How are oocytes arrested in meiosis until fertilisation?

Oocytes are arrested in meiosis until fertilisation through a process called meiotic arrest, regulated by cyclic adenosine monophosphate (cAMP).

In more detail, oocytes, or female germ cells, are produced in the ovaries during fetal development. These cells begin the process of meiosis, a type of cell division that results in four daughter cells each with half the number of chromosomes of the parent cell. However, the oocytes do not complete meiosis immediately. Instead, they are arrested in the first stage of meiosis, known as prophase I, and remain in this state until puberty.

This arrest is maintained by high levels of a molecule called cyclic adenosine monophosphate (cAMP) within the oocyte. cAMP is a second messenger, meaning it transmits signals from outside the cell to the cell's interior. In this case, the signal it transmits is to halt the progression of meiosis. The high levels of cAMP in the oocyte are maintained by the surrounding follicle cells, which continuously produce and supply the oocyte with cAMP.

At the time of ovulation, the oocyte completes the first meiotic division and progresses to the second stage of meiosis, metaphase II. However, it is again arrested, this time due to a decrease in cAMP levels. The oocyte remains in this state until it is fertilised by a sperm cell. Upon fertilisation, the oocyte completes the second meiotic division and becomes a mature egg cell, ready to begin the process of embryonic development.

In summary, the arrest of oocytes in meiosis until fertilisation is a complex process regulated by the levels of cAMP within the oocyte. This ensures that the oocyte is ready to be fertilised at the appropriate time and can then develop into a healthy embryo.