ANSC 300 - Ch. 7 - Lactation

Mammary gland

Evolved from modified secretory exocrine skin glands that have evolved to produce milk

There to nourish neonate by providing nutrients and immunity (colostrum)

Reproductive organ that develops over time from prenatal period through pregnancy, parturition and beyond

Monotreme

Early mammals simply secreted milk onto their skin

Marsupial

Developed teats, but still have relatively less developed mammary systems

Eutherians

Fully developed mammary gland producing complex milk

Mammogenesis

Growth of mammary gland (prenatal, prepubertal, puberty, pregnancy)

Lactogenesis

Transition from pregnancy to lactation (parturition)

Galactopoiesis

Maintenance of lactation (suckling, nursing)

Involution

Transition to non-lactating period (dry period, weaning)

Primary bud

Grows slowly in response to insulin

Mammary gland development before puberty

Cyclic changes in estrogen and progesterone promotes ductal growth, branching and lobule-alveolar development

What does cortisol synergize with in mammary growth?

E2/P4 and prolactin

What hormone blocks the onset of lactogenesis and is also required for mammary gland alveoli development?

Progesterone

Mammary ridges

Thickened epidermal tissue that give rise to the mammary gland

Forms from axillary to inguinal regions

Primary mammary bud (prenatal growth)

Thickened epidermal tissue begins to develop inward and penetrate into the mesenchyme (dermis)

Secondary primary bud (prenatal growth)

Primary mammary bud begins to send out branches that further penetrate into the dermis

Canalization

Fingerlike secondary buds begin to lengthen and branch out. Finally they begin to form canals or channels that will form the duct system of the gland. Myoepithelial cells surround the terminal portions of the developing gland

Puberty

Ovarian activity - increased estrogen and progesterone

Estrogen stimulates ductal growth and branching

Progesterone stimulates alveolar development. End bud formation

Multiple reproductive cycles cause further ductal growth, elongation, branching and end bud formation

Mammary alveoli

Secretory epithelium

Apical and basal side

Synthesizes milk components (protein, fat, sugar)

Secretes into lumen of alveoli

Water follows by osmosis

Myoepithelial cells contract to squeeze milk into ducts

Why is timing of parturition and lactation important?

There needs to be enough milk within a few hours between parturition and the arrival of the neonate to have enough milk for it to eat.

Prolactin

Removal of pituitary gland in pregnancy women or animals leads to a marked decrease in the subsequent lactation

Blocking secretion before parturition severely reduces milk yield

Synergizes with cortisol to induce lactation

When P4 secretion is blocked after mid-pregnancy the minimal hormone requirement for initiation of milk secretion is cortisol and prolactin

Stimulated by suckling: important in non-ruminants

Hormonal requirements for maintenance of lactation

Thyroid hormone, prolactin, cortisol and GH

Removal of milk from mammary gland is required for continuous milk synthesis

Thyroid Hormone

Ablation results in significant reduction in milk production

• Supplementing thyroid hormone will:

  • Increase milk production by 15-20%

  • Increase metabolic rate, heart rate, etc.

  • Only effective for a limited time

Growth Hormone

Enhances gluconeogenesis and suppresses fat synthesis in adipose tissue to support lactogenesis

How does GH orchestrate the metabolism of body tissues?

Coordination involves many tissue and physiological processes

Coordination involves all nutrient classes

Carbohydrate, lipid, protein, and minerals

IGF-1

Produced in many tissues in the body

Necessary for normal growth and development

High levels are associated with some cancers

rBST and milk safety

Protein hormones inactive when taken orally

rbST is species specific

No composition difference between rbST and non-rbST milk

Extensively researched in >3,000 scientific studies

Human safety declared by medical and health associations, scientific societies and government regulatory agencies