Reproduction: Folliculogenesis Flashcards
What are some of the processes/things that need to happen in order to successefully reproduce
- Differentiation into male or female
- Sexual maturation
- Production, storage and release of sufficient supply of eggs and sperm
- Gametes have to have correct number of chromosomes
- Eggs and sperm have to meet i.e. gamete transport
When and where are the primoridal germ cells first identifiable within an embryo?
- First identifiable within the epithelium of the yolk sac 3 weeks after conception
Describe the pathway primoridial germ cells take to migrate to the genital ridges
- Migrate from epithelium of yolk sac through the connective tissue of the hind gut
- They pass the developing kidney and then from there they enter the genital ridge
- PGC’s divide by mitosis as they migrate
When the primordial germ cells enter into the developing ovary what are they then called?
Oogonia
What eventually occurs to the Oogonia that will result in the formation of Primary Oocytes?
- Eventually Oogonia will stop dividing by mitosis and will then enter meiosis I
- At this point they become known as primary oocytes
What process decides the no. of eggs that a female will ever make? Why is this?
- The mitotic divisions of the primordial germ cells (Oogonia once they reach developing ovary) decide how many eggs a woman will ever have
- This is because once the Oogonia enter meiosis I and become Primary Oocytes no more division is able to occur
What 2 things can occur to a primary oocyte that will cause it to un-arrest itself from meiosis I?
- They are ovulated or they die
Give a brief overview of the process of meiosis
- DNA replication during S phase of cell cycle forms two copies of each chromosome
- During Meiosis I the homologous chromosomes line up at the equator of the cell into pairs and get attached to the mitotic spindle. They then get separated to opposite spindle poles
- This results in the formation of two cells with each cell containing one member of each homologous pair of chromosomes
- During Meiosis II the sister chromatids line up at the equator of the cell and get attached to the mitotic spindle. They then get separated to opposite spindle poles
- This results in the formation of 4 haploid cells
What is the difference in the way that the homologous pair of replicated chromosomes line up at the centre of the cell during meiosis and mitosis?
- In mitosis the homologous pairs of replicated chromosomes line up one behind the other to form a line
- In meiosis the homologous pairs of replicated chromosomes line up next to each other
Does Oogenesis result in the production of 4 mature haploid ovum similar to how spermatogenesis results in the production of 4 mature hapolid spermatozoa?
- No, Oogenesis only results in the production of 1 mature haploid ovum cell. The 3 other haploid cells produced during oogenesis are called polar bodies.
Why does Oogenesis only result in the production of one mature ovum?
- Because during meiosis I one of the 2 cells produced takes all of the cytoplasm this cell becomes known as secondary oocyte.
- Other cell produced as a result of meiosis I known as first polar body
- During Meiosis II when the secondary oocyte divides again one of the cells takes all the cytoplasm - this cell beomes mature ovum.
- Other cell produced due to division of secondary occye also forms polar body
- Because the first polar body divides into 2 during meiosis II you end up with 3 polar bodies and 1 mature ovum
Within the foetal ovary why do protective layers need to form around the primary oocyte?
Protective layers form around it to protect primary oocyte as it’s in a very vulnerable state as it’s arrested in meiosis I
Explain the formation of the protective layers that form around the primary oocyte
- Primitive sex cords that surrond the primary oocyte will condense around the oocyte and differentiate into Granulosa cells
- The granulosa cells then secrete an acellular layer called the basal lamina
- This whole structure is called the primordial follicle
Just before birth there is a massive decrease in the amount of primordial follicles present within the ovary. Why does this decrease occur?
- Primordial follicles that contain a primary oocyte with DNA damage are destroyed
- Primary oocytes that don’t form a primordial follicle are destroyed
Define folliculogenesis
The growth and development of the primordial follicles from earliest “resting” stages as laid down in the foetal ovary, through to ovulation
At any given time within the ovary are most of the primordial follicles growing?
- No, most of the primordial follicles within the ovary do not grow
- Instead what happens is that after puberty a small ampunt of primoridal follicles grow each day
What changes occur to the primordial follicles as they begin to grow?
- Granulosa cells multiply - results in enlargement of oocyte
- Primary oocyte secretes another acellular layer called Zona pellucida
- Second layer of cells called Theca then differentiate around the basal lamina
Are the early stages of Primordial follicle growth FSH-dependent or FSH-independent?
They are FSH-independent
How do we know that the early stages of primordial follicle growth are FSH-independent?
- Primoridial follicle growth still apparent in FSH-deficient patients or those with mutations of the FSH receptor
- Primoedial follicle growth still apparent in women when FSH is suppresed e.g. Women on the contraceptive pill
The growth of the theca results in the formation of a blood supply within the primoridal follicle. What does the creation of a blood supply mean for the follicle?
- Means that the follicle is now able to be influenced by external hormones
What physical change occurs to the primordial follicle as a result of the multiplication of the granulosa cells?
- Gaps begin to form in the granulosa cell layers
- These gaps get filled with follicular fluid which filters into these gaps from the follicular blood supply
- These fluid-filled gaps go on to form an antrum
What is the primordial follicle known as once the antrum develops?
Known as the Antral or Seconadry follicle
What is follicle initiation?
- Occurs after puberty when a small amount of follicles leave the resting pool within the ovaries and grow continuously
When do the follicles that go through follicle initiation stop growing?
- They stop growing once they form an antrum and as a result gain a blood supply
What will allow the antral follicles produced during follicle initation to continue to grow?
- Antral follicles have a blood supply and so are able to respond to circulatory hormones
- Because of this these antral follicles now need FSH in orer to continue to grow
What is follicle recruitment?
- Occurs when Antral follicles are exposed to and respond to changes in FSH that occur in the menstral cycle
- As a result of this they are then recruited into the menstral cycle
What is follicle selection?
- Occurs when one of the antral follicles recruited into the menstral cycle is selected to be ovulated
- Follicle selected becomes known as dominant follicle
Explain the 2-cell, 2-gonadotrophin theory
- 2 cells - Theca cells and the Granulosa cells
- 2 gonadotrophin hormones - LH and FSH
- The thory states that LH will bind to the LH receptor present on the theca cells
- This causes cholesterol within the theca cells to be converted into progesterone
- This progesterone is then converted into androgens (mainly androstenedione and testosterone)
- Both androstenedione and testosterone then travel to the Granulosa cells
- FSH then binds to FSH receptors on the granulosa cells
- This causes androstenedione and testosterone to be converted into Oestrogens (mainly estradiol) via the aromatase enzyme
- Estradiol then secreted by the granulosa cells into the follicle
