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 (ie. ovary or testis).
Describe the differentiation of the PGCs 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.
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.
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). The primary oocytes are packed into the outer layer of the ovary: the cortex.
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.
Describe how a primary oocyte becomes a primordial follicle.
The oocyte is arguably the most important cell in the body, and will be in its vulnerable first meiotic phase for many years. Therefore, each one becomes surrounded by protective layers and protective cells.
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.
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.
Most of the follicles in the ovary are not growing - after puberty, only a few grow each 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.
Once growth of the follicles has started, a second layer of cells then differentiates around the basal lamina, making the theca cells.
What regulates follicle growth?
The factors controlling the initiation of growth and the early stages are largely unknown.
FSH drives most of folliculogenesis, but early growth is independent of FSH ie. driven by local factors
- apparent in FSH-deficient patients/ 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 follicle growth to form an antrum.
As the follicle starts to grow, it increases rapidly in diameter and granulosa cell divisions increase, but gaps begin to form in the granulosa layers. The gaps consist of fluid-filled spaces which form an antrum and are filled with follicular fluid.
The 2 main phases of follicle growth are labelled by the absence of presence of an antrum. The follicles with an antrum are known as antral or secondary follicles.
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.
Briefly go over the stages of folliculogenesis.
A cohort of early follicles leave 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 th menstrual cycle. This is known as follice recruitment.
The human pelvis is designed to carry a single foetus, therefore, from the group that are recruited, only one (two, max) will subsequently be selected for ovulation.
Describe the 2-cell, 2-gonadotrophin 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.
