Lecture 9 - Reproductive endocrinology Flashcards
The hypothalamus is inherently female. Testosterone defeminizes the hypothalamus during
embryogenesis and “eliminates” the GnRH surge center in the male.
So, the Male hypothalamus lacks a developed surge center.
GnRH = gonadotropin releasing-hormone
Purpose of the hypothalamic “surge center”.
to release large amounts of GnRH
thus only exists in the female
The onset of puberty depends upon
the ability of specific hypothalamic neurons to produce sufficient amount of gonadotropin releasing hormone (GnRH) to promote and
support gametogenesis.
Onset of puberty in females can be defined in a variety of ways.
For example, The first day that serum progesterone exceeds
a threshold of 1 ng/mL.
(let’s you know that ovulation has occurred successfully)
Puberty starts with the development of hypothalamic neurons is influenced by: (3)
(these neurons produce GnrH)
– Development of threshold body size
– Exposure to a variety of environmental and social
cues
– The genetics of the animal
External factors that modulate the timing of puberty (4)
- Season during which the animal is born (sheep)
- The photoperiod that the animal is experiencing during the onset of puberty (sheep)
- The presence of the opposite sex during the
peripubertal period (swine and cattle) - The density of the groups (within the same
sex) in which the animals are housed (swine). (more females together reach puberty earlier, at least with sows)
The fundamental requirement for pubertal onset is
the secretion of GnRH at the appropriate frequency
and quantities to stimulate gonadotropin release by the pituitary.
In the female, the tonic center controls
basal levels of GnRH with the increase in pulse frequency. The surge center controls preovulatory release of GnRH.
In the prepubertal female, the surge center is sensitive to
the positive feedback of estradiol.
But, the surge center cannot release “ovulatory quantities” of GnRH because the ovary cannot produce high levels of estradiol.
At low levels of estrogen, the tonic center
has
a high sensitivity to negative feedback
and therefore does not produce high levels
of GnRH.
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.
It is the sensitivity to negative feedback that is
decreased and triggers the onset of puberty in
the female.
The decreased sensitivity to negative feedback by the tonic center means that
smaller and smaller quantities of
estradiol can stimulate the release of GnRH
and thus LH and FSH are secreted.
These gonadotropins then stimulate more follicles and more and more estradiol is produced until finally the surge center releases the preovulatory surge of GnRH.
In the male, the onset of puberty is brought about by
decreased hypothalamic sensitivity to negative feedback by testosterone/estrogen.
Body Maturation and a Certain Degree of
adipose tissue is Required for the Onset of Puberty in the Female
Metabolic signals that may stimulate GnRH neurons (3)
- blood glucose levels
- leptin from adipocytes
- blood fatty acids
Environmental and Social Conditions that Impact
the Onset of Puberty in the Female (3)
- Season of birth
- Presence of the male
- Size of the social group in which females are
housed
Phases of the estrous cycle
follicular phase
(end with ovulation, high estradiol conc.)
luteal phase
(progesterone high)
Pro- and estrus form the follicular phase.
Met- and diestrus form the luteal phase.
Stages of the estrous cycle
Proestrus (begins to show estrus signs, estradiol going up)
Estrus (heat occurs here, highest estradiol, progesterone low)
Metestrus (follows ovulation, estradiol down, progesterone up)
Diestrus (progesterone up)
Pro- and estrus form the follicular phase.
Met- and diestrus form the luteal phase.
Length of proestrus and estrus in dogs.
Both approx. 9 days.
Length of diestrus and anestrus in the dog.
An- approx. 5 months
Di- approx. 2 months
Describe proestrus in dogs.
Proestrus begins with the appearance of a blood-tinged vaginal discharge and by vaginal
swelling.
It ends when the bitch accepts the male for mating. The ovaries contain large follicles
at the onset of proestrus.
Estradiol gradually increases and peaks slightly before the onset of
estrus.
Describe estrus in dogs.
Shortly after peak estradiol, behavioral estrus begins. Both LH and FSH peak in early estrus.
Ovulation is completed at about the third day of estrus and fertilization is completed at about
the sixth day.
Progesterone increases during the latter part of estrus signifying luteinization.
Describe diestrus in dogs.
Both pregnant and open bitches are considered to be in diestrus. Pregnancy status does not alter the length of diestrus!
Progesterone peaks at about 15 days then decreases gradually.
Bitches that do not become pregnant are often considered to be pseudo-pregnant.
Describe the estrus of queens.
A queen enters estrus (about 9 days) every 17 days.
If copulation does not occur, the queen
enters a post-estrus phase and comes into estrus a few days later.
Since the queen is an induced ovulator, when mating does not occur, ovulation does not occur and a CL is not formed.
Describe the cycle of queens if mated.
When mating occurs during estrus, ovulation is induced, fertilization occurs and pregnancy takes place.
After ovulation corpora lutea are formed causing a marked elevation in progesterone.
After a 60 day gestation period, parturition occurs and lactation ensues.
Lactational anestrus does not occur in the cat because she will come into estrus while she is lactating.
Note P4 line missing from pregnancy part of graph.
Explain Progesterone priming.
To resume cyclicity Post partum, the brain needs to be “primed” by progesterone before an adequate estrus is produced.
In seasonal breeders: The breeding season is affected by (2)
– Light
– Temperature
Suckling Effecton Lactational anoestrus:
The physical stimulation of suckling that causes the release of endogenous opioids, such as endorphins, resulting in an inhibition of LH release, prolonging the postpartum anovulatory
interval.
High P4 suppresses the release of
estradiol to some extent.
Describe: Cyclicity, follicular phase.
P4 from CL down
GnRH up
FSH & LH up
Proestrul follicular development results.
The above increases inhibin conct.
Inhibin up makes FSH down.
Dominant follicle produces estradiol which induces LH surge.
What is inhibin?
a hormone.
purpose of inhibin is to suppress (duh).
It suppresses FSH.
secreted by the Sertoli cells in men and by the granulosa cells of a dominant follicle in women.
Hormonal changes during the follicular phase (proestrus and estrus).
Proestrus sees a steady increase in estradiol.
Estrus sees estradiol and LH peak.
What are Follicular waves?
Several follicles grow at the same time, in waves.
FSH “recruits” follicles to grow.
Most eventually become atretic.
Typically one, sometimes more, become dominant and continue to grow.
P4 increases at the same time and when its high follicles can’t complete maturation.
All of the 1st wave follicles become atretic.
2nd wave dominant follicle is able to complete maturation as at that point P4 begins to decrease.
2-3 even 4 follicular waves occur.
