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Course: How does the human body work? - Class 12 > Unit 1
Lesson 4: GametogenesisOogenesis and follicular development review
Oogenesis, development of follicles, and ovulation.
This article provides a review of:
- the stages of oogenesis,
- how oocytes develop into an ovum, and
- how follicles develop alongside oocytes.
Key terms
Term | Meaning |
---|---|
Oogonium | immature germ cell that gives rise to female gamete |
Oocyte | female gametocyte that divides to form egg cell |
Ovum | egg cell, female gamete |
Follicle | structure that encloses oocyte in a cavity, covered by granulosa cells |
Theca | covering of follicle |
Antrum | fluid-filled cavity in Graafian follicle |
Polar body | small daughter cell formed by unequal cell division of oocyte |
Oogenesis at a glance
Oogenesis is the process by which mature female gametes, or ova, develop from germ cells. The image below summarizes how this happens, and over what timeline it unfolds.
As the oocytes develop, they get enclosed in structures known as follicles. Let's first take a brief look at the timeline of oogenesis, and then at the different follicles involved.
Oogenesis: timeline and stages
Before birth
- a few million immature germ cells, or oogonia, are formed within each fetal ovary.
- some oogonia enter meiosis I, are arrested at prophase I, and are now known as primary oocytes.
- primary oocytes form primary follicles (more on what follicles are later).
Between birth and puberty
- several of the follicles degenerate over time, leaving only a few
follicles intact by the time puberty hits.
After puberty
- primary follicles develop into secondary follicles, and then into tertiary, or Graafian follicles.
- the primary oocyte within each tertiary follicle completes its first meiotic division, producing a secondary oocyte and a polar body.
- the Graafian follicle ruptures to release the secondary oocyte, in a process known as ovulation.
Follicular development in the ovary
Follicle is the term used to describe a group of cells that enclose a cavity, or other cells. Let's take a peek inside the ovary to see how follicles develop alongside the oocytes.
- Each primary follicle is made up of a primary oocyte, surrounded by a layer of granulosa cells.
- A few more layers of granulosa cells get added onto the primary follicle to form the secondary follicle. The secondary follicle also develop a covering, or a theca.
- Secondary follicles develop into tertiary, or Graafian follicles, characterized by a fluid-filled space called the antrum, and two layers of theca (theca interna and theca externa).
- The secondary oocyte develops a thick covering known as the zona pellucida. The Graafian follicle releases the oocyte through ovulation.
Want to join the conversation?
- In the schematic diagram of oogenesis, how does the first oogonia mitotically divide to produce three oogonia? Wouldn't mitosis produce two oogonia?(1 vote)
- I think the first part of the diagram is just showing that the 2n germ cell divides over and over, I don't think it's implying that 3 cells arise from mitosis.
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Originally, replying, I misunderstood your question and thought you were asking about the primary oocyte meiosis and I wrote this whole description. While it's not what you were asking, I'm going to leave it below in case it answers someone else's question in the future.
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The first meiotic division of an oogonium produces two cells: the primary oocyte and the first polar body. This first polar body basically is a way to discard half the DNA; most of the cytoplasm stays with the primary oocyte.
Unlike regular meiosis (such as with spermatogenesis), the first polar body never divides, and instead degrades through atresia. Only the primary oocyte undergoes Meiosis II (but only in the presence of a sperm!). If this happens, a second polar body is produced (which is also degraded).
So, from the initial oogonium, one egg cell (oocyte) is produced along with two polar bodies: 3 cells!(5 votes)