Oogenesis
Primordial germ cells migrate from the yolk sac to the developing ovary around week 5 of development. These cells become oogonia.
As the oogonia undergo mitotic division to increase their numbers, some begin to enter meiosis I becoming primary oocytes.
During prophase of meiosis I, primary oocytes, replicate the entire compliment of DNA to make 46 double stranded chromosomes. They remain suspended in this state, surrounded by a single layer of flattened epithelial cells to form primordial ovarian follicles. They remain in this state of suspension until puberty before resuming meiosis I.
By the 5th month of development, the number of primordial follicles in each ovary is estimated to be approximately 7 million. In the next two months, the majority of these undergo atresia (degeneration) so at birth, the total number of primordial follicles a newborn girl possesses is about 600,000.
Fig 1. Diagram showing the stages in oogenesis. Note that each cell division results in an even distribution of the chromosomes but and unequal division of the cytoplasm.
By the time a girl reaches puberty, the number has been further reduced to around 40,000, of which only about 500 will be ovulated during the reproductive life of the individual. Over production and die back are common themes in development.
At puberty, primordial follicles begin to mature at the rate of about 15 to 20 per month until menopause, around age 50. Of the 15 or 20 that begin maturation at the beginning of the cycle each month, usually only one is ovulated.
As the follicle grows and matures, the primary oocyte remains fixed in prophase of meiosis I, until about 3 hours prior to ovulation when it completes meiosis I, by division into a secondary oocyte and a smaller 1st polar body that are haploid, each possessing 23 double stranded chromosomes.
It is the secondary oocyte that is released from the follicle at ovulation. It only completes meiosis II to form an ovum and second polar body if fertilization takes place. These each possess one set of 23 single stranded chromosomes, the haploid number.
Since genetic females have two X chromosomes, all gametes produced by normal oogenesis will carry an X chromosome.
Whether the first polar body actually divides, as shown in the illustration, is a discussion for philosophers and conspiracy theorists.
Follicles
The four panels making up the slide show below review the development of the ovarian follicles using images of H & E stained sections of the ovary. They are value added for those who are interested in matching the histologic appearance of each follicle with the status of the oocytes but watching it is not crucial to understanding of the overall process of oogenesis.
You can advance the slide by a mouse click or using the arrows at the right or left side of the panel.
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Start Slide Show
Primordial Follicles
In primordial follicles, the follicular cells surrounding the primary oocyte are flat.
The primary oocyte is diploid possessing 46 chromosomes that are replicated during prophase of meiosis I, doubling the amount of DNA in the cell.
Primary oocytes remain suspended in prophase of meiosis I for up to 50 years, as they don't continue meiosis until after puberty and then only a few each month for the reproductive life of the woman.
Fig 2a. Primordial follicles in the ovary. Primary oocytes are arrested in prophase of meiosis I
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Primary Follicles
Beginning at puberty, 15 to 20 primordial follicles resume development each month, under the influence of follicle stimulating hormone (FSH), secreted by the anterior pituitary,
A follicle becomes a primary follicle when the surrounding follicular cells change from squamous to cuboidal.
Most of these primary follicles become atretic. Only one or two will progress to maturity.
Fig 2b. Primordial follicles become primary follicles when the surrounding follicular cells morph from being squamous to becoming cuboidal in response to FSH from the anterior pituitary.
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Secondary Follicles
The primary follicle becomes a secondary follicle with the appearance of a mucopolysaccharide layer around the primary oocyte; the zona pellucida and the accumulation of fluid, liquor folliculi.
Multiple layers of supportive cells surrounding the primary oocyte are called granulosa cells.
Granulosa cells provide nutritional and hormonal support for the developing oocyte.
The increase in liquor folliculi and granulosa cells contribute to enlargement of the follicle to the tertiary follicle stage.
Fig 2c. Primary follicles become secondary follicles when they have multiple layers of granulose cells and liquor follicle begins to accumulate in the follicular antrum.
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Tertiary (Graafian) Follicles
Within the tertiary follicle, the primary oocyte sits on a cushion of granulosa cells called the cumulus oophorous.
Cells of the cumulus oophorus in closest contact with the zone pellucida are called corona radiata.
About 3 hours prior to ovulation the primary oocyte completes meiosis I to form the secondary oocyte and first polar body. These are located on one side of the follicle, surrounded by the zona pellucida, corona radiata and cumulus oophorus, all of which will be shed at ovulation.
Fig 2d. Continued accumulation of liquor follicluli causes the follicle to swell and protrude from the surface of the ovary as a tertiary or griffin follicle. Prior to ovulation, the primary oocyte completes meiosis I. A surge of LH from the pituitary causes rupture of the follicle and ovulation.
Ovulation
The tertiary follicle will continue to enlarge to become a graafian follicle.
Following a surge of luteinizing hormone (LH) from the anterior pituitary, the follicle ruptures and ovulation occurs.
At ovulation, the secondary oocyte and polar body, contained within zona pellucida and a layer of granulosa cells called corona radiata are released into the peritoneal cavity. They are drawn into the uterine tube to await the arrival of spermatozoa. If viable sperm have been deposited in the female reproductive tract, fertilization may occur.
If fertilization occurs, the secondary oocyte completes meiosis II, without further replication of the chromosomes, producing an ovum and a second polar body, each haploid possessing 23 single stranded chromosomes (1N DNA).
If fertilization does not occur, the endometrium enters the menstrual phase of the menstrual cycle and the stratum functionalis will be shed.
Fig 3. At ovulation a secondary oocyte and first polar body, enclosed in zona pellucida, accompanied by a layer of granulose cells, the corona radiata, are released from the tertiary follicle.