ANSC 300 Lactation

Mammary Gland

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

• nourishes neonate by providing nutrients and immunity (colostrum)

• is a reproductive organ that develops over time from prenatal period through pregnancy, parturition, and beyond

Monotremes

• early mammals simply secreted milk on to their skin

• egg laying

Marsupials

• developed teats, but still have relatively less developed mammary systems

• metatherians

Eutherians

• fully developed mammary gland producing complex milk

• placental

Stages of Mammary Gland Development

1. Mammogenesis

2. Lactogenesis

3. Galactopoiesis

4. Involution

Mammogenesis (STAGE 1)

growth of gland

• prenatal

• repubertal

• puberty

• pregnancy

Lactogenesis (STAGE 2)

• transition from pregnancy to lactation

  • parturition

• initiation of lactation is tied to changes in hormones occuring at the end of pregnancy

• rapid onset from no milk production to high milk production in a matter of hrs/day

• E2/P4 are not enough to induce milk production, need other hormones

Galactopoiesis (STAGE 3)

• maintenance of lactation

  • suckling, nursing

• hormonal requirements for maintenance of lactation includes thyroid hormone, prolactin, cortisol, GH

• removal of milk from the mammary gland is required for continuous milk synthesis

Involution (STAGE 4)

transition to non-lactating period

• dry period, weaning

Mammary Growth

1. Primary Bud: grows slowly in response to insulin

2. Puberty: cyclic changes in estrogen and progesterone promotes ductal growth, branching and lobule-alveolar development

3. Cortisol synergizes with E2/P4 and prolactin

4. Progesterone blocks milk synthesis before parturition

Prenatal Development

1. Mammary Ridges

2. Primary Mammary Bud

3. Secondary Mammary Bud

4. Canalization

Mammary Ridges (STAGE 1)

are thickened epidermal tissue that give rise to the mammary gland

• forms from axillary to inguinal regions

• number and placement of glands is species specific

Primary Mammary Bud (STAGE 2)

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

Secondary Mammary Bud (STAGE 3)

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

Canalization (STAGE 4)

the fingerlike secondary buds begin to lengthen and branch out. Finally, they begin to form canals 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 and ends bud formation

• multiple reproductive cycles cause further ductal growth, elongation, branching, and ends bud formation

Mammary Alveoli

• secretory epithelium

• apical and basal side

• synthesizes milk components (protein, fat, sugar)

• secretes into lumen of alveoli

• water followed by osmosis

• myoepithelial cells contract to squeeze milk into ducts

Why is Timing of Parturition and Lactation Important?

there needs to be enough milk in order to feed neonate immediately after parturition

• surges in hormones keeps lactation stable

Profiles of Various Hormones During Pregnancy

Endocrine Changes Near Parturition

Progesterone (LACTOGENESIS)

• P4 blocks induction of PRL and GR receptors on mammary gland cells

• blocks transcription of a-lactablbumin and casein mRNA in mammary cells

• blocks protein synthesis of casein, a-lactalbumin and lactose synthesis

Prolactin (LACTOGENESIS)

• removal of pituitary gland in pregnant women or animal leads to a marked decrease in the subsequent lactation

• blocking prolactin secretion before parturition severely reduces milk yield

• prolactin 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 nonruminants

Cortisol and Prolactin are _______

synergistic

____ and ____ have No Direct Mammary Actions.

LH and FSH

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

• Why?

  • idk

Growth Hormone

• GH has no lactogenic activity in vitro

• no evidence of GH-R in mammary cells

• GH enhances gluconeogenesis and suppresses fat synthesis in adipose tissue to support lactogenesis

• acts indirectly through induction of IGF-1 and modulation of metabolism

GH Orchestrates the Metabolism of Body Tissues

• coordination involves many tissue and physiological processes

• coordination involves all nutrients classes

  • carbohydrate, lipid, protein, and minerals

Metabolic Effects of GH

Effect of Nutrition on IGF-1 Secretion to bST

Nutritional Status Modulates Response to GH

Insulin-Like Growth Factor-1

• 70 amino acid peptide with 50-60% sequence homology to insulin and 76% homology to IGF-II (human)

• human IGF-1 is identical to bovine IGF-1

• produced in many tissues of the body

• necessary for normal growth and development

• high levels have been associated with some cancers

• no difference in rbST and non-bST treated cows

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