PARTURITION AND LACTATION

PARTURITION

Active labor is characterized by a dramatic increase in the number of oxytocin receptors in the myometrium. Once begun, the process appears to be self-perpetuating. The level of maternal catecholamines increases, resulting in the liberation of free fatty acids, including arachadonic acid; there is also an increase in the level of maternal or fetal cortisol, which decreases the production of uterine smooth muscle prostacyclin. It is unlikely that oxytocin is the initiator of labor despite the fact that oxytocin receptors are present in the myometrium and increase before labor, and it stimulates decidual prostaglandin E2 and prostaglandin F2a production.

Therefore, the prostaglandins (PG) are thought to play the central role. For years, it has been known that rupture, stripping or infection of the fetal membranes, as well as instillation of hypertonic solutions into the amniotic fluid, results in the onset of labor. These facts have led to the hypothesis that a fetal-amniotic fluid-fetal membrane complex is a metabolically active unit that triggers the onset of labor. Evidence supporting a causative role of prostaglandins in the labor process is present since PGs induce myometrial contractions in all stages of gestation. However, direct evidence relating endogenous PGs to labor is not clear. Important to this hypothesis is the understanding that at least one mechanism in the onset of parturition is the release of stored precursors of PGs from the fetal membranes.

The major precursor for PGs is arachadonic acid, which is stored in glycerophospholipids. The fetal membranes are enriched with two major glycerophospholipids, phosphatidylinositol and phosphatidylethanolamine. As gestation advances, the progressively increasing levels of estrogen stimulate the storage, in fetal membranes, of these glycerophospholipids containing arachadonic acid.

A series of fetal membrane lipases, including phospholipase A2 and Phospholipase C control the release of arachadonic acid from storage in fetal membrane phospholipids. Once in a free state, arachadonic acid is available for conversion to PG. Additional factors that augment and accentuate the normal process of labor include the liberation of corticosteroid by the mother and fetus, resulting in a decrease in the production of myometrial prostacyclin, a smooth muscle relaxant.

There is no reduction in maternal or fetal progesterone levels during spontaneous labor. Undoubtedly, progesterone is important in uterine quiescence because in the first trimester removal of the corpus luteum leads rapidly to myometrial contractions . Likewise, labor ensues following the administration of progesterone receptor antagonists in the third trimester. The anti-progesterone agents occupy progesterone receptors and inhibit the action of progesterone, which is clearly essential for maintenance of uterine quiescence. Yet, progesterone administration to women, except for very large doses, does not suppress uterine contractions once begun.

The ratios of estradiol and progesterone in various animal models are closely related to the stimulation of myometrial gap-junction formation. With decreasing progesterone relative to estradiol, gap junctions permit cell-cell communication for the synchronized myometrial smooth muscle contractions required for labor. Progesterone and the estrogens are antagonistic in the parturition process. Progesterone produces uterine relaxation, stabilizing lysosomal membranes and inhibiting prostaglandin synthesis and release. By contrast, estrogens destabilize lysosomal membranes and augment the synthesis of prostaglandin and their release . Although gradual increase in umbilical cord DHEAS and maternal estriol occurs toward term, there is no corresponding drop in either fetal or maternal progesterone concentrations.

The roles of estrogen include regulation of events leading to parturition because pregnancies are often prolonged when estrogen levels in maternal blood and urine are low, as in placental sulfatase deficiency or when associated with anencephaly. Feto-placental estrogens are closely linked to myometrial irritability, contractility, and labor. In primates, estrogens ripen the cervix, initiate uterine activity, and established labor. Estrogens also increase the sensitivity of the myometrium to oxytocin by augmenting prostaglandin biosynthesis. Because placental release of estrogens is linked to the fetal hypothalamus, pituitary, adrenals, and placenta the fetal pituitary adrenal axis appears to fine-tune parturition timing in part through its effect on estrogen production.

In human studies, there is a correlation in uterine activity with circulating maternal estrogens and progesterone as labor approaches. There is firm evidence of increasing, rhythmical fetal adrenal and placental steroid output over the 5 weeks just before term that is important in preparing human pregnancy for the final cascade of oxytocin and prostaglandins that stimulate labor.

 

Parturition – control

· Parturition requires myometrial contractions and cervical softening.

· The myometrium is a functional syncytium (coupled by gap junctions) so contractions are coordinated. Contractions occur because action potentials cause Ca2+ influx; oestrogen-primed myometrium shows pacemaker potentials, which initiate APs if they are large enough. Prostaglandins liberate Ca2+ from intracellular binding sites. Oxytocin increases the rate of Ca2+ influx and lowers the excitation threshold of the myocyte.

· Cervical softening. The cervix has a high connective tissue content – collagen bundles in a proteoglycan matrix. Cervical softening occurs because there is a loss of collagen and an increase in glycosoaminoglycans (GAGs), specifically keratan sulphate. (This does not bind collagen well; dermatan sulphate does, so the increased keratan:dermatan sulphate ratio may explain the “loosening” of the collagen.) Prostaglandins (PGE2; PGF2alpha ) increase cervical compliance.

· The endometrium is probably the most important site of prostaglandin (PG) synthesis. Remember that PGs are local hormones. The enzyme PLA2 is critical in PG synthesis. The estrogen:progesterone ratio controls both PG synthesis (high E:P ------> high prostaglandin synthesis) and also PG release : estradiol    ­oxytocin receptors in the endometrium ------> more PG release.

 

· Oxytocin is a peptide synthesized in hypothalamic neurons and transported along their axons to the posterior pituitary. It is released in response to tactile stimulation of the reproductive tract, esp. the cervix, and increases myometrial contractility (the Ferguson reflex). This reflex is facilitated by a high O:P ratio.

· Relaxin is a polypeptide hormone made by the corpus luteum and secreted throughout most of pregnancy. It causes relaxation of the interpubic ligament, so the pubic symphysis can separate to some extent. (This can cause considerable discomfort during pregnancy.) Its precise action in the human is not certain, but in addition to ligamentous relaxation it may also inhibit myometrial activity, inhibit oxytocin secretion and facilitate cervical softening. It therefore helps to maintain pregnancy but gets things ready for parturition.

Parturition – timing

· Human pregnancy normally lasts 40 weeks from the date of the last menstrual period.1 Forty weeks is “full term”. Labor is considered premature if it occurs before 37 weeks’ completed gestation; after 42 weeks pregnancy is considered prolonged. (Prematurity is not the same as low birth weight, though obviously premature babies tend to be small.)

· Human: unclear. Simple anencephaly (= no head and no CRH) does not necessarily prolong pregnancy; nor does fetal adrenal hypoplasia; infusion of ACTH or synthetic glucocorticoids does not induce parturition. (Nevertheless, cortisol does rise in late pregnancy.) It seems that the onset of labor is not critically dependent on fetal adrenal activity

– and there are no clear and consistent changes in placental estrogen synthesis at term. The E:P ratio does rise, but the change is neither abrupt nor universal. Might local steroid receptor changes be having the same effect? Unproven. PGF2 levels do rise in amniotic fluid before labour, and rise throughout parturition. So the changes in cortisol and PGF2alpha are the same as in sheep and goats, but the control mechanisms are unknown.

Dates always refer to the LMP, which is a useful clinical convention, but note that this means the true gestation (from ovulation) is 38 weeks.

 

Parturition – mechanism

· From ~36 weeks, the uterus occasionally contracts, weakly – Braxton-Hicks contractions. They can be felt – the uterus hardens. The head descends into the pelvis late in pregnancy, and “engages” there.

· Cervical ripening consists of softening, effacement (its length is obliterated) and dilatation.

· Labor begins when (a) uterine contractions are regular and (b) there is progressive dilatation of the cervix. Labor is divided into three stages.

· First stage of labor. From the onset of labor to full dilatation of the cervix (10 cm). Mean duration ~8h (primagravidae2) or ~5h (multiparous woman). The myometrium contracts, generating intrauterine pressures of 50–75 mmHg. The wave of excitation spreads down from the fundus of the uterus. As the muscle contracts, it also retracts

– it does not relax back to its original length (brachystasis). The lower uterine segment is far less muscular than the rest; no point squeezing at the bottom.

· Pain relief in labour – nitrous oxide (tricky to use correctly because time to action is slow – need to inhale before it hurts), TENS (transcutaneous electrical nerve stimulation; doesn’t work well for serious pain), intramuscular pethidine (mild opiate: if the baby gets some, it will come out doped up and with respiratory suppression, and will need naloxone before it starts to breathe), epidural anaesthesia (excellent but tricky to put in during contractions, involving as it does inserting a large needle temporarily into the epidural space; a plastic cannula is then left behind to infuse local anaesthetic around the nerve roots). Women who have pethidine and then an epidural have a tendency to fall asleep! Why?

· Monitoring in labor – cardiotocography is routine (monitors uterine contractions and fetal heart rate). More invasive monitoring, such as fetal scalp blood sampling, possible but avoided.

· The fetus is metabolically vulnerable during parturition: its own systems are not yet functioning, and maternal support is dwindling. Prolonged labor can result in fetal distress.

· Second stage. From full cervical dilatation to delivery of the fetus. Average duration 40 min (primagravidae) or 20 min (multiparae). The uterus continues to contract and force the fetus through the cervix. The head has been flexing continuously throughout labor. It rotates internally (usually from facing sideways to facing backwards). The head then extends as it passes through the perineum, and rotates externally to face sideways again. The shoulders appear (anterior, then posterior) and finally, the trunk is delivered by lateral flexion.

· At this point, the umbilical cord would be clamped3 and cut, the baby slapped around a bit until it turns pink (operating on lungs now, so O2 tension rises to normal levels) and makes some noise, dried, weighed, tagged, injected with vitamin K, etc.

· Third stage. From delivery of the baby to delivery of the placenta. About 15 min.

· The puerperium is the 6 weeks following birth (hence puerperal fever, now rare).

Parturition – pathology

A good deal can go wrong, and I won’t cover it except to list the top causes of maternal mortality – hypertensive diseases (18%), pulmonary embolism (18%), anaesthesia (14%), amniotic fluid embolism (10%), abortion (8%), ectopic pregnancy (7%) and haemorrhage (7%). In the 1980s the absolute death rate was 8.6 per 100,000.

Perinatal mortality (stillbirths plus first-week neonatal deaths) are mainly due to congenital abnormalities (20%), low birth weight and asphyxia. Absolute rate is about 10 per 1000 births.

The Caesarean section rate is 5–13% of all labours4 and the maternal mortality is 0.33/1000.

LACTATION BIOLOGY

Lactogenesis - Initiation of Lactation

Lactogenesis is the term meaning the initiation of lactation. This it the process of functional differentiation which mammary tissue undergoes when changing from a nonlactating to a lactating state. This process is normally associated with the end of pregnancy and around the time of parturition. Because lactogenesis is particularly dependent upon a specific set of hormones (called the Lactogenic Complex of hormones), mammary tissue from most states of the nonlactating mammary gland also can be made to undergo some degree of lactogenesis by administration of high amounts of those hormones, even in nonpregnant animals.

Defining Principles of Lactogenesis

Lactogenesis is a series of cellular changes whereby mammary epithelial cells are converted from a nonsecretory state to a secretory state.

Lactogenesis occurs by a two stage process :

1. Cytologic and enzymatic differentiation of alveolar epithelial cells. This coincides with very limited milk synthesis and secretion before parturition. Cytological changes associated with stage 1 of lactogenesis are described below. Enzymatic changes include increased synthesis of acetyl CoA carboxylase, fatty acid synthetase, and other enzymes associated with lactation, and increases in the uptake transporter systems for amino acids, glucose, and other substrates for milk synthesis. Note that synthesis of a-lactalbumin, and therefore, lactose synthesis does not begin until stage 2 of lactogenesis. Stage 1 of lactogenesis coincides with the formation of colostrum and immunoglobulin uptake (see The Neonate and Colostrum Lesson).

2. Copius secretion of all milk components. In the cow this begins about 0-4 days before parturition and extends through a few days postpartum. It is not until the release of the inhibitory effects of progesterone on lactogenesis (about 2 days prepartum in many mammals) and the stimulation by the very high blood concentrations of prolactin and glucocorticoids associated with parturition, that copious milk secretion begins (stage 2 of lactogenesis). One exception to this timing is in women. The drop in blood progesterone concentration does not occur in women until parturition, so that the full impact of stage 2 of lactogenesis sometimes does not occur until about 2 days postpartum. In pigs and mice, stage 2 of lactogenesis is occurring immediately prior to and at the time of parturition. It is difficult to get any mammary secretion out of a sow until parturition, whereas in the cow, substantial mammary secretion volume can be collected up to several days prepartum.


Points to consider :

** Lactose synthesis is the key to milk secretion. **

Hormones associated with parturition (decreasing progesterone and increases in glucocorticoid and prolactin) lead to transcription of the a-lactalbumin gene (see diagram below). The a-lactalbumin mRNA is translated at the RER and the a-lactalbumin protein interacts with galactosyltransferase in the Golgi apparatus in synthesis of lactose (see Lactose Synthesis Lesson). Synthesis of lactose osmotically draws water into the Golgi apparatus and secretory vesicles. This process allows for secretion of large amounts of milk and is the most obvious manifestation of stage 2 of lactogenesis. At the same time, synthesis of other milk components is increased.

Hormonal Changes Associated With Lactogenesis

A number of hormonal changes are occurring in the mother's blood around the time of parturition. Some of these hormonal changes are specifically involved in lactogenesis. From the figure, progesterone decreases starting a few days prepartum. Estrogen starts to peak prepartum, which in turn stimulates the periparturitent prolactin secretion. The periparurient prolactin peak is very important to the entire process of lactogensis, especially in initiating copius milk secretion (stage 2 of lactogenesis). Glucocorticoids also peak at parturition. And, there is a growth hormone peak associated with parturition. The content of a-lactalbumin in the mammary tissue is an indicator of lactogenesis.

Progesterone Involvement in Lactogenesis

** Progesterone is the key negative regulator of lactogenesis.


In vivo, progesterone probably works by a) increasing the mammary threshold to response to PRL (there are progesterone receptors in the mammary gland during pregnancy, so it can have a direct effect on the mammary cells); b) altering secretion of PRL from the pituitary; and c) having a direct effect on the mammary cells by occupying glucocorticoid receptors.

The Lactogenic Complex of Hormones

There is considerable species variability in how specific hormones control lactogenesis. Actually, with few exceptions we are really referring to a lactogenic complex of hormones rather than a single hormone that "does it all." Much of what we know comes from in vitro studies where lactogenesis or some phase of lactogenesis is induced in tissue from mid-pregnant primiparous animals (mostly rats and mice are used as experimental models). From all of that work we can conclude that:

In vitro, the lactogenic complex consists of insulin, glucocorticoids and prolactin.

Insulin

Insulin is required in vitro to cause a lactogenic-like response in mammary tissue. Insulin causes the nonsecretory epithelia to undergo one cell division. This cell division seems to be necessary for lactogenesis to occur.

The role of insulin in lactogenesis in vivo is not clear. However, it is known that mammary cells in vivo undergo a large burst of cell division in late pregnancy.

In vivo, IGF-1 (insulin-like growth factor-1) may be the primary mitogen involved in this cell division leading up to lactogenesis, with insulin playing a minor role in this function.

Both insulin and the IGFs may be involved in glucose uptake by the mammary cells. This glucose uptake is of critical importance for lactose synthesis. Insulin also may be directly involved in expression of milk protein genes.

Glucocorticoids

Glucocorticoids are required in vitro for full initiation of milk secretion. There may be several roles of glucocorticoids in lactogenesis. They seem to be involved in development of the RER and other ultrastructural changes in the cells required for massive protein synthesis. They also may be directly involved in transcription of the casein and a-lactalbumin genes.

In vivo:

Adrenalectomy blocks casein synthesis and casein mRNA synthesis. Mammary glucocorticoid receptors increase 3 fold in the second half of pregnancy in mice. Adrenal corticoids, especially the glucocorticoids, synergize with PRL to initiate lactation and are essential in most species for PRL to have an optimal effect in its role in initiating lactation. The effects of administration of ACTH (adrenocorticotropic hormone from the pituitary gland) are mediated by its stimulation of glucocorticoid secretion from the adrenal.

Glucocorticoid concentrations in blood are fairly low during most of pregnancy, but increase markedly during the last few days prepartum. Another consideration when evaluating the effective concentration of glucocorticoids in blood is the concentration of corticoid-binding globulin (CBG), a blood protein that binds to corticoids and prevents them from having their actions on cells. Concentrations of CBG decrease in the prepartum period, thereby increasing available free hormone. Increased uptake of glucocorticoid by the mammary tissue coincides with lactogenesis, although a precise association with the first or second stage of lactogenesis has not been established. Glucocorticoid receptors in the mammary cells increase in numbers in late pregnancy. Both cortisol and PRL are required to maintain glucocorticoid receptor numbers.

Prolactin (PRL)

In vitro, PRL added to tissue cultures containing insulin and glucocorticoids causes transcription of casein and a-lactalbumin genes, translation of milk protein mRNAs, swelling of the Golgi apparatus, and milk protein secretion, as well as synthesis of lactose and milk fat.

From in vitro studies on mid-pregnant mouse and rat mammary tissue cultures we can generalize that:

In most species, PRL binding sites (receptors) in the mammary gland are low during pregnancy and increase coincident with the secretion of milk during the second stage of lactogenesis. So, both the availability of PRL (blood hormone concentrations) and the responsiveness of the mammary epithelial cells to PRL (receptors for PRL) increase about the time of the switch-over from the first to the second stage of lactogenesis (start of copious milk secretion).

Estrogen

Estrogen is not directly invovled in lactogenesis, but indirectly it may have an effect by increasing numbers of PRL receptors in the mammary cells. In vivo, it may also be one factor in controlling PRL secretion from the pituitary.

Growth Hormone

In vitro: There are no GH receptors on the mammary epithelial cells, therefore there should be no in vitro effect. Some reported lactogenic activities of GH (especially human GH, which has some prolactin-like activity) probably arise from the very close PRL-like structure of that GH. That is, GH in those cases is binding to the PRL receptor and acting as if it was PRL.

In vivo: The involvement of GH in lactogenesis is not known. IGF-1 (secreted from the liver in response to GH) is a mammary mitogen. Low concentrations of IGF-1 can replace the very high insulin concentratrions required for in vitro culture systems where lactogenesis is studied.

In hypophysectomized - adrenalectomized - ovariectomized rats (pretreated with E and P4), PRL, GH plus glucocorticoids gives nearly normal initiation of lactation. Again the GH effect may be acting by indirect effects mediated by IGFs.

Hormonal Induction of Lactogenesis

Lactation can be induced in nonpregnant animals by injection of appropriate hormones. This usually involves stimulating some degree of mammary growth by injections of high levels of estrogen and progesterone (see Mammary Gland Development Lesson for the role of estrogen and progesterone in mammary growth), followed by some combination of the glucocorticoid and prolactin component of the lactogenic complex.


Galactopoiesis

Maintenance of Lactation

Galactopoeisis is the maintenance of lactation once lactation has been established. The role of milk removal complicates interpretation of the hormonal requirements for milk synthesis. Without frequent emptying of the mammary gland, milk synthesis will not persist in spite of adequate hormonal status. Conversely, maintenance of intense suckling or milking stimulus will not maintain lactation indefinitely. Nevertheless, suckling or actual removal of milk is required to maintain lactation.

Changes in mammary cell numbers (by growth or by cell death) and yield per cell are regulated in part by galactopoietic hormones and in part by local mammary factors. Both the hormonal and the local factors are regulated by milk removal.

 

Prolactin

Prolactin is a primary component of the galactopoietic complex of hormones.

There is a milking-induced or nursing-induced release of PRL (see graph below; adapted from Tucker 1994). This surge of PRL (green line in the graph) is small compared with the peripartum surge of PRL associated with lactogenesis; about a 3-fold increase over non-stimulated PRL concentrations (blue hatched line). However, the milking-induced PRL surge is a direct link between the act of nursing or milk removal and the galactopoietic hormones involved in maintaining lactation. The surge occurs over a period of about a half of an hour after milking or nursing. This compares with the oxytocin surge which only lasts about 5 to 10 minutes (red stippled box in the figure).

Suckling or milking probably works by decreasing prolactin inhibiting factor (PIF) from the hypothalamus, and therefore increasing PRL levels. It may also act to increase the response of the pituitary to prolactin releasing factors. The effect of suckling on PRL declines with advancing lactation, even if nursing stimulus is kept equivalent throughout lactation.

Prolactin is not the only hormone released in response to suckling. Somatostatin (inhibits growth hormone release from the pituitary), insulin, glucagons, and vasoactive intestinal polypeptide (VIP; a neurogenic peptide occurring in smooth muscles and secretory glands) are transiently increased in response to suckling or teat massage. In the case of VIP, which can act as a vasodilator, release of the peptide in the mammary gland during suckling may affect the tone of the larger ducts which relax during milk ejection. In addition to a potential role in milk ejection, VIP release from the mammary gland at suckling may also affect secretion of pancreatic hormones (insulin and glucagons).

Growth Hormone

Nursing causes increased release of GH from the pituitary. Blood GH decreases with advancing lactation. Thyroid Releasing Hormone (TRH) is the hypothalamic hormone which stimulates release of Thyroid Stimulating Hormone (TSH) from the pituitary. TRH can also release GH from the pituitary. The release of GH from the pituitary in response to TRH administration also decreases with advancing lactation.

Adrenal Corticoids

*** are essential for maintenance of lactation.

Adrenal glucocorticoids can inhibit lactation at high doses. However, at lower doses (resulting in more physiological blood concentrations) exogenous glucocorticoid stimulates milk yield in humans  in early lactation and prevents the expected decline in milk yield in later lactation. 

Thyroid Hormones

*** are essential for maximal secretion of milk.

Feeding thyroprotein (iodinated casein) to cows increases milk yield by 10 % in early lactation and by 15-20% in late lactation.

Maintenance of Lactation

Role of Milk Removal

Galactopoeisis is the maintenance of lactation once lactation has been established. The role of milk removal complicates interpretation of the hormonal requirements for milk synthesis. Without frequent emptying of the mammary gland, milk synthesis will not persist in spite of adequate hormonal status. Conversely, maintenance of intense suckling or milking stimulus will not maintain lactation indefinitely. Nevertheless, suckling or actual removal of milk from the gland is required to maintain lactation.

Milk Removal

*** Milk removal is required for maintenance of lactation.

Nursing or milking stimulus triggers release of galactopoietic hormones which may stimulate the next round of secretory activity, especially prolactin (see the section on Maintenance of Lactation - Hormones). If milk removal is not maintained there is no stimulation of prolactin release.

Acute accumulation of milk in the gland causes an increase in intra-mammary pressure. This increase in pressure activates the sympathetic nerves in the gland, which acts peripherally to decrease mammary blood flow. As mammary blood flow declines, the availability of hormones (eg. PRL) and nutrients to the gland is reduced.

If milk is not removed, the Feedback Inhibitor of Lactation (FIL) accumulates in the alveolar lumen, inhibiting further synthesis and secretion of milk

Autocrine Control of Lactation:

If one nipple side is milked 2X/day and the other side milked only once per day, or if milking is incomplete from one side, milk yield is decreased only in the less frequently emptied gland.

These unilateral effects cannot be attributed to systemic (hormonal) control because both sides of the nipples are exposed to the same concentrations of galactopoietic hormones.

These types of observations gave rise to the hypothesis that a milk constituent acts as an inhibitor of milk secretion and that removal of this inhibitor at milking regulates the rate of milk secretion.

Each time milk is removed:

Prolactin release is stimulated
Intra-mammary pressure is relieved
FIL is removed from the alveoli

If milk is not removed:

There is no stimulation of PRL release
There is an acute accumulation of milk in the gland, resulting in:
 

Increased intra-mammary pressure
Activation of sympathetic nerves
Decreased mammary blood flow
Decreased availability of hormones and nutrients to the gland

Rate of milk secretion declines

The gland is under the influence of the systemic factors shortly after milking and maximal secretion rate is achieved. This gradually slows as the role of the local factors becomes dominant. If milk is not removed, then the secretion rate will eventually drop to zero (see below), however, under normal nursing or milking intervals the secretion rate does not go to zero. Once milk is removed, the cycle begins again.

Milk Secretion Rate

Milk yield is dependent on (1) the amount of secretory tissue and (2) the rate of milk secretion (per unit of time). Secretion rate is affected by the accumulation of milk in the alveolar lumen. Accumulation of milk in the lumen increases the intra-mammary pressure. Once the intra-mammary pressure reaches a certain level (probably about 8 to 10 hours after the last milking), secretion rate declines.

Milking Interval

Milk composition is different a various intervals after the last milking

Milking Frequency

Stage Time Mechanism Response
1 Immediate
(hours to days)
removal of chemical feedback inhibitor increased milk secretion
2 Short Term
(days to weeks)
stimulation of cell differentiation increased milk secretion
3 Long Term
(weeks to months)
stimulation of cell proliferation increased milk secretion

Milk Ejection

Oxytocin

Oxytocin is a 9 amino acid long peptide [Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-Gly]. It has a molecular mass of 1007 daltons. Oxytocin has a disulfide bond between the two cysteines. Reduction of the disulfide bond inactivates oxytocin. One IU (international Unit) is approximately 2 micrograms of pure peptide.

Hypothalamic Nuclei and Oxytocin Synthesis

Oxytocin is syntheized in the hypothalamus in specific nuclei, the paraventricular nucleus and the supraoptic nucleus in the hypothalamus. [A cluster of nerve cells in the brain is often called a nucleus. This is different from the nucleus of a single cell.] Neurons in these hypothalamic nuclei synthesize the oxytocin precursor and package it into vesicles. Oxytocin is initially synthesized as a large molecular weight precursor which also consists of the oxytocin-carrier peptide neurophysin. The precursor is proteolytically cleaved in the neuron in the oxytocin-containing vesicle to yield oxytocin bound to neurophysin. The oxytocin-neurophysin complex is the intracellular storage form of oxytocin. The oxytocin-containing vesicles are transported from the cell body (which is in the hypothalamus), down the axons to the neuron endings in the posterior pituitary. This is called the hypothalamo-neurohypophysial tract. The oxytocin-neurophysin complex is stored in neurosecretory granules called herring bodies in the axon ending.

The synthesis of oxytocin in the cell bodies and its transport to the axon endings occur separately from the milk ejection reflex.

Milk Ejection Reflex

The milk ejection reflex is a neuroendocrine reflex. The reflex has an afferent pathway (conducted from the teats to the brain via neurons) and an efferent pathway (conducted from the pituitary to the mammary gland via blood-borne hormones).

Afferent Pathway:

The greatest amount of innervation in the mammary gland is in the teats, where there are pressure sensitive receptors in the dermis. Mechanical stimulation of the teats activates pressure sensitive receptors in the dermis where the pressure is transformed into nerve impulses that travel via the spinothalamic nerve tract to the brain. These nerves synapse in the paraventricular nucleus and in the supraoptic nucleus in the hypothalamus. When the cell bodies of the oxytocin-containing neurons are stimulated by these impulses originating in the teat, an action potential moves down the oxytocin-containing neurons from the cell body in the hypothalamus down the axon to the neuron ending in the posterior pituitary. This causes release of oxytocin and neurophysin into the blood. The efferent pathway starts at this point.

Efferent Pathway:

The efferent pathway begins with the release of oxytocin into the blood. The oxytocin then travels to the mammary gland via the blood, binds to oxytocin receptors on the myoepithelial cells, causing the myoepithelial cells to contract, and resulting in increased intra-lumenal (intramammary) pressure and ejection of milk from the alveolar lumen.

Oxytocin receptors are associated with the myoepithelial cells, not the smooth muscle of the mammary gland.

Involvement of Autonomic Nervous System and Stress

The autonomic nervous system is part of the central nervous system. It mainly controls viseral function. The autonomic nervous system is made up of two types of nerves, the parasympathetic nerves and the sympathetic nerves.

Parasympathetic nerves: The neurotransmitter of parasympathetic nerves is acetylcholine.

There is no parasympathetic innervation in the mammary gland.

Sympathetic nerves: The neuroendocrine components of sympathetic nerves are epinephrine and norepinephrine. Epinephrine (adrenaline) is primarily from adrenal medulla. Norepinephrine is a neurotransmitter from peripheral nerves and nerves in the brain. Norepinephrine also can from the adrenal medulla.

The effect of sympathetic nerves on milk ejection depends upon the type of neurotransmitter receptor:
Generally,

- alpha-receptors are vasoconstrictive.
- norepinephrine can stimulate milk ejection via brain alpha-receptors.
- norepinephrine can inhibit milk ejection via brain beta-receptors.
- it is the location of these receptors that is the most important thing.

Most sensory receptors (neurons) are located in the teat. There are pressure-sensitive neurons around the cisterns and the large ducts.

*** There is no direct innervation of alveoli or myoepithelial cells!

If you electrically stimulate the cut end of the mammary nerves, you do not get milk ejection.

Norepinephrine and epinephrine can inhibit oxytocin-induced contraction of myoepithelial cells.

Stressful stimuli will inhibit milk ejection. This occurs via epinephrine or norepinephrine derived from the adrenal gland or the sympathetic nerves by the following mechanisms :

Emotional disturbances can cause inhibition of the CNS part of the milk ejection reflex


Colostrum vs. Milk

Colostrum is produced by the udder immediately after parturition. The composition of colostrum is considerably different from the composition of normal milk. Three to 5 days immediately postpartum is needed for the secretions to change to the composition of milk. During this period the total solids are elevated, especially the immunoglobulins. Newborn calves are practically devoid of immunoglobulins, the antibodies against various disease organisms. Calves must ingest the immunoglobulins from colostrum to acquire a passive immunity against common calfhood diseases. Feeding colostrum after birth is especially critical during the first 24 hours of a calf's life. After this time, enzymes in the digestive tract degrade the antibodies and the permeability of the gut to antibodies decreases.

Lactose content is depressed in colostrum compared with milk, whereas fat and casein percentage is rather variable. High lactose in the intestine can cause scours in calves, and presumably the reduced lactose content of colostrum helps to prevent this disease.

Calcium, magnesium, phosphorus, and chloride are high in colostrum, potassium is low. Iron is 10 to 17 times greater in colostrum than in normal milk. This high level of iron is needed for the rapid increase in hemoglobin in the red blood cells of the newborn calf.

Colostrum is a good source of vitamins for the neonate. The newborn calf is practically devoid of vitamin A which provides a degree of protection against various diseases. Colostrum contains 10 times as much vitamin A and 3 times as much vitamin D as that found in normal milk.


Milk Composition
 

Water

Water content of milk is dependent upon the synthesis of lactose. Without some water in the milk, milk would be a viscous secretion composed mostly of lipid and protein and would be extremely difficult to remove from the gland. Upon birth, the mammalian neonate is not able to seek out its own water supply and would dehydrate rapidly without the water component of milk.

Carbohydrate

Lactose is the major carbohydrate in the milk of most species. Lactose is a disaccharide composed of the monosaccharides D-glucose and D-galactose, joined in a ß-1,4-glycosidic linkage. The chemical name for lactose is 4-0-ß-D-galactopyranosyl-D-glucopyranose. It is essentially unique to milk, although it has been identified in the fruit of certain plants. Of the mammalian species where information is available, only some marsupials have an alternative sugar other than lactose, and those sugars are generally trisaccharides of glucose and galactose.

Lactose plays a major role in milk synthesis. It is the major osmole in milk and the process of synthesis of lactose is responsible for drawing water into the milk as it is being formed in the mammary epithelial cells. Because of the close relationship between lactose synthesis and the amount of water drawn into milk, lactose content is the least variable component of milk (see the table below).

Lactose is not as sweet as other disaccharides such as sucrose (a fructose-glucose disaccharide), or the monosaccharides fructose or glucose. Lactose is cleaved to glucose and galactose in the intestine of the neonate by an enzyme activity called lactase (or ß-galactosidase). The galactose is then converted to another glucose by a different enzyme. Lactose is a major, readily digestible source of glucose which provides energy for the neonate. Lactose intolerance can occur in adult animals or animals who do not have lactase activity in their intestines.

Milk Fat

The fat component of milk is composed of a complex mixture of lipids. Triglycerides are the major type of lipid in milk fat. Triglycerides are composed of three fatty acids covalently bound to a glycerol molecule by ester bonds. Milk fat is the major source of lipid used by the neonate mammal for accumulating body adipose in the initial days after birth. Most mammalian neonates are born with little body adipose that might be used for insulation or as a source of stored energy. A few days after birth most neonates begin to be able to metabolize milk fat as an energy source.

Milk fat is secreted from mammary epithelial cells as fat globules which are primarily composed of a globule of triglyceride surrounded by a lipid bilayer membrane with some similarities to the apical membrane of the epithelial cells. This fat globule membrane helps to stabilize the fat globules in an emulsion within the aqueous environment of milk . Lipid has a lower buoyant density than water, so when raw milk is centrifuged the fat rises to the top resulting in the cream layer. There are so many fat globules that they also carry some of the milk protein to the top, so cream also contains a small amount of protein in addition to the milk fat component; this protein component contributes to the whipping characteristics of cream.

Milk Protein

The total protein component of milk is composed of numerous specific proteins. The primary group of milk proteins are the caseins. There are 3 or 4 caseins in the milk of most species; the different caseins are distinct molecules but have similarities in structure. Caseins have an amino acid composition which is important for growth and development of the nursing young. Caseins are fairly easily digestible in the intestine compared with many other food proteins available. This high quality, easily digestible protein in cow milk is one of the key reasons why milk is such an important human food.

Casein is composed of several similar proteins which form a multi-molecular, granular structure called a casein micelle. In addition to casein molecules, the casein micelle contains water and salts (mainly calcium and phosphorous). Some enzymes are associated with casein micelles, too. The micellar structure of casein in milk is an important part of the mode of digestion of milk in the stomach and intestine, the basis for many of the milk products industries. Casein is one of the most abundant organic components of milk, along with the lactose and milk fat. Individual molecules of casein alone are not very soluble in the aqueous environment of milk. However, the casein micelle granules are maintained as a colloidal suspension in milk. If the micellar structure is disturbed, the micelles may come apart and the casein may come out of solution, forming the gelatinous material called the curd. This is part of the basis for formation of all non-fluid milk products. Because the casein micelle is in suspension, it can be separated from the rest of milk by centrifugation at a very high speed.

There are many whey proteins in milk and the specific set of whey proteins found in mammary secretions varies with the species, the stage of lactation, the presence of an intramammary infection, and other factors. The major whey proteins are ß-lactoglobulin and a-lactalbumin. a-Lactalbumin is an important protein in the synthesis of lactose and its presence is central to the process of milk synthesis. ß-Lactoglobulin's function is not known. Other whey proteins are the immunoglobulins (antibodies; especially high in colostrum) and serum albumin (a serum protein). Whey proteins also include a long list of enzymes, hormones, growth factors, nutrient transporters, disease resistance factors, and others.

Caseins are highly digestible in the intestine and are a high quality source of amino acids. Most whey proteins are relatively less digestible in the intestine, although all of them are digested to some degree. When substantial whey protein is not digested fully in the intestine, some of the intact protein may stimulate a localized intestinal or a systemic immune response. This is sometimes referred to as milk protein allergy and is most often thought to be caused by ß-lactoglobulin. Milk protein allergy is only one type of food protein allergy.

Minerals

The major minerals found in milk are calcium and phosphorous. These minerals are required in large quantities by the rapidly growing neonate for bone growth and development of soft tissues. They are both mostly associated with the casein micelle structure..

Other Components in Milk

Milk contains all the major vitamins. The fat soluble vitamins A, D, E, and K, are found primarily in the milk fat; milk has only limited amounts of vitamin K. The B vitamins are found in the aqueous phase of milk

Milk always contains leukocyte cells, also known as somatic cells. The concentration of leukocytes in milk varies with the species (human milk has relatively high somatic cell counts; cow milk from healthy glands has low somatic cell counts), infection status of the gland, and stage of lactation.

Milk has numerous other components, many of which are grouped in the major biochemical components listed above. These may include bioactive factors such as hormones and growth factors, enzymes, cellular proteins, and others.

Dr Mahmoud Ahmad Fora

Last Updated Mar 25, 2006