Nervous system, Milk Composition, and Functional Differentiation of Mammary Epithelial Cells

from which embryonic tissue does the nervous system derive?

ectoderm

what is the functional cell of the nervous system?

neuron

describe the neuron

polarized & specialized for transmitting signals 

what does it mean that neurons are polarized?

have distinct structures for receiving and sending signals

receiving signal structure

dendrite

sending signal structure

axon

what are dendrites?

extensions of the neuron that receive chemical signals to activate or suppress neuron activity 

what is an axon?

long projection of a neuron that conducts electrical impulses away from the cell body

action potential

wave of electrical depolarization that travels along the axon, leading to neurotransmitter release at axon terminals

what happens when neurotransmitters are released at axon terminals?

affect nearby nerves, tissues, glands, & muscles by either exciting or inhibiting activity

what are the two main divisions of the nervous system?

CNS & PNS

what is the CNS comprised of?

brain & spinal cord

what is the PNS comprised of?

somatic & autonomic divisions

what is the somatic nervous system?

-part of PNS

-responsible for voluntary control of skeletal muscles

what does “somo” mean in somatic?

body

what is the autonomic nervous system?

-part of PNS

-responsible for involuntary control of smooth & cardiac muscle, and glands

which branch of the nervous system innervates the mammary gland?

sympathetic branch of the ANS

what are efferent nerves?

-transmit signals from CNS to tissues

-motor function

what are afferent nerves?

-transmit signals from tissues to CNS

-sensory function

how do mammary afferent nerves exist?

in pairs

define pre-ganglionic nerves in the mammary gland

nerves with dendrites in mammary gland and axons in spinal ganglia

define post-ganglionic nerves in the mammary gland

nerves with dendrites in the spinal ganglia and axons that project to the hypothalamus

which nerves are responsible for initiating the milk ejection reflex?

mammary afferent nerves

what triggers the milk ejection reflex?

-mechanical stimulation

-offspring nuzzling or udder preparation

list the steps of milk ejection reflex

1. Mechanical stimulation of mammary gland

2. Activation of pre- and post-ganglionic neurons

3. Hypothalamus releases oxytocin into blood

4. Oxytocin travels through blood to mammary gland

5. Myoepithelial cells contract → alveolar secretions forced into ducts

what do pre-ganglionic nerves do in milk ejection?

generate impulses after udder stimulation or nuzzling

what do post-ganglionic nerves do in milk ejection?

activated by pre-ganglionic nerves to signal hypothalamus for oxytocin release

can oxytocin release be triggered without mechanical stimulation?

-yes → conditioned reflexes

-sights, sounds, or thoughts related to milking

what hormone is responsible for milk letdown?

oxytocin

what factors reduce oxytocin release & milk letdown in cows?

stress → inhibits hypothalamic oxytocin release

what is the half-life of oxytocin in blood?

about 5 minutes

where are myoepithelial cells found?

surround alveoli of mammary gland

what role do myoepithelial cells play in milk ejection?

contract in response to oxytocin → squeezing milk from alveoli into ducts

what is the primary component of milk?

-water

-with dissolved & suspended solids

what are dissolved solids in milk?

-whey proteins

-lactose

-minerals

-vitamins

what are suspended solids in milK?

-casein micelles

-milk fat globules

what are curds?

-solid portion used for cheese-making 

-casein, lipid droplets, calcium

what is whey?

-liquid portion

-soluble proteins, lactose, minerals, vitamins

what macronutrients are in milks?

-carbs

-protein

-fat

-minerals → ash

What is the main carbohydrate in milk?

 Triglycerides → short- & medium-chain fatty acids unique to milk

What are the two types of proteins in milk?

Casein proteins and whey proteins

What makes milk proteins unique compared to other tissues?

Caseins and most whey proteins are only produced by mammary epithelial cells

Why are unique milk structures important for neonates?

They provide improved digestion and disease resistance beyond basic nutrition

Define functional differentiation

process where mammary epithelial cells activate pathways to produce milk components

one example of functional differentiation in milk production?

activation of enzymes that convert acetate into milk fat

what percentage of milk fat is synthesized in mammary epithelial cells from acetate?

about 50%

what drives functional differentiation of mammary cells?

-hormones

-growth factors → gestation/parturition

-physical removal of milk

what is the role of milk removal in differentiation?

Autocrine/paracrine growth factors triggered by milk removal push differentiation to completion

when is differentiation complete?

several days into lactation

what are tight junctions?

cell connections that regulate the movement of substances between alveoli and surrounding fluid

what is the state of tight junctions before parturition (prepartum)?

Leaky, allowing equilibration between milk in alveoli and interstitial fluid

what is the state of tight junctions after parturition (postpartum)?

Tight, preventing equilibration between milk and interstitial fluid

what is α-lactalbumin?

whey protein produced only by mammary epithelial cells

where is α-lactalbumin secreted?

Into the alveolar lumen

if α-lactalbumin is found in the blood, what does it indicate?

Leaky tight junctions allowed it to escape

Why is plasma α-lactalbumin zero at day -200 in heifers?

Early gestation → mammary cells are forming (mammogenesis), but lactogenesis hasn’t started

why does α-lactalbumin rise slowly from day -160 to -40?

Lactogenesis is beginning at a low level

Why does α-lactalbumin increase sharply between day -40 and 0?

Late gestation causes a surge in lactogenic activity

Why does α-lactalbumin drop after calving?

Milk leaves the gland and tight junctions tighten, reducing leakage