Lactation

Mammary Gland and its Development

The development of mammary glands, mammogenesis, begins along the ventral surface of the developing conceptus. Beginning as mammary ridges, they develop into the primary mammary bud which pushes into the underlying dermis, creating the secondary mammary buds. As the secondary mammary buds lengthen and branches, the branches canalize and form tiny ducts in each bud which later develop into a lumen. At birth, this structure is known as lactiferous ducts which are larger ducts that empty in the teat. Between birth and puberty, the mammary glands undergo isometric growth and no alveoli are present. During puberty, the glands experience allometric growth and a duct and alveolar framework is constructed, yet no alveoli develop yet. The ducts branch and the diameter increases due to estradiol present from estrous cycles and progesterone allows the terminal portions of each branch to initiate alveolar development. Prolactin and growth hormone are necessary alongside estradiol to allow for rapid duct development. The presence of prolactin and growth hormone increase during the onset of puberty. Since the mammary framework is not yet complete during puberty, complete alveolar development occurs during pregnancy. Lobules begin to develop from the terminal alveoli during the middle of pregnancy. In the late stage of pregnancy, the lobulo-alveolar structures develop. Prolactin, adrenal cortisol and placental lactogen all play a role in this stage for milk synthesis. Around this time, the two stages of lactogenesis also occur with stage I including differentiation and stage II including secretory activity. When parturition occurs, the neonate will be able to suckle and derive the necessary nutrients from the milk (1). Hormones such as prolactin, GnRH, growth hormone, thyroid hormone and the chemicals responsible for their release are all found within milk. Along with that, colostrum, or the first mammary gland secretions, are necessary to the neonate since immunoglobulins can be found in colostrum, which is provided two to three days after parturition. Cows have four mammary glands each respectively leading into a teat with a canal present in each. Neural sensory stimulations such as the sight of a calf, the sound of the milking machine in a parlor, or massaging of the udder all offer enough stimulation to cause the hypothalamus to release oxytocin into the bloodstream. Oxytocin causes the myoepithelial cells in the mammary glands to contract and squeeze the alveoli. This allows milk to flow from the cavity of the alveoli and into the ducts which lead into the gland cistern and the teat, shown in figure 15.