Wagner-Lactation

Front 1

describe the anatomical considerations of the lactating breast

Back 1

The functional milk-producing unit of the breast is the alveolus; the inner lining of this structure is composed of epithelial cells that secrete milk into the alveolar lumen; the milk is the drained into a collecting duct and transported to the nipple Alveoli have an external layer of myoepithelial cells that play a role in milk ejection; alveoli are arranged in clusters known as lobules

Front 2

describe the hormonal inputs associated with lactation

Back 2

During gestation, the maternal hormonal environment converts the breast into an internal glandular structure that is capable of milk production midway through gestation
a. estrogen-promotes breast duct development
b. progesterone-stimulates alveolar-lobular development
c. hCS-stimulates the synthesis of enzymes involved in milk production
e. cortisol, growth hormone & insulin-these hormones potentiate the lobuloalveolar development induced by estrogen, progesterone, prolactin and hCS

Front 3

describe the role of prolactin in lactation

Back 3

secreted by the anterior pituitary; induces the synthesis of enzymes necessary for the production of breast milk
receptor-mediated signal transduction-upon activation by prolactin, the receptor dimerizes in the plasma membrane and initiates intracellular signaling through its tyrosine kinase capability

Front 4

describe the JAK-Stat pathway

Back 4

JAK is a tyrosine kinase associated with the prolactin receptor; activation of the prolactin receptor leads to phosphorylation of tyrosine residues on JAK, which activates JAK’s inherent tyrosine kinase activity
Activated JAK phosphorylates tyrosine residues on Stat (signal transducer and activator of transcription); phosphorylated Stat dissociates from the receptor, dimerizes and translocates into the nucleus, where it activates the transcription of specific genes

Front 5

describe the regulation of prolactin secretion

Back 5

estrogen stimulates the synthesis and secretion of prolactin from lactotrophs in the anterior pituitary; prolactin is under tonic inhibition by dopamine released from tuberoinfundibular nerves in the mediobasal hypothalamus
Dopamine is carried via the hypophysial portal vasculature to the anterior pituitary, where it activates dopamine D2 receptors on the lactotroph plasma membrane; this leads to an increase in K+ efflux and a reduction in Ca2+ influx, resulting in a membrane hyperpolarization and a decrease in prolactin release

Front 6

describe the role of prolactin in the maintainence of lactation after pregnancy

Back 6

After parturition, maternal levels of estrogen and progesterone decline; these hormones antagonize the stimulatory effects of prolactin on breast milk production; clearance of progesterone removes the inhibitory effect on the production of alpha-lactalbumin in the RER of the alveolar epithelial cells
Prolactin stimulates alpha-lactalbumin production, which stimulates lactose synthase and increases milk lactose
Prolactin stimulates milk secretion into the alveolar lumen; afferent stimulation of the nipple caused by infant suckling triggers prolactin release by decreasing the excitability of tuberoinfundibular nerves, thereby decreasing the release of dopamine and its inhibitory influence over prolactin secretion

Front 7

describe the suckling reflex and the role of oxytocin in the maintainence of lactation after pregnancy

Back 7

suckling reflex-afferent stimulation of mechanoreceptors caused by infant’s suckling of the nipple triggers a neuroendocrine reflex that results in an increased secretion of prolactin (anterior pituitary) and oxytocin (posterior)
role of oxytocin-released from magnocellular neurons of the hypothalamic supraoptic and paraventricular nuclei in response to afferent stimulation of the nipple by suckling; causes milk ejection by eliciting contraction of the alveolar myoepithelial cells, pushing the milk through the ducts toward the nipple (milk letdown); this reflex can be conditioned to other stimuli, such as the cry of the infant, causing milk to leak from the nipple

Front 8

describe the composition of breast milk

Back 8

1. Nutrients-water, carbohydrates (lactose), protein, fat and minerals
2. Immunological factors-contains B and T cells, macrophages, and neutrophils; confers protective advantage to breast-fed infants
a. secretory IgA-helps protect infants against enteric infections (E. Coli); IgA disrupts the ability of intestinal bacteria from adhering to the cell surfaces of luminal epithelial cells, thereby preventing systemic invasion
b. mucus-helps to emulsify bacteria residing in the intestinal lumen of the infant, thereby hindering bacterial attachment to epithelial cell membranes and crossing of the intestinal mucosa
c. lactoferrin-sequesters iron present in the infant’s intestinal lumen, thereby decreasing the availability of this mineral essential for the growth and replication of intestinal pathogens
d. bifidus factor-helps promote the growth and development of Lactobacillus bifidus; provides the newborn beneficial intestinal flora
e. colostrum-secreted for the first 5 days postpartum; lemon-yellow in color, and contains higher concentrations of minerals (Na and Cl) and immunoglobulin proteins, but less lactose, and fat content

Front 9

describe the effects of lactation on the reproductive cycle

Back 9

1. GnRH secretion-episodic prolactin pulses that occur in response to the suckling of the infant serve to inhibit the reproductive cycle of the nursing mother; due to prolactin’s ability to decrease GnRH release from the hypothalamus and antagonize the effects of GnRH on gonadotropin secretion from the anterior pituitary
2. antagonizes the ovarian actions of gonadotropins
3. steroid hormone secretion and ovulation-compromised ovarian effectiveness of the gonadotropins results in a decreased output of estrogen and progesterone, thereby minimizing the possibility of ovulation (anovulation in 1st 6 months postpartum)
4. Hastens involution-the suckling-induced increase in oxytocin release expedites involution due to the increased contractility of the uterine myometrium

Front 10

describe cessation of lactation

Back 10

As the infant is weaned, the suckling-induced increases in prolactin and oxytocin secretion occur w/less frequency; as a result, there is less milk production, ejection and letdown; the breast milk begins to accumulate in the lumen of the alveoli, and the resultant increase in hydrostatic pressure decreases further milk production