Reproductive Cycle and Hormonal Control

Standing Heat

• Standing heat is a primary sign looked for in animals.
• Standing heat is also referred to as "standing to be mounted."

Estrogen (E2)

• Estrogen, abbreviated as E2, triggers a surge of gonadotropin-releasing hormone (GnRH) from the surge center.

Gonadotropin-Releasing Hormone (GnRH)

• Two parts of the hypothalamus release GnRH: the tonic center and the surge center.
• The tonic center maintains a daily release of GnRH, pulsing approximately every 1.5 hours, varying slightly by species.
• When GnRH is released, it stimulates the release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH).

Luteinizing Hormone (LH) and Follicle-Stimulating Hormone (FSH)

• LH and FSH drive the normal function of the reproductive cycle.
• They stimulate follicle development, ovulation, and the formation of corpora lutea (CLs).
• These hormonal changes are responsible for changes in the reproductive tract, mainly due to estrogen and progesterone produced by the follicles.

Positive Feedback of Estrogen

• Estrogen provides positive feedback to the surge center, leading to a significant release of GnRH, which in turn causes a surge in LH and FSH.
• LH primarily causes ovulation by inducing changes in the body that cause the follicle to rupture and release the egg.

Estrus and Ovulation

• Estrus activity occurs, and ovulation occurs shortly afterward.

Timing of Insemination

• The lifespan of frozen and thawed sperm is shorter compared to fresh sperm due to damage during the freezing process.

Ovulation and Fertilization

• During ovulation, the follicle ruptures, releasing the egg.
• Fertilization can occur approximately a day later.
• The site of fertilization is the ampullary-isthmus junction (AIJ) within the oviduct.

Corpus Luteum Development

• The corpus luteum starts to develop, becoming functional by approximately day 5.
• The theca and granulosa cells of the follicle transform into luteal cells, which produce progesterone.