• The menstrual cycle is divided into 2 components:
► Ovarian cycle
0 Follicular Phase (10-14 days): The period from a single dominant follicle to the formation of a follicle ready for ovulation
0 Luteal Phase (14 days): The period from ovulation to the onset of menses
► Uterine cycle
0 Proliferative Phase
0 Secretory Phase
ovarian cycle
Follicular Phase
► The follicular phase refers to the period in which the dominant follicle develops until the preovulatory follicle that will ovulate.
Follicular developmental stages
Primordial follicle
- Latent primordial follicle
- Active primordial follicle (single layer primary follicle)
Preantral follicle (multi-layered primary follicle)
Antral follicle (secondary follicle)
Preovulatory follicle (Graaf's follicle)
Primordial Follicle
0 At birth, follicles in the ovarian cortex are primordial follicles. There is a primary oocyte (paused in the diplotene stage of prophase of I. meiosis) within each primordial follicle. These oocytes are surrounded by a single layer of squamous pregranulosa cells. These primordial follicles, whose oocyte nucleus is smaller than 20 µm in diameter, are follicles that are in the resting phase (latent phase).
Since there is no FSH receptor in granulosa cells in primordial follicles, FSH has no role in the activation of follicles in the latent phase. Therefore, the selection of the primordial follicle group and the development of the selected ones are independent of gonadotropins.
0 Locally released intraovarian factors from granulosa cells (especially kit ligand and its receptor c-kit and growth factors) are thought to be effective in the activation of latent primordial follicles.
0 Activated primordial follicles become a single layer primary follicle.
Signs of activation of the latent primordial follicle are:
- Oocyte nucleus equal to or greater than 20 µm
- Cubicization of single layer squamous granulosa cells
During this period, gap-junctions made of connexin proteins begin to form between the oocyte and the granulosa cells. Thus, a connection is established between the oocyte and the granulosa cells. A basement membrane (Slavjanski's membrane) forms around the cubic granulosa cells, which separates them from the stromal cells.
Preantral Follicle (Multilayer Primary Follicle)
Growth accelerates and the granulosa cell layer becomes stratified. Meanwhile, the diameter of the oocyte increases and is surrounded by a glycoprotein membrane synthesized from granulosa cells (zona pellucida). This membrane prevents fertilization of the oocyte by more than one sperm.
With the effect of gonadotropins, theca cells begin to differentiate from the stroma outside the basal lamina.
In granulosa cells, FSH receptors first appear in the preantral follicle stage.
The continuity of development of the preantral follicle depends on the ability of that follicle to convert its androgenic microenvironment to an estrogenic microenvironment. The transition from the androgenic microenvironment to the estrogenic microenvironment is also FSH dependent.
The progression of the preantral follicle is completely dependent on FSH, without FSH, the development of the preantral follicle is not possible and the follicles undergo apoptosis (atresia).
Androgens, at low concentrations, positively regulate follicle development. While low concentrations of androgens increase granulosa cell proliferation and aromatase activity, they decrease programmed theca cell death.
However, high concentrations of androgens impair follicle maturation and development by reducing FSH secretion. In addition, as the aromatase capacity is exceeded, androgens accumulated behind are converted to 5a-reduced androgens, not estrogens, by preantral granulosa cells. These androgens also inhibit the aromatase enzyme.
Follicles in the highly concentrated androgenic microenvironment undergo atresia.
A significant amount of estrogen is made in the granulosa cells in the preantral follicles. With the effect of increasing estrogen in the follicular microenvironment, the level of FSH receptors in the follicles increases. After this stage, estrogens stimulate follicular development synergistically with FSH.
Antral follicle (secondary follicle)
0 While the follicle fluid (liquor follicle) formed by the synergistic effect of estrogen and FSH forms a cavity (antrum), the follicle turns into antral follicle.
0 As the follicle continues to grow, the surrounding stroma becomes prominent into two layers. While the outer layer theca externa is mostly in the structure of vascular connective tissue and acts as the capsule of the developing follicle; The inner layer of the theca interna is epithelioid just like granulosa cells and can synthesize steroid hormones.
0 At this stage, the dominant follicle is selected (between the 5th and 7th days of the cycle). There are 2 main features that distinguish the dominant follicle from other follicles:
- Contains more FSH receptors.
- Aromatase capacity (production of estrogen) is higher.
In the meantime, the release of FSH from the pituitary is suppressed by the negative feedback effect of the high amount of estrogen and inhibin given to the systemic circulation by the dominant follicle. In the other follicles, which are less mature due to the suppression of FSH in the mid-follicular period and cannot respond adequately to the decreased FSH, estrogen production decreases and their microenvironment remains androgenic and undergoes atresia over time. Since this atresia is a programmed cell death, it is called apoptosis.
The first stage of the atresia process in the follicles is the reduction of FSH receptors in the granulosa cells.
0 In the late antral follicle (9th day of the cycle), LH receptor begins to appear in the granulosa cells under the influence of FSH.
0 Anti-Müllerian hormone (AMH) is released from granulosa cells in follicles. While this oscillation is very low in primordial follicles, it reaches very high levels in preantral and small antral follicles. It is a glycoprotein belonging to the transforming growth factor-B family.
Circulating AMH level is directly proportional to developing follicles and is an important indicator of fertility prognosis by determining ovarian reserve independent of gonadotropins.
Preovulatory Follicle (Graaf's Follicle)
0 In more advanced stages, vascularity increases in the dominant follicle. The size of the follicle reaches 16-20 mm. In this period, the oocyte is in the cumulus oophorus, surrounded by granulosa cells, pushed to one pole of the follicle.
The structures surrounding the oocyte in the follicles are respectively; zona pellucida, granulosa and theca interna
0 Increased estrogen levels (>200 pg/ml, >50 hours) originating from the preovulatory follicle pull the LH trigger with a positive feedback effect. LH, which begins to increase in the environment, attaches to the LH receptors in the granulosa cells in the preovulatory follicle and initiates progesterone release on the 10th day of the cycle. The progesterone that starts to be released pulls the FSH trigger with a positive feedback effect.
FSH and LH, which start to rise 2-3 days before ovulation, provide the final maturation of the preovulatory follicle.
As a result of the LH discharge just before ovulation, the I. meiosis in the oocyte is completed and the secondary oocyte is formed.
Ovulation
• When estrogen exceeds the level of 200 pg/ml and remains above this level for 50 hours, the LH trigger is pulled with a positive feedback effect.
Ovulation-producing effects of the resulting LH peak:
► It increases the wall tension of the follicle by increasing the blood flow into the follicle.
► It initiates preovulatory progesterone synthesis and the resulting progesterone activates collagenase and causes connective tissue degradation in the follicle wall.
► It increases the synthesis of PgE in the follicular fluid, and accordingly it helps the degradation of connective tissue in the follicle wall by increasing the plasminogen activator and lysosomal activity.
► Increases Pgf2a synthesis in follicular fluid, which leads to contraction of smooth muscle fibers around the follicle and ensures ovulation
► Inhibition of prostaglandin synthesis prevents the follicle from cracking (luteinized ruptured follicle). Therefore, NSAIDs should not be used in infertile patients.
► After all; 24-36 hours after the onset of the LH surge, or 10-12 hours after the LH peak, the follicle wall cracks and ovulation occurs. At the time of ovulation, the cumulus oophorius and antral fluid from the follicle are excreted into the abdominal cavity. At this time, tubal activity is at its peak.
Indicators of ovulation
1. Increased basal body temperature (BBT)
2. Thick cervical mucus, Spinnbarkeit and Ferning (-)
3. Demonstration of LH surge
4. Demonstration of secretory endometrium in endometrial biopsy
5. Midluteal serum progesterone measurement >3 ng/ml
6. Disappearance of the existing follicle in USG
7. The surest indicator of ovulation is pregnancy
Estrogen level begins to decrease just before ovulation and continues in the early luteal phase and continues to decrease until the midluteal phase. As a result of the secretion of the corpus luteum, it starts to rise again in the midluteal period.
Luteal Phase
► After follicle rupture and oocyte expulsion, corpus luteum is formed as a result of luteinization in granulosa cells with the effect of LH. The continuation of the function of the corpus luteum depends on the continuation of LH production. Meanwhile, the capillaries formed by VEGF, whose release is increased, penetrate the granulosa cells and 8-9 days after ovulation, maximum vascularization and progesterone and estrogen levels are reached. Vascularization allows greater amounts of LDL cholesterol to be transported into cells, thus storing precursors for increased steroidogenesis.
The corpus luteum is the major source of ovarian sex steroids and inhibin A in the postovulatory phase of the cycle.
► Progesterone level rises and reaches its highest level approximately 1 week after the LH peak (midluteal phase). This period is also the ideal period for the blastocyst to implant into the endometrium.
► Inhibition of new follicle development in the luteal phase:
0 Increasing progesterone suppresses new follicle development with both local and central effects.
0 Low levels of gonadotropins due to the (-) feedback effects of both progesterone and estrogen also inhibit the initiation of the development of new follicles in the luteal phase.
0 Inhibin A, which is secreted from the luteal granulosa cells and whose level rises, causes FSH levels to decrease to the lowest level in the luteal period.
► 10-12 of the luteal phase. The corpus luteum starts to lose its LH receptors on the 24th day of the cycle and if pregnancy does not occur, it regresses on the 24th day of the cycle and is called the corpus albicans. In case of pregnancy, hCG released from trophoblasts constantly stimulates the corpus luteum to secrete progesterone by mimicking the LH effect. The support of the corpus luteum is essential until the 5th week for the continuation of the pregnancy. The placenta begins to produce enough progesterone after the 5th week (luteal placental shift).
► The mechanism of the degeneration of the corpus luteum is not known exactly, but it is thought that the released prostaglandin F2a has a luteolytic effect. PGF2a reaches its highest levels in the cycle during menstruation.
► With the decrease in inhibin A levels and the associated increase in FSH levels 2 days before menstruation, the follicles begin to prepare for the next cycle. With the decrease in corpus luteum steroids and inhibin levels, the GnRH-gonadotropin system is reactivated; FSH and LH begin to rise.
Since the life span of the corpus luteum is 14 days (12-16) on average, the luteal phase is constant in the cycles and lasts about 14 days. Therefore, the differences between the cycle lengths are due to the follicular phase.
Uterine Cycle
• The endometrium consists of two functional layers.
► Stratum functionale; (upper 2/3) is the superficial layer of the endometrium that is shed with each menstruation and is fed by spiral arterioles. This layer is the region where proliferation, secretion and degeneration occur. It is divided into two parts; stratum compacta (superficial) and stratum spongiosa (intermediate)
► Stratum basale; (bottom 1/3) is the deep layer of the endometrium, which proliferates under the influence of estrogen and regenerates the functional endometrial layer shed during menstruation and is fed by the bacillary arteries.
Proliferative Phase
► At the end of each menstruation, all layers of the endometrium are shed except the basal layer. Under the influence of estrogen, the basal layer regenerates and the functional layer is formed again. The dominant change seen in this period is the transformation of the endometrial glands, which initially appear straight, narrow and short, to longer and curved ones.
► Numerous mitoses (reparations occur) are seen in the glands proliferating with the effect of estrogen from the developing follicles and a transition from single-layered columnar epithelium to pseudostratification occurs. The stroma is densely compact. Vascular structures are rarely encountered and spiral arterioles begin to elongate.
► The endometrium, which is 1-2 mm at the beginning of the proliferative phase, reaches a thickness of 5-8 mm at the end of the proliferative phase.
Secretory Phase
► Progesterone released from the corpus luteum after ovulation reduces mitosis and DNA synthesis, increases vascularization, and the lumens of the glands are filled with eosinophilic protein-rich secretion. in the 17-18 days of the cycle in glandular epithelium. characteristic PAS-positive subnuclear glycogen vacuoles begin to appear (this is the first histological sign of ovulation).
► in the 6-7 days Postovulatory (20-21 of the cycle. ) intraluminal vacuoles containing glycogen become prominent and secretion reaches its highest level. This is the most suitable period for blastocyst implantation. The mean endometrial thickness before implantation is 8-10 mm.
► While there is no change in the stroma until the postovulatory 7th day, progressive edema starts from the 7th day. Spiral arteries become prominent and convoluted.
► On the 24th day of the cycle, the eosinophilic staining pattern in the perivascular stroma becomes evident. On the 26th day of the cycle, pseudodecidualization becomes widespread, and on the 28th day (2 days before menstruation) there is PMN leukocyte infiltration and a pseudo-inflammatory appearance in the stroma. Stromal collapse is the harbinger of menstruation.
Menstruation
The first day of menstrual bleeding is the first day of the menstrual cycle.
The decrease in progesterone levels due to the regression of the corpus luteum eliminates the inhibitory effect on prostaglandin synthesis in the endometrium and increases local prostaglandin levels.
PgF2a. It causes spasm and vasoconstriction in the spiral arteries that feed the functional layer of the endometrium. The blood flow to the capillaries is disrupted and the tissue becomes ischemic and necrotic. With tissue fragmentation, the functional layer begins to shed (desquamation).
Loss of transmembrane proteins called cadherin, which connects endometrial cells, is an important part of endometrial cell destruction.
Prostaglandin synthesis occurs in the endometrium during the menstrual cycle and reaches its peak during menstruation. The resulting PgF2a is vasoconstrictor and increases ischemia. It also increases myometrial contractions. Due to this, it allows the menstrual blood to be thrown out. This condition causes pain during menstruation (dysmenorrhea) in some patients.
Menstruation was a progesterone withdrawal bleeding.
Normal menstrual blood does not clot because the fibrinolytic activity is too high.