Reproductive Physiology Flashcards
Ovaries
site of oogenesis and most hormone production, contain the follicles that are central determinants of the estrous cycle
vulva/vagina
site of spermatoazoal deposition
uterus
site of oocyte fertilization and embryo implantation
Cervix
“gate” between the vulva/vagina and uterus, barrier for natural insemination, but not AI since it goes through cervix
oocyte
(ovum) the component of the follicle that is released at the end of the follicular phase of estrous cycle
corpus luteum
a remnant of the follicle that facilitates the luteal phase, after the oocyte has been ovulated
granulosa and theca cells
cells in the follicle and later on, in the CL that regulate the estrous cycle, hormone production and regulation. Work in concert to stimulate production and release of estrogens from the granulosa cell.
LH activates receptors on the tehca and granulosa cells while FSH activates receptors only on the granulosa cells.
They remain in the ruptured follicle, which is important for the impending luteal phase
Theca cells
periphery of the follicle, “outside”
LH stimulates the synthesis of testosterone in the theca cells. The testosterone diffuses toward the granulosa cell where it is converted to estrogen by an enzyme (aromatase) that is activated by FSH.
The estrogen then diffuses into the blood.
Granulosa cells
the cells within the follicle, “inside”, near the ovum
Also synthesize P4 in response to LH, but they can make P4 even in the absence of LH.
They cannot convert P4 to testosterone so the progesterone diffuses toward the theca cells where it is converted to testosterone that diffuses back to the granulosa cell.
The only role of FSH is to convert testosterone to estrogen via aromatase in the granulosa cells.
The FSH-estrogen activity causes the granulosa cells to proliferate and secrete fluid. This fluid antrum of the follicle. The first follicle to develop a full antrum (one that surrounds the ovum) is the follicle that is ovulated.
Estrous
adj. a series of temporal transitions that define the female reproductive cycle
Estrus
noun, a specific phase of the estrous cycle in which many mornes peak and sexual receptivity is highest, hormone levels fall precipitously at ovulation, standing estrus, can be AI 6 hours later to time with ovulation
Estrus coincides with ____
ovulation, thus increasing the change of an oocyte becoming fertilized by a spermatozoa
Polyestrous
estrous cycle repeats throughout the span of the reproductive age of the female, cattle and swine
seasonally polyestrous
estrous cycle repeats during a specific portion of the calendar year
cats, horses, sheep, and goats
monoestrous
estrous cycle occurs only one time in succession.
Certain wildlife are monoestrus such that offspring arrive in warm weather and are not neonates when winter arrives.
Dogs can have 2-3 cycles/yr but are monoestrus since it occurs 1 cycle at a time
The cycle can repeat within the same calendar year, but not immediately
Anestous
period in between estrous cycles (mono- and diestrous seasonally polyestrous), a condition in which estrus does not return in a polyestrous animal (especially cattle)
Diestrus
a specific phase of the estrous cycle, two cycles of conjunction,.
The stage with peak progesterone and maximally CL activity. Ends when the CL has invented completely.
follicular phase
the phase in which follicular development predominates. Most of the female reproductive hormones (except for progesterone) peak. Includes estrogens, LH, and FSH. Culminates with ovulation.
luteal phase
the phase in which the corpus luteum predominates. Most of the female reproductive hormones (except progesterone) are at a trough. Progesterone, inhibin and Prostaglandin F2 alpha are high. Ends when the corpus luteum undergoes involution (death).
Proestrus
the stage within the follicular phase in which follicles start to mature and hormones (except progesterone) start to rise
Metestrus
the stage within the luteal phase in which the CL matures and most hormones are at a trough
Estrogens
lipoidal hormones that are derivatives of cholesterol
Estrone (E1) and beta estradiol (E2) are the main estrogens, especially the latter
Both are produced from the male-associated hormones (androstenedione and testosterone, respectively in the granulosa cells
Interact with and activate receptors that are present inside the cell
Activation of these receptors leads to changes in gene expression in specific cells. Ultimately, the result is increased or decreased synthesis of various proteins involved in the female reproductive cycle.
estrogen binding
Estrogen binding and receptor homodimerization. Influences gene expression in many cell types. Synthesis of some proteins decrease in other proteins.
Gonadotropins
LH and FSH peptide hormones released from the anterior pituitary, released in response to GnRH released from the hypothalamus
GnRH
Gonadotropin releasing hormone, also a peptide. The release of GnRH is regulated by several different factors, ie hormones
hormones of the pituitary hypothalamic axis
LH, FSH, and GnRH: interact with and activate seven transmembrane-spanning G protein coupled receptors (GPCRs)
Each peptide is recognized by a specific GPCR, ie the GnRH receptor will not recognize LH or FSH.
Gonadotropin receptors
GnRH receptor: anterior pituitary
LH receptor: theca interna cells
FSH receptor: granulosa cells
When activated by its cognate ligand (hormone) a GPCR undergoes a conformational change. This conformational change “attracts” intracellular enzymes that subsequently alter biochemical cascades and gene expression within the cell.
GPCR-based effects tend to be faster than those observed with the activation of intracellular receptors, ie the estrogen-R.
Causes opening of ion channels to be faster
Progestins
lipoidal hormones that are derivatives of cholesterol
Progesterone is the main progestin
This hormone promotes gestation. Whereas estrogen is what the estrous cycle is generated by.
Interact with and activate receptors that are present inside the cell; these receptors are different than the strogen-R. Activation of these receptors lead to changes in gene expression in specific cells. Ultimately, the result is increased or decreased synthesis of various proteins involved in the female reproductive cycle.
Progesterone receptors
Progesterone binding and receptor homodimerization. Influences gene expression in many cell types. Synthesis of some proteins decrease in other proteins.
Prostaglandin F2 alpha
a lipid released from the uterus,
Released during diestrus if a fertilized ovum has not implanted in the uterus. The hormone responsible for continuing the estrous cycle if the animal is not pregnant
Although it is a lipid, PGF1 alpha interacts with a GPCR on the surface of cells. There are many prostaglandins and each one activates a specific GPCR.
For example, PGE does not activate the PGF2 alpha receptor and, likewise, PGF2 alpha does not activate the PGE receptor
Inhibin
a peptide released from the granulosa cells. Mildly inhibits the release of GnRH from the hypothalamus. Activates a cell surface receptor that is not a GPCR. But, just like a GPCR, activation of this receptor leads to biochemical and gene expression changes within the cell.
Inhibin binding receptor
Hypothalamus: Inhibin binds and receptor activated, activated receptor in turn activates another protein. The protein upregulates specific gene expression. Genes encode for proteins that trap GnRH in the hypothalamic cell.
Anterior Pituitary cell: Inhibin binding and receptor activation; activated receptor in turn activates another protein. The protein upregulates specific gene expression. Genes encode for proteins that trap FSH in the anterior pituitary cell.
Hormone receptors in the estrous cycle
Estrogen and progesterone are lipid-soluble so they can cross biologic membranes and interact with their intracellular receptors. PGF2-alpha is lipid while LH and FSH and GnRH are peptides that activate G protein coupled receptors. Inhibin is a peptide that activates a non-GPCR cell surface receptor.
Aromatase deficiency and virilization of females
Aromatase is the enzyme needed for the conversion of testosterone to estrogen. Conversion and activation primarily occurs in the granulosa cells. Genetic mutations can lead to a failure to produce the enzyme or the production of a dysfunctional enzyme. In some granulosa cell tumors, the enzyme is repressed leading to virilization/ masculazation.
For the genetic mutation, the net result can be pseudohermaphrodism and transient virilization of the mother during pregnancy. Maternal virilization (hirsutism and acne) can be subtle and undetected in many species (except humans). Offspring are often infertile because they do not have the estrogen surge/ peak needed for ovulation. Mild deficiency identified in a very few high-level female athletes.
Hormone replacement therapies
Estrogen and progesterone are lipid-soluble so they can cross biologic membranes and interact with their intracellular receptors. Because they are lipid soluble, they can be administered orally and then cross the intestinal barrier and enter the portal and systemic circulations.
LH, FSH, and GnRH are peptides that do not cross biologic membranes. Because of their physicochemical characteristics and the presence of peptidases in the intestinal tract, they must be administered by injection (IM or sub Q).
Hormone changes during Proestrus
P4 plummets and estrogen and LH and FSH all start to rise.
P4 hits a trough at about mid-proestrus, coinciding with a rapid ascent of estrogen, and LH and FSH levels.
The mostly inverse relationship between P4 and estrogen and LH and FSH is based on negative control mechanisms discussed later.
The decline in P4 is a result of the involution (death) of the corpus luteum, the major supplier of P4 during the estrous cycle.
The CL is the post-ovulatory “follicle” site (more discussion in the luteal phase lecture)
Sources of hormones
Estrogen: follicle
P4: CL
LH: anterior pituitary
FSH anterior pituitary
natural insemmination
precedes ovulation by 1 day
artificial insemination
For artificial insemination, the delivery of the semen is targeted closer to ovulation- usually 8-12 hrs after the 1st signs of estrus.
Hormone changes during estrus
Estrogen, LH, and FSH, peak as the animal demonstrates estrus activity (receptivity). The hormonal peak occurs about 1 day prior to ovulation.
Estrogen, LH and FSH levels rapidly descend from the time of estrus to ovulation. All four of the major hormones (even P4) are at their lowest levels at ovulation and for a few days after. This low hormone status is due to: cellular changes in the follicle at and after ovulation (estrogen); negative control mechanisms preventing gonadotrope release (LH and FSH); and the immature CL (P4).
Physical progression of the follicle
The ovaries are loaded with primordial follicles.
A group (cohort) of follicles are then physically grouped (recruited) and mature into small follicles. A portion of these follicles undergo atresia (deeneration) while another portion (cohort) matures into medium follicles (selection).
A very small number of medium follicles mature into large follicles and the rest undergo atresia. One of the large follicle exerts a dominant effect (via hormones and physical interactions) on all other large follicles.
The dominant follicle becomes the follicle that is ovulated from the ovary.
Follicular development occurs in waves whereby the maturation process occurs two to three times per cycle.
Ultimately one follicle becomes the ovulatory follicle.
As the follicle develops, a fluid-filled sac enlarges and encompasses the ovum.
This sac is termed the antrum.
Once the antrum surrounds the ovum and occupies most of the follicle the ovum is discharged from the ovary.
Follicular Dynamics: hormonal progession
Maturation of follicles is initiated by estrogen produced by the primordial follicles. The estrogen causes a release of FSH and LH via GNRH release from the hypothalamus. The medium follicles then release more estrogen plus inhibin.
More estrogen leads to more GnRH release but inhibin prevents the further release of FSH.
The dominant follicle releases large amounts of estrogen plus inhibin.
GnRH is thus released in large quantities.
LH is thus released in large quantities (the LH surge) while inhibin continues to hamper FSH release.
P4 from CL decreases As the CL dies. GnRH increases Since P4 was repressing GnRH release.
FSH and Lh increase. Leads to Estrual follicular development, inhibin increases causing FSH decrease. Estradiol increases to threshold causing Preovulatory LH surge.
Feedback mechanisms: P4 prevents GnRH release at the hypothalamus
Progesterone binding and receptor homodimerization. Influences gene expression in the hypothalamus. Genes encode for protein that tap GnRH in the hypothalamic cells.
Feedback Mechanisms: GnRH promotes LH and FSH release at the anterior pituitary
GnRH travels from the hypothalamus to the AP. At the AP, GnRH activates its receptor.
The activated GnRH receptor initiates biochemical cascades resulting in the release of FSH and LH
Feedback Mechanisms: LH and FSH promotes estrogen synthesis in the follicle
LH stimulates the synthesis of testosterone in the theca cells. The testosterone diffuses toward the granulosa cell where it is converted to estrogen by an enzyme (aromatase) that is activated by FSH.
The estrogen then diffuses into the blood, leading to more GnRH and more LH and FSH released.
Feedback Mechanisms: estrogen promotes GnRH release from the hypothalamus
Estrogen binding and receptor homodimerization.
Influences gene expression in the hypothalamus.
Genes encode for proteins that export GnRH out of the hypothalamic cells.
Feedback Mechanism: estrogen promotes its own synthesis at the follicle (granulosa cell)
Estrogen binding and receptor homodimerization.
Upregulate FSH- receptor gene expression.
More FSH receptors lead to more estrogen produced.
Feedback mechanism: inhibin prevents FSH release from the anterior pituitary (mildly does the same for GnRH)
Inhibin binding and receptor activation; activated receptor in turn activates another protein. The protein upregulates specific gene expression. Genes encode for proteins that trap FSH in the anterior pituitary cell.
Feedback mechanism: estrogen promotes the LH surge from the anterior pituitary at ovulation, by exporting GnRH from the hypothalamus
Estrogen has complete control of GnRH release after P4 remains absent for 1-2 days
Since P4 is no longer around, GnRH is released in larger quantities.
Net result is an LH surge since FSH release is still hampered by inhibin at the anterior pituitary.
superovulation
The process in which the ovary is pharmacologically “nudged” to ovulate more than one ovum.
Chemical analogues of LH and FSH are given to the animal, ultimately overriding the inhibitory effects of inhibin.
Used in embryo transfer to maximize the number of ova that can be fertilized.
Cystic ovary
Pathologic situation in which the follicle fails to ovulate.
Often related to metabolic deficits in the lactating dairy cow.
Treatment involves a GnRH analogue +/- manual rupture via palpation.
Cystic follicles are much larger than natural follicles about to ovulate.
Cystic corpus luteum will be really firm.
Metestrus
P4 starts to rise after ovulation. P4 peaks during diestrus.
The other major hormones (E2, LH, FSH, and GnRH) are at insignificant levels during metestrus and diestrus. The lack of these other major hormones is due to the ability of P4 to inhibit both the release and activity of GnRH
Physical progression of the corpus luteum
At ovulation, the follicle ruptures the serosal surface of the ovary, releasing the antral fluid and the ovum. Theca and granulosa cells remain in the ruptured follicle, which now becomes the corpus luteum (CL)
corpus hemmorhagicum
Blood vessels also rupture, thus the early CL is called the called the corpus hemmorhagicum.
This stage is physically evident since a bloody vaginal discharge can be observed.
The theca and granulosa cells undergo a transformation into luteal cells, designated as such because cholesterol and lutein infiltrate the cells thus providing a yellow appearance.
corpus luteum
Consolidation of blood vessels and transformed cells. The CL continues to grow as the LLCs increase in size and the SLCs increase in number.
CL size is maximized at about mid-cycle (day 14 after ovulation)
LLC
The granulosa cells become large luteal cells (LLC) that have gone through hypertrophy as they fill cholesterol
SLC
The theca cells undergo hyperplasia (increase in number but not size) thus transforming into the small luteal cells
luteolysis
After day 14 of ovulation, due to the finite lifespan of the LLCs and SLCs, and a hormone that kills the LLCs and SLCs.
Once the corpus luteum has degenerated to a small state, the cycle will restart at the follicular phase.
The process in which the CL is minimized in both size and function.
P4 levels drop sharply during luteolysis, since the dead/dying LLCs and SLCs cannot produce P4.
As the CL degenerates and losses the lutein and cholesterol, the yellow color abates and the structure is now called the corpus albicans
Both a passive and active process that results in apoptosis in the luteal cells. Results in calcium-mediated mitochondral and DNA damage in LLCs and SLCs.
The passive process is a default activation of the intrinsic apoptosis pathway that is innately “pre-programmed” into luteal cells
corpus albicans
white body, the very small corpus luteum
This structure further disintegrates over time, otherwise the ovary would expand because of each new corpus albican per cycle.
Progesterone synthesis
Both the theca and granulosa cells are capable of synthesizing P4. This synthesis is controlled by LH in theca cells (and thus the SLCs).
For granulosa cells and thus LLC, P4 synthesis is an ongoing (constitutive) process that does not require LH.
Thus the LLs are a major source of P4 from the CL, since LH levels are very low during the luteal phase
Progesterone in the luteal phase
P4 is the “quiescent” hormone that minimizes many of the other hormones during the luteal phase. P4 also minimizes uterine proliferation and contractility, while promoting glandular secretions.
The CL has a finite time period in which it can synthesize P4 in the absence of LH or an analogue.
active facet of luteolysis
The active facet of luteolysis involves the hormones oxytocin and PGF2 alpha.
The dying CL releases oxytocin, a peptide hormone.
As apoptosis increases in the CL, oxytocin release also increases.
Oxytocin acts locally on the uterus via the vascular counter-current exchange. P4 increases the number of oxytocin receptors in the endometrium.
Oxytocin then stimulates the synthesis of PGF2 alpha in the uterus.
PGF2 alpha directly activates apoptosis by activating its cognate GPCR that subsequently facilitates a calcium influx.
Additionally, the number of PGF2 alpha receptors on the CL remains static even though the CL shrinks in size.
The PGF2 alpha activity speeds the degeneration of the CL and a return to the follicular phase.
vascular counter- current exchange
PGF2 alpha goes to the CL using the vascular counter-current exchange.
For oxytocin, the peptide moves from the ovarian vein into the uterine artery
progesterone prevents GnRH release at the hypothalamus
Progesterone binding and receptor homodimerization.
Influences gene expression in the hypothalamus. Genes encode for proteins that trap GnRH in the hypothalamic cells.
P4 prevents GnRH receptor synthesis in the AP
Progesterone binding and receptor homodimerization
Influences gene expression in the anterior pituitary
Inhibits the expression of the gene encoding for the GnRH receptor.
P4 stimulates oxytocin receptor synthesis in the endometrium
Progesterone binding and receptor homodimerization. Influences gene expression in the endometrium.
Activates the expression of the gene encoding for the oxytocin receptor.
Oxytocin stimulates PGF2 alpha synthesis in the endometrium
Oxytocin travels from the CL to the endometrium.
At the endometrium, oxytocin activates its receptor (whose expression has been promoted by P4).
The activated oxytocin-receptor initiates a biochemical cascade (involving PLA2) resulting in the removal of PGF2 alpha from the lipid bilayer.
The PGF2 alpha then leaves the endometrial cell and goes to the CL.
PGF2 alpha promotes oxytocin release from the CL
PGF2 alpha induces apoptosis in the CL resulting in more oxytocin release.
Oxytocin travels to the endometrium, where it induces more PGF2 alpha synthesis.
The loop continues as more PGF2 alpha is released from the endometrium, causing more oxytocin release from the CL
Uterine anomalies and anestrous
In a truly anestrus situation, the animal does not exhibit estrus and thus does not appear to have an estrous cycle, there is also no bloody vaginal discharge (corpus hemmorragicum (CH) release) observed during the cycle.
Often associated with the metabolic drain of producing high volumes of milk in dairy cattle.
Can be associated with anatomical anomalies involving the uterus.
Excessive scar tissue in one uterine horn, or the absence of a uterine horn, can lead to anestrus.
If the scar tissue or anatomic deficiency occurs on the uterine horn ipsilateral to the CL, luteolysis will be slowed because the CL will die from the passive apoptosis.
In these cases, the estrous cycle is typically doubled (42 days vs the normal 21 days) since the active facet of luteolysis is not contributing to the death of the CL.
But if the damaged or incomplete the uterine horn is contralateral to the CL, the estrous cycle proceeds as normal.
That is, the active facet of luteolysis (involving PGF2 alpha and oxytocin) is still viable if the uterine horn pathology is on the opposite side of the CL.
Randomness dictates which ovary (left or right) has the ovulatory follicle and the resulting CL. To ensure maximal fertility, remove the ovary on the bad side.
pharmacologic luteolysis
For dairy farmers, it is imperative that cows get pregnant ASAP.
At day 6 post estrous, the CL is sensitive to the effects of PGF2 alpha. In the normal estrous cycle, the uterus does not make significant amounts of PGF2 alpha until day 17 post-estus.
To short cycle the cow, PGF2 alpha can be administered at day 6 post-estrus. The cow should then demonstrate estrus in about 3 days post-injection, with artificial insemination ensuing shortly thereafter.
For cows that did not demonstrate estrus or estrus was missed by the person in charge of monitoring estrus, the corpus hemorragicum will prequently appear on the tail at day 3 post estrus. The animal can then be administered PGF2 alpha bout 3-4 days later.
This short cycling process can shave 11 days off of the cycle.
this practice can also be used to “synchronize” a gorup of animals, usually a group of heifers. All animals in the group receive the PGF2 alpha injection twice with a 10 day interval in between. Most of the animals will exhibit estrus at 3 days following the second injection.
For the animals in day 0 to day 5 post-estus group (PGF2 alpha insensitive to the first injection), they will be sensitive to the second injection since they will now be at day 10 to day 15.
For animals in the day 6 to day 21 post-estrus group, the first injection will “resent” their estrus cycle to day 0 thus they will be at day 10 upon the second injection.
This synchronization is used in embryo transfer to generate a group of fertile recipients; and to synchronize the calving of heifers as a management tool.
The disadvantage to the short-cycling process is the possibility for an abortion if the animal is pregnant. This can arise because of improper record keeping, especially in the lactating animal. Therefore it is best to check for pregnancy, via rectal palpation, before administering PGF2 alpha.
cystic CL
Occurs an occasion when the theca and granulosa cells make an incomplete transition to SlCs and LLCs respectively.
The CL fills with antral-like fluid, does not undergo apoptosis, and is PGF2 alpha insensitive.
The animal will exhibit anestrus, and manual rupture (via rectal palpation) is a potential course of action.
Testes
site of spermatogenesis and hormone production
Penis
delivers spermatozoa
Bull Accessory glands:
prostate, bulbourethral (Cowpers) gland, and seminal vesicles (vesicular gland), provide components needed in the ejaculate
Boar Accessory glands
prostate, bulbourethral gland, and seminal vesicle, provide components needed in the ejaculate
Horse accessory glands
prostate, bulbourethral gland, and seminal vesicle, provide components needed in the ejaculate
Dog accessory glands
prostate, bulbourethral gland, and seminal vesicle, provide components needed in the ejaculate
Seminiferous tubules
site of spermatogenesis and hormone production
Spermatozoa are formed from spermatogonia lining the epithelium of the seminiferous tubules
Spermatozoa are formed from spermatogonia lining the epithelium of the seminiferous tubules.
Sertoli cells (not depicted) are within the tubules, and facilitate sperm maturation.
Leydig cells are adjacent to the tubules in the interstitium (support structure or scaffold)
epididymis
storage tubules for spermatozoa
vas deferens
transport tubule for exiting spermatozoa
spermatogenesis
production of sperm, overall process that lead to the production of sperm (spermatogonia to spermatozoa)
Timeframe is species-dependent (61 days in bovine)
There is correlation between production and scrotal circumference.
No yet practical way to pharmacologically or otherwise enhance daily sperm production.
spermatocytogenesis
the conversion of spermatogonia to spermatocytes
Spermatogonia are attached to the epithelium of the seminiferous tubules.
Sertoli cells then propel the spermatogonia towards the central part of the lumen of the seminiferous tubule.
Hormones from the Sertoli cells then cause the spermatogonia to mitotically divide and become become primary spermatocytes.
The mitotic processes can occur up to 6 times, with incremental steps of maturation ocurring at each stage.
The number of divisions is species-dependent.
Each division moves the spermatogonia towards the lumen.
spermiogenesis/ spermatidogenesis
the conversion of the spermatocyte to the spermatozoa (ie. spermatidogenesis + differentiation)
A primary spermatocyte then undergoes meiosis to form two secondary spermatocytes.
A secondary spermatocyte then undergoes meiosis to form two spermatids that mature.
Each primary spermatocyte undergoes two steps of meiosis to form four spermatids, with each step moving toward the lumen.
Each spermatid matures into a single mature spermatid.
Spermatids
mono-chromosomal cells containing a single chromosomes from the paternal parent (one autosome and one sex chromosome, either X or Y).
Mature spermatids are a hybrid of an epithelial cell and a sperm.
spermiogenesis-differentiation
Mature spermatids differentiate into spermatozoa that “float” in the lumen of the seminiferous tubules.
The differentiation process leads to the formation of components that allow for motility (the tail) and the ability to penetrate the ovum (the acrosome). The Golgi apparatus becomes the acrosome. It produces granules that form the proacrosome. At the same time the centrioles migrate to the opposite side of the nucleus, and align perpendicularly to form the proximal and distal centrioles.
acrosome
“leading” surface of the spermatozoa, which was formed from the golgi apparatus.
Contains enzymes necessary for entering the surface of the ovum.
One such enzyme is hyaluronidase shich breaks apart hyaluronic acid structures present on the surface of the ovum.
Also contains proteolytic enzymes that break apart proteins on the surface of the ovum.
Also contains receptors that recognize chemicals releases from the ovum. These receptors participate in the chemotaxis of sperm.
Cap phase
part of differentiation in spermiogenesis.
a. The Golgi moves toward the distal ople.
b. The inner and outer acrosomal membranes are formed.
c. The acrosome forms from the distal centriole.
Acrosomal phase
part of differentiation in spermiogenesis.
a. The nucleus elongates.
b. the proximal centriole becomes the neck
c. the mitochondria migrate to the distal pole.
maturation phase
part of differentiation in spermiogenesis.
a. Mitochondria become trapped in the middle piece by the neck (proximal) and annulus (distal).
b. The principle piece, containing the motility proteins (microtubules), becomes evident.
Spermiogenesis- tail formation
The microtubules that contain proteins that create a whip-like action in the tail.
Dyein, kinesin, tubulin, and nexin are microtubular proteins that coordinately move, causing a bend in the tail of the sperm. Tubulin is anchored (like actin) while dyein and kinesin move in opposite directions along the tubulin polymer, resulting in a tubulin “wave”
storage of sperm
Spermatozoa are mature spermatids that are released into the lumen and have moved to the epididymis and vas deferens to storage.
These tubes secrete substances that keep sperm motility to a minimum.
Motility is activated during and after ejaculation.
Ejaculation
Peristalsis of the vas deferens moves sperm from the epididymis to the urethra. Urethral contractions move the sperm out of the penis. Along the way, the epididymis, vas deferens and accessory glands add various components to the sperm, thus creating semen.
Prostate gland
The prostate adds alkalinity and a clotting enzyme.
The alkalinity immobilizes the sperm that are optimally motile at pH 6.
The clotting enzyme converts fibrinogen (added by the seminal vesicles) to fibrin that is cross-linked and clumps the sperm.
The prostrate also adds profibrinolysin that is activated in the vagina.
Vesicular glands
The vesicular glands (seminal vesicles in the horse) add fructose, citric acid, prostaglandins, and fibrinogen to the sperm.
Fructose and citric acid serve as nutrients for the sperm.
Fibrinogen is the precursor of fibrin, a protein that is important in blood clotting but it can also coagulate any type of cell.
bulbourethral gland
adds mucus to the ejaculate. This mucus, along with cholesterol (provided by the seminiferous tubules) that coats the sperm, protects the sperm and prevents excessive motility that wastes energy.
Semen thus contains sperm plus protective substances (mucus, fibrin, and cholesterol) plus nutrients and hormones.
cytoplasmic droplet
remnant of the spermatozoal cytoplasm arising from the acrosome.
During the nuclear condensation/elongation process, this droplet is “squeezed” through the mid-piece and to the tail while in the epididymis.
Lost during ejaculation
sperm morphology
The head is loaded with DNA and its shape is species-dependent. The acrosome absorbs stain (eosin-nigrosin) when the sperm is dead.
Cryptorchidism
failure of descent of one or both testes
Spermatozoa are extremely sensitive to the increased temperature inside the body cavity, so no viable sperm are produced from an undescended testes.
However, Leydig cells prefer the elevated temperature and thus the patient can produce an adequate amount amount of testosterone from an undescended testes (assuming that the testes are structurally normal).
Unfortunately, the Leydig cells in the undescended testes are overstimulated by the body heat.
This overstimulation can lead to proliferation resulting in cancer.
corpus cavernusum
erectile tissue of the penis
corpus spongiosum
spongy area that surrounds the urethra and prevents occlusion of the urethra during tumescence
Tumescence
penile erection
The flaccid penis becomes tumescent via the corpus cavernosum.
Nitric oxide (released by the parasympathetic system) causes helicine artery dilation allowing blood to move through loosened interendothelial junctions.
The blood fills the corpus cavernosum causing turgor pressure that causes tumescence.
The NO causes the vasodilation by relaxing the vascular smooth muscle (VSM) of the helicine (aka central) arteries in the corpus cavernosum.
NO is released by NANC (non-adrenergic/non-cholinergic) neurons (parasympathetic). NO activates guanylate cyclase which converts GTP to cGMP.
cGMP inactivates myosin light chain kinase (MLCK) in VSM cells.
Phosphodiesterase 5 (PDE5) recycles cGMP to GMP, thus halting tumescence by returning the VSM to the contracted state (restoration of sympathetic tone via norepinephrine).
VSM relaxation allows for the extravasation of blood from the helicine artery into the corpus cavernosum.
Blood cells “trickle” between the endothelial cells and the VSM cells.
Upon VSM contraction, veins drain the blood from the corpus cavernosum as part of detumescence.
Tumescence and ejaculation-differing autonomic control
Tumescence is driven by the parasympathetic system involving release of No that relaxes the VSM of the helicine artery in the penis (detumescence is therefore sympathetic).
Ejaculation is driven by the sympathetic system that promotes the release of mucus and the coordinated contraction of the epididymis, vas deferens, and urethra.
Androgens
the major hormones regulating male reproductive physiology.
Includes: testosterone, dihydrotestosterone, and androstenedione.
Primarily produced by the Leydig cells that are in the interstitium of the seminiferous tubules.
Regulate male reproduction by interacting with nuclear receptors that promote various functions in the target cell.
Dihydrotestosterone has the greatest affinity for the androgen receptor and many cells convert testosterone to dihydrotestosterone via n enzyme designated as 5 alpha reductase.
Since androgens regulate gene expression, these hormones tend to regulate long-term functions and are not typically involved in immediate responses:
In utero development of male sex organs, descent of the testes, growth of male sex organs, puberty, body hair, libido, etc.
main androgen produced by the Leydig cells
Testosterone, although all three are produced
Nuclear androgen receptor mode of action
Androgen binding and receptor homodimerization.
Translocation to nucleus.
Bind to DNA.
Upregulate specific gene expression.
androgens and gonadotropins
Androgen production and release is controlled by two gonadotropins: gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH), both of which are peptides.
GnRH is released by the hypothalamus and it stimulates the release of LH from the anterior pituitary.
LH is released into the systemic circulation and it reaches the Leydig cells that have an abundance of LH receptors.
Activation of the LH receptors leads to the synthesis of testosterone (from cholesterol) by the Leydig cells.
The LH receptor is a 7-transmembrane-spanning G protein-coupled receptor.
GnRH also stimulates the release of follicle-stimulating hormone (FSH) from the anterior pituitary. FSH stimulates spermatogenesis by the Sertoli cells by promoting the elevation of the spermatocytes towards the lumen of the seminiferous tubules.
FSH is also a peptide that activates a G protein-coupled receptor.
Testosterone works in concert with FSH to promote spermatogenesis at the Sertoli cell.
Prolactin
another gonadotropin released by the anterior pituitary in response to GnRH.
Prolactin stimulates the synthesis of LH receptors on the Leydig cells.
Prolactin is also a peptide but it activates a receptor that is not a G protein-coupled receptor.
negative feedback of testosterone
Testosterone controls its own over-production at the level of the hypothalamus and anterior pituitary.
Does so either by directly interacting with the hypothalamus/pituitary, or through the actions of inhibin.
Inhibin travels to the brain where it prevents the release of FSH and GnRH.
Inhibin activates its receptor that promotes the synthesis of proteins that trap FSH in the anterior pituitary cells or GNRH in the hypothalamus.
The inhibin receptor is a serine-threonine kinase type of receptor
Testosterone-mediated control of GnRH, LH, and FSH release
Androgen binding and receptor homodimerization
Upregulate specific gene expression
Genes encode for proteins that trap LH and FSH in the AP cell.
Testosterone-mediated Activation of Inhibin release (Sertoli)
Androgen binding and receptor homodimerization.
Upregulate inhibin gene expression.
Inhibin released from the Sertoli cell.
Inhibin-mediated control of FSH release
Inhibin binding and receptor activation; activated receptor in turn activates another protein.
The protein upregulates specific gene expression
Genes encode for proteins that trap FSH in the AP cell
Testosterone
produced by Leydig cells in response to LH. Sertoli cells convert it to DHT.
Both testosterone and DHT inhibit and release of GnRH from the hypothalamus.
Inhibin is produced by sertoli cells and it inhibits the release of FSH from the anterior pituitary.
Inhibin also prevents the release of GnRH from the hypothalamus.
Hormone control-summary of receptors
Testosterone is lipid soluble so it can cross biologic membranes and interact with its intracellular receptor.
FSH, LH, and GnRH are peptides that activate G protein-coupled receptors.
Inhibin is a peptide that activates a serine-threonine kinase receptor (prolactin activates a similar type of receptor).
Penis
Dogs, cats, and non-human primates have an os penis or penis bone or baculum.
Helps to maintain tumescence in species in which coitus is protracted.
May also be an evolutionary adaptation such that tumescence is maintained even through the male may be distracted by competing males during coitus.
bulbus glandis
Dogs have a bulbus glandis that enlarges during coitus
helps to maintain the interaction with the female
known as the “tie” or “lock”
cantharadine and increased libido
Cantharadine is a defensive compound found in blister beetles, aka Spanish fly.
Cantharadine blisters in the skin, but if ingested it will cause inflammation in the urethra, prostate, vas deferens, and testes.
Inflammation in these regions leads to an increased libido as a means of expelling the inflammation.
occasionally dead blister beetles are trapped in alfalfa hay.
Cattle fed this hay can demonstrate hypersexual activity.
This type of hypersexual activity from reproductive system inflammation has also been noted for humans infected with the rabies virus.
Precocious puberty
a disease in which males exhibit puberty at a very young age (1/4th of normal). Due to a genetic mutation involving the gene encoding the LH receptor.
As normal male enters puberty at an appropriate age, LH is produced and the LH receptor is activated leading to testosterone synthesis.
In PP, the mutation yields an LH receptor with a unique ability to be active even in the absence of LH (constitutively active receptors).
These males produce enough testosterone to instigate puberty at a very early age.
Besides LH, what other types of receptors could be constitutively active and lead to precocious puberty?
GnRH-R
Testosterone-R
Prolactin-R (probably not since LH would be absent)
The absence of what receptor could also lead to precocious puberty?
Inhibin-R
Testosterone, PEDs, and cancer
Androgen binding and receptor homodimerization. Upregulate specific gene expression.
Genes encode for proteins that promote cell growth.
PEDs (anabolic steroids) are sometimes used by athletes (horses) to increase muscle mass and performance.
Testosterone, or a structural analog that activates the androgen receptors are used in this capacity.
This practice can permanently impair the ability of the Leydig cells to produce testosterone (leading to infertility).
However, moderate supplementation of testosterone, or an anabolic steroid, has been used in emaciated patients.
Excess testosterone can cause cancer
Androgen binding and receptor homodimerization.
Upregulate specific gene expression.
Genes encode for proteins that promote cell growth and division (hyperplasia)
Sperm in the Female Reproductive Tract
During natural insemination, sperm deposition occurs in the vagina.
During artificial insemination, sperm deposition can occur in the uterus or a specific uterine horn.
For natural insemination, the cervix is the major obstacle preventing the sperm from reaching the site of fertilization (uterus)
A few sperm are rapidly transported to the uterus within a few moments after arrival in the female.
These sperm are non-viable and may serve as “reconnaissance” sperm that clear the path for their viable counterparts.
The vast majority of viable sperm are delivered by sustained transport mechanisms that move the sperm to the uterus in a uniform pattern over time.
In some species, vaginal plugs prevent sperm loss.
Vaginal plugs can be created by the female or be part of semen.
The most important obstacle for sperm is the cervix (natural insemination).
Sperm travel through the cervix via basal channels that contain less abundant and thinner mucous.
Sulfomucins are chemically more dense than sialomucins, and thus the sperm travel through the sialomucin-containing channels.
Sperm are lost or inactivated by:
- Retrograde transport (gravity) that expels the serum away from the uterus.
- Phagocytosis by leukocytes (especially neutrophils)
- microbes (especially bacteria) that adhere to sperm and decrease motility-introduced during copulation
Sperm are actively transported towards the uterus by
- Myometrial contractions induced by prostaglandins and estrogen from semen.
- Estrogen from the female.
Bidirectional myometrial contractility
Towards the ovary during insemination.
Toward the vagina during menstruation and parturition.
The direction is determined by the actin:myosin ratio and the type of myosin present.
Actin is more abundant than myosin most of the time in myometrial cells.
Transient expression of a phosmo-myosin in a portion of the myometrial cells will even out the ratio and cause a change in contractile direction
Capacitation of Sperm
Sperm in the epididymis are nonmotile and covered with inhibitors.
Ejaculated sperm are motile but covered with inhibitors.
The female reproductive tract removes the inhibitors, facilitating motility and viability.
Mucus, cholesterol, fibrin, profibrinolysin, and other proteins coat the sperm.
The female reproductive tract removes the mucus and cholesterol.
It also activates profibrinolysin that becomes fibrinolysin which breaks apart the fibrin.
Ultimately, the capacitation process reveals zona pellucida-binding proteins and chemotaxis receptors on the acrosome of the sperm.
Capacitation occurs over several hours and is reversible.
That is, some sperm can be de-capacitated and then re-capacitated over 1-2 days.
Review of Ovulation
Ovulation releases a ovum surrounded by the corona radiata.
The corona radiata is made up of granulosa cells.
The corona radiate nourishes the ovum after ovulation.
The zona pellucida is interior to the corona radiata.
Fertilization
The released ovum then encounters sperm in the ampulla of the oviduct.
Sperm are randomly hypermotile in the ampulla, and one penetrates the ovum.
The penetration leads to a calcium influx into the ovum, causing a release of granules.
These granules alter the biochemistry of the surface of the ovum such that no more sperm will attach (polyspermy inhibition)
steps of fertilization
- a sperm attaches to zona pellucida
- a sperm acrosome binds to receptors on zona pellucida
- Acrosome reaction
- Penetration of a sperm in zona pellucida.
- Fusion of a sperm with the plasma (vitelline) membrane of the oocyte
6a. Cortical reaction that releases calcium across the perivitelline space to the zona pellucida
6b. The zona reaction that biochemically alters the zona pellucida to make it impervious to additional sperm.
Fertilization- attachment
driven by:
acrosomal receptors that recognize chemotactic chemical released from the ovum
Acrosomal enzymes (proteases) that break apart proteins on the corona radiata
Binding of acrosome to receptors on zona pellucida
The acrosome surface contains molecules (ZBR and ARPR) that physically bind to a molecule (ZP3) on the zona pellucida.
ZBR and ARPR are ligands, while ZP3 is the receptor
Acrosome reaction
this reaction is initiated by the ZBR-ZP3 binding event.
The OAM and IAM fuse, thus causing an exocytosis of the acrosomal contents.
Hyaluronidase and acrosin are two enzymes released from the acrosome, and these enzymes promote the penetration of the zona pellucida (fusion)
Fusion
also aided by sperm motility.
The sperm traverses the zona pellucida and settles in the perivitelline space.
The IAM fuses with the oocyte (vitelline) membrane.
The fusion causes the release of calcium from cortical granules, initiating the zona reaction.
The excess calcium in the perivitelline space leaks into the zona pellucida, preventing any further penetration (vitelline block)
Fertilization-introduction of sperm DNA into the oocyte
Sperm nuclear material must decondense from its supercoiled state to a more linear state.
The decondensation process involves reduction of disulfide linkages in the histone proteins holding the DNA.
syngamy
fusion of pronuclei. After the sperm DNA is decondensed, it goes through structural changes to become the male pronucleus. The unpaired male and female pronuclei then align to form paired chromosomes. The resulting zygote then divides and moves toward the uterus.
summary of zygotogenesis
Sperm hypermotility goes in random directions, thus relying on a chance encounter with the ovum.
But all of the other steps involve specific molecular interactions:
Acrosomal enzymes (attachment)
ZBR/ARPR-ZP3 binding
Acrosomal fusion proteins that bind to the oocyte membrane
Disulfide reductions: disulfide bond between histones breaks.
DNA base-pairing by hydrogen bonding: AT and CG
Fertilization-zygote transitions
The zygote transitions to a morula then to a blastocyst.
The morula contains 4-64 cells (depending on species).
To become a blastocyst, the cells in the morula condense to one end leaving an open space called the blastocoele.
Cells are designated as blastomeres.
The blastocoele pressure forces the blastocyst out of the zona pellucida.
This excytosis process is termed hatching.
The hatched blastocyst contains the ICM, the blastocoele, and the trophoblasts.
blastogenesis
The outer blastomeres (trophoblasts) of the morula adhere because of tight junctions between the cells.
The inner blastomeres of the morula adhere together because of gap junctions between the cells.
Trophoblasts release Na+ that osmotically attracts water, thus pushing the central blastomeres eccentrically into the intracellular mass (ICM)
The blastocyst is the entity that is implanted into the uterus.
Implantation occurs at the ICM locus of the blastocyst.
ectopic pregnancy
Fertilization and/or implantation occurs somewhere besides the uterus.
The abdominal type is the rarest but possible since the ovary-oviduct junction is not secure.
Parturition can be very precarious, with C-section and hysterectomy likely outcomes.
Ectopic fertilization can occur randomly but ectopic implantation requires either:
a. Endometriosis-uterine pathology that halts that production of PGF2 alpha
b. Ectopic production of CG (c`horionic gonadotropin)
twins
Identical twins arise when one oocyte is fertilized but, at the two-cell stage, the cells separate and each one independently goes through blastogenesis.
That is the blastocyst has two ICMs.
Fraternal twins are the result of two separately fertilized oocytes.
Pre-Implantation Changes in the zygote:
Development of the embryo within the zona pellucida
The single-celled zygotes become two-celled (biblastomeric) which becomes the morula.
Morula blastomeres segregate into the trophoblasts (periphery) and the ICM (eccentric), forming the blastocyst
The 2, 4, and 8 cell stages are totipotent, meaning that they can differentiate into any type of cell.
At the ICM stage, cells are pluripotent meaning that they can differentiate into one of the three germ layers (ie. embryonic stem cells).
Blastomeres get progressively smaller during this pre-hatch phase.
That is, there is no net increase in size of the blastocyst.
Hatching- Trophoblast cells produce:
i. Fluid that causes pressure within the zona pellucida.
ii. Proteolytic enzymes that break apart the zona pellucida, thus releasing the blastocyst.
Post-hatching embryogenesis:
The hatched blastocyst undergoes a series of changes just prior to implantation: The blastocyst increases in size. A number of membranes form, which are needed for implantation.
These membranes include the yolk sac, the chorion, the amnion, an the allantois.
The hatching embryogenesis: The primitive endoderm forms within the blastocoele, adjacent to the trophoblasts. The yolk sac begins as an evagination of the primitive endoderm on the ventral side of the ICM, which now is synonymous with the embryo. While the endoderm and yolk sac are forming, the mesoderm forms on the ventral side of the embryo (ICM). Note that the zygote begins to expand in size as mitosis occurs.
The mesoderm expands laterally surrounding the embryo. Simultaneously, the yolk sac and primitive endoderm replace the blastocoele. The trophoblasts become the trophectoderm.
Next the yolk sac dissects the mesoderm into two distinct parts, while also breaching the embryo. The dorsal part of the trophectoderm then collapses around the embryo and mesodermal sections, forming the amniotic folds.
Next the chorion is formed as a fusion of the trophectoderm and the mesoderm. Simultaneously, the amniotic folds move toward the dorsal pole while the allantois forms from the embryo.
Ultimately the amniotic folds embrace, forming the amniotic cavity. simultaneously, the allantois expands and part of the allantoic membrane comes in close contact with the chorion, thus forming the allantochorion or chorioallantoic membrane.
Maternal recognition of pregancy:
In ruminants, the hatched blastocyst releases interferon-tau (IFN-tau) prior to implantation.
IFN-tau diminishes the expression of oxytocin receptors in the endometrium.
Recall that the dying CL releases oxytocin that travels to the uterus and causes a release of prostaglandin F2 alpha that further activates apoptosis in the CL.
So, in ruminants, the IFN-tau prevents luteolysis. In the sow, the hatched blastocyst releases estrogen that causes PGF2- alpha to be released into the lumen of the uterus and not into the counter-current vasculature.
The PGF2-alpha is destroyed in the lumen, thus preventing the PGF2 alpha- mediated luteolysis in the sow.
Unclear mechanisms in the horse, dog, and cat.
Implantation
The embryo physically adheres to the endometrium. Adherence takes place on the amniotic pole of the embryo. Trophoblastic cells, now part of the chorion, invade the surface of the endometrium. this invasion process triggers biochemical events that enable maintenance of the pregnancy.`
Maintenance of Pregnancy- the extended luteal phase and early pregnancy
Chorionic gonadotropin (CG) is release from the trophoblast cells that have invaded the endometrium.
CG temporarily prevents CL involution and it promotes progesterone and estrogen synthesis by the CL.
CG is a structural analog of LH and thus it activates LH receptors on the CL. CG activation of the LH receptor leads to maintenance of the CL and progesterone synthesis by the CL.
Progesterone is the pro-gestation hormone and the CL is the main source during the beginning of pregnancy.
After 2-8 months (in a few species), the CL undergoes its inevitable involuiton and the placenta is now large enough to supply the progesterone.
Progesterone prevents the return of the follicular phase during pregnancy.
Accessory corpora lutea in the few species
In some species, accessory CLs are formed in order to support the pregnancy. In most mammals, the CLs (primary plus accessory) and the placenta cooperatively produce the progesterone all the way through pregnancy.
In humans, cattle, sheep and horses the placenta takes over at some point.
In horses, the placenta takes over at the relatively earliest stage but it is very inefficient at making progesterone.
One of the reasons for the high failure rate of full term pregnancies in horses.
Humans are equally dependent upon placental progesterone in early pregnancy, but the efficiency of synthesis is greater.
Late gestation
Progesterone is the dominant hormone that maintains uterine contractility quiescence.
A decrease in progesterone signifies the nearing of parturition (~4 day before)
freemartinism
occurs in the bovine with fraternal twins of each sex
The female is typically infertile because she has been androgenized by male DNA. These females are a chimera of XX and XY.
There is a device that determines if a freemartin is infertile. The device is inserted into the vagina and the animal is fertile if it does not encounter resistance.
Describe the four steps of embryonic preparation between syngamy and implantation.
Development of the embryo, hatching, post-hatching embryogenesis and prevention of luteolysis.
key point: post-hatching embryogenesis
the amnion is formed on the “implantation pole” while the choriallantoic membrane is formed on the distal pole of the embryo
Describe how ruminants and swine embryos differentially signal to the uterus that implantation is imminent, and how this signaling influences the ovary.
Ruminant embryos release IFN-tau that decreases oxytocin receptors, thus preventing the release of the luteolytic hormone PGF2-alpha. Swine embryos release estrogen that reroutes PGF2 alpha to the uterine lumen where it is destroyed, thus preventing ovarian luteolysis.
What is the role of CG in maintaining the early part of pregnancy
CG is an LH-like hormone released from implanted trophoblasts and this hormone maintains progesterone release from the CL.
Describe the importance of accessory CL in the maintenance of early to mid-pregnancy.
Accessory CLs are formed to assist in progesterone release as the primary CL undergoes its inevitable involution.
Describe the hormonal importance of the placenta in the maintenance of mid-pregnancy and beyond.
In some species the placenta assumes the responsibility of producing most of the progesterone needed for the maintenance of pregnancy
Metabolic functions of the placenta
The placenta serves as a metabolic organ by supplying nutrients to the fetus while also removing wastes from the amnion.
Depending on the species, there are a number of parts to the blood-placental barrier (B-P-B).
In some species, the nutrients must cross a basement membrane after exiting the endometrial (maternal) capillary.
But in most species, the nutrients must also cross a basement membrane to get from the chorion (placenta) to the chorionic capillary.
Water and gases (such as O2 and CO2) cross the B-P-B by simple diffusion. Unfortunately the same for CO.
Glucose and amino acids cross the B-P-B by facilitated diffusion, meaning that there are dedicated channels (pores) in the B-P-B that allow these molecules to specifically cross.
Sodium and potassium cross through active transport, indicating that there are dedicated channels that allow these molecules to cross against a gradient in an energy dependent process.
Lipid hormones (like testosterone, estrogen, etc.) cross the B-P-B via simple diffusion.
Most all other compounds are excluded. Immunoglobulins cross by facilitated diffusion.
Unfortunately, a few toxic substances (including drugs) and microbial pathogens can cross the B-P-B, resulting in abortions, teratogenesis (birth defects, fetal deformities, etc). or milder fetal syndromes.
Endocrine functions of the placenta
- Serves as a stimulator of ovarian function
- Maintains pregnancy
- Influences fetal growth and development
- Stimulates the mammary gland
- Assists in parturition.
stimulation of ovarian function
The placenta releases CG. Especially in the mare, where it is released from endometrial cups- invaginations of the placenta into the endometrium.
eCG has luteotrophic activity, and recall that it is a peptide that activates the LH receptor on SLCs in the CL.
LH-R (a GPCR) activation leads to P4 synthesis, and recall that P4 is the progestational hormone.
eCG has FSH-like activity in other species, and thus it is used for superovulation in embryo transfer.
maintenance of pregnancy
The placenta takes over as the primary source of progesterone at various stages of gestation in the bovine, ovine, equine, and human
