Structure of the oocyte
Minute 1 - look at structure of oocyte
- oocyte contains a haploid nucleus (mother material - X chromosome only)
- lots of cytoplasm
- during meiosis there is an uneven division of cells in the body - one cell will keep all the cytoplasm and some DNA, and a polar body that has just discarded DNA.
- cell surface membrane is where the sperm needs to fuse to fertilise the egg
- around the oocyte and polar body is the zona pellucida = an extracellular matrix where sperm bind to and undergo acrosome reaction, releasing their enzymes that penetrate it. Allows species specificity. Stays during the whole process of implantation.
- reminiscent cells from the granulosa cells = cumulus cells
Oocyte journey in the female tract
- made in the ovary
- oocyte is ovulated out during the menstrual cycle after they have matured, leaving the somatic cells behind
- oocyte will be captured by the oviduct, which has ciliated cells, aiding transportation of the egg which is immotile
- fertilisation occurs in the first part of the oviduct, closer to the ovary = ampulla
- if fertilisation occurs then the embryo will go down using the ciliated cells to the uterus, where it can be implanted.
Process of fertilisation
- egg waits for the sperm to find it
Requirement for fertility
The core process relies on the creation and fusion of male and female gametes.
- normal production of sperm (spermatozoa)
- normal production of eggs (oocyte, ova)
- sperm traverse female tract to reach the egg - time restrain: sperm lasts 5 days in tract but egg once ovulated only lasts 24 hours
- sperm penetrate and fertilise an egg
- implantation of the embryo into the uterus
- normal pregnancy
What are the differences in gametogenesis between the male and female?
The steps of gametogenesis are the same in the male and female.
Timing and outcome differ sharply.
Gametogenesis in females
- oogenesis starts in the embryo (prebirth)
- oogensis is not continuous (2 meiotic arrests)
- each diploid cell develops into only 1 haploid cell
- oogenesis continues after puberty and stops at menopause (male continues)
Process of oogenesis
- primordial germ cells will differentiate into oogonium (diploid cells)
- mitosis in the embryo (pre-birth), expanding number of oocyte, creating a maximum amount of primary oocytes
- during embryogenesis the foetus will undergo meiosis one, but there will be an arrest in prophase I, until the point of ovulation during puberty where meiosis I resumes
- secondary oocyte is formed and the polar body degenerates
- meiosis II is started and are arrested at metaphase II.
- the eggs are ready to be fertilised and only if fertilisation occurs will meiosis II resume to form a fully haploid ovum
Meiosis in females (resumed at ovulation)
- one parent cell undergoes meiosis I, creating an uneven division
- the primary oocyte divides the chromosomes evenly but the cytoplasm and organelles remain in one of the two daughter cells which becomes the secondary oocyte. The other daughter cell becomes a polar body, which will degenerate and die.
- the same thing happens for the second meiotic division, creating a haploid ovum
What does the formation of a polar body allow?
It allows the primary oocyte to reduce its genome by half and conserve most of its cytoplasm in the secondary oocyte.
Full process of oogenesis
- Diploid oogonia will undergo mitosis to make the primary oocyte
- Primary oocyte undergoes meiosis I (all pre-birth) which arrests until puberty
- At puberty meiosis I resumes at the time of ovulation, to create a secondary oocyte and the first polar body
- Meiosis II is started and then arrested at metaphase II
- If fertilisation occurs, meiosis II resumes to form the ovum and polar bodies
What is meiosis?
The process of the actual oocyte going from diploid to the haploid ovum.
What is folliculogenesis?
The development of the follicles
