ENDOCRINE CHANGES DURING ESTROUS CYCLE

The estrous cycle is controlled by the interaction of FSH, LH, estrogen and progesterone. These hormones are common to most domestic animals, however, their secretory patterns and relative effects vary among the species.

( During early development, prior to sexual maturity , very little gonadotropic hormones are secreted and the ovaries, therefore, remain inactive.

( However, around the age of 6 months and above, the pituitary begins to secrete higher levels of the gonadotropic hormones called follicular stimulating hormone (FSH) and luteal hormone (LH).  The rise in FSH and LH will initiate the sexual cycle and this cyclical increase and

decreases in FSH and LH in turn, control the cyclic ovarian changes and, as such, are responsible for the physiologic events in the normal reproductive cycle of the female animals. The female animal has two ovaries that will produce the ova (eggs) which are contained within follicles that grow toward the surface of the ovary.

( When FSH and LH from the pituitary gland begin to be secreted in high quantities during onset of sexual maturity, the ovaries and the follicles within them will begin to grow.

( Within these follicles, a follicular fluid hormone, secreted by the ovary, called estrogen, surrounds the ovum. This hormone is a biologic chemical that produces physiologic and social/behavioral effects within the female that will signal a readiness to mate.

( Two days prior to ovulation, there is a surge in the secretion of LH by the pituitary gland preceded by rapid swelling of the follicle. This LH surge is of critical importance because in its absence, even with the other hormonal physiologic effects taking place, ovulation will not occur.

( Additionally, the LH surge causes the ovarian cells to switch over to secreting progesterone hormone rather than estrogen.

( Within two days of the LH surge, the follicle reaches the surface of the ovary and bursts, thereby releasing the ovum into a capsule that surrounds the ovary. This process is referred to as ovulation.

( In the meantime, the ruptured follicles from which each ovum was developed will begin to produce a rapidly dividing mass of cells called luteal bodies, which will make up the corpus luteum.

( In addition to producing progesterone, which will maintain the pregnancy, the corpus luteum will also produce inhibin, the hormone that will signal the pituitary gland to decrease production of FSH and LH.