REPRO: Folliculogenesis Flashcards
What has to be achieved to reproduce?
- differentiation of the foetus into male or female
- sexual maturation
- production, storage and release of sufficient supply of eggs and sperm
- the correct number of chromosomes in eggs and sperm
- eggs and sperm have to meet ie. gamete transport
- creation of a new individual with genes from both parents
- to nurture individual until capable of ‘independent life’
Describe the germ cells (that are going to produce gametes) as they enter the gonads.
Cells that will become eggs or sperm are called primordial germ cells (PGC). The PGCs are first identifiable in the yolk sac of the developing foetus at 3 weeks after conception.
They undergo many cycles of mitosis, They then migrate to the genital ridge in the foetus (which will become the gonad). The further differentiation of the PGC depends on the development of the gonad (i.e. ovary or testis).
Describe the differentiation of the primary germ cells to oocytes.
Germ cells become oogonia when in the ovary.
Oogonia are egg-precursors, diploid and multiply by mitosis. Once the mitosis stops and they enter meiosis, they are known as primary oocytes.
At birth, they have been surrounded by a layer of granulosa cells, and are now known as the primordial follicle.
What is an Ooctye?
An oocyte is the very beginning of human life – in the simplest of terms, it is an immature egg cell. Throughout the process of ovulation, this immature egg cell eventually matures and becomes an ovum, or egg.
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. Therefore, the mitotic divisions are critical. The mitotic division lays down the future reserves.
Once the oogonia enter the first stage of meiosis, no more division occurs and they become primary oocytes.
The primary oocyte remains in the first phase of meiosis until it is ovulated (or dies). This means that an egg can stay in an oocyte can remain in meiotic arrest for decades (40-50 years).
The primary oocytes are packed into the outer layer of the ovary: the cortex.
How are the primary oocytes protected?
An oocyte is arguably the most important cell in the body and is 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 - what you are born with. Then there is the establishment of the true ‘ovarian reserve’.
Explain the duplication of the chromatids in mitosis/meiosis.
Both mitosis and meiosis start the same way. In the S phase of the cell cycle (during interphase), the chromosome will replicate. Therefore each chromosome is represented as having 2 chromatids which are attached at the centromere. This is still drawn as one chromosome, but it contains 2 identical chromatids known as sister chromatids.
There is then the breakdown of the nuclear membrane during prophase and formation of the spindle with the duplication of the centrioles (centrosomes - consist of 2 centrioles).
Give an overview of Mitosis.
Mitosis is a process where a single cell divides into 2 identical diploid daughter cells (cell division).
During mitosis one cell divides once to form two identical cells. The major purpose of mitosis is for growth and to replace worn out cells. If not corrected in time, mistakes made during mitosis can result in changes in the DNA - this can lead to genetic disorders. There is the duplication of the chromosomes interphase, and the mitosis occurs consisting of PMAT.
- Interphase:
The DNA in the cell is copied in preparation for cell division, this results in 2 identical full sets of chromosomes.
Outside of the nucleus are 2 centrosomes, each containing a pair of centrioles. During interphase, microtubules extend from these centrosomes.
- Prophase:
The chromosomes condense into X-shaped structures that can be easily seen under a microscope.
Each chromosome is composed of 2 sister chromatids, containing identical genetic information.
The chromosomes pair up so that both copies of chromosome 1 are together, both copies of chromosome 2 are together, and so on. The nuclear membrane breaks down, releasing the chromosomes. The mitotic spindle, consisting of the microtubules and other proteins, extends across the cell between the centrioles as they move to opposite poles of the cell.
- Metaphase:
The chromosomes line up neatly end-to-end along the centre (equator) of the cell such that the homologous chromosomes are one after the other (e.g. maternal, paternal, maternal, paternal for all 46 chromosomes.)
The centrioles are now at opposite poles of the cell with the mitotic spindle fibres extending from them. The mitotic spindle fibres attach to each of the sister chromatids at the centromere.
- Anaphase:
The sister chromatids are then pulled apart by the mitotic spindle which pulls one chromatid to one pole and the other chromatid to the opposite pole.
- Telophase:
At each pole of the cell a full set of chromosomes gather together. A membrane forms around each set of chromosomes to create two new nuclei. The single cell then pinches in the middle to form two separate daughter cells each containing a full set of chromosomes within a nucleus. This process is known as cytokinesis.
Give an overview of Meiosis.
Meiosis is a process where a single cell divides twice to produce 4 haploid daughter cells that are not genetically identical. These cells are our sex cells (gametes) – sperm in males, eggs in females.
Meiosis can be divided into nine stages. These are divided between the first time the cell divides (meiosis I) and the second time it divides (meiosis II):
Meiosis I
1. Interphase:
The DNA in the cell is copied in preparation for cell division, this results in 2 identical full sets of chromosomes.
Outside of the nucleus are 2 centrosomes, each containing a pair of centrioles. During interphase, microtubules extend from these centrosomes.
- Prophase I:
The copied chromosomes condense into X-shaped structures that can be easily seen under a microscope. Each chromosome is composed of 2 sister chromatids containing identical genetic information.
The chromosomes pair up so that both copies of chromosome 1 are together, both copies of chromosome 2 are together, and so on.
The chromosomes line up ADJACENT to each other. This allows for the pairs of chromosomes to exchange bits of DNA in a process called recombination or crossing over.
The nuclear membrane breaks down, releasing the chromosomes
The meiotic spindle, consisting of microtubules and other proteins, extends across the cell between the centrioles.
- Metaphase I:
The chromosome pairs line up next to each other along the centre (equator) of the cell. The centrioles are now at opposites poles of the cell with the meiotic spindles extending from them. The meiotic spindle fibres attach to one chromosome of each pair.
- Anaphase I:
The pair of chromosomes are then pulled apart by the meiotic spindle, which pulls one chromosome to one pole of the cell and the other chromosome to the opposite pole.
In meiosis I the sister chromatids stay together. This is different to what happens in mitosis and meiosis II.
- Telophase I and cytokinesis:
The chromosomes complete their move to the opposite poles of the cell.
At each pole of the cell a full set of chromosomes gather together.
A membrane forms around each set of chromosomes to create two new nuclei. The single cell then pinches in the middle to form 2 separate daughter cells each containing a full set of chromosomes within a nucleus. This process is known as cytokinesis.
At the end of meiosis 1 we have reduced the number of chromosomes, though they are still as chromatids. at the end of meiosis 2 we’re going to reduce our number of chromatids (like in mitosis) by the spindle.
Meiosis II
- Prophase II:
Now there are two daughter cells, each with 23 chromosomes (23 pairs of chromatids).
In each of the two daughter cells the chromosomes condense again into visible X-shaped structures that can be easily seen under a microscope.
The membrane around the nucleus in each daughter cell dissolves away releasing the chromosomes.
The centrioles duplicate.
The meiotic spindle forms again. The chromosomes line up one after the other like in mitosis.
- Metaphase II:
In each of the two daughter cells the chromosomes (pair of sister chromatids) line up end-to-end along the equator of the cell. The centrioles are now at opposites poles in each of the daughter cells. Meiotic spindle fibres at each pole of the cell attach to each of the sister chromatids.
- Anaphase II:
The sister chromatids are then pulled to opposite poles due to the action of the meiotic spindle.
The separated chromatids are now individual chromosomes.
- Telophase II and cytokinesis:
The chromosomes complete their move to the opposite poles of the cell.
At each pole of the cell a full set of chromosomes gather together.
A membrane forms around each set of chromosomes to create two new cell nuclei. Cytokinesis will occur producing 4 granddaughter cells, each with half a set of chromosomes (haploid):
in males, these four cells are all sperm cells in females, one of the cells is an egg cell while the other three are polar bodies (small cells that do not develop into eggs).
Briefly describe the splitting of the chromosomes during mitosis.
DNA replication during interphase forms two sister chromatids, which are banded together to form a chromosome.
During mitosis, sister chromatids separate and move to opposite ends of the cells.
During cytokinesis, the parent cell divides, forming two daughter cells. Each daughter cell has two copies of each chromosome (homologous pairs).
Briefly describe the splitting of chromosomes during meiosis.
The diploid parent cell contains two homologous pairs of chromosomes.
DNA replication during interphase forms two copies of each chromosome, and the copies remain attached.
During meiosis 1, homologous chromosomes separate into two haploid cells, each containing one member of each homologous pair.
During meiosis 2, sister chromatids separate and four haploid cells are formed that will develop into gametes.
What happens to the oocyte in Meiosis?
The oocyte is arrested in meiosis 1. Chromosomes will line up on the spindle in metaphase 1 and remain arrested until ovulation.
What are the products of meiosis in a female?
In females, the oogonium enters the ovary and becomes a primary oocyte. At ovulation it splits into 2 daughter cells after the first meiotic division. However, there is UNEQUAL division of cytoplasm during cytokinesis - one of the daughter cells contains almost all of the cytoplasm (secondary oocyte) whereas the other daughter cell (first polar body) just has the chromosomes and a very little bit of cytoplasm. The secondary oocyte produces produces a mature ovum and a polar body. The first polar body produces 2 more polar bodies.
The primary oocyte is arrested in meiosis 1 (prenatal) and will remain this way until ovulation (post puberty).
Males start with one diploid cell and 4 haploid gametes (sperm).
Describe folliculogenesis.
Folliculogenesis is defined as the growth and development of follicles from the earliest ‘resting’ stages as laid down in the foetus, through to ovulation.
In the foetal ovary, the surrounding cells condense around the oocyte and differentiate into the granulosa cells. The granulosa cells then secrete an acellular layer called the basal lamina. The whole structure is called the primordial follicle.
Most of the follicles in the ovary are not growing - after puberty, a cohort of follicles initiate growth every day.
As the follicles start to grow, the granulosa cells multiply and the oocyte secretes another protective acellular layer called the zona pellucida, which stays attached after ovulation. The zona pellucida is structurally within the basal lamina.
Once growth of the follicles has started, a second layer of cells then differentiates around the basal lamina, making the theca cells, which is vascularised.
What regulates follicle growth?
The factors controlling the initiation of growth and the early stages are largely unknown, but granulosa cells multiply and 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/ those with mutations of FSHR
- also means that even when FSH is suppressed, eg. on contraception pill (COCP), the follicles will still continue early growth, but then die.
