Lactation Flashcards
Define mammogenesis
Mammary growth at puberty or during pregnancy
Define lactogenesis
Initiation of milk secretion at parturition (production of colostrum)
Define galactopoeisis
Continued milk secretion during lactation
Describe the embryological development of the mammary glands
- originate from epidermis
- Arise along 2 lateral lines on ventral surface of developing foetus
- Called mammary ridge, thickened epidermal tissue, give rise to primary mammary bud
- Extend from axillary region to inguinal region
- Primary bud pushes into dermis to grow
- Branch into secondary mammary bud
- Will then branch out and canalise to form lactiferous ducts
- Myoepithelial cells from mesoderm
Outline the significane of lactation
- Free neonate from dependency on other foods
- Allows offspring to continue development outside uterus
- Sovial development and bonding
- Natural contraception
- Foetus born with minimal fat for ease of birth
Outline the adaptation allowing neonates to suckle and breath at the same time
Long hard palate, not present in birds
Describe the anatomy of the mammary gland
- Glandular tissue, secretory epithelial cells and excretory ducts
- Excretory ducts empty into gland cistern
- Epithelial cells surrouding spherical lumen = alveolus (aka alveolar cells)
- Myoepithelial cells around each alveolus and ducts
- Groups of alveoli form lobules
- Groups of lobules form lobes
- As each gland develops, get development of a teat sinus
- Some have internal sphincter in teat/streak canal
Where is the majority of control in the mammary gland?
In the teat/streak canal
What is the function of the myoepithelial cells?
Contraction will propel milk out of alveoli and into ducts
What speecies have thoracic mammary glands?
- Man and other primates
- Elephant
What species have abdominal mammary glands?
- Dog
- Cat
- Sow
- Rat
- Rabbit
What species have inguinal mammary glands?
- Goats, sheep
- Cow
- Guinea pig
- Mare
Which species have 2 mammary glands?
- Man and other primates
- Elephant
- Goats, sheep
- Guinea pig
- Mare
How many mammary glands do dogs have?
~10
How many mammary glands to cats have?
8
How many mammary glands to pigs have?
8-18
How many mammary glands do rats have?
~10
How many mammary glands do rabbits have?
10
How many mammary glands to cows have?
4
How many teat canals per teat do humans, elephants and rabbits have?
8-10
How many teat caals do dogs have per teat?
8-22
How many teat canals per teat do cats have?
4-8
How many teat canals per teat do sows and mares have?
2
Which species have 1 teat canal per teat?
- Rats
- Goats, sheep
- Cows
- Guinea pigs
Compare teat sinuses between species?
- In horses and cattle branches anastomose to form one teat sinus
- In bitch, no coming together of ducts
Describe the mammary glands of marsupials
- Within pouch
- Young forms direct seal around teat
Describe the mammary glands of monotremes
- Mammary tissue essentailly a band of sweat glands
- No teats - milk runs out along hair shafts
Describe the suspensory apparatus of the bovine mammary gland
- Suspensory ligament divided into 2 parts
- Medial suspensory ligaments
- Lateral suspensory ligament
- Like 2 palstic bags in one hand - where they touchh is medial ligament, outsides are lateral ligaments
Describe the medial suspensory ligament of the bovine mammary gland
- Arises from symphyseal tendon (attaches prepubic tendon to pubic symphysis)
- Mainly yellow elastic connective tissue
Describe the lateral suspensory ligaments of the bovine mammary gland
- Run from symphyseal tendon and external crus of the inguinal ring
- Assisted by femoral fascia
- Then run ventrally over lateral aspects of udder
- Inelastic connective tissue
Describe the connective tissue in the bovine mammary glands
- Runs through glands
- Come together to form 4 bands
Expain the lateral deviation seen in the engorged udder or older ruminants
- Medial ligament stretchier
- Laterals cannot stretch
Describe the innervation of the mammary glands of the bitch
- Only sympathetic supply
- No PSNS
- Controls vascular elements only
How are the myoepithelial cells stimulated?
Oxytocin, no neural signals
Describe the lymphatic drainage of the mammary glands in the bitch
- Cranial glands: axillary and axxessory axillary lymph nodes
- Caudal: superficial inguinal (mammary) nodes
- Middle glands either drain cranially or caudally, no mixture
- Can have more cranial glands draining caudally and more caudal glands draining cranially, no way to tell
Describe bovine lympatic drainage of the mammary glands
- Lymphatic drainage arises in lobules, not alveoli
- Leaves gland via mammary lymph nodes (upper rear of udder in cattle)
Describe the innervaiton of the bovine mammary gland
- Somatic sensory afferent from tactile receptors in skin
- Sympathetic motor efferent along blood vessles (from lumbar and sacral sympathetic chains)
- 4 spinal nerves in cattle (1st, 2nd lumbar, inguinal and perineal)
- No PSNS
- Myoepithelial cells controlled by oxytocin, not innervated
What is the blood supply to thoracic mammary glands?
Mammary arteries are branches of thoracic and pectoral arteries
What is the blood supply to abdominal mammary glands?
Branches of cranial and caudal superficial epigastric arteries
What is the blood supply to inguinal mammary glands?
Branches of external pudendal artery
Describe the blood supply to the mammary glands of cats and dogs
- Anterior glands: lateral and internal thoracic, cranial superficial epigastric arteries
- Caudal glands: caudal superficial epigastric arteries (from external pudendal)
Describe the venous drainage of the bovine mammary gland
- External pudendal veins into external iliacs then caudal vena cava
- Superficial epigastric veins (aka milk veins) into internal thoracic v then cranal vena vaca
- Perineal veins into internal pudendal vein then caudal vena cava
What controls mammary gland growth?
Endocrine mediated
Describe the growth of mammary glands between birth and puberty
- Isometric (like any other tissue)
- No ability to secrete milk
Describe mammary gland growth after puberty
- Allometric growth
- During oestrus or menstrual cycles ducts begin to branch under effect of oestrogen
- More alveoli formed under influence of P4
- Complete and grapid growht of ducts in presence of prolactin and GH (increase at puberty)
Describe mammary gland growth in pregnancy
- Terminal alveoli grow into lobules
- Prolactin, adrenal cortical hormones adn placental lactogen
- Relaxin from CL in pigs
- Allows synthesis of milk
What changes occur in mammary gland following lactation?
- Atrophy and regression back to close to original size
- Never same as before
- Each successive lactation will get bigger, so more lactations means more capacity for milk
Describe the process of mammary gland involution
- Less milk needed, pressure increases in gland
- Cells less function - pressure atrophy
- Cells atrophic and die by apoptosis
- Immune cells (lymphocytes and macrophages) invade tissue
- Cells remain non-functional until next pregnancy
What is the role of immune cell invasion in mammary gland involution?
Participate in production of IgG for next lactation
Describe mammary gland involution in polytocous species
- Occurs soon after birth
- e.g. 8 teats but 5 young
- Match amount of milk to number of young
What are the 2 types of milk?
- Regular milk
- Colostrum
Describe colostrum
- Secretion accumulated at end of last pregnancy and secretion formed during first 24 hours after parturition
- More protein, fat, minerals and vitamins
- Supplies newborn with immunoprotection
When does colostrum need to be taken in by the neonate?
- Ideally within first 6 hours
- At the lates 24-48 hours after birth
- After this will no longer absorb antibodies
Why is colostrum produced for 4 days after birth?
- Will not absorb antibodies in this time
- However will bind out viruses etc in abomasum of neonate
Which species has milk that is appropriate for most species?
Goats
What component of milk is the principle contributor to osmolarity?
Lactose
- Milk has same osmolarity as blood
What proteins are present in milk?
- Caseins
- Lactalbumin
- Lactoglobulin
- Immunoglobulins
Where is milk synthesised?
Epithelial cells of alveolus
Describe the organelles that are used in milk synthesis
- Mitochondria: ATP for producction, synthesis of fatty acids and non-essential amino acids
- RER: synthesis of secretory proteins
- SER: synthesis of phospholipids and TAGs
Describe the secretion of fat from the alveolar cells
- Bunch out of cytoplasm with small layer of cytoplasm around the edge
- Membrane-limited lipid droplets
- Helps keep milk as an emulsion
How are proteins and lactose secreted?
- Granules by exocytosis
- Lactose secreted with protein
How is the water component added to milk?
Follows lactose by osmotic pressure
Describe the synthesis of lipid in the mammary gland
- TAG made from fatty acids in chylomicrons and LDL in blood
- And from glucose (non-ruminants) or acetate and beta-hydroxybutyrate (in ruminants i.e. ketone bodies)
- Same pathway as in tissues
- Body fat broken down by liver
Describe the synthesis of lactose by the golgi
- Series of conversions starting with glucose
- Glucose + ATP -> glucose-6-P and ADP
- Glucose-6-P -> glucose-1-P
- Glucose-1-P + uridine triphosphate -> UDP-glucose +PPi
- UDP-glucose -> UDPgalactose
- UDP-galactose + glucose via lactose synthase to make lactose and UDP
Describe the cellular components of milk
- Leukocytesnad dislodged mammary epithelial cells
- Normal: 30,000-300,000 cells/ml
- Inflammation can iincreas 10-100 times due to neutrophil invasion
- Cell density is parameter of milk quality
List the hormones involved in control of galactopoeisis
- Prolactin
- Oestrogen
- Progesterone
- Cortisol
Describe the role of prolactin in milk synthesis
- Regulates production of alpha-lactalbumin
- Density of prolactin receptors regulated by thyroid, adrenal and ovarian hormones
Describe the role of cortisol in galactopoeisis
Necessary for growth and differentiation of golgi apparatus
Describe the role of progesterone in the control of galactopoeisis
- Blocks prolactin receptors during pregnancy, blocking milk synthesis
- Stimulates glandular development
- Remove P4, produce a-lactalbumin, lose inhibition and gain stimulation
List the factors regulating continuous milk secretion
- Hormones
- Local conditions in alveoli
- Secretion depends on prolactin
- Release depends on oxytocin
- Insulin and growth hormone
- Accumulation of milk in alveoli
- Feedback inhibitor of lactation
- Suckling action of neonate
Describe the role of insulin and growth hormone in galactopoeisis
- Growth hormone can substitute for prolactin in ruminants
- Insulin and GH regulate metabolism and ensure adequate energy supply to udder
Describe the feedback inhibitor of lactation
- Modulates synthesis of lactose and milk protein
- Inhibits formation of secretory vesicles in golgi apparatus
Describe mammogenesis during pregnancy and preparation for lactation
- Placenta produces hromones to induce mammary growth
- Higher in twin than singleton
- Placental lactogen in synergy with oestrogen
- Progesterone and oestrogen still present
- P4 induces storage of arachidonic acid during pregnancy
- E2 induces phospholipase A to allow conversion of arachidonic acid to PGF2a to lyse CL, reduce P4 and expose oxytocin receptors
- mammary growth continues during milk secretion
Describe placental lactogen
- Prolactin and some GH activities
- Produced by binucleate cells of placenta
- Mainly sheep
What hormone do sows use instead of placental lactogen?
Relaxin
Describe the role of progesterone in mammogenesis
- Provides growth element to glands
- Grows in preparation, inhibits milk synthesis
- Gland growth but no synthesis
- Induces storage of arachidonic acid during pregnancy
Describe the role of E2 in mammogenesis
- Induces phospholipase A, thus arachidonic acid converted to PGF2a
- Leads to luteolysis in species with CL
- Softens cervix
- PGF2a induces myometrial contractions
- E2 exposes myometrial oxytocin receptors
What stimulates lactogenesis to begin?
Fall in progesterone removing prolactin inhibition
Describe the onset of lactogenesis
- Increase in udder volume during pregnancy follows increase in oestrogen and placental lactogen (mimicing prolactin)
- Onset of milk secretion coincides with drop of P4 at parturition
- Peak in cortisol stimulates increased conversion from P4 to E2
Describe the role in control of lactogenesis by alpha-lactalbumin
- Binds to galactosyltransferase and UDP-galactose to form lactose synthase
- Needed to produce lactose
- P4 inhibits this
- Prolactin induces this
Describe the mechanisms for the maintenance of galactopoeisis
- Suckling stimulus -> PVN in hypothalamus
- Tactile stimulation of mammae maintains prolactin production
- Removal of milk
- 2-3x daly suckling needed
- Only tactile stimulationworks, oxytocin admin does not
Describe how the suckling stimulus to the PVN of the hypothalamus acts to maintain galactopoeisis
- Reduces dopamine which releases inhibition on prolactin release from ant. pit
- Increases vasoactive intestinal polypeptide (VIP) which stimulates prolactin release
What is required for milk ejection?
- Sensory activation (not just suckling)
- Neural activation
- Oxytocin release
- Contraction of alveoli and ducts
- Mechanical transfer of milk
Describe the milk ejection reflex
- Stimulation of sensory neurones (suckling, smell, vision, sound), to hypothalamus
- PVN nerves stimulated, release of oxytocin from neurohypophysis
- Increases pressure in alveoli
- Reduce resistance in small excretory ducts
- Reduce resistance in teat canal
- Myoepithelial cells surrounding alveoli - squeeze milk out of alveoli into ducts
- Contract in response to oxytocin
Describe the effect of suckling on parturition
- Cervix stretches due to oxytocin
- Sucking stimulates release of oxytocin
- Thus suckling drive parturition of rest of litter
Describe suckling in pigs
- Set meal times
- Mother lies down, calles young over
- Allowed to feed for 2 minutes then shake off young
Describe the metabolism during lactation
- Changes in metabolism in many tissues
- Coordinate redircection of metabolic resources
- Homeorhesis occurs - reduced fat syntehsis and increased lipolysis, decreased uptake of glucose in muscle and adipose tissue, increase mammary uptake
- Increased uptake of amino acids in mammary gland (breakdown of muscle protein insufficient)
What regulates changes in metabolism during lactation
- Prolactin
- Growth hormone
- Insulin
Describe the endocrine control of mammary involution
- After weaning
- Loss of suckling = drop in prolactin
- Local factors in milk that block secretion
- Build up of pressure due to loss of milk removal
How does lactational anoestrus occur?
- Lactation reduces gonadotrophin secretion
- Suckling induces opioid production
- Blocks GnRH secretion preventing ovulation
What is embryonic diapause?
Delayed implantation
- Extension of the period between blastocyst development and implantation
How does lactation lead to embryonic diapause?
Reduces metabolic stress on animal that is simultaneously lactating and supporting a pregnancy
List species that that have delayed implantation but not due to lactational delay
- Badger
- Stoat
- Roe deer
- Rat, mouse
- Grey seal
- Camels
Give the roles of oxytocin
- Contraction of myometrium
- Milk ejection
- Part of feedback cycle for luteolysis
- Maternal and social bonding
- Stimuli for parturition and milk let down
List conditions affecting lactation
- Pseudopregnancy
- Galactostasis
- Agalactia
- Mastitis
What is galactostasis?
Congestion of gland combined with oedema and inflammation
What is agalactia?
- Lack or reduced milk production
- Inadequate nutrition or liquid intake, mastitis, other bacterial infections, mineral deficiencies
- MMA in sows
