Why does meiosis stop at phase II for the female and does not finish until after it has been fertilized by a sperm?
Meiosis is a specialized type of cell division that produces gametes, such as eggs and sperm, which are necessary for sexual reproduction. In females, meiosis stops at phase II until fertilization occurs, while in males it continues uninterrupted. This is due to the unique biological processes associated with oogenesis (the development of eggs) and fertilization in females.
To understand why meiosis stops at phase II in females, let's take a closer look at the steps involved in oogenesis.
1. Primordial Germ Cells: During embryonic development, primordial germ cells (PGCs) are formed. These cells later develop into oogonia, the precursors of eggs.
2. Meiotic Arrest: Before birth, the oogonia undergo several divisions to form primary oocytes. These primary oocytes then enter a state of "meiotic arrest," during which meiosis halts at prophase I. Most primary oocytes remain in this arrested state until puberty.
3. Ovulation and Resumption of Meiosis: Each month after puberty, a few primary oocytes are stimulated to resume meiosis and complete prophase I. However, only one primary oocyte proceeds to the next phase, called metaphase II, as part of the menstrual cycle.
4. Fertilization: If fertilization occurs, the secondary oocyte (ready for fertilization) completes meiosis and forms a mature egg. During fertilization, the sperm fuses with the mature egg, triggering the completion of meiosis by stimulating the secondary oocyte to proceed to the final stages of cell division.
The reason for the meiotic arrest in females is related to the formation of the ovarian reserve, which involves preserving a pool of primary oocytes. This mechanism ensures that females are born with a supply of potential eggs that can be used throughout their reproductive lifespan. By preserving the primary oocytes in a dormant state, the female reproductive system maintains the capacity to produce eggs throughout a woman's life.
In summary, meiosis stops at phase II in females as part of the natural process of oogenesis. The pause in meiosis allows the formation of a reservoir of primary oocytes, while the resumption of meiosis and completion of cell division occur only when fertilization takes place. This unique mechanism ensures the production of viable eggs for successful reproduction.