Folliculogenesis Flashcards
What has to be achieved to reproduce?
- Differentiation into male or female
- Sexual maturation
- Production, storage and release of sufficient supply of eggs and sperm
- correct number of chromosomes in eggs & sperm
- Eggs and sperm have to meet ie. gamete transport
- Creation of new individual with genes from both parents
- To nurture individual until capable of “independent life”
Germ cells enter the gonad
• Cells that will become eggs or sperm originate from primordial germ cells (PGC)
• PGCs first identifiable in the yolk sac of the developing foetus at 3 weeks after conception
Undergo many cycles of mitosis
They migrate to the genital ridge in the foetus
The genital ridge becomes the gonad
Further differentiation of the PGC into male/female gametes depend on the sexual differentiation/development of the gonad i.e. into ovary or testis
If PGCs enter the ovary they become oocytes
- Germ cells become oogonia when in the ovary
- Oogonia are egg-precursors, diploid and multiply by mitosis
- Once mitosis stops and they enter into meiosis, known as primary oocytes
Germ cells to eggs
- All the eggs that a woman will ever have are made at this stage
- The mitotic divisions are therefore critical
- Once the oogonia enter the 1st stage of meiosis no more division occurs and they become primary oocytes
- The primary oocytes remains in the first phase of meiosis until it is ovulated (or dies)….maybe for 52 years!
Primary oocytes primordial follicle part 1
- Oocyte arguably the most important cell in the body and will in the vulnerable 1st meiotic phase for many years
- Therefore each one becomes surrounded by protective layers and cells
- In the foetal ovary, the surrounding cells condense around the oocyte and differentiate into the granulosa cells (GC)
- The granulosa cells then secrete an acellular layer called the basal lamina (BL)
- The whole structure is called the primordial follicle
Primary oocytes primordial follicle part 2
- Germ cells become oogonia when in the ovary
- Oogonia are egg-precursors, diploid and multiply by mitosis
- Once mitosis stops and they enter into mieosis, known as primary oocytes
- Surrounded by layer of GC – now known as primordial follicle
- Establishment of the true “ovarian reserve”
Duplication of chromatids
- Chromosomes replicate during S-phase of cell cycle
- Remain attached at the centromere
- Each copy known as a chromatid → the 2 copies are identical to each other → “sister” chromatids
- Exact copy of original chromosomes
Folliculogenesis: the growth of the follicle
- Defined as the growth and development of follicles from the earliest “resting” stages as laid down in the foetus, through to ovulation
- Most of the follicles in the ovary are not growing – after puberty a cohort of follicles initiate growth each day
- As the follicles start to grow, the granulosa cells (GC) multiply and the oocyte secretes another protective acellular layer called zona pellucida (ZP)
- Once growth of the follicles has started a second layer of cells then differentiate around the basal lamina (BL): the theca, which is vascularised
Follicle growth part 1
• The factors controlling initiation of growth and the early stages are largely unknown, but granulosa cells multiply & oocyte enlarges (though still in meiotic arrest)
• FSH drives most of folliculogenesis but early growth is independent of FSH i.e. driven by local factors
Apparent in FSH-deficient patients or those with mutations of FSHr
Follicle growth part 2
Also means that even when FSH is suppressed eg. on COCP, the follicles will still continue early growth but then die
• As follicle starts to grow it increases rapidly in diameter and granulosa cells divisions increase, but gaps begin to form in the granulosa cell layers
• These gaps consist of fluid-filled spaces which form an antrum & are filled with follicular fluid
• 2 main phases of follicle growth labelled by absence or presence of antrum
• Follicles with an antrum are known as antral or secondary follicles
Antral follicle
- Characterised by a cavity or “antrum”
- Contains fluid formed as exudate of plasma containing secretory products of oocyte & GC
- Known as “follicular fluid”
- As follicular fluid volume and antrum expands, oocyte is displace to one side
Folliculogenesis
- A cohort of early follicles leaving the resting pool and grow continuously
- This is known as follicle initiation
- They will NOT continue to grow unless they reach the size at which they respond to changes in FSH that occur in the menstrual cycle
- This is known as follicle recruitment
- Human pelvis designed in general to carry a single foetus therefore,
- From the group that are recruited only one will subsequently be selected for ovulation
Control of folliculogenesis
Hypothalamus - GnRH to pituitary
Pituitary - FSH/LH stimulates ovary
Ovary releases oestrogen and progesterone and stimulates hypothalamus and pituitary
