4- 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”
Where do cells that will become sperm or egg cells originate from?
They originate from primordial germ cells (PGCs)
When are PGCs first identifiable?
They are first identifiable in the yolk sac of the developing foetus at 3 weeks after conception
What happens when germ cells enter the gonads?
- They undergo many cycles of mitosis
- They migrate to the genital ridge in the foetus
- The genital ridge becomes the gonad
What does the further differentiation of the PGCs depend on?
Further differentiation of the PGC into male/female gametes depend on the sexual differentiation/development of the gonad i.e. into ovary or testis
What are PGCs converted to in the ovary?
- 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
Why are the mitotic divisions of the egg during foetal life critical?
All the eggs that a woman will ever have are made at this stage
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.
The primary oocytes are packed into the outer layer of the ovary (the cortex)
What is the primordial follicle and what is its function?
It acts as a protective layer for oocytes
In the foetal ovary oocytes are surrounded by granulosa cells, the granulosa cells then secrete an acellular layer called the basal lamina (BL)
The whole structure is called the primordial follicle
Describe the differentiation of PGCs to 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, oogonia are known as primary oocytes
- Surrounded by layer of GC at birth– now known as primordial follicle
- Establishment of the true “ovarian reserve”
When does the duplications of chromatids occur in the cell cycle?
- Chromosomes replicate during S-phase of cell cycle (2 chromatids per chromosome)
- 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
What are the 4 stages of mitosis?
- prophase
- metaphase
- anaphase
- telophase
What is Folliculogenesis?
Defined as the growth and development of follicles from the earliest “resting” stages as laid down in the foetus, through to ovulation
Describe Folliculogenesis
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
What factors regulate follicular growth?
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
- Also means that even when FSH is suppressed eg. on COCP, the follicles will still continue early growth but then die
Describe the growth of the follicle to form an antrum
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
What are the 2 main phases of follicular growth?
2 main phases of follicle growth labelled by absence (pre-antral follicular growth) or presence of antrum (antral follicular growth)
Follicles with an antrum are known as antral or secondary follicles
How are antral follicles characterised?
- Characterised by a cavity or “antrum”
- Contains fluid formed as exudate (fluid leaking from blood vessels) of plasma containing secretory products of oocyte & GC, known as the ‘follicular fluid’
Describe antral development
The antrum (the fluid-filled space) expands due to an increase in fluid volume. This causes the oocyte to be displaced to one side.
The oocyte becomes surrounded by cumulus cells, which form the cumulus oophorus, surrounded by the antrum. These cells are very sticky, and they are what the sperm has to fight through to get to the oocyte.
Describe and explain the steps in 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
- this is known as the dominant follicle
What is the theca layer in the antral ovarian follicle composed of?
2 layers
- theca interna
- theca externa
What are the 3 stages in Folliculogenesis and which stages are gonadotropin dependent?
1- Initiation
2- Recruitment
3- Selection
Stages 1 is gonadotropin independent and stages 2 and 3 are gonadotropin dependent
What is the importance of the vasculature to the follicle?
The follicle is open to influences by surrounding circulation and anything produced by the follicle has the ability to enter the circulation
What is the 2-cell, 2-gonadotropin theory?
According to the theory, luteinising hormone (LH) stimulates the thecal cells to produce androgens, and follicle-stimulating hormones (FSH) stimulates granulosa cells to produce oestrogens from androgens via aromatase (activated by FSH).
What receptors are always present on theca cells?
LH receptors
Theca cells never have FSH receptors
