**2. Physiology of folliculogenesis**

Ovaries are almond-shaped organs located in the pelvis on either side of the uterus. In addition to production of ova the ovaries are also a distinct endocrine organ producing hormones primarily estrogen and progesterone that are very important for normal reproductive function. The adult ovary can be divided into 3 main regions superficial to deep. These are the a. Cortex that consists of tunica albuginea ovarian follicles (primordial, primary, secondary, small medium, large Graafian follicles, and corpus luteum, atretic follicles. The B. medulla consists of blood vessels and nerves in connective tissue. The innermost hilum contains large spiral arteries and Leydig cells.

Normally the ovary produces a single dominant follicle in each menstrual cycle that undergoes maturation and results in ovulation (**Figure 1**). The granulosa cells of the growing follicles produce estradiol during the first half of the menstrual cycle or the follicular phase. The folliculogenesis begins with recruitment of a primordial follicle into a pool of growing follicles and it is a long process almost a year for a primordial follicle ultimately resulting in ovulation of the follicle or death by atresia. The process of folliculogenesis can be divided into 2 phases. First phase is "preantral" or "gonadotrophin independent" phase, where there is growth and differentiation of oocyte. The preantral phase mainly controlled by local factors growths factors by autocrine and paracrine processes. The second phase is the "gonadotrophin dependent" phase or the "antral phase" when there is a fast growth in the size of the follicle itself and it is under the control of gonadotrophin. The Antral phase is regulated by FSH and LH and also by paracrine growth factors that cause intracellular signaling and multiplication of cells.

The process of this folliculogenesis goes through multiple sub phases too. These include: 1. primordial follicle recruitment; 2. preantral follicle development; 3. Selection and growth of antral follicle; and 4. follicle atresia. The primordial follicle is the reproductive unit of ovary. It gives rise to dominant follicle. In human female fetus, primordial follicles are formed 6 and 9 months of gestation and many of the primordial follicles undergo apoptosis. The number of primordial follicles decreases progressively due to recruitment and apoptosis and very few of them are present after menopause. Primordial follicle recruitment is completely dependent on paracrine mechanism largely controlled by growth factors especially TGF-beta superfamily.

A primary follicle is in the next sequence that has the presence of one or more cuboidal granulosa cells arranged in a single layer around the oocyte. The most important event at this stage is the FSH receptor expression and oocyte growth and

**63**

**2.1 Follicular phase**

gonadotropin action.

*Ovarian Reserve*

*DOI: http://dx.doi.org/10.5772/intechopen.89772*

factor for granulosa cell proliferation.

P450AROM gene expression by FSH in granulosa cells.

plays a role in follicular development in early part of the cycle.

maturation. Granulosa cells express receptors for FSH at this stage and the stimulus for this is FSH itself. Further development of primary follicle into preantral follicle is FSH dependent. The development of primary follicle is associated with striking change in oocyte too. Even there is a development of intimate connections between oocyte and granulose cells through the transzonal oocyte processes and gap junctions (cumulus oophorus complex). FSH is an important factor in supporting follicle growth after antrum formation and in also preventing apoptosis thus it is a survival factor for antral follicles. Thus FSH plays a very important role in selection and dominant follicle development. The proliferation of granulosa cells is a very important feature for development of dominant follicles. FSH is the most important

As the granulosa cells proliferate and a dominant follicle develops it acquires the capacity to produce estradiol. FSH mediates the granulosa cells to acquire the above potential. The progressive increase in estradiol production by the dominant follicle from Day 7-Day 12 of menstrual cycle is only possible due to increased levels in

The physiologic mechanism by which dominant follicle produces estradiol is the two cells two-gonadotrophin concept. When FSH recruits follicles to preovulatory development, their granulosa cells develop LH receptors, start undertaking aromatization and also inhibin production. Inhibin has the capability to increase LH stimulated thecal androgen production. LH receptors are present on theca cells throughout the menstrual cycle. LH acts on theca cells to produce primarily androstenedione and to a lesser extent testosterone. Theca cell synthesized androstenedione is transported by paracrine circulation into the granulosa cells. In granulosa cells androstenedione and testosterone are aromatized to estrone and finally into estradiol by 17-β-hydroxysteroid dehydrogenase type I. This is known as the twocell, i.e., theca cell and follicular cell, two-gonadotropin, i.e., FSH and LH and two hormone, i.e., estradiol and progesterone theory of regulation of estrogen synthesis in the human ovary as shown in **Figure 2**. Granulosa cell derived inhibin takes part in a paracrine mechanism communicating with theca cells and thus amplifying androgen synthesis. This theca cell derived androgen is converted to estradiol in granulosa cells of preovulatory follicles. Inhibin B has an early follicular phase elevation and lower values after ovulation. Thus it suggests Inhibin B is a granulosa cell product and can be a marker for follicular function, oocyte number and thus

The follicular phase starts from the first day of menstrual cycle D1 until ovulation. Basal body temperature chart normally shows lower values during this phase. Development growth trajectories of the ovarian follicles characterize this first phase of the cycle. Folliculogenesis starts during the last few days of the preceding menstrual cycle and continues till the release of the mature follicle at the time of ovulation. The primary development of the follicle to the preantral follicle is not gonadotropin dependent, and further follicular growth beyond this point requires

The secretion of gonadotropins from anterior pituitary is regulated by gonadotropin releasing hormone (GnRH), steroid hormones, and various peptides released by the dominant follicle. The growth of follicular size and number of granulosa cells in each follicle leads to an increase in estradiol serum concentrations in the early follicular phase. FSH receptors exist only on the granulosa cell membrane. There is increase in the number of FSH receptors with the gradual rise in serum FSH levels during the late follicular phase. The rise in serum FSH along with the rise in FSH

**Figure 1.** *Folliculogenesis, ovulation and corpus luteal formation.*

### *Ovarian Reserve DOI: http://dx.doi.org/10.5772/intechopen.89772*

*Innovations in Assisted Reproduction Technology*

cause intracellular signaling and multiplication of cells.

spiral arteries and Leydig cells.

3 main regions superficial to deep. These are the a. Cortex that consists of tunica albuginea ovarian follicles (primordial, primary, secondary, small medium, large Graafian follicles, and corpus luteum, atretic follicles. The B. medulla consists of blood vessels and nerves in connective tissue. The innermost hilum contains large

Normally the ovary produces a single dominant follicle in each menstrual cycle that undergoes maturation and results in ovulation (**Figure 1**). The granulosa cells of the growing follicles produce estradiol during the first half of the menstrual cycle or the follicular phase. The folliculogenesis begins with recruitment of a primordial follicle into a pool of growing follicles and it is a long process almost a year for a primordial follicle ultimately resulting in ovulation of the follicle or death by atresia. The process of folliculogenesis can be divided into 2 phases. First phase is "preantral" or "gonadotrophin independent" phase, where there is growth and differentiation of oocyte. The preantral phase mainly controlled by local factors growths factors by autocrine and paracrine processes. The second phase is the "gonadotrophin dependent" phase or the "antral phase" when there is a fast growth in the size of the follicle itself and it is under the control of gonadotrophin. The Antral phase is regulated by FSH and LH and also by paracrine growth factors that

The process of this folliculogenesis goes through multiple sub phases too. These

A primary follicle is in the next sequence that has the presence of one or more cuboidal granulosa cells arranged in a single layer around the oocyte. The most important event at this stage is the FSH receptor expression and oocyte growth and

include: 1. primordial follicle recruitment; 2. preantral follicle development; 3. Selection and growth of antral follicle; and 4. follicle atresia. The primordial follicle is the reproductive unit of ovary. It gives rise to dominant follicle. In human female fetus, primordial follicles are formed 6 and 9 months of gestation and many of the primordial follicles undergo apoptosis. The number of primordial follicles decreases progressively due to recruitment and apoptosis and very few of them are present after menopause. Primordial follicle recruitment is completely dependent on paracrine mechanism largely controlled by growth factors especially TGF-beta

**62**

**Figure 1.**

*Folliculogenesis, ovulation and corpus luteal formation.*

superfamily.

maturation. Granulosa cells express receptors for FSH at this stage and the stimulus for this is FSH itself. Further development of primary follicle into preantral follicle is FSH dependent. The development of primary follicle is associated with striking change in oocyte too. Even there is a development of intimate connections between oocyte and granulose cells through the transzonal oocyte processes and gap junctions (cumulus oophorus complex). FSH is an important factor in supporting follicle growth after antrum formation and in also preventing apoptosis thus it is a survival factor for antral follicles. Thus FSH plays a very important role in selection and dominant follicle development. The proliferation of granulosa cells is a very important feature for development of dominant follicles. FSH is the most important factor for granulosa cell proliferation.

As the granulosa cells proliferate and a dominant follicle develops it acquires the capacity to produce estradiol. FSH mediates the granulosa cells to acquire the above potential. The progressive increase in estradiol production by the dominant follicle from Day 7-Day 12 of menstrual cycle is only possible due to increased levels in P450AROM gene expression by FSH in granulosa cells.

The physiologic mechanism by which dominant follicle produces estradiol is the two cells two-gonadotrophin concept. When FSH recruits follicles to preovulatory development, their granulosa cells develop LH receptors, start undertaking aromatization and also inhibin production. Inhibin has the capability to increase LH stimulated thecal androgen production. LH receptors are present on theca cells throughout the menstrual cycle. LH acts on theca cells to produce primarily androstenedione and to a lesser extent testosterone. Theca cell synthesized androstenedione is transported by paracrine circulation into the granulosa cells. In granulosa cells androstenedione and testosterone are aromatized to estrone and finally into estradiol by 17-β-hydroxysteroid dehydrogenase type I. This is known as the twocell, i.e., theca cell and follicular cell, two-gonadotropin, i.e., FSH and LH and two hormone, i.e., estradiol and progesterone theory of regulation of estrogen synthesis in the human ovary as shown in **Figure 2**. Granulosa cell derived inhibin takes part in a paracrine mechanism communicating with theca cells and thus amplifying androgen synthesis. This theca cell derived androgen is converted to estradiol in granulosa cells of preovulatory follicles. Inhibin B has an early follicular phase elevation and lower values after ovulation. Thus it suggests Inhibin B is a granulosa cell product and can be a marker for follicular function, oocyte number and thus plays a role in follicular development in early part of the cycle.
