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The Follicular Phase of the Estrous Cycle - Coggle Diagram
The Follicular Phase of the Estrous Cycle
Gonadotropin
Release is controlled by ovarian estrogen and hypothalamic GnRH
Produced by the hypothalamus
Also responsible for stimulating the release of the gonadotrophins FSH and LH
The tonic and surge centers in the hypothalamus control GnRH release
Episodic profile: the profile of tonic GnRH release is characterized by having many small pulses or episodes. These pulses have various frequencies and amplitudes depending on the degree of nervous activity (rate of firing) in the tonic center
GnRH release from the tonic center appears to be spontaneous
Stimulates the release of FSH and LH from anterior pituitary, causing growth and development of ovarian follicles
The surge center is responsible for the preovulatory release of GnRH that stimulates a surge of LH, causing ovulation
GnRH release from the surge center is controlled by estrogen
Responsible for release of large quantities of GnRH, thus causing a surge of LH that causes ovulation
Positive feedback: the stimulation in response to rising concentrations of estradiol
Negative feedback: when estradiol levels are low and progesterone is high
elevated GnRH is essential for initiating the follicular phase of the estrous cycle
Secretion of GnRH is controlled by two separate areas in the hypothalamus
Hypothalamic nuclei: areas composed of clusters of nerve cell bodies which represent anatomically discrete regions
Follicular Preparation for Ovulation
Growth of the antral follicles is stimulated by FSH and LH and involves both growth and death of these follicles
Antral follicles of various sizes develop in response to tonic levels of FSH and LH and these antral follicles are always present
The dynamics of antral follicles involve three events: recruitment, selection and dominance
Recruitment: the phase of follicular development in which a group (cohort) of small antral follicles begins to grow and produce estradiol. Following recruitment, a group of growing follicles which have not undergone atresia are selected
Some of the recruited follicles undergo atresia
Selection: involves the emergence of dominant follicles (potentially ovulatory) from the cohort of previously recruited antral follicles
Selected follicles may become dominant or they may undergo atresia
As they proceed toward dominance, they continue to produce increasing amounts of estrogen as well as the hormone inhibin
Inhibin is a protein hormone produced by the follicle that selectively inhibits the release of FSH from the anterior pituitary
Monotonous species: (care and mare) gives birth to a single offspring. There are several follicles, but only one will develop into the dominant follicle
Polytomous species: (litter bearers) have multiple dominant follicles
Dominance is characterized by one or more large preovulatory follicles exerting a major inhibitory effect on other antral follicles from the recruited and selected cohort
Atresia
Involves far more follicles than does he process of dominance
Refers to the closure or disappearance of the antrum, which accompanies the degenerative changes of an antral follicle
Generally short-termed processes when compared to the preantral stages
The follicular phase comprises only about 20% of the estrous cycle, but the process of follicular growth and degeneration (known as follicular dynamics) occurs continuously throughout the entire estrous cycle
Sexual Receptivity
Follicle preparation for ovulation occurs under a set of endocrine conditions that is different from the first two waves
FSH and LH are at higher concentrations than during the time of previous waves because the inhibition of GnRH by progesterone has been removed
Elevated levels of FSH induce recruitment of follicles from the gonadotrophin sensitive pool within the ovary
When follicles are recruited, they begin to produce estradiol and small quantities of inhibin
As inhibin levels increase (during selection), the degree of negative feedback on the anterior pituitary increases.
FSH begins to decline and LH begins to become more important than FSH to follicular development
The stage of dominance is characterized by continued decreasing FSH levels and increasing LH levels
The LH surge occurs when the estradiol levels of the dominant follicle have reached threshold
The estrogen secretion of the dominant follicle decline abruptly once the preovulatory surge of LH occurs
Estrogen induces reproductive behavior
Elevated progesterone coupled with low progesterone induces significant behavioral changes in female
During the follicular phase the female becomes sexually receptive and copulation can take place
The dominant follicle begins to produce progesterone before ovulation
Following the LH surge, the cells of the theca interna begin to produce progesterone instead of testosterone
Collagenase: causes the breakdown of collagen, which is a major component of connective tissue. This happens due to local elevation of progesterone
Ovulation
Results from a cascade of events starting with the LH surge
Preovulatory surge of LH is critically important because it sets in motion a series of biochemical events that lead to ovulation
Brought about by elevated blood flow, breakdown of connective tissue, ovarian contractions
There are two types of ovulators
Spontaneous ovulators ovulate with a regular frequency and do not require copulation
Ovulation is brought totally because of hormonal changes
Reflex ovulators requires stimulation of the vagina and/or cervix for ovulation to occur
Folliculogenesis and ovulation can be artificially induced using various hormones
Being able to understand the follicular dynamics and ovulations allows us to manipulate the timing of ovulation for management and convenience purposes
Two main approaches to this manipulation
Induced ovulation: requires premature luteolysis or controlled luteolysis. This can be accomplished with the administration of exogenous prostaglandin F 2 alpha.
You can also treat animal with progestational compounds
Superovulation requires the administration of exogenous gonadotrophins which cause abnormally high numbers of follicles to be selected
Includes injections of equine chorionic gonadotrophin or FSH followed by administration of LH, GnRH or human chorionic gonadotrophin several days later to induce ovulation