Pig

The pig reaches puberty at age 6 9 months.[2,3] Its estrous cycle is 21 days in length. During the latter part of the

Phases of the Estrous Cycle

-6-5-4-3-2-10 I 2 3 4 5 6 7 8 9 10 II 12 13 14 15 16 17 18 19 20 0 I 2 3 4 5 6

Day of Cycle f ■^ The luteal phase begins after ovulation and

-6-5-4-3-2-10 I 2 3 4 5 6 7 8 9 10 II 12 13 14 15 16 17 18 19 20 0 I 2 3 4 5 6

Day of Cycle

The follicular phase begins after luteolysis that causes the decline In progesterone. Gonadotropins (FSH and LH) are therefore produced that causes follicles to produce estrogen (E2). The follicular phase is dominated by E2 produced by ovarian follicles. The follicular phase ends at ovulation. Estrus is designated as day 0.

f ■^ The luteal phase begins after ovulation and includes the development of corpora lutea that produce progesterone (P4). The luteal phase also includes luteolysis that is brought about by prostaglandin F2„.

Fig. 1 Phases of the estrous cycle. The follicular phase begins after luteolysis, which causes a decline in progesterone. Gonadotropins (FSH and LH) are therefore produced that stimulate follicles to produce estrogen (E2). The follicular phase is dominated by E2 produced by ovarian follicles. The follicular phase ends at ovulation. Estrus is designated as day 0. The luteal phase begins after ovulation and includes the development of corpora lutea that produce progesterone (P4). The luteal phase also includes luteolysis, brought about by prostaglandin F2a. (From Pathways to Pregnancy and Parturition 2nd Edition, with permission from Current Concepts, Inc., Pullman, WA.) (View this art in color at www.dekker.com.)

estrous cycle, rapidly growing ovarian follicles secrete large amounts of estradiol-17p about 48 hours prior to the LH surge. The pig displays estrus for 48 72 hours with ovulation occurring at the end of estrus or about 36 hours after onset. The LH surge peaks at the time of estrus and returns to basal levels about 10 hours later. FSH increases about 2 3 days after the initation of estrus and then decreases due to an increase of inhibin secretion from ovarian follicles. Following ovulation, a CL forms and progesterone secretion reaches its maximum around day 12 of the cycle and decreases after day 15. This decrease is due to episodic releases of PGF2a from the uterus that cause the demise of the CL.

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