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Oogenesis

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Oogenesis occurs in the ovary.
Primordial germ cells migrate from the wall of the
yolk sac during embryonic development and enter the developing ovary. These differentiate into oogonia. Some of the oogonia are arrested in the prophase of meiosis I and become primary oocytes. The primary oocytes are surrounded by a single layer of flattened epithelial cells to form primordial follicles. They remain in this state until the individual reaches puberty.
At puberty primordial follicles begin to mature into
primary follicles at the rate of about one per month for the entire reproductive life of the woman (to about age 50).
As the follicle matures, the
primary oocyte completes its first meiotic division to give rise to a secondary oocyte and a smaller polar body.
After ovulation, if fertilization takes place, the
secondary oocyte undergoes the second meiotic division to become an ovum and a second polar body.
Both the ovum and the polar body are contained within a glycoprotein shell called the
zona pellucida.
The follicle cells of the ovary develop into a
corpus luteum, an endocrine gland that produces hormones to maintain the wall of the uterus in its progestational state.

Follicles

  • Primordial Follicle
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    Some oogonia are arrested in prophase of meiosis I to become primary oocytes while the individual is still in utero. The primary oocytes are surrounded by special supportive cells called follicular cells. Initially the supportive cells are flat and form primordial follicles. As follicular cells assume a more cuboidal shape, the follicle becomes a primary follicle. The primary oocyte contains 46 double structured chromosomes, or the diploid number.

  • Primary Follicle

    The primary follicle, containing a primary oocyte begins to grow at puberty. The primary oocyte contains 46 chromosomes, two of which are the sex chromosomes. Since the individual is female, the sex chromosomes are both X chromosomes. Each of the 46 chromosomes consists of a pair of chromatids.

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  • Secondary Follicle
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    As the follicle enlarges, fluid begins to accumulate within it. the follicle is now known as a secondary follicle. The primary oocyte also develops a glycoprotein shell called the zona pellucida. Continued accumulation of liquor folliclli results in enlargement of the follicle. The primary oocyte and surrounding granulosa cells remain on one side of the follicle.
  • Tertiary Follicle (Graafian Follicle)
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    In the tertiary follicle, the primary oocyte completes meiosis I about 3 hours prior to ovulation to form the secondary oocyte and first polar body. These are located on one side of the follicle, surrounded by the zona pellucida and a layer of granulosa cells called the corona radiata. The oocyte also sits on a cushion of granulosa cells called the cumulus oophorous. A thick layer of granulosa cells lines the inside of the follicular space which is filled with a large accumulation of fluid known as liquor folliculi. The outer layer of the follicle wall is called the theca and it is divided into a theca interna and a theca externa. The tertiary follicle will continue to enlarge to become a graafian follicle. Eventually, following a surge of follicle stimulating hormone (FSH) form the anterior pituitary, the follicle will rupture and ovulation occurs.