FOLLICULOGENESIS GAMETOGENESIS

FOLLICULOGENESIS GAMETOGENESIS

1.2.2.     Folliculogenesis

The development of ovulatory follicles throughout the reproductive life of a female at a fixed interval called folliculogenesis. The release of mature oocyte known as ovulation, ovulation take place in every 28 days.

Construction of the zona pellucida and the formation of product required for the fertilization and early embryonic development events take place during that extended phase. Follicles ultimately had two fates either ovulate as a mature oocyte or undergo atresia (atresia is known as the death of ovarian follicle), which can occur at any point during follicular development.

Primordial follicles

In newborn females small primordial follicles (fundamental developmental unit) are present and in the adult ovary, it is the prevailing follicle type. The concerned follicular epithelial cells were derived from the coelomic epithelium and when they combine with oocyte commonly known as a primordial follicle. Oocyte and follicular epithelium constitute the Primordial follicle. A single flat layer of follicular epithelium or granulosa cells or pregranulosa cell present on primordial follicle. This granulosa cells layer surround the oocyte.

Primordial follicles are positioned in the peripheral cortex of the ovary. As the follicles and oocyte start to grow, they move deeper into the cortex of the ovary.

Primary follicles

The primordial follicle increase in size proliferate. This stage is known as the primary follicle. The oocyte of the primary follicle is known as primary oocyte which enters into the first meiotic division, at this stage development of Follicular stimulating hormone (FSH receptor) also take place on primary follicle.

The primary oocyte is an arrest in the diplotene stage of prophase I. During the arrest different type of developmental changes occurs within the primary oocyte. The zona pellucida begins to form during the arrest of 1° oocyte. Oocyte not completely surrounded by the zona pellucida until the follicle reaches the late preantral stage. Groups of oocytes cyclically restart meiosis along with the onset of puberty. Thus the first part of meiosis begins in the embryo in human female and then arrest and wait for the signal to restart the meiosis, which comes at the onset of puberty roughly 12 years later. It is also a fact that some oocytes are upheld in meiotic prophase for nearly 50 years. At the time of birth, millions of primary oocytes present but only about 400 mature during a woman's reproductive life span.

Secondary Follicles

When puberty comes, the pituitary gland secretes the FSH which receives by the FSH receptor present on the primary follicle. In response to FSH the cell resume and complete the meiosis I and enter into meiosis II and the break of arrest occur and resume the meiosis takes place. At this stage 11 to 12 primary follicles are selected to further developmental process. The granulosa cells divide mitotically, the follicles are now called secondary follicles. The secondary follicles are bigger in size, has a new outer new outer layer of connective tissue, blood vessels, and theca cells. The cell work with the granulosa cells to produce estrogen. This primary follicle recruits the stroma like theca cells after oocyte signalling. The theca cells cover the granulosa outermost layer, basal lamina and differentiate the whole capsule into theca internal and theca external.

At this stage meiosis first get complete and the formation of secondary oocyte along a polar body takes place and chromosome no divide in half.

Tertiary or Antral Follicles

As the formation of antrum takes place and mark the formation of the tertiary follicle or (or late antral follicles). The antrum is a cavity in which follicular fluid get filled.

The granulosa cells first acquire the LH receptor. During the final stages of follicle maturation, the blood supply of the theca layer increases drastically.

Several follicles reach the tertiary stage at the same time, and most of these will undergo atresia.

The secondary oocyte or tertiary follicle having the largest size of the antrum and present nearby the periphery of ovary get selected to further developmental process at the seventh day of menstrual cycle and rest undergo atresia. Thus at eight days, only one secondary oocyte remain. This tertiary follicle grows further and called is Graafian follicle. The Ovulation takes place on the 14th day of the menstrual cycle. FSH stimulates the growth of a tertiary follicle, and LH stimulates the production of estrogen by granulosa and theca cells. Once the follicle is mature, it ruptures and releases the oocyte. Cells remaining in the follicle then develop into the corpus hemorrhagicum. Corpus hemorrhagium is a temporary structure, and within four days of ovulation, it is converted into corpus luteum. Progesterone is released by corpus luteum.       

The trophoblast cell of blastocyst secretes human chorionic gonadotropin (hCG) hormones.  Human chorionic gonadotropin signals the corpus luteum to relax progesterone continuously. Corpus luteum is essential for the maintenance of pregnancy. It also releases relaxin hormones. Relaxin causes softening of pubic symphysis which helps in parturition (Delivery of the child). When there is no fertilization. The corpus luteum stops releasing progesterone after 10 days and converted into corpus albicans. When the egg is fertilized and implantation of the embryo occurs.

1.2.3.     Developmental competence

Oocyte ability to produce normal, viable and fertile offspring after fertilization. During follicular development, the acquisition of developmental competence is a gradual process.

The percentage of oocytes that can develop to the blastocyst stage usually expressed the developmental competence. Developmental competence also evaluates by morphological evaluations such as the number of blastomeres or the ratio between trophectoderm cell numbers and inner cell mass.

Apoptosis

Throughout embryonic and neonatal life apoptosis plays an important role in the massive loss of oogonia and oocytes.

The vast majority of follicular populations go through atresia rather than ovulation.

1.2.4.     Oocyte Maturation

Oocyte maturation takes place when oocyte progresses from the diplotene to the metaphase II stage and it is a complex process because nuclear maturation also takes place in this phase. The transition from the diplotene stage to metaphase is called diakinesis. In response to the ovulatory LH, surge oocyte resumes its meiosis or removal of the secondary oocyte from the follicle takes place. During diakinesis, folding of nuclear membrane get starts, the disappearance of nuclear pores occur and then the fragmentation of nuclear membrane occur before quickly disappearing to leave only small sacs with double walls and all these events are identified as germinal vesicle breakdown (GVBD), which is the first visible sign of meiotic continuation. When nucleolus comes in the contact with the cytoplasm get disappears.


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