Ovarian Cycle

Question 1

what is ovarian cycle?

Answer 1

 Ovarian cycle- constitute development and maturation of a follicle, ovulation and formation of corpus luteum and its degeneration

Question 2

state the cell types found in a follicle

Answer 2

the functional unit of the ovary is the follicle which contains 3 cell types:
 Surface epithelium: derived from celomic epithelium
 Germ cells: gives rise to the oocyte
 Sex cord stroma (supporting cells): granulosa cells, theca cells, fibroblasts

Question 2

at what stage are oocytes present in females?

Answer 2

 The oocytes are present in the female fetus from an early stage of development.
 By the end of second trimester in utero the number of oocytes has reached a maximum and they arrest at the first prophase step in meiotic division.
 No new oocytes are formed during the female lifetime.
 With the onset of menarche, the primordial follicles containing oocytes will activate and grown in a cyclical fashion, causing ovulation and subsequent menstruation in the event of nonfertilization

Question 2

phases of ovarian cycle

Answer 2

 Ovarian cycle go through 3 phases:
1. Follicular phase
2. Ovulation
3. Luteal phase.

Question 2

outline the follicular phase of the ovarian cycle

Answer 2

FOLLICULAR PHASE
 Maturation of oocyte is key event in this phase.
 Within the follicles:
 FSH stimulates the theca cells to produce androgens from cholesterol.
 FSH also stimulates the granulosa cells to convert the androgens to estrogen (estradiol) (by aromatization).  Estrogen begin to rise as the follicular development occurs
 Estrogen stimulates growth the follicle and the proliferative phase of the endometrium.
 As the follicles grow and estrogen secretion increases theirs is negative feedback on the pituitary to decrease FSH secretion, this assists in the selection of 1 follicle to continue in its development towards ovulation- the dominant follicle.
 The follicle with the most efficient aromatase activity and highest concentration of FSH induced LH receptors will be the most likely to survive as FSH levels drop, while smaller follicles will undergo atresia.
 The dominant follicle will go on producing estrogen and also inhibin which enhance androgen synthesis under LH control.
 The follicular phase lasts an average of 14 days.
 The follicular phase coincides with the proliferative phase of the endometrium.

Question 3

outline the ovulation phase of the ovarian cycle

Answer 3

Ovulation phase
 By the end of the follicular phase, the dominant follicle has grown to approximately 20mm in diameter.
 Dominant follicle, shortly before ovulation reaches the surface of the ovary.
 Cumulus detaches from the wall, for ovum with surrounding cells (corona radiata) to freely float in liquor folliculi.
 Oocyte completes its 1st meiotic division which was arrested in diplotene stage of meiotic prophase 1, with
extrusion of 1st polar body which is pushed to the perivitelline space.
 Follicular wall near ovarian surface becomes thinner, begins to bulge locally, and at the apex, an avascular spot, the stigma, appears.
 Androgens synthesized in the theca cells also rises around the time of ovulation and this is
thought to have an important role in stimulating libido, ensuring that sexual activity is likely to occur at the time of greatest fertility.
 The LH surge is one of the best predictors of imminent ovulation and this is the hormone detected in urine by most over the counter ‘ovulation predictor’ tests.
 The LH surge is also thought to stimulate the resumption of meiosis in the oocyte just prior to its release.
 ↑LH increases collagenase activity, resulting in digestion of collagen fibers surrounding the follicle & also increase PG levels which cause local muscular contractions in ovarian wall which extrude the oocyte,
 Some of cumulus oophorus cells then rearrange themselves around the zona pellucida to form corona radiata
 Around midcycle (14th day), few hrs after LH surge, dominant follicle which has reached a size of 20mm ruptures leading to ovulation.
 If not fertilized within 12- 24hrs the human ovum dies.
 Women wishing to become pregnant must thus avoid prostaglandin synthetase
inhibitors such as aspirin and ibuprofen which may inhibit oocyte release

Question 4

outline the luteal phase of the ovarian cycle

Answer 4

Ovalatory phase- Luteal phase
 After ovulation, the ruptured Graafian follicle develops into corpus luteum, whose life cycle is divided into 4 stages: Proliferation, vascularization, maturation and regression
 The granulosa cells have a vacuolated appearance with accumulated yellow pigment hence the name corpus luteum (‘yellow body’).
 Theca cells on the ovary form the corpus luteum, which undergoes extensive vascularization in order to supply granulosa cells with a rich blood supply for continued steroidogenes, after the release of the oocyte.
 The collapsed follicle after rupture undergoes a metamorphosis into corpus luteum which produces progesterone, estrogen and inhibin.
 Ongoing pituitary LH secretion and granulosa cell activity ensures a supply of progesterone which stabilizes the endometrium in preparation for pregnancy (coincides with the secretory phase of the endometrium).  Progesterone levels are high during this phase.  These high levels of progesterone have an inhibitory effect on FSH and LH production thus preventing further follicular growth in the ovary during that cycle.
 The total life span of corpus luteum is 14 days (luteal phase lasts 14 days)
 In the absence of βHCG produced from an implanting embryo, corpus luteum regress in a process known as luteolysis.
 Mature corpus luteum is less sensitive to LH, produces less progesterone causing shedding of the endometrium and thus menstruation.
 This period is constant irrespective of the length of menstrual cycle.
 This is why menstruation occurs 14 days after ovulation.