Ovarian function Flashcards
Where do the primordial germ cells arise?
From the epiblast of the yolk sac at week 3
How do the PGCs migrate to the gonadal ridges?
Due to amoeboid movement and guided by chemotaixs from the gonadal ridges. As they travel they proliferate. Motility is lost after week 7.
What will result from a loss of PGC migration?
Teratomer
What leads to the development of the female reproductive system?
The absence of Y chromosome leads to the default pathway. Absence of androgens and AMH.
What are the 3 cell types present in the gonadal ridges?
PGCs - oogonia Granulosa cells (somatic) Theca cells (somatic)
Where do the cells develop from?
Sex cord cells cluster around PGCs to form primordial follicles. The sex cords mature into granulosa cells.
Theca cells and vasculature arise from mesenephros.
What is Turner’s syndrome?
xo genotype - oocyte development cannot occur without both x chromosomes. Without germ cells it leads to ovarian dysgenesis (progressive loss of PGCs) and a streak ovary (hypoplastic and fibrous tissue)
What does the development of female organs require?
Absence of androgens and AMH. Normal germ cells for ovary development.
Are post pubertal ovaries homo or heterogeneous?
Heterogeneous as the follicles are all at different stages of development
What is the function of a ovary?
To produce oocytes and hormones
Describe the process through oogenesis.
1) PGCs proliferate through mitosis whilst travelling to ridges
2) Within the ridges oogonia undergo mitosis to produce primary oocyte
3) Primary oocyte undergoes 1st meiotic division to produce secondary oocyte
4) Secondary oocyte undergoes 2nd meiotic division to produce mature tertiary oocyte
When does female meiosis begin and how does it compare to males?
Male begins at puberty.
Female begins during fetal period but is arrested during the same period.
What controls entry into meiosis in both males and females?
Female is controlled via Stra8 (stimulated by retinoic acid 8 gene). Retinoic acid builds in the ovary during fetal development for expression of Stra8.
Males metabolises RA to prevent entry into meiosis before puberty. It is controlled by cytochrome p450.
Where does the first meiotic block occur?
Primary oocyte enters 1st meiosis and is blocked at PROPHASE I. It remains arrested until ovulation.
When does the second meiotic block occur?
On ovulation, meiosis is restarted and completed to produce a secondary oocyte that enters meiosis II, getting blocked at METAPHASE II.
When is meiosis completed?
After fertilisation.
What is the problem with oocytes being arrested?
Makes them vulnerable to damage and fertility declines with age, the longer they are arrested. After 35 years egg quality declines and the risk of trisomy increases due to damage of the spindles needed for chromosomal segregation.
What happens to the number of oocytes throughout life, starting from fertilisation?
Women have a finite number of oocytes as all stem cells enter meiosis before birth. Usually run out around 50yrs.
A fetus has 7 million at 20 weeks but at puberty there is 400,000 but only 500 will be ovulated. Atresia of oocytes begins after 20 weeks gestation due to selection of the best eggs.
What is different about meiotic divisions in females compared to males?
Divisions are asymmetrical, producing polar bodies that contain little cytoplasm and excess genetic material.
When are the polar bodies released?
1st body is released before ovulation on completion of meiosis I. The 2nd body signifies fertilisation and the completion of meiosis II.
Why do polar bodies have little cytoplasm?
Oocytes must compensate for the lack of cytoplasm coming from the sperm, by keeping most of theirs.
Where do oocytes develop within the ovaries?
Within follicles that are supported by the somatic cells, granulosa and theca.
What are the three stages of follicle development?
Primary (preantral)
Secondary (preantral)
Tertiary (Antral / Graafin)
What is a primordial follicle?
A primary oocyte surrounded by a flattened layer of granulosa cells.
