Oogenesis

Question 1

What happens during the process of germ cell migration?

Answer 1

During the process of migration, they increase in number through mitosis and undergo a reduction in chromosomes via meiosis. Then this results in the formation of mature gametes.

They increase so much in number that around 6-7 million germ cells will be reached in humans.

Question 2

Approx when is the first site of an oocyte observed (humans)?

Answer 2

Around 20 weeks.

Question 3

Where are germ cells formed? How do they arrive at the gonads?

Answer 3

Germ cell precursors will be formed outside the embryo and will migrate into the embryo until they reach the gonadal ridge.

Question 4

Describe the structure of a primordial follicle.

Answer 4

• Approx 20 microns.
• Covered by a layer of flat follicular cells.
• Inside cytoplasm: balbiani bodies, annulated lamellae, mitochondria.
• Basal lamina
• Surrounding stromal cells that have not yet differentiated.
• Oocyte has started producing the zona pellucida but it is not yet fully formed.

Question 5

Describe the structure of the ovary.

Answer 5

Has a germinal epithelium made by cuboid simple epithelium.

Below is the tunica albuginea made by connective tissue.

The outer section is the cortex and the inner is the medulla.

The ovary is bound to the peritoneum by an ovarian ligament.

Blood vessels come through the illium to the medulla.

Different types of follicles are localised in the cortex.

Question 6

Describe the structure of a follicle during maturation.

Answer 6

Initially there is an oocyte in the middle surrounded by one layer of follicle cells, which increases in number during development.

A basal membrane encloses the follicle cells, this is surrounded by stromal cells.

Follicle cells differentiate to granulosa cells, while stromal cells differentiate to theca cells.

During development, the oocyte secretes glycoproteins outside its membrane, creating another layer called zona pellucida.

Question 7

Does gonadotropin regulate all the processes?

Answer 7

No, one stage is gonadotropin independent while the other is gonadotropin dependent.

Question 8

Describe the features of a primary follicle.

Answer 8

• The zona pellucida is better formed.
• Cells have become cuboid.
• Basal lamina surrounds the follicle.
• Observe an increase in oocyte size.

Question 9

Describe the features of a late primary follicle.

Answer 9

• The layers of follicular cells have increased, they differentiate into granulosa cells.
• Granulosa cells has microvilli that will communicate with the oocyte.
• Communication between the cytoplasm of the oocyte and the granulosa cells through gap junctions.
• The oocyte accumulates cortical granules below the PM.
• Stromal cells begin differentiating into theca cells.

Question 10

Describe the features of a secondary follicle.

Answer 10

• Antrum starts forming, this is formed by a series of proteins and liquid secreted by the granulosa cells.
• Oocytes will have reached a diameter of around 125 micrometers, the full size of the oocyte.
• Zona pellucida fully formed.
• Cells increase in size and start expressing the FSH receptor.
• Cells of the theca will start expressing the LH receptor.
• The theca has become bigger, 2 types are distinguished: theca interna and theca externa.

Question 11

Describe the features of a mature graafian follicle.

Answer 11

• There is the theca and surrounding basal lamina.
• With the formation of the antrum, we differentiate granulosa cells from the mass of cells called cumulus oophorus. These cells form the corona radiata.
• Can reach 2cm.
• Also called the dominant follicle as only 1 will become this size.
• Produces estrogen.

Question 12

What happens in the pre-antral phase of ovulation?

Answer 12

• A growth of the follicle is observed from the primordial follicle maturing into the graafian follicle.
• Follicles cells increase in numbers and differentiate into different types of cells.
• Different levels of hormones - hormones involved are LH, FSH, estradiol and progesterone.

Question 13

What happens in the antral phase of ovulation?

Answer 13

• When the antrum is starting to be seen.
• Follicles start to die.
• Secretion of follicular fluid.
• Formation of cumulus oophorus and corona radiata.
• Increase in expression of receptor of LH.

Question 14

What happens in the pre-ovulation phase of ovulation?

Answer 14

• High LH level.
• Oocyte will start meiosis.
• The germinal vesicle will break down.
• The cortical granuli will be highly expressed.
• Increased size of follicle.

Question 15

What happens at ovulation?

Answer 15

• The follicle is released from the ovary.
• Stigma is formed.
• What remains of the follicle will undergo a luteinizing process.

Question 16

How many follicles mature every month?

Answer 16

1 mature every month while many others degenerate.

Question 17

What happens when the follicle breaks?

Answer 17

The wall of the follicle can be seen to appear thinner and thinner. All the cells inside will be released outside. The oocyte will be released surrounded by cells of the corona radiata.

Some muscle cells will contribute to this release, there will be an accumulation of hyaluronic acid around the cells of the cumulus oophorus.

Question 18

What happens to the oocyte after ovulation?

Answer 18

Once ovulation has occurred, the oocyte has a maximum of 24 hours to survive and be fertilised.

Otherwise there will be the degeneration of the oocyte which is eliminated with the menstrual cycle.

Question 19

What happens to the remaining follicle cells after ovulation?

Answer 19

After ovulation, the remaining follicle is called a corpus luteum. This releases progesterone. If fertilisation occurs, the corpus luteum will keep producing progesterone.

If no fertilisation, the corpus luteum becomes corpus albicans and consequently a decrease in the production of progesterone, there will be no inhibition in the synthesis of gonadotropins and so the cycle will start again.

Question 20

Describe the positive feedback loop between the hypothalamus, pituitary gland and the follicles.

Answer 20

The hypothalamus releases gonadotropin releasing hormone that acts on the pituitary gland to promote the release of FSH and LH.

These 2 hormones will induce the growth of follicles, which will also be hormone producing structures. Then estrogen will be produced. High levels of estrogen will have a feedback on the hypothalamus to stimulate it to produce more LH and FSH to cause a peak of these hormones and thus causing ovulation.

Question 21

What happens to the hormones after ovulation?

Answer 21

After ovulation, the progesterone produced by the corpus luteum will have an inhibitory action on the hypothalamus, inhibiting the gonadotropin releasing hormone, thus stopping the release of FSH and LH.

If progesterone is still present, no release of FSH and LH is observed during the time of embryo development.

Question 22

What are the 3 layers in the uterine wall?

Answer 22

The uterine wall is composed of a lining mucosa (endometrium), a layer of baso cells called myometrium, and a part called perimetrium.

Question 23

What is the structure and function of the endometrium?

Answer 23

The endometrium is the main part in the increase in size of the uterine lining. It can be divided into 2 layers, the stratum functionalis and the stratum basalis.

Question 24

Describe the proliferative phase of the menstrual cycle.

Answer 24

Estrogen acts on the uterine walls to result in a proliferative phase. The uterine wall becomes thicker to replenish what was lost in the previous phase.

Question 25

Describe the secretory phase of the menstrual cycle.

Answer 25

After ovulation is the secretory phase, where a receptor of progesterone is present on the endometrium. They can reply to the secretion of progesterone, this causes the stimulation of the glands to produce mucus.

Question 26

What happens if there are no progesterone released after ovulation?

Answer 26

If there is no release of progesterone, there would be a drastic reduction of PR causing menstruation.