Throughout the adult reproductive years of the female, the structural composition and hormonal activity of the ovaries are continually changing. Hormones, called gonadotropic hormones, produced by the anterior pituitary gland within the brain initiate such changes. During early development, prior to sexual maturity of the bitch, very little gonadotropic hormones are secreted and the ovaries, therefore, remain inactive. However, around the age of 6 months, 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 in the bitch will initiate the sexual cycle sometime between the age of 6 months and 14 months. Cyclical increases 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 bitch.

Two days prior to ovulation, there is a surge in the secretion of LH by the pituitary gland proceeded 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. As a result, there is an increase in progesterone levels, and a decrease in estrogen levels. 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. If the bitch is bred, then sperm will subsequently fertilize each of the released ova. The ova will then move down the oviducts, which connect each ovary to each of the two uterine horns, and the fertilized ova will eventually implant in the walls of the uterus where they will develop into fetuses. 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. When enough inhibin has eventually been secreted, this will end the mating period.

Puberty

Age of onset of puberty:

Male ……….. 6 – 9 months

Female ……… 6-8 months

REPRODUCTIVE CYCLE

The  reproductive cycle of the bitch is comprised of four stages: proestrus, estrus, diestrus, and anestrus.

Proestrus: (average duration = 9 days; range = 3-17 days) Swelling of the vulva, the external tissue of the vaginal opening, and bloody discharge marks the beginning of the proestrus stage, also known as the follicular stage. During proestrus, the ovarian follicles, each containing ova, increase in size. Increasing amounts of estrogen hormone, secreted by the ovarian follicles, cause the cells of the vaginal walls to take-on a distinctive shape, a process known as cornification. Both the level of estrogen and vaginal cornification are useful indicators of proestrus.

Estrus: (average duration = 9 days; range =3-21 days) Receptivity to mating marks the beginning of the estrus stage. Physiologically, estrus coincides with the predominant presence of cornified vaginal epithelial cells and an increase in serum progesterone levels to 2 ng/ml. Ovulation usually occurs 2 days following this increase in progesterone and hence, monitoring the levels of progesterone is an excellent indicator for timing breeding.

Diestrus: (average duration = 2 months) Approximately 6 days after ovulation, the cornified vaginal epithelial cells will revert to a non-cornified state. This condition marks the beginning of diestrus. This stage ends when progesterone levels fall to less than 1 ng/ml just prior to whelping in the pregnant bitch or approximately 2 months after ovulation in the nonpregnant bitch.

Anestrus: (average duration = 4-4.5 months) The beginning of this stage is marked by the drop in serum progesterone levels to less than 1 ng/ml. The beginning of proestrual bleeding marks the end of this stage. Duration of anestrus is quite variable among bitches and may be governed by both genetic and environmental variables.