Lecture 3 - Ovary and Folliculogenesis

What are the 4 major events of the follicular phase?

1. Elevated gonadotropin release from the anterior lobe of the pituitary

2. Follicular growth and preparation for ovulation

3. Sexual receptivity

4. Ovulation

How is the follicular phase initiated?

After luteolysis that results in a marked reduction in progesterone

What are the primary steps leading to the preovulatory LH surge?

1. Decreased progesterone from CL breakdown

2. Increased GnRH

3. Increased FSH and LH

4. Proestrual follicular development

5a. Increased inhibin and estrogen from follicles that results in decreased FSH

5b. Increased estrogen from follicles to threshold which causes the preovulatory LH surge

How does ovarian estrogen impact gonadotropin release?

-Positive feedback on the surge center

-Negative feedback on FSH release from the anterior pituitary

What are characteristics of GnRH release from the tonic center?

-Constant low levels

-Appears to be spontaneous, but may be influenced by progesterone

What are characteristics of GnRH release from the surge center?

-Controlled by high estrogen

-Accompanied by low progesterone

What parts of the estrus cycle are considered the follicular phase?

Proestrus and estrus

Explain hormone concentrations during proestrus.

-FSH and LH rise, which causes the production of estradiol by ovarian follicle

-Estrogen then begins rising

-Progesterone decreased

Explain hormone concentrations during estrus.

-Estrogen rises and when it peaks, it causes the preovulatory surge of LH and induces ovulation

-FSH decreased due to inhibin

What are the primary ovarian structures?

-Primary follicles

-Secondary follicles

-Antral follicles

-Corpora lutea

-Corpora albicantia

When are follicles found in the ovary? When is a developing/functional CL found?

-Follicles: any point

-CL: depends on estrus cycle (found in estrus/metestrus)

What is follicular recruitment?

-Small antral follicles are recruited from the ovarian pool and produce small amounts of estrogen

-High FSH, low LH, no inhibin, low estradiol

What is follicular selection?

-Follicles are selected from previously recruited small follicles and become either atretic or develop further

-Low FSH, moderate LH, low inhibin, moderate estradiol

What is follicular dominance?

-Selected follicles that do not become atretic become dominant follicles that produce large quantities of estrogen

-Dominant follicles will ovulate

-Low FSH, high LH, high inhibin, high estradiol

What is the difference in ovulation between monotocous and polytocous species?

-Monotocous only a single follicle is selected

-Polytocous more than one follicle is selected

What is atresia?

Degeneration of follicles

How are follicular dynamics different in cattle?

-They have 3 follicular waves; one in metestrus, one in diestrus, and the typical proestrus wave that results in ovulation

-In the first 2 waves, the follicles selected will become atretic

How is estrogen synthesized? (4 steps)

1. LH binds to its receptor on the theca cell which converts cholesterol to testosterone

2. Testosterone diffuses into the granulosa cell

3. FSH binds to its receptor on the granulosa cell and causes the synthesis of aromatase, which converts testosterone into estradiol

4. Estradiol then enters circulation

How does estrogen affect the reproductive tract?

-Increased blood flow

-Genital swelling

-Leukocytosis

-Increased mucosal secretion

-Initiation of uterine gland growth

-Elevated myometrial tone

How does estrogen affect the brain?

Influences reproductive behavior

How does induced ovulation occur? (4 steps)

1. Copulation stimulates sensory nerves in the vagina and cervix

2. Impulses are relayed to the spinal cord

3. Impulses are relayed to the surge center of hypothalamus

4. If the stimulus has sufficient magnitude, neurons fire and cause a GnRH surge and consequently an LH surge

What animals are modified induced ovulators?

Camelids

What is the theorized stimulation in modified induced ovulators?

Presence of seminal plasma in the female reproductive

What is the function of PGE2?

It increases blood flow to the ovary and dominant follicle, which causes edema, and then increases follicular pressure to contribute to ovulation

What is the function of PGF2a?

-It increases contraction of ovarian smooth muscle, which increases follicular pressure to contribute to ovulation

-It also releases lysosomal enzymes to weaken the follicle wall

What does the preovulatory LH surge cause?

-Increase in PGE2 and PGF2a

-Shift from estrogen to progesterone secretion by dominant follicle

-Increased blood flow to dominant follicle and ovary

-Gap junction breakdown between granulosa cells and oocyte

What are the 4 phases of oocyte maturation?

1. Mitotic division of primordial germ cells (prenatal)

2. Nuclear arrest

3. Cytoplasmic growth

4. Resumption of meiosis (after puberty)