Ovaries: Estrogen and Progesterone

ovaries

produce oocytes and hormones

oviducts

site of fertilization and embyro development. Contains infundibulum, ampulla, and isthmus

uterus

site where embryo attaches and fetal development occurs. Horns, body, and cervix

vagina

copulatory organ

where are the ovaries located?

in the dorsal abdominal cavity, just caudal to the kidneys

what are the two functional ovarian structures?

the follicles and the corpus luteum

ovary cortex

follicular growth and corpora luteal location

ovary medulla

very irrigated and rich in nerve endings

what is the difference between the menstrual cycle and the estrous cycle?

The menstrual cycle is a monthly cycle in humans characterized by the shedding of the uterine lining, while the estrous cycle occurs in many non-human mammals and involves a period of receptivity to mating and does not include a menstrual phase.

Does the endometrium shed in an estrous cycle?

No, the endometrium does not shed; instead, it is reabsorbed if pregnancy does not occur.

Estrous cycle

a cyclical pattern of ovarian activity that transitions from reproductive receptivity to non-receptivity enabling pregnancy after mating. The main event is the cyclic appearance of behavioral sexual activity

lordosis

a sexual behavior in female mammals characterized by an arched back and elevated hindquarters, signaling readiness to mate.

what will trigger sexual receptivity?

high estrogen levels

what are the two cells that act together in ovarian follicles to produce estrogen?

Granulosa and theca cells.

what phases of the estrous cycle are in the follicular phase?

proestrus, estrus

proestrus phase

very short or non-existent in queens; more than a week in bitches. Characterized by active follicular growth and estrogen production

estrus phase

female shows sexual receptivity to the male. Estrogen is very high and induce estrus

what phases of the estrous cycle are in the luteal phase?

metestrus, diestrus, anestrus

metestrus phase

in some species (mare, ruminants, sow) there is a transitional phase between estrus and diestrus where the corpus lutea are developing

diestrus phase

longest period of the cycle and characterized by the presence of corpora lutea and high levels of progesterone

anestrus phase

reproductive quiescence

ovarian follicles

vesicle like structures that contain the oocyte and develop from primordial to more mature, pre-ovulatory stages. As they grow they produce increasing amounts of estrogen and upon ovulation it transforms into corpus luteum

ovulation

local inflammatory process in the pre-ovulatory antral follicle that results in the release of a mature oocyte in most domestic animals, except for dogs, where an immature oocyte is release

corpus luteum

a temporary endocrine organ that produces progesterone during pregnancy or part of the estrous cycle

what main hormone is produced in the ovarian follicle?

Estrogen

how many carbons are in estrogen?

18

what is the main hormone secreted in the corpus luteum?

progesterone

in what phase of the estrous cycle of the bitch is estrogen the highest?

proestrus

in what phase of the estrus cycle of the bitch is progesterone the highest?

diestrus

in what phase of the estrus cycle of a bitch is LH its highest?

end of proestrus, beginning of estrus

In most other species when is LH at its highest concentration?

end of estrus phase

What are follicular dynamics?

describes how follicles grow in waves and the sequence of these waves in the reproductive cycle

When will the most mature follicles be ovulated?

during the estrus phase when LH surges occur

what hormones does folliculogenesis depend on?

FSH and LH

theca cells

stimulated by LH and produce androstenedione

granulosa cells

stimulated by FSH and concery androstenedione to estrogen

what is the effect of estrogen on the uterus?

increases blood perfusion, increases smooth muscle motility, growth of uterine glands

what is the effect of estrogen on the vagina?

increases blood perfusion, increases smooth muscle motility, increases migration of leukocytes into its lumen

what is the effect of estrogen on the vulva?

edema of the vulva, redness

what is the effect of estrogen on the brain?

stimulate brain centers that induce sexual receptivity, mating posture, phonation and increased physical activity

what is the effect of estrogen on hypothalamus and pituiatary?

very high levels of E2 create a strong positive feedback on the hypothalamic-pituitary axis during estrus, leading to a massive release of GnRH and a surge of LH, which induces ovulation

What does the final increase in estrogen by fully dominant follicles reach a critical threshold?

it activates the hypothalamic surge center triggering GnRH release and subsequent LH surge.

What does the LH surge stimulate?

histamine, proteolytic enzymes, and prostaglandins causing follicular swelling and rupture, releasing the oocyte

what is spontaneous ovulation in animals?

rupture of the mature follicle

what happens after spontaneous ovulation?

follicular walls collapse, bleeding occurs and luteinization begins, producing progesterone

what will terminate estrous behavior?

lack of estrogen

what is induced ovulation?

stimulation of vaginal mechanoreceptors in the wall of the vagina need to be stimulated by physical copulation with the male to trigger ovulation in certain species, such as cats and rabbits.

In animals that use induced ovulation, what follows after copulation?

once the receptors are stimulated, they will send impulses through the sensory neurons to the spinal chord and the hypothalamus, which will then release GnRH to stimulate the release of luteinizing hormone (LH) from the pituitary gland, triggering ovulation.

why do luteal cells have LDL cholesterol receptors on their surface?

To uptake cholesterol for progesterone synthesis.

Why does P4 exert a strong negative feedback on the hypothalamic pituitary axis?

Because high levels of progesterone inhibit the release of GnRH, LH, and FSH, preventing further ovulation and maintaining pregnancy which inhibits estrus behavior

why can human chorionic gonadotropin be used as an inducer of ovulation?

it binds to LH receptors

what is luteinization?

follicular granuloas and theca cells transform into luteal cells, forming the corpus luteum, which produces progesterone. LH is essential for CL development

Why does high progesterone suppress the surge center in the hypothalamus?

to reduce LH pulses, preventing preganancy

what are the effects of progesterone on the uterus?

stimulates endometrial glands to secrete products that support the development of the embryo and it inhibits motility of the myometrium to provide calming conditions for the embryo to attach

what is the effect of progesterone on the mammary gland?

promotes alveolar development in the mammary gland in preparation for lactation. Prolactin is also important for milk production

What is luteolysis at the end of diestrus?

the process whereby the corpus luteum undergoes regression and cell death. It is induced by prostaglandin F2 alpha.

what animals have an active luteal destruction process activated to end the cycle and start a new estrous cycle?

cows, sows, mares

what hormone initiates the PGF alpha 2 release from the endometrium?

oxytocin

what does PGF2a inhibit?

progesterone synthesis and opens calcium channels leading to apoptosis in luteal cells, promoting luteolysis.

How does declining progesterone levels start a new cycle?

As progesterone levels decline, the lack of its inhibitory effect on the hypothalamus allows for the release of gonadotropin-releasing hormone (GnRH), stimulating the pituitary gland to produce follicle-stimulating hormone (FSH) and luteinizing hormone (LH), thus initiating a new estrous cycle.

what will maintain the corpus luteum for preganancy to continue?

prevent the release of endometrial PGF2a to avoid luteolysis