Repro 2 Female Physiology

Front 1

Describe how testosterone de-feminizes the brain and why females have a surge center

Back 1

-during prenatal development in the male, testosterone from the fetal testis “defeminizes” the brain
-the female fetus has no testis to produce testosterone and she therefore develops a GnRH surge center in the hypothalamus
-in order for testosterone to “defeminize” the hypothalamus, it must be first converted to estradiol
-in the female a protein called “alpha-fetoprotein binds to estadiol and prevents it from crossing the blood brain barrier
-therefore estradiol cannot affect the hypothalamus
-alpha-fetoprotein is a glycoprotein is a glycoprotein synthesized by the embryonic yolk sac & later the fetal liver
-in the male, testosterone crosses the blood brain barrier, is converted to estradiol in the brain and the estradiol “defeminizes” the hypothalamus, thus minimizing surge function

Front 2

What is the definition of puberty in females?

Back 2

*Age at first estrus (heat) – age that the female become sexually receptive & displays her first estrus
-relatively easy to determine because the female shows outward behavioral signs
-however first ovulation may not be accompanied by behavioral estrus in heifers & ewes → termed silent ovulation
*Age at first ovulation – difficult to determine
*Age at which a female can support pregnancy without deleterious effects
-acquisition of a threshold body size is important in controlling the onset of puberty
-the metabolic costs of pregnancy & lactation are high, thus it makes biologic sense that the female cross a “metabolic threshold” before puberty occurs

Front 3

What is the definition of puberty in males?

Back 3

*Age when behavioral traits are expressed
-generally, males or most species acquire reproductive behavioral traits (mounting & erection) well before they acquire the ability to ejaculate and produce spermatozoa
*Age at first ejaculation
-generally the ability to ejaculate substantially precedes the ability to produce sufficient spermatozoa to achieve fertilization
*Age when spermatozoa first appear in the ejaculate
-to determine when first spermatozoa are available, one must collect ejaculate once per week, which is difficult in large animals & requires special equipment and therefore has its limitations
*Age when spermatozoa first appear in the urine
-frequent collection of urine is difficult in large domestic animals and requires special equipment
*Age when the ejaculate contains a threshold number of spermatozoa
-the presence of a threshold (minimum number) of spermatozoa is required
-reflects the minimum seminal characteristics required to achieve pregnancy following copulation

Front 4

Describe the changes that occur in the hypothalamus secretion of GnRH before & after puberty in the FEMALE

Back 4

-before puberty in the female & male, GnRH neurons in both the tonic center & surge center of the hypothalamus release low amplitude and low frequency pulses of GnRH
-after puberty in the female, the tonic center controls basal levels of GnRH but they are higher than in the prepubertal female because the pulse frequency increases
-before ovulation can occur, full neural activity of the surge center must be achieved
-such activity results in sudden bursts of GnRH known as the preovulatory GnRH surge
-the GnRH neurons must fire frequently and release large quantities of GnRH in order to cause the preovulatory LH surge
-the preovulatory GnRH surge is a series of rapid, high amplitude pulses
-prior to puberty, low frequency GnRH pulses provide an insufficient stimulus to cause the anterior lobe of the pituitary to release FSH & LH at high levels
-therefore, follicular development (even though it does occur before puberty), cannot result in high circulating estradiol concentrations
-estradiol therefore remains below the minimum threshold that is necessary to trigger firing of GnRH neurons in the surge center
-the surge center responds primarily to a positive feedback stimulus
-failure to ovulate occurs because the ovaries do not produce enough estrogen to activate the highly sensitive surge center
-in a sense, the surge center lies dormant in the prepubertal female even though it is capable of responding to estradiol
-the reason it lies dormant is that the prepubertal ovary does not produce sufficient quantities of estradiol to stimulate the surge center to secrete high amplitude pulses of GnRH
-at low levels of estrogen, the tonic center has a high sensitivity to negative feedback and therefore does not produce high levels of GnRH and gonadotropins remain low
-during the pubertal transition, however, the negative feedback sensitivity by the tonic center to estradiol decreases and consequently higher and higher amounts of GnRH are produced that stimulate the ovary to produce more and more estrogen
-when estrogen levels reach a certain threshold, it now causes massive discharge of GnRH from the surge center (positive feedback)
-ovulation can now take place and puberty ensures

Front 5

Describe the changes that occur in the hypothalamus secretion of GnRH before & after puberty in the MALE

Back 5

-it involves a negative feedback loop only
-the negative feedback in the male is due to some testosterone and mostly to estradiol because testosterone is converted to estradiol within the brain by aromatization
-in the male the GnRH neurons become less and less sensitive to the negative feedback of testosterone & estradiol as puberty approaches
-this means larger and larger quantities of testosterone and estradiol are needed to inhibit the GnRH neurons
-with this decreased sensitivity to the negative feedback of testosterone/estrogen, the hypothalamus can produce more and more GnRH and thus more and more LH/FSH and the male reaches puberty

Front 6

What is the follicular phase?

Back 6

FOLLICULAR PHASE
-the follicular phase is the period from the regression of the corpora lutea to ovulation
-the follicular phase is relatively short – 20% of the estrous cycle
-during the follicular phase the primary ovarian structure are growing dominant follicles that produce the primary reproductive hormone, estradiol

Front 7

What is the luteal phase?

Back 7

LUTEAL PHASE
-is the period from ovulation until corpora lutea regression
-the luteal phase is much longer than the follicular phase and in most mammals is about 80% of the estrous cycle
-during this phase, the dominant ovarian structures are the corpora lutea (CL) and the primary reproductive hormone is progesterone