Ch 8 - The Follicular Phase
Four Major Events
Elevated gonadotropin secretion from the anterior lobe of the pituitary
Follicular growth and preparation for ovulation
Sexual receptivity
Ovulation
Ovulation
Luteolysis
Driving force for the initiation of the follicular phase
Causes the CL to become nonfunctional; marked reduction in progesterone secretion as a result
Proestrus
Estrus
Follicles begin to produce more and more estradiol
Becomes closely associated with but precedes ovulation
Estradiol
Governed by the hypothalamus, the anterior lobe of the pituitary, and the ovary through the secretion of estradiol in the absence of progesterone
Hypothalamus
Tonic Center
GnRH secretion appears to be spontaneous but is influenced by progesterone
Surge Center
Secretes basal levels of GnRH until it receives appropriate positive stimulus
Consists of 3 nuclei
Preoptic nucleus
Suprachiasmatic nucleus
Anterior hypothalamic area
Surge center responds dramatically to high blood concentrations of estradiol and low concentrations of progesterone
Tonic and surge centers control GnRH release
Surge = once per cycle
Tonic = consistent
GnRH
Controls the onset of the preovulatory LH surge that causes ovulation
Induces reproductive behavior in non-primate mammals
Dominant hormone secreted by the developing follicles and causes profound changes that prepare the reproductive tract for copulation
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Positive and Negative Feedback
Positive Feedback: stimulation in response to rising concentrations of estradiol
Negative Feedback: during the middle part of the cycle when estradiol is low and progesterone is high
- Progesterone acts on the preovulatory center to reduce the frequency of GnRH pulses
Once estradiol reaches a threshold concentration, or peak (during estrus), the preovulatory center is "turned on" and secretes large quantities of GnRH that stimulate the anterior lobe of the pituitary to secrete a preovulatory surge of LH
Anterior Lobe of the Pituitary
Stimulates secretion of FSH and LH
Causes growth and development of ovarian follicles
Follicular Dynamics
The process of follicular growth and degeneration
Occurs continuously throughout the entire estrous cycle
Antral follicles are always present and are stimulated by FSH and LH
Dynamics of antral follicles
Selection
Dominance
Recruitment (emergence)
Atresia
Degeneration of follicles
Phase of follicular development in which a cohort (group) of small antral follicles begins to grow (emerge) to secrete estradiol
- Most recruited follicles undergo atresia
A group of growing follicles that have not undergone atresia
- Multiple in dogs and cats
- One in horse and cow
Produce increasing amounts of estradiol and inhibin
- Inhibin is a protein hormone secreted by antral follicles that inhibits release of FSH from anterior pituitary
- ONly follicles receiving a large blood supply grow and ovulate
Occurs continuously throughout folliculogenesis
High FSH and low LH pulse frequency, low inhibin and low estradiol
Low FSH, moderate LH and low inhibin
Low FSH, high LH pulse frequency and high inhibin
FSH and LH are controlled differently
Estradiol and inhibin secreted by growing follicles decreases FSH
GnRH pulse frequency increases LH
Major effects on reproductive tract
Increased mucosal secretion
Initiation of uterine gland growth
Leukocytosis
Change in tissue electrical conductivity
Genital swelling
Increased blood flow
Elevated myometrial tone
Behavior culminates with the female standing to be mounted by the male
A cascade of physiological and biochemical changes that cumulate in rupture of preovulatory follicles and release of the oocyte from the ovary
- Occurs in response to LH
Brought about by
Breakdown of connective tissue
Ovarian contractions
Elevated blood flow
The dominant follicle begins to secrete progesterone before ovulation
This stimulates collagenase to be released by the theca interna cells
- Causes the breakdown of collagen
LH surge causes prostaglandin E2 and F2a to be synthesized and secreted locally by the ovary
PGE2
PGF2a
Lysosomal enzymes cause further connective tissue deterioration at the apex of the follicle
Causes contractions of the myoid (smooth muscle) components of the ovary
Causes lysosomes within granulosa cells to rupture, releasing their enzymes
Intermittent contractions will increase local rpessure to force the stigma to protrude even more dramatically from the surface of the ovary
Secretes plasminogen which is converted to plasmin by an activator
Helps the follicle remodel itself into a corpus luteum after ovulation by directing the organization of the tissue structure
Plasmin is the active enzyme that helps deissolve the coagulum of the corpus hemorrhagicum and aids in remodeling the follicle into a corpus luteum
Types of Ovulators
Spontaneous Ovulators
Reflex/Induced Ovulators
Ovulate with a regular frequency and do not require copulation
Ovulation is brought about totally in response to hormonal changes
Require stimulation of the vagina and/or cervix for ovulation to occur
Most have a long copulation time or copulate with intense frequency to ensure adequate neural stimulation will take place and cause ovulation
The tactile stimulation is converted to action potentils that innervate the hypothalamus to cause a preovulatory surge of GnRH
This causes LH to be released prompting the cascade of events leading to ovulation
Camelids are a modified version of this - presence of seminal plasma in reproductive tract is more important than tactile stimulation in inducing ovulation
Artifical Folliculogenesis and Ovulation
Superovulation - many follicles ovulate
Hormonally induced ovulation - achieved via PGF2a administration
Four Phases of Oocyte Maturation
Nuclear arrest (dictyotene)
Cytoplasmic growth
Mitotic division of primordial germ cells (prenatal)
Resumption of meiosis
Ovulation growth involves the formation of a large cytoplasm and a zona pellucida