Lactation Physiology(part 2)

By: A. Riasi (PhD in Animal Nutrition &

Physiology)

فیزیولوژی تولید و ترشح شیر

At the end of this section students will be able to reply

How is the blood flow of mammary gland?

What is the importance of the udder lymphatic network?

What is the neuroendocrine reflex of milk ejection?

How is Immunoglobulin transport in the mammary gland?

What is galactopoeisis?

How change the mammary gland physiology during the dry period?

What are the allometric and isometric growth of mammary gland?

What is the role of hormones in mammogenesis?

Mammary blood supply

Milk synthesis rate depend to the rate of blood flow of udder.

Blood flow in the mammary gland increase before parturition.

The efficiency of extraction of the components from the blood

while it passes through the udder is more important.

Mammary blood supply

Mammary lymphatic network

Mammary nervous system

The efferent innervation of the mammary gland is entirely

sympathetic in origin.

Innervation of the udder is sparse compared with other tissues.

Sensory nerves are involved in milk ejection and found in the teats

and skins.

There is no parasympathetic innervation to the gland.

Sympathetic nerves are associated with the arteries but not with

alveoli.

There is no innervation of the secretory system.

Few nerves go to the interior of the udder.

Milk ejection

Oxytocin has the main role in milk ejection and causes

contraction of the myoepithelial cells.

The time from the start of a tactile stimulation until the

occurrence of milk ejection is different.

Milk ejection reflex actually is a neuroendocrine reflex.

The reflex has two pathways:

Afferent Pathway (neural)

Efferent Pathway (hormonal, blood-borne)

Milk ejection

Other mechanisms of milk ejection:

Myoepithelial cells will also contract in response to vasopressin

(ADH or antidiuretic hormone).

Milk ejection may be a condition response.

Stimulation of the genital tract such as vaginal distention causes

release of large amounts of oxytocin.

The mechanical tap stimulus does not involve oxytocin.

Effect of stress on milk ejection

Various stressful stimuli that inhibit milk ejection are associated

with increased activity of the sympathetic nervous system.

Role of autonomic nervous system

Sympathetic nerves, The neuroendocrine components of

sympathetic nerves are:

Epinephrine

Norepinephrine

Colostrum production

Colostrum has larger amounts of specific proteins than milk:

Immunoglobulins

Antimicrobial peptides (lactoferrin and lactoperoxidase)

Other bioactive molecules, including growth factors

Under certain circumstances, the maternal antibodies may attack

and destroy the newborns red blood cells (neonatal

isoerythrolysis).

Immunoglobulin transport in the mammary gland

The IgGs make up the majority of immunoglobulin in cow

colostrum.

Most of the IgA and IgM that are transported into colostrum are

synthesized by the plasma cells (B lymphocytes).

Transport of the immunoglobulins occurs through the epithelial

cells by a process involving small transport vesicles.

Bioactive factors in colostrums and milk

Colostrum and milk contain many factors that can influence cell

growth, differentiation, and function:

Glutamine

Polyamines

Nucleotides

Galactopoeisis

Galactopoeisis is the maintenance of lactation once lactation has

been established.

Two key interrelated components for maintenance of lactation:

Galactopoietic hormones

Prolactin

Growth hormone

Removal of accumulated milk

Galactopoeisis

Role of local mammary factors in regulating milk secretion.

Feedback inhibitor of lactation (FIL) found in milk.

FIL is thought to be produced by the mammary cells as they

synthesize and secrete milk.

Physiology of mammary gland during the dry period

During dry period the gland has three distinct functional states:

The period of active involution

The period of steady state involution

The period of lactogenesis and colostrogenesis:

Regeneration and differentiation of secretory epithelial cells

Selective transport and accumulation of immunoglobulin

The onset of copious secretion

Physiology of mammary gland during the dry period

The mammary gland undergoing transition at two stages:

At the beginning of the dry period

At the end of the dry period

Physiology of mammary gland during the dry period

Reducing the length of the dry period of dairy cows may affect:

Postpartum health

Reproduction performance

Milk production

Physiology of mammary gland during the dry period

Intra-alveolar pressure triggers the events of active involution:

The appearance of lysosomes in the secretory epithelial cells.

Macrophages enter the mammary tissue and secretion.

The rate of synthesis of major milk constituents decrease:

Fat

Casein

Lactose *

Citrate *

β-lactoglobulin

α-lactalbumin

Physiology of mammary gland during the dry period

By 7 days involution, the concentration of serum proteins in

mammary secretion is significantly elevated.

The permeability barriers are not totally destroyed and the

mammary gland maintains a degree of control.

Physiology of mammary gland during the dry period

The concentration of the iron biding protein lactoferrin (Lf)

dramatically increase.

The major site of synthesis of the Lf found in bovine mammary

secretions is thought to be the secretory epithelial cell.

Lf is a major protein in the secretion of the non-lactating

mammary gland.

Lactoferrin is bacteriostatic by virtue of its ability to bind iron

with great affinity.

Development of the Mammary Gland (Mammogenesis)

Mammary gland has allometric and isometric growth

The development of mammary growth has five phases:

Fetal phase

Prepubertal phase

Postpubertal phase

Pregnancy

Lactation

Development of the Mammary Gland (Mammogenesis)

Timeline for the development of the mammary gland in bovines

Day 30, condensing ectodermal cells

Day 35, mammary line

Day 43, mammary bud

Day 65, teat development

Day 80, sprout

Day 150, channel formation

Development of the Mammary Gland (Mammogenesis)

Development of the Mammary Gland (Mammogenesis)

Prepubertal mammary growth begins as isometric growth, and

before puberty becomes allometric.

A large portion of mammary growth before puberty is an

increase in:

Connective tissue

Ductal growth

Growth of the fat pad

Development of the Mammary Gland (Mammogenesis)

Feed restricted heifers have >30% larger mammary glands at

puberty.

Feeding high energy diets during the prepubertal period

suppresses serum bovine somatotropin (bST) levels.

Development of the Mammary Gland (Mammogenesis)

Through the first several estrous cycles after puberty, rapid

mammary growth continues.

Most of the growth is lost through regression during the luteal

phase of each estrous cycle.

Nutrition plays an important, though controversial, role in

postpubertal mammary development.

Development of the Mammary Gland (Mammogenesis)

Mammary growth is a continuous, exponential process from

conception to parturition

The greatest increase occurs in mass of parenchymal tissue in

late pregnancy.

The increasing udder size during the fifth and sixth months of

pregnancy is due to:

The elongation of mammary ducts

The formation of alveoli

The reduction of identifiable fat cells in the fat pad

Development of the Mammary Gland (Mammogenesis)

Mammary growth continues in early lactation.

Persistency of lactation (maintaining peak milk yield) depends

on the continual survival of those milk-secreting cells.

In rats, increases in total mammary DNA was seen from

parturition until weaning.

Hormonal control of mammogenesis

The ovarian steroids are important for mammogenesis.

The ovarian activity appears to mediate the actions of GH, specifically

through changes in IGF-I. 

During cyclic activity, there is no significant exposure to

estrogens and progesterone together.

This takes place during late pregnancy when the CL produces large

amounts of progesterone and the feto-placental unit generates elevated

levels of estrogens. 

Hormonal control of mammogenesis

In vitro studies showed that estrogen plus prolactin and growth

hormone stimulated mammary growth.

Subsequently, estrogen was observed to induce secretion of

growth factors from pituitary, kidney, and mammary tumor cells.

Thus, it was postulated that growth factors secreted from

extramammary tissues into serum may act via an endocrine mechanism

to mediate the mammogenic effects of estrogen.

Hormonal control of mammogenesis

Growth factors secreted locally from mammary tissue may

mediate, via a paracrine or autocrine mechanism, estrogen effects

on mammogenesis.

Prolactin was discovered to be critically important for initiation

of lactation in the periparturient period in several species, including

cattle.

Indeed in cattle, lactogenesis is the only function of prolactin clearly

established to this day.

Hormonal control of mammogenesis

Mammogenesis depends not only on hormonal concentration but

also on:

Receptor availability within the mammary tissue

The presence of transport proteins and intracellular lipids that are

capable of making steroids unavailable to the tissues.

 

Hormonal control of mammogenesis

Several other hormones play a permissive and supportive role in

mammary growth:

Placental lactogens

Adrenal gland hormones

Thyroid hormones

Relaxin

Parathyroid hormone

Effect of parathyroid hormone-related protein (PTHrP)

Hormonal control of mammogenesis

Other factors that may affect mammogensis:

Insulin-like growth factors (IGF)

Epidermal cell factors (ECF)

Transforming growth factors (TGF)

Fibroblast growth factors (FGFs)