Lecture 9: Histology and Physiology of the Ovarian Follicle Flashcards
Histologically, what are the two parts of the Ovary and what does each part cotain?
1) Cortex - houses follicles w/ oogonia
2) Medulla - highly vascular, contains nerves, interstitial cels and lymphatics
What gives rise to more than 80% of cases of ovarian cancer and why?
Surface epithelial cells, are highly mitogenic
Explain the 6 steps of Oocyte maturation from a primary oocyte?
1) Primary Oocyte (4N DNA)
2) Primordial follicle/Unilaminar primary follicle (arrested at end of meiotic prophase I)
3) Secondary follicle/Multilaminary primary follicle
4) Early Tertiary follicle (Secondary follicle)
5) Graafian follicle
6) Ovulated ovum
Production of a female gamete capable of fertilization by sperm begins where?
The fetal ovary
What stage of follicular development is this?
Primordial Follicle
What stage of follicular development is this?
Primordial Follicle
What stage of follicular development is this?
Primordial Follicle
How do Granulosa cells aid in the development and maturation of oocytes?
Provide nutrients (AA’s, nucleic acids, and pyruvate)
What is the primary way that Primordial follicles are lost; what do we start with as a woman enters repro. maturity and end with at ovulation?
- Death due to follicular atresia; a small subset of primordial follicles will enter follicular growth in waves
- Start with 300,000 primordial follicles and end with <500 dominant follicles
Where is the gamete in primordial follicles derived from; what stage is it at; where is the nucleus of the oocyte?
- Oogonia that have entered the 1st meiotic division and now are called 1° oocytes
- Arrested in prophase I (can remain for 50 years)
- Nucleus of the oocyte (germinal vesicle) remains intact at this stage
What is the endocrine function of the Primordial follicle?
Release paracrine factors ; do not produce ovarian steroid hormones
Label A and B
A) Primordial follicles
B) Primary (Unilaminar) follicle
What stage of follicular development is this?
Unilaminar (1° in physiology) follicle
What are the histological characterisitcs of a Primary (unilaminar) follicle?
- Central Oocyte
- Single layer of simple cuboidal follicular (granulosa) cells
- Zone pellucida separates the oocyte and follicular cells
What are the histological characteristics of a secondary (multilaminar primary) follicle?
- Follicular (granulosa) cells proliferate into a stratified cuboidal epithelium known as the zona granulosa
- Zone pellucida enlarges and is clearly seen
- Theca interna
- Theca externa
What stage of follicular development is this and label A-B?
- Multilaminar primary (2° in physiology) follicle
A) Zona Pellucida
B) Follicular cells = Zona Granulosa
Once a follicle becomes 2° by acquiring several layers of cuboidal granulosa cells what does it secrete and what do these secretions cause?
- Secretes paracrine factors, which induce nearby stromal cells to differentitate into epitheliod thecal cells
- Fibrous capsule-like Thecal cells are on the outside of the follicle’s basement membrane
Once a thecal layers forms, the follicle is referred to as?
Mature preantral follicle
How do 2° follicles get blood supply; what do they release during this time and describe how they move to increase access to blood supply?
- Progression to 2° follicles involves the formation of capillaries and an increase in the vascular supply to developing follicular units
- Follicle release angiogenic factors that induce the development of 1 or 2 arterioles, generating a vascular wreath around the follicle
- Movement of the follicle from the outer cortex to the inner cortex, closer to the vasculature of the ovarian medulla
How do preantral (2°) follicles form the Zona Pellucida; what is the function of the Zona Pellucida?
- Oocyte begins to grow and secrete extracellular matrix glycoproteins (ZP1, ZP2, and ZP3) that form the Zona Pellucida
- Zona Pellucida provides a species-specific binding site for sperm during fertilization
How is contact between granulosa cells and oocyte maintained?
- Cytoplasmic processes penetrate the zona pellucida and form gap junctions at oocyte surface
- Gap junctions form between adjacent granulosa cells (basis for intercellular communication)
Which receptor does the growing preantral follicle express; what are they dependent on for growth; are they producing ovarian hormones at this stage?
- Express FSH receptors
- Dependent primarily on paracrine factors from the oocyte to grow
- Do NOT produce ovarian hormones at this stage
What is the major product of the Thecal cells and what receptor do they express; what is the production of this product like during the preantral follicle stage?
- Major product = androstenedione; express LH receptor
- Thecal cells do NOT express high levels of 17β-hydroxysteroid dehyrogenase (so only small amounts of testosterone formed)
- Androstenedione production at this stage is absent or minimal
How does the antrum develop and what phase does this mark?
- Follicular fluid from granulosa cells secretions begin to appear
- Fluid drops coalesce to form a singular follicular antrun
- Marks the beginning of the antral phase and gonadotropin dependency
What stage of follicular development is this and label A-D?
- Secondary (3° in physiology) follicle
A) Theca Interna
B) Antrum
C) Zona Granulosa
D) Zone Pellucida
What is a Graffian Follicle; what stage is this; ploidy of the oocyte; and size of the follicular antrum?
- Stage after the first meiotic division has completed but before ovulation
- Oocyte is now a 2N haploid
- Follicular antrum reaches maximum size
- 3 classes of Granulosa Cells
What are the 3 classes of Granulosa Cells seen in the Graffian Follicle; important functions of each?
1) Cumulus oophorous
- Anchors oocyte to wall of follicle
- Nutrient delivery channel
2) Mural granulosa cells
- Lining the wall of the follicle
3) Corona radiata
- Granulosa cells firmly anchored to zona pellucida
- Secrecte hyaluronic acid-rich product that aids in fallopian tube uptake
What stage of follicular development is this; label A-D?
- Graffian follicle (preovulatory follicle)
A) Zona Granulosa
B) Zone Pellucida
C) Corona Radiata
D) Antrum
Early antral follicles are dependent on what for normal growth; large antral follicle become highly dependent on?
- Early antral follicles are dependent on FSH for normal growth
- Large antral follicle become highly dependent on FSH for their growth and sustained viability
What is the function and location of mural granulosa cells seen in the growing antral follicle?
- Outer wall of the follicle
- Become highly steroidogenic
- Remain in the ovary after ovulation to differentiate into the corpus luteum
What is the function and location of the Cumulus oophorus and corona radiata in the growing antral follicle?
- Innermost layer (relative to oocyte) of granulosa cells
- Maintains gap and adhesion junctions with the oocyte
What happens to corona radiata cells during ovulation; importance of this event?
- Released from the ovary w/ the oocyte
- Crucial for the ability of the fimbriated end of the oviduct to grab and move the oocyte along the length of the ovidcut to the site of fertilization
Growth during the antral stage causes follicular cells to become responsive to gonadotropins; which hormones do Theca interna and Granulosa cells respond to?
Theca Interna = LH
Granulosa = FSH
When does the oocyte become competent to complete meiosis I at ovulation; what does it synthesize during this time?
- At the antral stage
- Synthesizes sufficient amount of cell cycle components (i.e., cyclin-dependent kinase-1 and cyclin B)
When do larger antral follicles gain meiotic competence, when do they finish meiosis?
- During the antral stage, but…
- Maintain meiotic arrest until the midcyle LH surge
How is meiotic arrest achieved in larger antral follicles?
Elevated levels of cAMP in the mature oocyte
Thecal cells of large antral follicles produce significant amounts of?
Androstenedione and some testosterone
What does FSH binding to its receptor on mural granulosa cells cause?
- Stimulates proliferation of Granulosa cells
- Induces the expression of CYP19-aromatoase
- Androgens are converted to estradiol-17β by the Mural Granulosa Cells
What hormone do the mural granulosa cells of the large antral follicles produce increasing quanitities of during the early follicular phase?
Inhibin B (analagous to Sertoli cells)
What do low levels of estrogen and inhibin from mural granulosa cells lead to?
- Exert negative feedback on FSH secretion, thereby contributing to the selection of the follicle with the most FSH-responsive cells
- Lets us find the follice which is most sensitive to FSH (dominant follicle)
How is the dominant follicle selected?
- End of the monthly cycle, several large antral follicles are recruited to begin rapid development
- As FSH levels decline, the rapidly growing follicles progressively undergo atresia, until one follicle is left
- The largest follicle with the most FSH receptors (highest sensitivity) of the recruited crop becomes the dominant follicle
- By midcycle, the dominant follicle becomes a large preovulatory follicle (Graffian follicle)
Thecal cells of the dominant follicles express which receptors and produce what?
- LH receptors and produce andorgens
- Basal LH levels stimulate production of steroidogenic enzymes (3β-HSD, CYP17, CYP11A1), LDL receptors in Thecal cells
What happens to the androstenedione and testosterone produced by Thecal cells of the dominant follicle?
- Diffuse from the Thecal cells into the Mural Granulosa cells or can enter the vasculature surrounding the follicle
- Inside the Mural Granulosa cell the androstenedione will be converted to testosterone or estrone w/ the final product being estradiol-17β
Mural granulosa cells of the dominant follicle express which receptor; what are they very sensitive to and what does binding of this hormone cause?
- Express FSH receptors and are very sensitive to FSH
- FSH up-regulates aromatase gene expression and activity
- Aromatase converts androstenedione to estrone and testosterone to estradiol-17β
- FSH also induces the expression of LH receptors in the mural granulosa cells during the 2nd half of the follicular phase
What is the improtance of Mural Granulosa cells of the dominant follicle becoming responsive to both FSH and LH?
- Allows these cells to maintain high level of CYP19 (aromatase) in the face of declining FSH levels
- Acquisition of LH receptors also ensures that mural granulosa cells will respond to the LH surge
What is the periovulatory period; how long does it last; what occurs?
- Time from onset of LH surge to ovulation (usually 32-36 hours)
- Change in the steroidogenic function of the thecal and mural granulosa cells (aka lutenization)
- Prepares cells for luteinization
- Formation of corpus luteum
- Increased production of progesterone
What does the follicle do inside the ovary just prior ovulation; what does the LH surge induce?
- Follicle presses against wall of ovary to form bulge called stigma
- LH surge induces the release of inflammatory cytokines and hydrolytic enzymes from the theca and granulosa cells
- Lead to the breakdown of the follicle wall, tunica albuginea, and surface epithelium near the stigma
What is released once the Cumulus cell-Oocyte complex detaches from the granulosa cell and is freed within the antral cavity; why is this important?
- Released (TGF-β)-related factor, GDF9
- GDF9 stimulates the cumulus cells to secrete hyaluronic acid and other extracellular matrix components, causing expansion of the oocyte complex.
- Expansion makes it easier for capture by the oviduct and easier for sperm to locate
What induces the 1° oocyte to complete meiosis I?
LH surge induces release from arrest and completion of meiosis I w/ extrusion of the first polar body
Once the 2° oocyte is formed after being released from arrest what does it progress to?
Progresses to metaphase II where it arrests again until fertilization
What happens to the basal lamina of the mural granulosa cells upon release of the culumus-oocyte complex?
- Basal lamina of mural granulosa is enzymatically degraded
- Angiogenic factors released to increase blood supply to the new corpus luteum (i.e., VEGF, angiopoietin-2, and bFGF)
- Increased blood supply to follicle/corpus luteum
What does Corpus Hemorrhagicum refer to?
- Blood vessels of the theca interna invade the antrum and basement membrane (basal laimina) breaks down
- Blood from the theca interna fills the antrum
- Clotting and fibrosis will occur over the following weeks
The LH surge induces shifts in the steroidogenic activity of the mural granulosa cells leading to the luteal phase, causing inhibition of, which leads to a rapid decline in what?
- Inhibits aromatase expression and consquently, estrogen production
- Rapid decline in estorgen helps to turn off the positive feedback on LH secretion
What does the LH surge induce in the granulosa lutein cells and what does this increase during the luteal phase?
- Vascularization of granulosa cells to make cholesterol available for steroid synthesis and increase expression of LDL/HDL receptors
- Increases expression of proteins and enzymes to produce progesterone
What phase is this and how do you know; what is each arrow pointing to?
- Luteal Phase
- The blood clot has formed
- Top arrow is at Thecal Cells
- Bottom arrow is Granulosa Cells
The corpus luteum continues to enlarge during the luteal phase and fill with what; what stage does it enter
Fills with Cholesterol Esters and enters an involution stage about 14 days after ovulation
What does regression of the corpus luteum (luteolysis) lead to?
- Formation of corpus albicans, which is stromal CT replacing degenerating luteal cells
- Will decrease in size, but seldom dissapears
What is seen in this image?
Corpus albicans (luteal phase)
What is indicated by A and B?
A) Theca lutein cells
B) Granulosa lutein cells
How long is the corpus luteum programmed to live for; what occurs if fertilization happens?
- Programmed to live for ~14 days
- If fertilization occurs, will be rescued by hCG and remain viable for the duration of the pregnancy
What is progesterone and estrogen production by the corpus luteum production like from the onset of the LH surge?
- Progesterone production by the corpus luteum increases steadily and peaks during the midluteal phase
- Estrogen production transiently decrease in response to LH surge but then rebounds and also peaks at midluteal phase
- Progesterone and estrogen levels decline during the 2nd half of the luteal phase unless and increase in circulating hCG compensates for the decreased sensitivity to LH
What is the importance of the increased progesterone from the corpus luteum for pregnancy?
- Transforms uterine lining into adhesive and supportive structure
- Important for implantation and early pregnancy
Luteal hormone output is absolutely dependent on what; what is progesterone output closely correlated with?
- Basal LH levels
- Progesterone output is closely correlated to the pulsatile pattern of LH release
What happens to FSH and LH levels during the luteal phase and how?
- Both FSH and LH are reduced to basal levels during the luteal phase by the negative feedback from progesterone and estrogen
- Granulosa lutein cells secrete inhibin A, which further suppresses FSH secretion
What is follicular atresia, when does it occur, and which cell populations live and die?
- Demise of an ovarian follicle; can occur at any time during development
- Predomiant process in the ovary
- Granulosa cells and oocytes undergo apoptosis
- Thecal cells persist and repopulate the cellular stroma of the ovary
During follicular atresia the Thecal cells survive and retain which receptors and the ability to produce; they are referred to as?
- Retain LH receptors and the ability to produce androgens
- Collectively are referred to as the interstitial gland of the ovary
During follicular atresia the oocyte degenerates and the basement membrane separates from the oocyte and forms?
Thickens to become the glassy membrane
What is this image of?
Corpus Albicans
