Female Reproductive System

Ovary 
(Assigned reading: pp 461-472 in text chapter)
Gwen V. Childs, Ph.D.

 Test yourself:  How much do you know about the female reproductive system?

1)  Identify the anatomical parts of the female reproductive system.

2)  Define the life stages of the ovary, including events during fetal life, menarche, menopause.

3)  When do oogonia stop dividing mitotically and in what meiotic stage are they  when they stop?   

4)  Why is sufficient nightly sleep important to normal onset of puberty?

5)  The pulse frequency of gonadotropin releasing hormone (GnRH) favors LH over FSH.  When would the slower pulses (favoring FSH) be important?  When would rapid pulses (favoring LH) be important?

6)  What is menopause and perimenopause?  When does it begin?  Why would the number of oocytes and maturing follicles affect future cycles?

7)  Describe the general organization of the ovary.  What is “germinal epithelium” and why is it a misnomer?

8)  Define the stages of follicular development.  Distinguish a primordial from a unilaminar primary follicle. 

9)  Distinguish a multilaminar primary follicle from a secondary follicle.

10) What stimulates the growth of primary follicles (division of granulosa cells?)

11) Where does follicle stimulating hormone act and on what cells?

12) Describe the partnership between the theca interna and the granulosa cells in the primary and secondary follicles.  Which cell type is stimulated by LH?  How does FSH support this function?

13) What route and structures are used for the communication between the granulosa cells and the oocytes?  Describe all regions in the follicle.

14) How might you tell if a follicle will mature and ovulate?  What is the “dominant follicle”?

15) How does the dominant follicle promote its own ovulation?

16) Define the crucial development steps in the oocytes and the follicle just before ovulation. How does LH stimulate ovulation?  What final maturation step is stimulated by LH?

17) If only one follicle can be dominant, why begin with so many?

18) When does the primary oocyte become a secondary oocyte?  What is meant by "germinal vesicle breakdown (GVBD)"?

19) Define the formation and function of a corpus luteum.  What causes it to degenerate? What is a corpus albicans?

20) Review the types and sources of hormones involved at each stage of development:

Primordial to Primary follicle

Unilaminar Primary to multilaminary secondary

Secondary to Graafian follicle

Corpus luteum 

new crop of follicles for next cycle

 Organization of the female reproductive system:

    1. Two ovaries
    2. Two oviducts + uterus
    3. Vagina
    4. External genitalia

 

Parts of the female reproductive system.  Sagittal section (left) shows uterus, vagina and one oviduct and ovary.  Note juxtaposition of uterus to bladder.  Above view shows the passage way from vagina to uterus. Note the thick wall of latter. Two oviducts with small darkly colored ovaries are shown.

 

Life Stages of the ovary:

Fetal development:

a.    Oogonia develop after the first month and undergo several mitotic divisions.  After 6 weeks, they migrate from the yolk sac to populate the germinal ridge and eventually the ovarian cortex.

b.   Mitoses continues during fetal life until there are about 5-7 million oogonia. Only a fraction  become surrounded by granulosa cells (1 million).  The rest undergo atresia and die.  Oogonia + granulose cells=primordial follicle.

c.   This 1 million is reduced further by atresia (apoptosis) so that only 600,000 oogonia remain at birth.   Some continue to undergo atresia.

 

Photo shows fetal ovary with numerous oogonia .  Those that are surrounded by a layer of simple squamous epithelia are “primordial follicles” . The rest are destined to die.

Childhood:

Pulses of hormones from the brain (gonadotropin releasing hormone—GnRH) and pituitary (gonadotropins) are low; probably held in check by pineal secretion of melatonin; destruction of pineal causes precocious puberty.

Peripubertal

GnRH from the brain increases in pulse frequency and amplitude during early hours of sleep. This stimulates LH and FSH (gonadotropins from pituitary) and subsequently stimulates the ovary to produce testosterone and estrogen several hours later.  Very important that young child get enough sleep. This early stimulation at night sets in motion the pulses that will begin the cycles.

Menarche:

   Beginning of menses (first cycle); may not be ovulatory, however.    12.7 years of age average (9-15 yrs)   400,000 oocytes by the time of menarche

What happens at menarche?

1)  Three important conditions must be met: sufficient body fat (16-24%; 85-104 lbs); adequate sleep; vision (sunlight) (anorexia and blind young people have delayed puberty).

2)  Why sleep?

a. Peaks of GnRH and gonadotropins increase in amplitude during the first hours of sleep; synthesis of sex steroids follows.  This sets in motion the feedback needed for first ovulation.

b. Later, the peaks occur during waking hours as well.

3)  Frequency of pulses of GnRH may initially be slow, which favors secretion of follicle stimulating hormone from the pituitary.  1 pulse every 1.5-3 hours.   FSH is important in initiating development of the secondary follicles.

4) As they develop and mature, Follicles produce estrogen (see below for more complete description of this cycle) which promotes an increase in GnRH receptors on pituitary gonadotropes. 

5) Estrogen is also important in regulating GnRH neurons and pulse frequency increases to one 15 min pulse/hour.  This favors the secretion of pituitary luteinizing hormone (LH) which rises to a sharp peak (called the LH surge). LH is critical for ovulation and post-ovulatory events. 

6)  First cycle occurs when follicles produce enough estrogen to stimulate GnRH pulses and the LH surge. This LH surge stimulates first ovulation.

 

Basically, the ovary is initially given a hormone signal from the pituitary (via Growth hormone and FSH) to jump start follicular development to the primary-secondary stage. Usually, only about five follicles are allowed to mature to the point of ovulation, however others may also mature (to help with the estrogen production).  This state of maturation signals back to the pituitary by sending out higher and higher levels of estrogens.  These estrogens feedback positively on the pituitary gonadotrope and stimulate it to produce the receptors needed to receive the extra GnRH pulses.  This stimulates more LH in a big surge which stimulates ovulation of the designated dominant oocyte.

Menopause:

a.  Cessation of menses

b.  Perimenopause: may begin in mid 30’s; At that point, woman may have only 100,000 oocytes left.

c.  During reproductive life, only 450 eggs will be released during ovulation (assuming a 28 day cycle).  The rest will undergo atresia.

d. Of all follicles present at the beginning of menarche, only 0.1-0.2% will develop and undergo ovulation.

e. Menopause may actually be the result of the rapid drop in oocytes (which drops precipitously after age 35).  The low oocyte number means there are fewer developing follicles to produce estrogens that drive the pre-ovulatory events in the brain and the pituitary. 

f.  This eventually causes the cycles to cease or become irregular.  The lack of adequate gonadotropin stimulation further accelerates atresia of the follicles.

Organization of the ovary

1)   Attached to uterus by broad ligament via mesovarium which conveys blood supply to ovary

2)   Peritoneum covers outer surface: simple cuboidal (mesothelium) that is misnamed “germinal epithelium”. 

3)   Next layer is ct, called “tunica albuginea” (dense irregular ct)

4)   Cortex is next layer: contains stroma and follicles

5)   Medulla is inner layer: contains large blood vessels that enter at hilus.

 

 

Ovarian Histology

Define the stages of follicular development.

1)  “Follicular phase of the menstral cycle”.

2)    Begins on day 1 (of menses)

 

Primordial: = oocytes+ follicular cells

a.   Regulation not certain: Suggestions include Growth hormone and/or Insulin like growth factor-1 (IGF-1-- locally produced).  Girls with no LH or FSH have primordial and primary follicles

b.   Contains Primary oocyte, arrested in diplotene of prophase ( Meiosis I).

c.   Protected by simple squamous follicular cells (1 layer);

d.   Oocyte is 25 µm diameter

 

 

Primordial follicles:

Primordial follicles with one layer of squamous follicular cells around the oocyte.  Note prominent nucleus  with its dark nucleolus. Oocyte fills entire follicular space.

 

Outside follicle is stroma.

 

 

 Primary follicle: unilaminar

1)   Also may be regulated by GH and/or IGF-1

2)    Primary Oocyte may have more prominent nucleus (called a “germinal vesicle”)

3)   One layer of granulosa cells, have become cuboidal

4)  100 µm diameter

5)      Forms a “zona pellucida” layer around oocyte

a)      Glycoprotein rich zone (ZP 1, 2, 3 )

b)      Zone of contact and communication between oocyte microvilli and granulosa cell processes.

c)      Develop gap junctions (connexon molecules)

 

Unilaminar Primary follicle (early)

 

Early unilaminar Primary follicle shows the granulosa cells becoming cuboidal. Note prominent nucleus (germinal vesicle).

 

Primary follicle: multilaminar (see following figure)

a.   Still may be regulated by GH or IGF-1

b.   Oocyte produces a hormone called “activin” .

c.   Activin signals mitotic activities of granulosa cells and they divide, making the follicle multilayered.

d.   Zona pellucida is more prominent (ZP)

e.   Stromal cells begin to be organized into two layers (TF)

                                i.      Internal: theca interna

                                ii.      External: theca externa

f.    150 µm in diameter.

 

 

 

Functions of Primary follicle:

1)      To signal its presence and state of maturation to pituitary and brain

2)      To help support the maturation of the oocytes

3)      To concentrate gonadotropins and steroids (and other hormones) to be used for maturation.

 

In other words, primary follicles are beginning to signal the body and pituitary that it is maturing. It secretes estrogens via the following mechanism:

1)  Thecal cells develop receptors for luteinizing hormone (LH) (stimulated by estrogens or FSH)

2)  LH stimulates theca cells  to produce androstenedione (and, to a lesser extent, testosterone), which are both androgens.

3)   Androstenedione and testosterone move to granulosa cells where they are converted to estrogens (estradiol or estrone) by the enzyme aromatase. 

 

This figure shows the pathway.  The enzymes for the pathway leading up to androstenedione or testosterone steroids are produced by the thecal cells.  Granulosa cells contain the aromatase that produces the final product (estradiol or estrone).

 

The pathway that involves DHEA is the one used during the follicular phase of the estrous cycle.

 Secondary Follicle

1)  Follicle continues to grow to 200 µm.

2)   Layers of granulosa cells increase

3)  Intercellular spaces develop within the mass of granulosa cells and become filled with “liquor folliculi”

a.      Contains steroid and peptide hormones (an extracellular storage site???”

b.      Estradiol, progesterone (see above pathway) are steroids

c.      Inhibin, follistatin, and activin are peptides.

d.      Eventually fuses to form one large antrum in larger secondaries

e.      The immediately distinguishes it as a “secondary” or “antral follicle”.  (Note: primary follicles are sometimes called “pre-antral” follicles).

4)      What kind of oocyte is found in the secondary follicle?  PRIMARY Oocyte

5)      A single layer of granulosa cells surrounds the zona pellucida, again communicating via gap junctions with the microvilli of the oocyte. This single layer is called the “corona radiata” 

 

Early secondary follicle. 

 

Oocyte grows larger as the layers of granulosa cells increase.  There are two pools of “liquor folliculi”

Note also, that the thecal cells are better organized around the granulosa cells.

 

For practice, find the: oocyte, zona pellucida, liquor folliculi, theca interna and theca externa.

 

6)   As follicle grows, the need for vascularization increases, so vessels grow into thecal area to allow  adequate oxygen and nutition for avascular follicle.

7)  FSH is responsible for continued proliferation of granulosa cells in all antral follicles; FSH also stimulates granulosa cell production of estrogens (from androgens produced by thecal cells). 

8)  FSH stimulates LH receptors on thecal cells to allow LH to regulate androgen production.

9)  The organization of later stages of the secondary follicle show that the granulosa cells form a “hill” on which the oocyte and corona radiata cells sit.  This is called the cumulus oophorus. 

 

A more mature secondary follicle showing a single antrum.

 

The oocyte with its corona radiata projects into the antrum, sitting on a collection of granulosa cells called the “cumulus oophorus”. 

 

Identify: antrum, granulosa cells,

Thecal cells, corona radiata, zona pellucida, primary oocyte.

 

 Atresia :

We begin with up to 7 million oocytes and by menarche we have only 400,000.  By age 30, we have only 100,000.  In our lifetime, a woman will ovulate only 450 oocytes or 0.1% of those she had at puberty.  What happens to the rest?

 

Those follicles not “chosen” undergo atresia (programmed cell death). Before puberty, most are primordial or primary follicles.  After puberty, atresia can happen at all levels. Atretic follicles can be seen as degenerating (often the oocyte is degenerating) and often a remnant is evident in the cortex.

 

Atretic follicle

In this atretic follicle, only a remnant of the zona pellucida is left.

 What selects the follicles that are destined to go on?  Survival of the fittest?

During active development (after puberty) of follicles, multiple secondary follicles will develop at the same time to the graafian follicle stage, and only the dominant follicle will ovulate.  This alternates so that one ovary produces the dominant follicle one cycle and the other produces it the next cycle.  Workers have sought possible inhibitory factors that may be blood borne, which would signal the ovary as well as the follicles.  An important distinguishing feature of a "successful follicle" is a high concentration of estrogens relative to androgens in the follicular fluid (liquor folliculi).  If the estrogen:androgen ratio in the follicular fluid becomes <1, that follicle will become atretic.  The dominant follicle is the one that has the highest estrogen:androgen ratio (the fittest follicle).  It thus drives the estrogen levels up, which has the vital positive feedback on LH secretion and also causes an increase in GnRH receptors.

In addition, dominant follicle produces inhibin which  inhibits pituitary FSH synthesis and secretion.  The combination of inhibin and estrogen lowers FSH before ovulation.  The lower FSH causes the non-dominant graafian follicles to atrophy. 

With the surge of LH, however, the inhibin secretion is blocked, thus permitting the FSH to rise once more. FSH therefore has a small peak, lagging behind the LH surge.  This helps stimulate the growth of primary follicles receptive to FSH. 

Some other considerations:

1)   Sometimes two secondary follicles are allowed to go to maturity and ovulate, and if both are fertilized, fraternal twins develop.

2)  Secondary follicles do contribute to the estrogen and other hormone levels and help the chosen dominant follicle signal the body that a cycle is ongoing.

 Graafian Follicle (mature)

Oocyte has grown to maximum diameter of 100 mm.

These follicles are huge and not often seen in a section of an ovary, because they take up nearly the entire ovary.  They are 2.5 cm and can be seen as a transparent bulge on the surface of the ovary. 

 

There are critical developmental steps that must be met before the dominant oocytes can be ovulated.

What happens to the dominant oocyte?

1)   Sufficient estrogen must have reached the pituitary to allow a drop in FSH and a surge of LH.

2)   LH then stimulates the oocytes to resume meiotic prophase, several hours before ovulation.  It   stimulates the production of meiotic inducing substance (a local factor)

3)   The nucleus moves to a position beneath the plasma membrane and completes the latter stages of meiotic prophase.  The nuclear membrane breaks down and chromosomes assemble on the metaphase plate.

4)   The division begins and, because of the unequal division of cytoplasm, it ends as a large oocytes and a daughter polar body (first polar body remains stuck to the zona pellucida).

5)   In in vitro fertilization studies, this process heralds the maturation of the oocytes and is called the “germinal vesicle breakdown  (Recall the nucleus was called the germinal vesicle).  GVBD is the acronym for what happens.  

6)   The oocyte is now a SECONDARY OOCYTE.

7)   It proceeds to metaphase of Meiosis II and is arrested again until after ovulation and fertilization.

What happens to the follicle so it can release the oocyte?

1)   Matrix attracts fluid so that it bulge and allow internal reorganization

2)   Cells reorganize and rearrange inside resulting in the detachment of the oocytes and its corona radiate with a few adherent granulosa cells. 

3)   Bulging outer pole appears pale.  Called the “stigma or macula pellucida”. Pale due to local cessation of blood flow in the theca capillaries.

4)  Thinning of tissue caused by enzymes.  Collagenase is higher in follicular fluid.  Also, LH stimulates granulosa cells to produce plasminogen activator which converts the proenzyme to plasmin, an enzyme capable of degrading the basal lamina around the follicle and converting procollagenase to collagenase.  These enzymes are important in the breakdown of the connective tissue needed for ovulation. 

Corpus luteum 

This begins the Luteal phase of the ovarian cycle. 

After ovulation and loss of the liquor folliculi, the wall of the follicle collapses and is infolded.  The granulosa and thecal cells tend to mix and there appears to be extravasation of blood from the capillaries of the theca to form a clot.  This is the first stage of the formation of the corpus luteum, called the corpus hemorrhagicum.  LH will then convert this structure into the corpus luteum in order to provide vital hormones for the support of the uterine endometrium and the developing placenta.

 

Corpus hemorrhagicum

 

Note blood clot in the center.

 

Granulosa cells form the bulk of the cells in this body.

 Corpus luteum granulosa cells produce progesterone and also convert the androgens produced by the thecal lutein cells into estrogens.   Granulosa lutein cells are larger, paler and about 80% of the cell population.  Theca lutein cells are darker staining and small and about 20% of the population.  They produce the same hormones, only they specialize in the production of androgens.  A low power view of a corpus luteum is shown in the next figure. Most of the lighter staining cells are the granulosa cells.

 

 

 

 

 

 

 

 

Corpus luteum

 Progesterone feeds back on LH and FSH and shuts these hormones down.  If pregnancy occurs, then the corpus luteal cells are maintained by a gonadotropin secreted by the placenta (called chorionic gonadotropin—which is the hormone they detect in pregnancy tests).  It pregnancy does not occur, the absence of any gonadotropin (LH) leads to degeneration of the corpus luteum.  A degenerated corpus luteum is called a corpus albicans (white body).  Most of the body becomes fibrotic and is phagocytized by macrophages.  A corpus albicans is shown in the following figure.

 

  Ovarian Medulla

Mainly connective tissue and blood vessels.  May include interstitial cells/glands that secrete androgens. Similar types of interstitial cells are found in the Hilus.  Similar to Leydig cells.

 Summary: Sequence of hormones and their actions:

Primordial to Primary follicles:  GH, IGF-1 (?) and activin

1)    Primordial follicles need receptors for GH or IGF-1 to allow them to become primary follicles.  Thereafter, GH and IGF-1 are protective against atresia.

2)    Activin production by oocytes of primary follicle stimulates granulosa cells to proliferate and become multilaminar.  Also estrogen from neighboring antral follicles (secondary follicles) may stimulate granulosa cells to develop LH receptors (a step to a secondary follicle). 

Primary follicles to secondary follicles:  FSH + estrogens and androgens

1)        High progesterone and estrogen from previous luteal phase cause slower pulses of GnRH early in the cycle.

2)        This favors a higher FSH:LH ratio (once every 1.5-3 hours).  FSH promotes development of antral follicles (secondary follicles) and the production of estrogen.

3)        FSH also stimulates the production of LH receptor on Theca cells.   Theca cells produce androgens which are used by granulosa cells (in the presence of FSH) to produce estrogens.

4)        Estrogen rises as secondary follicles develop and mature.  Dominant follicle especially has high estrogen levels (estrogen:androgen ratio>1).   

Secondary follicle to Graafian follicle and ovulation

  1. Pre-ovulatory estrogen rise stimulates increased GnRH pulse frequency and amplitude

  2. Pre-ovulatory estrogen rise increases GnRH receptors on gonadotropes.   This sets up a condition that favors LH.  Increases LH:FSH ratio

  3. Granulosa cells are also stimulated to produce inhibin, especially in dominant follicle, which inhibits FSH.  In addition, increased estrogen and increased GnRH pulses suppress FSH.   So, just before ovulation, FSH goes down. This promotes atresia of non-dominant follicles.

  4. LH surge causes final maturation steps of follicle including completion of first meiotic division and formation of first polar body, production of enzymes needed for breakdown of connective tissue.

  5. LH surge also inhibits inhibin which allows a parallel rise in FSH that lags behind LH.  This rise stimulates the next crop of follicles for the next cycle. 

Corpus luteum development

  1. LH stimulates progesterone production (along with estrogens) which maintains uterine lining, preparing it for pregnancy. 
  2. Progesterone feedback to pituitary slows GnRH pulses and suppresses LH and FSH for that 2 week period.
  3. As Corpus luteum degenerates, progesterone falls. With the falling progesterone, the slower pulses of GnRH brought about by luteal phase promotes the higher FSH:LH ratio. This helps support the growth of follicles in the next cycle.
  4. Corpus luteum degenerates with no gonadotropin support (assuming no pregnancy) and this allows another slow rise in FSH. 

New Crop of Follicles

Rising LH at midcycle inhibits the release of inhibin from ovary.  This transitional inhibition allows a mid-cycle rise in FSH (a small peak right at midcycle) which helps stimulate the next crop of growing follicles.  As soon as LH surge is over


Text © Gwen V. Childs, Ph.D.
childsgwenv@uams.edu

12/06/2002 last edited

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