Reproductive cycles

Stages and phases of the estrous cycle

Changes in hormones during early follicular phase From ovary to hypothalamus –Decreased production of Progesterone by CL results in decreased blood progesterone concentrations. –Decrease in blood progesterone causes increased pulse-like release of GnRH from hypothalamus. GnRH then enters bloodstream that circulates from hypothalamus to pituitary gland. Progesterone GnRH Hypothalamus Pituitary gland

Changes in hormones during early follicular phase From hypothalamus to pituitary –Decrease in blood concentrations of progesterone Pituitary gland becomes more sensitive to GnRH Release LH in response to increased concentrations of GnRH in the circulation. GnRH LH Hypothalamus Pituitary gland

Changes in hormones during early follicular phase From hypothalamus to pituitary –LH Released in a pulse-like manner from pituitary gland in response to pulsatile secretion of GnRH GnRH LH Hypothalamus Pituitary gland

Changes in hormones during early follicular phase From pituitary gland to ovary –LH Stimulates development of large follicle on the ovary –Production of large amounts of estradiol. LH Estradiol Hypothalamus Pituitary gland

Changes in hormones during follicular phase Further decrease in blood progesterone concentrations causes further increase in number of GnRH pulses released by hypothalamus. This increase in GnRH pulses in turn causes further increase in secretion of LH by anterior pituitary gland and subsequently estradiol by the large follicle. LH FSH Estradiol Progesterone GnRH Hypothalamus Pituitary gland

Changes in hormones during preovulatory period Estradiol and Progesterone –Production of estradiol by the preovulatory follicle Extremely high –Progesterone production Very low (often undetectable by the assays we use).

Changes in hormones during preovulatory period Preovulatory LH and FSH surges –Estradiol concentrations Reach a threshold (peak). –Estradiol peak Associated with a large period of enhanced release of GnRH Prolonged elevation of blood LH concentrations over a period of hours –Refer to this prolonged, elevation in LH as the LH surge. Estradiol Estrus LH GnRH

Changes in hormones during preovulatory period Preovulatory LH and FSH surges –The preovulatory LH surge Physical rupture of the follicle and oocyte release (ovulation) Transformation of cells in the follicle wall –Progesterone producing luteal cells –Blood concentrations of FSH also reach a peak Much smaller compared to the LH surge. Estradiol Estrus LH GnRH FSH

Changes in hormones during early luteal phase Estradiol and Progesterone –Shut-down of estradiol production Immediately after preovulatory LH surge Shift to produce progesterone (Transformation into luteal cells) –Luteinization of follicular cells

Changes in hormones during early luteal phase Estradiol and Progesterone –Ovulation Approximately 30 hours after preovulatory LH surge in cattle Beginning of luteal phase –Released oocyte Picked up by the oviduct and transported to the site of fertilization where it will be fertilized by the sperm Ovulation

Changes in hormones during early luteal phase Estradiol and Progesterone –Blood concentrations of progesterone Low 24 to 48 hours post ovulation –Differentiation of cells that were the in the preovulatory follicle into progesterone-producing luteal cells Begin to increase rapidly 3 to 4 days after ovulation Early CH

Changes in hormones during early luteal phase Estradiol and Progesterone –Transformation of CH into CL A large, highly vascularized structure. Blood concentrations of progesterone begin to increase rapidly 3 to 4 days after ovulation CH CL

Changes in hormones during early luteal phase LH –Begin to decrease and reach basal level within hours after the plateau of the LH surge. LH

Changes in hormones during early luteal phase FSH –Begin to increase immediately after ovulation The secondary FSH surge. Stimulates growth of a group of antral follicles (a cohort) –Start of a new follicular wave FSH

Changes in hormones during early luteal phase FSH –Begin to decrease As follicles begin to grow –Reach baseline FSH

Changes in hormones during mid luteal phase Estradiol and Progesterone –CL reaches its maximum size (20 to 25 mm in diameter in cattle) Maximum production of progesterone –Production of progesterone by CL remains fairly constant Reflected in relatively stable concentrations of progesterone in blood

Changes in hormones during mid luteal phase Estradiol –Elevated during early stage From the growing largest follicle of the first follicular wave after ovulation. Far less than blood estradiol concentrations during estrus.

Changes in hormones during mid luteal phase Estradiol –Decrease because the large follicle (from the first follicular wave) eventually stops growing and begins to shrink in size –Increase again during growth of the largest follicle emerging from the subsequent follicular wave.

LH –Fairly low and accompanied by infrequent episodic pulses (approximately 1 per hour). –Progesterone is A potent inhibitor of the high frequency LH secretion observed during the preovulatory period –Growing large follicle does not produce enough estradiol to trigger LH surge and hence does not ovulate. LH

Changes in hormones during late luteal phase FSH –Increase as the large follicle begins to shrink in size Stimulate growth of another cohort of follicles. –Like during early stage, one follicle begins to outgrow the rest Decline in concentrations of FSH FSH

Changes in hormones during late luteal phase Estradiol and Progesterone –Blood concentrations of progesterone Elevated until around 17 days after estrus No pregnancy –Decline due to destruction of the CL (luteolysis) Luteolysis –Beginning of follicular phase

Changes in hormones during diestrus LH and FSH –The large follicle that emerged from this FSH surge may become the preovulatory follicle if it begins to grow just prior to luteolysis. Or, it may grow and shrink like the first one, which triggers the onset of a third follicular wave and growth and development of another large follicle. –When cows fail to conceive, they will undergo luteolysis and enter proestrus. The entire events described here will be repeated again. These events are repeated until animals become pregnant.