In the schematic diagram of oogenesis, how does the first oogonia mitotically divide to produce *three* oogonia? Wouldn't mitosis produce *two* oogonia?

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. // // // 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. // // // 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!

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Course: Class 12 Biology (India) > Unit 3

Lesson 4: Gametogenesis

Oogenesis and follicular development review

Google Classroom

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 $100, 000$ ‍ 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.

test your understanding

Which of the following statements about oogenesis is true?

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.

Test your knowledge

Several women across the world suffer from polycystic ovary syndrome (PCOS). A common side way to diagnose PCOS is to look for enlarged ovaries with multiple follicles that have failed to release their oocytes. In other words, ovulation often fails to occur.

Which follicle ruptures to release the oocyte during ovulation?"

Test your knowledge

Tertiary follicles can be identified by the presence of a large antrum.

Which of the following options best describes the antrum?

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waterlily008
Posted 3 years ago. Direct link to waterlily008's post “In the schematic diagram ...”
In the schematic diagram of oogenesis, how does the first oogonia mitotically divide to produce three oogonia? Wouldn't mitosis produce two oogonia?
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Answer
- Zoe Buryska
  Posted 2 years ago. Direct link to Zoe Buryska's post “I think the first part of...”
  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.
  // // //
  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.
  // // //
  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!
  Comment on Zoe Buryska's post “I think the first part of...”
  (5 votes)