Luteal Phase

What are the two stages of the luteal phase? What is the major

The luteal phase extends from ovulation until CL regression

Two stages:

• Metestrus - Luteinization

• Diestrus

  • Characterized by fully functional CL

  • Luteolysis (CL regression)

What is luteinization?

A process whereby cells of ovulatory follicle differentiate into luteal tissue

• Theca interna and granulosa cells of follicle undergo dramatic

• This transformation is largely governed by LH

  • PGE2 plays a role in remodeling too

Describe the corpus luteum. Mention the two cell types

• Large luteal cells (LLC) originate from granulosa cells

  • 20-70 microns

  • 3x increase in size and volume (hypertrophy)

• Small luteal cells (SLC) originate from theca interna

  • < 20 microns

  • 5x increase in cell number (hyperplasia)

Both large and small luteal cells are steroidgenic (able to produce steroids)

• Synthesize and secrete P4

• SLC are LH-dependent, LLC produce P4 constitutively

Describe the functional capactiy of the corpus luteum

• Blood flow to CL= 6-10mL/min/g tissue

  • Most luteal cells are near capilaries

  • High metabolic demand; CL consumes 2-6 times more oxygen per unit weight than the liver, kidney, or heart

Non-steroidogenic cells proliferate too:

• Endothelial cells, fibrobasts, white blood cells

• Adds CL enlargement

• Proliferation of cells in developing CL has a growth rate that rivals rapidly growing tumors

Insufficient CL function = Decrease P4

What is required for progesterone synthesis?

• Basal LH secretion

• Cholesterol

What are the steps of progesterone biosynthesis?

1. Receptor-mediated endocytosis - cholesterol enters cell

2. Cholesterol transported from cytoplasm into mitochondria by StAR (rate limiting step)

3. Cytochrome P450 SCC converts cholesterol to pregnenolone

4. Pregnenolone converted to P4 in smooth endoplasmic reticulum via 3-beta hydroxysteroid dehydrogenase (3B-HSD)

Describe specifically the first step of progesterone synthesis

Cholesterol is dlivered to the luteal cell primarily via lipoprotein (LDL or HDL)

• LDL-CHOL binds to membrane-bound receptor (R)

• LDL-CHOL-R complex is internalized

• Cholesterol is released from the receptor complex as cholesterol esters

• Receptor is “recycles” back to plasma membrane

Describe steps 2-4 for progesteron synthesis

2. LH binds to LH receptor (LHR) on plasma membrane

3. LHR activates G protein (G) that activates membrane-bound adenylate cyclase (AC)

4. ATP is converted to cyclic AMP (cAMP), the second messenger

What is the 5th step in progesterone synthesis

cAMP activates protein kinases, which:

• Increase rate of LDL-CHOL receptor internalization

• activates cholesterol esterase (cleaves cholesterol from its ester)

• Promote entry of cholesterol into mitochondria

  • Example: catalyzing synthesis of StAR

What is the 6th step of progesterone synthesis

In mitochondria, cholesterol makes pregnenolone

• Enzyme: cytochrome P450 side-chain cleaveage (CYP450scc)

What is the 7th of progesterone synthesis?

Pregnenolone leaves the mitochondria and is converted to progesterone in the smooth endoplasmic reticulum

• Enzyme: 3-beta hydroxysteroid dehydrogenase (3B-HSD)

What are the primary target organs of progesterone? What effect does it have on the brain?

• Hypothalamus (brain)

• Female Tract

• Mammary gland

Inhibitory effects on the hypothalamus:

• Decreases GnRH pulse fequency

  • Prevents preovulatory LH surge

  • Prevents behavioral estrus

What are the major effects of progesterone on the female tract and mammary glands

• Stimulates endometrial gland growth

• Stimulates nutrient secretions by the oviducts and endometrium

• Blocks myometrial contractions

  • Quiteting effect on myometrium in cow, ewe, sow

  • Allows for proper attachment of conceptus

• Stimulates production of thick cervical mucus

Mammary glands:

• Causes final alveolar development of mammary glands prior to parturition

What is Luteolysis? What are the two types?

Luteolysis - destruction of the corpus luteum

• 1st Functional luteolysis:

  • Decreased progesterone (P4) production

• 2nd Structural luteolysis

  • Physical degradation, shrinkage, and apoptosis of luteal cells

    • Programmed cell death - ordered biochemical and morphological changes in the cell (not necrosis)

What are the hormones that control luteolysis?

• Oxytocin (from LLC, hypothalamus)

• Progesterone (from CL)

• Prostaglandin F2-alpha (from endometrium)

  • Exceptions: primates, dog, cat

What is required in many species for luteolysis?

The uterus is required (in many species)

• CL and endometrium must communicate for successful luteolysis

• If luteolysis does not occur, animal remains in a sustained luteal phase

Describes what happens if we remove different parts of the uterus

• Remove uterus after ovulation prolongs CL lifespan; mimics pregnancy

• Removal of contralateral horn: CL lifespan unaffected

• Remove ipsilateral horn: Increased CL lifespan

What is the luteolytic hormone in sheep

• PGF2a

What is the vascular countercurrent exchange?

• Where PGF2a is transported to the ipsilateral ovary via the uter-ovarian vascular countercurrent transport system

  • Present in cow, ewe, and sow

• Relatively high concentrations; no dilution in circulation

PGF2a leaves endometrium via UOV → Passively diffuses from UOV into closely apposed OA → Transported directly to ovary → Luteolysis

What are the steps resulting in loss of luteal progesterone secretion

1. PGF2a binds to receptor on plasma membrane

2. PGF2a-R complex opens Ca++ channels

3. PGF2a-R complex activates protein kinase C (PK-C) - Inhibits P4 synthesis

What does luteolysis result in

• Cessation of P4 secretion

• CL structural regression to form corpus albicans

• Removal of negative feedback by P4 on GnRH

Describe the first half of the estrous cycle and Late luteal phase

• First half of the estrous cycle: P4 prevents PGF2a secretion by blocking oxytocin receptors in uterus

  • After 10-12 days, P4 loses its blocking ability; precise mechanism is unknown

• Late luteal phase: oxytocin and PGF2a stimulate each other in a positive feedback manner

  • Injection of oxytocin → PGF2a secretion from uterus

  • Injection of PGF2a → rapid release of ovarian oxytocin

A critical number of PGF2a pulses within a given timespan is required to induce complete luteolysis

What is the proposed mechanism of luteolysis in primates

The uterus is not required for luteolysis in humans/primates:

• Uterectomy does not affect cyclicity

• Rhythmic pattern of folliculogenesis, luteal develpment, and luteolysis will occur even if uterus is removed

• Proposed mechanism:

  • oxytocin from posterior pituitary acts on ovary, binds receptor

    • Small amounts of PGF2a are released, act directly on CL

  • Endometrial PGF2a causes localized vasocontriction of endometrial arterioles, which initiates menstruation

What can be used to synchronize estrus in cattle (introvaginal)

EAZI-BREED CIDR

• Controlled internal drug release

• Progesterone intravaginal insert

• CIDR is inserted for 7 days

• Administer Lutalyse on 6th day

• Cows and heifers will enter estrus within 2-3 days of CIDR removal

What does a normal estrus cycle look like? When is estrus?

What does the giving a PGF2a incjection in day 0-6 have?

• No effect

What does giving a PGF2a injection between day 6 and 17 effect?

• Cow enters estrus within 3 days after injection

What are the different ways to detect estrus?

• Mount detection aids:

  • Kamar pressure-sensitive mount detector

• Heat detector animals

  • Vasectomized bull

• Activity monitors/pedometers

  • Increased activity during estrus

“AM-PM rule” → breed 12 hours after observing cow in standing heat

Describe the steps of Ovsynch

• GnRH is administered into cows that are eligable to be inseminated (must be cycling)

  • If there is a dominant follicle on the ovary, the cow will ovulate in response

  • If the cow doesn’t have a dominant follicle, GnRH will promote follicular growth

• 7 days later - Administer PGF2a to induce luteolysis; cow will enter follicular phase

• 48 hours later - Administer second injection of GnRH to induce ovulation

• 16 hours later - Perform timed artificial insemination (without estrus detection)