Lactation Physiology

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Functional Units of the Mammary Gland

The functional units of the mammary gland are the al-veoli (Illustration 6.1). Each alveolus is composed of a cluster of cells (secretory

cells) with a duct in the center, whose job it is to secrete milk. The ducts are arranged like branches of a tree, each smaller duct leading to six to ten larger collecting ducts. These branchlike collecting ducts lead to the nipple. Myoepi-thelial cells surround the secretory cells. Myoepi-thelial cells can contract under the influence of oxy-tocin and cause milk to be ejected into the ducts.

Mammary Gland Development

During puberty, the ovaries mature and the release of estrogen and progesterone increases (Table 6.3). The cyclic release of these two hormones governs pubertal breast development

(Il-lustration 6.2). The mam-mary gland develops its lobular structure (lobes)

Mammary Gland The source of milk for offspring, also commonly called the breast. The presence of mammary glands is a characteristic of mammals.

Alveoli A rounded or oblong-shaped cavity present in the breast.

Secretory Cells Cells in the acinus (milk gland) that are responsible for secreting milk components into the ducts.

Myoepithelial Cells Specialized cells that line the alveoli and that can contract to cause milk to be secreted into the duct.

Oxytocin A hormone produced during letdown that causes milk to be ejected into the ducts.

Lobes Rounded structures of the mammary gland.

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Skin

Adipose tissue

Ribs

Intercostal muscles Lobe

Lactiferous duct Opening of lactiferous duct Nipple Areola

Lobule containing alveoli Pectoralis major

(muscle)

Suspensory (Cooper’s) ligament

Illustration 6.1 Breast of a lactating female.

This cut away view shows the mammary glands and ducts.

under the cyclic produc-tion of progesterone and is usually complete within 1 2 t o 1 8 m o n t h s a f t e r menarche. As the ductal system matures, cells that can secrete milk develop, the nipple grows, and its pig-mentation changes. Fibrous and fatty tissues increase around the ducts.

In pregnancy, the luteal and placental hormones (placental lactogen and chorionic gonadotropin) allow further preparation for breastfeeding (Illustration 6.2).

Estrogen stimulates development of the glands that will make milk. Progesterone allows the tubules to elongate and the cells that line the tubules (epithelial cells) to duplicate.

Table 6.3 Hormones contributing to breast development and lactation

Hormone Role in Lactation Stage of Lactation

Estrogen Ductal growth Mammary gland differentiation with

menstruation

Progesterone Alveolar development After onset of menses and during pregnancy Human growth hormone Development of terminal end buds Mammary gland development

Human placental lactogen Alveolar development Pregnancy Prolactin Alveolar development and milk

secretion

Pregnancy and breastfeeding (from the third trimester of pregnancy to weaning) Oxytocin Letdown: ejection of milk from

myopithelial cells

From the onset of milk secretion to weaning

Lactogenesis

Breast milk production, or lactogenesis, is classically described as occurring in three stages.¹¹ The first stage, or lactogenesis I, begins during the last trimester of preg-nancy; the second and third stages (lactogenesis II and III) occur after birth. Lactogenesis I may be impacted by premature delivery, method of delivery, and other fac-tors. These may explain why mothers who deliver pre-maturely are often unable to develop a full milk supply (25–35 ounces per day).

Lactogenesis I.

● During the first stage of milk production, milk begins to form, and the lactose and protein content of milk increase. This stage extends through the first few days postpartum.

Lactogenesis II.

● This stage begins 2–5 days post-partum and is marked by increased blood flow to the mammary gland. Clinically, it is considered the onset of copious milk secretion, or “when milk comes in.” Significant changes in both the milk composition and the quantity of milk that can be produced occur over the first 10 days of the baby’s life.

Lactogenesis III.

● This stage of breast milk

production begins about 10 days after birth and is the stage in which the milk composition be-comes stable.¹²

Hormonal Control of Lactation

Prolactin and oxytocin are necessary for establishing and maintaining a milk supply. Prolactin is a hormone that stimulates milk production. Suckling is a major stimulator of prolactin secretion: prolactin levels double with suck-ling.¹³Stress, sleep, and sexual intercourse also stimulate prolactin levels. To prevent milk production in the last 3 months of pregnancy, prolactin activity is suppressed by a prolactin-inhibiting factor that is released by the hypothalamus. This inhibition of prolactin allows the Lactogenesis Another term for human

milk production.

Prolactin A hormone necessary for milk production.

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During puberty, a system of ducts, lobes, and alveoli develops.

This system remains inactive until pregnancy.

During pregnancy, growth proliferates, with ductal branching and lobular-alveolar development proceeding at a spectacular rate, yet in an orderly fashion.

Illustration 6.2 Breast development from puberty to lactation.

mother’s body to prepare for milk production during pregnancy. The actual level of prolactin in the blood is not related to the amount of milk made, but prolactin is nec-essary for milk synthesis to occur.¹⁴

Oxytocin release is also stimulated by suckling or nip-ple stimulation. Its main role is in letdown, or the ejection

of milk from the milk gland (acinus) into the milk ducts. Women may experience tingling or sometimes sharp shooting pain that lasts about a minute and corresponds with con-tractions in the milk ducts. Oxytocin also acts on the uterus, causing it to contract, seal blood vessels, and shrink its size.

Secretion of Milk

Although the process of milk pro-duction is complex, understanding the basic mechanisms of milk secre-tion is important to understanding how factors such as nutritional sta-tus, supplementation, medications, and disease may affect breastfeeding or milk composition. As described by Neville et al., the secretory cell in the breast uses five pathways for milk secretion (Illustration 6.3).¹⁵ Briefly, some components like lactose are made in the secretory cells and se-creted into ducts. Water, sodium, po-tassium, and chloride are able to pass through alveolar cell membranes in either direction (passive diffusion). Milk fat comes from triglycerides from the mother’s blood and from new fatty acids produced in the breast. Fats are made soluble in milk by addition of a protein carrier to form milk-fat globules.¹⁶ The milk-fat globules are then secreted into the ducts. Immunoglobulin A and other plasma proteins are Illustration 6.3 The pathways for secretion of milk components.

Lactose Ca²⁺, PO₄ Citrate Milk protein

Closed in full lactation

Open in pregnancy

Lipids H₂O

Na K Cl

IgA Other plasma proteins?

Cells Na

Plasma protein?

Transcellular pathways

Capillary BM

Paracellular pathway

II III IV V

? Golgi

RER MFG

MFG = milk-fat globule SV = secretory vesicle ER = rough endoplasmic reticulum BM = basement membrane

source: From Lactation: Physiology, Nutrition, & Breastfeeding by J. C. Allen and C. Watters, pp. 49–102.

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captured from the mother’s blood and taken into the al-veolar cells. These proteins are then secreted into the milk ducts.

The Letdown Reflex

The letdown reflex stimulates milk release from the breast. The stimuli from the infant suckling are passed through nerves to the hypothalamus, which responds by promoting oxytocin release from the posterior pituitary gland (Illustration 6.4). The oxytocin causes contraction of the myoepithelial cells surrounding the secretory cells.

As a result, milk is released through the ducts, making it available to the infant. Other stimuli, such as hear-ing a baby cry, sexual arousal, and thinkhear-ing about nurs-ing, can also cause letdown, and milk will leak from the breasts.

Dalam dokumen Through the Life Cycle (Halaman 189-192)