L17: Ovaries, Estrogen, & Progesterone

Ovary

the female reproductive organ that produces eggs (oocytes) and secretes hormones like estrogen and progesterone

Follicle

structure in the ovary that contains an immature egg (oocyte) and produces estrogen. It develops through stages from primordial to mature follicles

Corpus Luteum

structure formed after ovulation from the ruptured follicle; it secretes progesterone to maintain the uterine lining during early pregnancy

Ovulation

local inflammatory process resulting in the release of a mature egg (oocyte) from the ovary, triggered by a surge in LH during the estrous cycle

Proestrus

hormones: FSH & estrogen increase

key events: follicles mature, estrogen rises, uterus thickens, vaginal bleeding/swelling

Estrus (heat)

hormones: peak estrogen levels

key events: female is sexually receptive, ovulation occurs at the end

Metestrus

hormones: progesterone rises

key events: CL forms & secretes progesterone, no longer sexually receptive

Diestrus

hormones: high progesterone

key events: CL secretes progesterone to maintain uterine lining, regresses if pregnancy doesn't occur

Anestrus

hormones: low estrogen & progesterone

key events: reproductive inactivity

In what phase of the estrous cycle do ovarian follicles mature, estrogen levels rise, and the female is not yet sexually receptive?

proestrus

Oviducts

site of fertilization & embryo development

What is the main function of the ovaries in female mammals?

to produce eggs (oocytes) and hormones like estrogen & progesterone

Two main ovarian structures

follicles & corpus luteum

The ovarian cortex is

the site of follicular growth & CL location

The ovarian medulla is

irrigated & rich in nerve endings

Menstrual cycle

the endometrial lining of the uterus is shed

Estrous cycle

defined by the appearance of reproductive behavior & the endometrium does not shed

When does ovulation occur during the estrous cycle?

at the end of estrus & beginning of metestrus

Which species do not have a metestrus phase of the estruus cycle?

cats & dogs

What triggers sexual receptivity during the estrus phase?

high estrogen levels

What secondary physical manifestations can appear during estrus?

edema of the vulva, standing to be mounted, reproductive posture (lordosis)

What are the two cells that act together in the ovarian follicles to produce estrogen?

theca & granulosa cells

In which species is the proestrus phase very short or non-existent?

cats (queens)

Proestrus is characterized by

follicular growth & estrogen production

Follicular phase of the estrous cycle consists of

proestrus & estrus

Luteal phase of the estrous cycle consists of

metestrus, diestrus, anestrus

Metestrus is characterized by

development of the corpus lutea

Diestrus is characterized by

fully functional CL and high progesterone

Anestrus is characterized by

reproductive quiescence (absence)

Ovarian follicles

- contain oocytes in various stages of development, follicular cells, and granulosa cells

- produce estrogen

Main hormone produced in ovarian follicles

estrogen

Main hormone produced in corpus luteum

progesterone

How are progesterone & estrogen transported in the blood?

bound, lipid-soluble with intracellular receptors

In what phase of the estrous cycle of the female dog are the estrogen concentrations higher?

proestrus

In what phase of the estrous cycle of the female dog are the progesterone concentrations higher?

diestrus

In what phase of the estrous cycle of the female dog does the LH reach its higher peak?

between proestrus & estrus

What hormones are involved in pseudopregnancy?

- abrupt drop in progesterone

- prolactin

What could be given to stop pseudopregnancy?

prolactin inhibiting hormone - dopamine

What are the pre-ovulatory follicles called?

dominant follicles

Follicular dynamics

process of follicular growth and degeneration in waves

Folliculogenesis depends on

FSH & LH from the hypothalamus

Gonadotropin-independent growth

follicular development up to formation of antrum is independent of FSH or LH

Gonadotropin-dependent growth

secondary to antral follicle development requires FSH & LH to develop further

Theca cells

stimulated by LH to produce androstenedione (estrogen precursor)

Granulosa cells

stimulated by FSH to convert androstenedione into estrogen

Estrogen effects on uterus

- increased blood perfusion

- increased smooth muscle motility

- growth of uterine glands

Estrogen effects on vagina

- increased blood perfusion

- increased smooth muscle motility

- increased migration of leukocytes into lumen (prevent infection)

Estrogen effects on external genitalia

- edema of the vulva

- redness

Synthesis of estrogen (E2) requires

cholesterol

Estrogen effects on brain

increased sexual receptivity, mating posture, increased physical activity

Estrogen effects on hypothalamic-pituitary

strong positive feedback on HPA, leading to massive release of GnRH & LG surge, inducing ovulation

In spontaneous ovulation, rupture of the mature follicle releases the oocyte how long after the LH surge?

12-24 hours

LH stimulates

- histamine, proteolytic enzymes, prostaglandins

- follicular swelling, pressure increase, spontaneous ovulation

In what species does induced ovulation occur?

queens, rabbits, camels

How does induced ovulation occur?

1. vaginal mechanoreceptors are stimulated by physical copulation

2. impulses are sent through sensory neurons to the spinal cord & hypothalamus

3. hypothalamic surge center releases GnRH

4. LH surge released

5. ovulation occurs

What can hCG (human chorionic gonadotropin) be used as an inducer of ovulation in cows?

hCG binds directly to LH receptors

What type of hormones are LH & hCG?

peptide hormones

What do luteal cells have on their surface?

LDL-cholesterol receptors

Luteinization

the formation of a corpus luteum in the ruptured follicle

Increasing progesterone concentrations are important for

preparing the endometrium for a potential pregnancy

High progesterone levels suppress the surge center in the hypothalamus, which

reduces LH pulses & prevents further ovulation

P4 stimulates endometrial glands to secrete products that

support the development of the embryo

P4 inhibits

smooth muscle contractions of the myometrium

In the mammary gland, P4 promotes

alveolar development in preparation for lactation

Luteolysis

regression of the corpus luteum

What induces luteolysis?

PGF2a

Luteolysis steps

1. oxytocin initiates PGF2a release from endometrium

2. PGF2a inhibits progesterone synthesis & opens Ca channels - apoptosis

3. declining progesterone activates HPA

4. new cycle starts (proestrus)

Maternal recognition of pregnancy

- CL (progesterone) must be sustained for pregnancy to continue

- embryo prevents the release of endometrial PGF2a to avoid luteolysis