretory lactiferous duct that opens at the nipple.  The lactiferous ducts empty at the nipple, which is a highly innervated, hairless protrusion of the breast designed for suckling by an infant.  Lobules contain secretary structures called alveoli (lactocytes) that have the ability to synthesize and secrete milk during lactation.  The epithelium of the alveoli and ducts contains myoepithelial cell layer. Contraction of the surrounding myoepithelial cells ejects the secreted milk out through the duct. PREPARATION OF THE BREASTS FOR LACTATION  All through pregnancy, the large quantities of estrogens secreted by the placenta cause the ductal system of the breasts to grow and branch.  whereas the high level of progesterone stimulates abundant alveolar- lobular formation.  Simultaneously, the stroma of the breasts increases in quantity, and large quantities of fat are laid down in the stroma.  Other important hormones for growth of the ductal system are: 1. Growth hormone. 2. The adrenal glucocorticoids. 3. insulin. 4. Thyroid and Parathyroid hormones. PREVENTION OF LACTATION DURING GESTATION  The mammary glands are fully capable of producing milk by the middle of pregnancy. However, milk secretion does not occur until parturition.  Prolactin is the primary stimulant of milk secretion.  High estrogen and progesterone concentrations during the last half of pregnancy block prolactin’s action on milk secretion.  Initiation of Lactation—Function of Prolactin  This hormone is secreted by the mother’s anterior pituitary gland.  its concentration in her blood rises steadily from the fifth week of pregnancy until birth of the baby.  The abrupt decline in estrogen and progesterone that occurs with loss of the placenta at parturition initiates lactation.  This secretion of milk requires an adequate background secretion of most of the mother’s other hormones as well, but most important are growth hormone, cortisol, parathyroid hormone, and insulin.  These hormones are necessary to provide the amino acids, fatty acids, glucose, and calcium required for milk formation. maintaining lactation  Once milk production begins after delivery, two hormones are critical for maintaining lactation: 1. prolactin, which promotes milk production and secretion. 2. oxytocin, which causes milk ejection.  Release of both of these hormones is stimulated by a neuroendocrine reflex triggered by suckling called The Milk Ejection Reflex. Hypothalamic Control of Prolactin Secretion  The hypothalamus mainly stimulates production of all the other hormones, but it mainly inhibits prolactin production by prolactin- inhibiting hormone(PIH).  PIH is now known to be dopamine, which also serves as a neurotransmitter in the brain.  Throughout most of the female’s life, PIH is the dominant influence, so prolactin concentrations normally remain low except During lactation.  prolactin also inhibits release of GnRH, and consequently, nursing can be associated with lactational amenorrhea.  The Milk Ejection Reflex  The infant cannot directly suck milk out of the alveolar lumen. Instead, milk must be actively squeezed out of the alveoli into the ducts and hence toward the nipple by contraction of specialized myoepithelial cells.  The infant’s suckling of the breast stimulates sensory nerve endings in the nipple, initiating action potentials that travel up the spinal cord to the hypothalamus.  The hypothalamus triggers release of oxytocin from the posterior pituitary.  The oxytocin is carried in the blood to the breasts, where it causes myoepithelial cells (which surround the outer walls of the alveoli) to contract, thereby expressing the milk from the alveoli into the ducts at a pressure of +10 to 20 mm Hg.  within 30 seconds to 1 minute after a baby begins to suckle, milk begins to flow.  This process is called milk ejection or milk let-down.  Suckling on one breast causes milk flow not only in that breast but also in the opposite breast.  It is interesting that fondling of the baby by the mother or hearing the baby crying often gives enough of an emotional signal to the hypothalamus to cause milk ejection.  In contrast, psychological stress, acting through the hypothalamus, can easily inhibit milk ejection. References:  GUYTON AND HALL TEXTBOOK OF MEDICAL PHYSIOLOGY, THIRTEENTH EDITION  Ganong’s Review of Medical Physiology twenty fourth edition. Thank you