Folliculogenesis

Question 1

What has to be achieved to reproduce?

Answer 1

• 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”

Question 2

What cells do the sperm and egg originate from?

Answer 2

Primordial Germ cells

Question 3

Where do the primordial germ cells migrate?

Answer 3

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

Question 4

Describe the formation of primary oocytes (PRIMORDIAL FOLLICLE)

Answer 4

1. Primordial Germ cells become oogonia when in the ovary
2. Oogonia are egg-precursors, diploid and multiply by mitosis
3. Once mitosis stops and they enter into meiosis, known as primary oocytes

Question 5

Describe the importance of the PGC entering the ovary

Answer 5

• 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 remain in the first phase of meiosis until it is ovulated (or dies) …. maybe for 52 years!

Question 6

Where are the primary oocytes located?

Answer 6

Primary oocytes are packed into the outer layer of the ovary: the ovarian cortex

Question 7

As oocytes are very important how are they then protected?

Answer 7

• 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

Question 8

Summarise how we form the primary follicles (just after the primary oocytes)

Answer 8

• 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
• Surrounded by a layer of GC – now known as primordial follicle
• Establishment of the true “ovarian reserve”

Question 9

Describe the process of mitosis and meiosis

Answer 9

Watch videos

Question 10

Give a brief overview of the meiotic process that occurs in women

Answer 10

On image

Question 11

Define Folliculogenesis

Answer 11

Defined as the growth and development of follicles from the earliest “resting” stages as laid down in the foetus, through to ovulation

Question 12

Do all the follicles grow?

Answer 12

• Most of the follicles in the ovary are not growing – after puberty a cohort of follicles initiate growth each day

Question 13

What forms when the primordial follicles start to grow?

Identify these cells on the diagram

Answer 13

• 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

On image

Question 14

How does the primary follicle grow?

Answer 14

granulosa cells multiply & oocyte enlarges (though still in meiotic arrest)

Question 15

What drives Folliculogenesis?

What is deficient FSH mainly caused by?

What happens when FSH is suppressed?

Answer 15

• FSH drives most of folliculogenesis but early growth is independent of FSH i.e. driven by local factors
o Apparent in FSH-deficient patients or those with mutations of FSHr
o Also means that even when FSH is suppressed eg. on COCP, the follicles will still continue early growth but then die

Question 16

What happens when the primary follicle starts to grow and what does it form? (fluid formation)

Answer 16

• 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

Question 17

The preantral and primary follicles do not contain any fluid.

Does the secondary follicle contain fluid?

Describe the structure of the antral follicle/ secondary follicle and then the pre-ovulatory follicle

Answer 17

Yes

Granulosa cells are pushed towards the edge and now are known as mural granulosa cells and the cells around the egg are now known as cumulus cells.

They then form a pre-ovulatory follicle or graffian follicle. All the granulosa cells are found towards the edge. The egg is surrounded by differentiated granulosa cells called cumulus cells. Granulosa cells attach it to the wall of other granulosa cells

Question 18

What is the follicular fluid characterised by?

What is it formed form?

What does it do to the occyte?

Answer 18

• 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
• Exudate = fluid leaking from blood vessels

Question 19

Why is an antra/ secondary follicle visible on ultrasound?

Answer 19

• AF are visible on ultrasound, because of the fluid filled space, which doesn’t absorb the ultrasound waves, so they appear black. Here we have a picture of an ovary we have a big dominant follicle which will ovulate and smaller antral follicles. We also have Atretic follicle: where it is starting to die, so the walls are then and maybe 1 or 2 small antral follicles

Question 20

What is follicle initiation?

What is follicle recruitment?

Answer 20

A cohort of early follicles leaving the resting pool and grow continuously

• 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

Question 21

Why is only 1 follicle recruited for ovulation?

Answer 21

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

Question 22

Give a summary up to one follicle being selected for ovulation

Answer 22

1. First, we have primordial follicle, very small 0.2mm. only visble under microscope. They intiate cohorts of this follicles everyday – grow very slow to form preantral follicles, consists of multiple layers of granulsa cells, zona pellucida (ZP), basal lamina and theca, but no antrum and this process is very slow and prolonged
2. Does not need gonadotrophin.
3. It will then form antral. In order to proceed along from this, it needs FSH. One follicle is selected to become the dominant follicle in the menstrual cycle which goes on to ovulate.

Question 23

Describe the timeline of each step folliculogenesis

Which stages are gonadotrophin dependant and indepndant?

Answer 23

On image

Question 24

How is the theca divided in antral ovarian follicle?

Answer 24

1. We have the theca being formed which rests on the basement membrane. This theca is very well vascularised.
2. The theca is divided into 2 parts: interna and externa.

Question 25

What is the importance of the theca blood supply?

Answer 25

Follicle now open to circulatory influences

Question 26

Describe the two-cell–two-gonadotropin theory

What enzyme converts androgens to oestrogens?

Answer 26

According to the two-cell–two-gonadotropin theory, luteinizing hormone stimulates thecal cells to produce androgens, and follicle-stimulating hormone stimulates granulosa cells to produce oestrogens from androgens.

The enzymes responsible for this production of androgens will exist only in the theca cells. the key enzyme needed for the conversion of androgens to oestrogens is the aromatase enzyme.

Also, the conversion of androgens from androgens to oestrogens is also driven by FSH, it binds to its receptor activates downstream signalling events which activate aromatase. They are only from on granulosa cells.

This pathway is also driven by LH binding to its receptors – found on theca cells
Steroids enter the circulation

Question 27

Describe the steroid pathway

Answer 27

The starting point for steroid biosynthesis is the conversion of cholesterol in pregnenolone by P450scc.

One route for pregnenolone metabolism is the delta-5 pathway (red arrows) through CYP17 (P450c17). Pregnenolone hydroxylation at the C17a position forms 17-hydroxypregnenolone, and subsequent removal of the acetyl group forms the androgen precursor dehydroepiandrosterone (DHEA). Tgis is then converted to androstenediol then converted to testosterone.

An additional route for pregnenolone metabolism is the delta-4 pathway (purple arrows), in which pregnenolone is converted to progesterone by 3b-HSD (an irreversible conversion). Progesterone is then converted to 17-hydroxyprogesterone by CYP17. In humans, 17-hydroxyprogesterone cannot be further metabolized. Importantly, CYP17 is exclusively located in thecal and interstitial cells in the ovary extrafollicular compartment, whereas CYP19 (aromatase), which converts androgens to estrogens, is expressed exclusively in GCs, which are in the intrafollicular compartment. Androgen aromatization to estrogens is a distinct activity that occurs in the granulosa layer, and it is induced by FSH via P450 aromatase (P450arom) gene activation.

Question 28

How is folliculogenesis controlled?

Answer 28

Negative feedback by oestrogens and progesterones on the hypothalamus