Female Tract, Oogenesis and Endocrine Control 1 Flashcards
In women the ovaries have no physical connection to the ________
Fallopian tubes.
Therefore postioning of the uterus, fallopian tubes etc are very important as the oocyte needs to be swept up by this system.
What is oogenesis?
Formation and development of ovum/oocyte to the point where it can be fertilised.
Developed in a ‘nursery’ called a follicle (oocyte + surrounding support cells). This is located near the surface of the ovaries in the cortex.
Eventually oocyte is released (ovulation) and the remaining follicle is important for ongoing pregnancy.
Where do the oogenesis germ cells come from, explain their embryonic origin!
- Have the embryo itself (developed from the inner cell mass of the blastocyt prior to implantation)
*lots off cells develop first in the yolk sac (important for cellular development), before the embryo has the ability to develop them in other places
- Ooegonia develop in the yolk sac, then migrate through the embryo quite a distance, past the hind gut, through to the genital ridges where they develop into either female/male gonads
- Gonad is undifferentiated up till 6weeks.
If only ~100 oogonia germ cells migrate to the genital ridges, BUT females have ~6million oogonia/ocytes for life, how can this occur?
When the oogonia reach the genital ridge they start dividing, so that from the few 100 germ cells there is a massive expansion of oogonia during the first few months of pregnancy till about 6 months!
This is Mitosis!
Then at 6 months there is now a huge peak of 6-7 mill cells, and all cells are made prior to birth, and they immediately start dying.
At birth you then only have 1 million ooctyes, seems wasteful!
Describe the meiosis of the oocyte, during embryonic development.
Following the huge expansion of oogonia germ cells via mitotic divisions in the developing ovary, meiosis then begins, but is not completed!
Meiosis halts just prior to metaphase 1 at the end of prophase.
Oogonia now called oocytes.
They stay like this until ovulation (12-50yrs)
What is a polar body?
As a result of meiotic division 1, when the secondary oocyte is produced, the other cell is the first polar body.
This is not much of a cell, has the other set of chromosomes in it, and it doesn’t usually divide.
Potentially could look at the polar body, to check if chromosomes are normal to confirm a healthy baby, without distrupting the ovarian cycle.
Describe the Ovarian Cycle…
- Following puberty, where FSH and LH is released in large quantities, the waves if ovarian primordial follicles become activated.
* -activated follicles can be recognised by the changes in the morphology and number of granulosa cells that surround the oocyte.
- takes ~85 days from the time of activation of a follicle to when it starts to form an antrum (containing follicular fluid) and be capable of ovulation* - With each menstrual cycle; groups of developing follicles are stimulated to grow very rapidly, the ‘follicular wave(s)’
SO prior to menstrual cycle, these groups of follicles become active, then they recieve hormonal stimulation from gonadotrophs, and produce a wave of some of those follicles, and some of those develop and some die, and then ONE of those is ovulated.
- * 3. During the follicular phase of the cycle one follicle will dominate over the others in terms of growth.
- *Usually in women there’s only one dominant follicle, whereas in other species there is multiple ovulated and fertilised*
What are the Phases of the Ovarian Cycle?
Follicular Phase:development of the follicle in response to follicle stimulation hormone (FSH). As luteinizing hormone (LH) and FSH levels increase they stimulate ovulation, or the release of a mature oocyte into the fallopian tubes.
Luteal Phase: the corpus luteum forms on the ovary and secretes many hormones, most significantly progesterone, which makes the endometrium of the uterus ready for implantation of an embryo.
What is Follicular atresia?
Periodic process in which immature ovarian follicles degenerate and are subsequently re-absorbed during the follicular phase of the menstrual cycle. Typically around 20 follicles mature each month but only a single follicle is ovulated.
The rest undergo atresia.
What are four main the stages of follicular growth?
- Primordial Follicles
- Primary Follicles
- Secondary Follicles
- Tetiary or Graafian Follicles
Describe the stages of follicular growth
- Primordial Follicles: developed in fetus, and sit until recruited for ovulation. Something triggers the development of those follicles in a relatively continuous fashion to become….
- Primary Follicles: when the follicle becomes active and spends 2-3 months growing
- Secondary follicles: Have lots of granulosa cells and are responsive to FSH, grow rapidly
- Tetiary or Graafian Follicle: dominant follicle which is ovulated
Primordial Follicle
An oocyte surrounded by a singular layer of flattened granulosa cells. Help the follicle to develop and are in permanent communication with the oocyte.
These tend to develop in nests on the edge of the ovary.
Post-puberty, these are activated and the granulosa cells become cuboidal!
How is it that the granulosa cells are so interconnected with the oocyte?
The granulosa cells have projections into the oocyte, allowing constant bi-directional chat between the cells.
Once puberty has activated the primordial cell, and its flattened granulosa cells become cuboidal, what is follicle now called?
A primary follicle!
Secondary Follicle
- Defined by an oocyte covered by multiple layers of cuboidal granulosa cells.
- These cells present FSH receptors, which causes proliferation and production of hormones.
These produce oestrogen (talk to pit. and hypo.), inhibin (talks to pituitary) and AMH (talks to a number of proceses)
- Theca Cells (like leydig cells): start to see a layer forming in the stroma, which have LH receptors to produce androgens
What is oestrogen formed from?
Progesterones (from cholesterol) are the first step in the steroid hormone pathway.
Progesterone ⇒ androgen ⇒ oestogens
So oesstrogens (female hormone) comes after the male androgen hormone.
Zona Pellucida
The thick transparent membrane surrounding a mammalian ovum before implantation.
Made up of 3 proteins (ZP-1, ZP-2, ZP-3).
ZP-1: present in primordal follicles.
ZP-2 and ZP-3 added to activated Follicles.
Important for filtering normal sperm and blocking polyspermy
Tertiary Follicle
- Large antrum, filled with follicular fluid
- Well developed granulosa cells in the wall
- oocyte surrounded by zona pellucida, theca int/ext
- Cumulous oophorus: oocyte + granulosa cells directly around it, with a plane of weakness
At ovulation the stalk will break. so that the oocyte is free and ready for implantation out of the ovaries. The tertiary follicles push right up against the serosa of the ovary until they burst free.
When is there endocrine control of follicular development?
First half (primordial-secondary): Gonadotrophin-independent growth of follicles, not controlled by FSH. We don’t clearly understand this bit! One we do know is AMH (anti-mullerian hormone)
Second Half (Antral- preovulatory):
How is the egg released?
Not in an explosive way! A very slow, controlled process of tetiary oocyte released and swept up by the fallopian tubes
What happens to the follicle after the egg is released?
It becomes the corpus luteum (‘yellow body’) and secretes progestetone.
Corpus Luteum: cells left behind rapidly proliferate and progress, and becomes very very thick walled. Eventually becomes, less yellow, and regress into corpus albicans .
Contain lutenised granulosa and lutenised theca cells that produce Progesterone
Function of oestrogen in the ovarian cycle
Upto 14 days: Decreases the levels of GnRH, FSH and LH and causes them to decrease!
Around 12 days: when oestrogen becomes too high, it switches and becomes positive feedback that stimulates the production of luteinsing hormone (which then acts on the follicles and stimulates ovulation ~12-24 hrs later)
Inhibins function during follicular development?
Also produced by the granulosa cells, but negatively regulate only FSH (not LH).
