When does maturation of oocytes begin?
Where do germ cells arise from?
The yolk sac
What happens after germ cells have arisen from the yolk sac of the female?
They colonise the gondal cortex and differentiate into oogonia
What happens once germ cells have differentiated into oogonia?
They proliferate rapidly by mitosis, and some enter meisois
What is oogonias entry into meisois 1 stimulated by?
What are mesonephric cells?
Flattened epithelial cells, also called Follicular Cells
What happens to the oogonia in meisosis 1?
They arrest in the diplotene stage (a resting stage) of prophase 1
What causes oogonia to arrest in the diplotene stage of prophase 1?
Due to Oocyte Maturation Inhibitor
What secretes Oocyte Maturation Inhibitor?
What are the oogonias that are arrested in the diplotene stage now known as?
When is the maximum number of germ cells reached?
What happens to the number of germ cells at mid-gestation?
Cell death begins, and many oogonia and primary oocytes degenerate
What is the process of degeneration of oogenia and primary oocytes known as?
What has happened to the oogonia by the 7th month of gestation?
The majority of oogonia have degenerated
How many primary oogonia survive by the 7th month of gestation?
What has happened all surviving primary oocytes by the 7th month of gestation?
They have entered meisosis I, and are surrounded by a flat layer of epithelial cells- follicular cells
What are the primary oocytes surrounded by follicular cells now called?
How long to the primordial follicles remain in meiosis I?
At least until puberty
What happens to most oocytes during childhood?
Most undergo atresia
How many oocytes remain by puberty?
What is the clinical relevance of a woman having all the oocytes she will ever have at birth?
Some may remain arrested for 50 years before further development, increasing the chance of cell damage and thus accounting for the increased risk of foetal chromosomal abnormalities in pregnancies of older women
What happens to oocytes from puberty onwards?
˜15-20 oocytes start to mature each month
What stages do oocytes go through when maturing?
What happens in the pre-antral stage of maturation?
- Primary oocyte grows dramatically, but does not re-start meiosis
- Flat follicular cells become cuboidal Granulosa cells, and proliferate, producing stratified epithelium
What do Granulosa cells do in the pre-antral stage?
What is the purpose of the glycoprotein secreted by the Granulosa cells in the pre-antral stage?
It surrounds the oocyte with a Zona Pellicuida
What do surrounding connective tissue (stroma) cells form in the pre-antral stage?
A Theca Folliculi
What does the Theca Follucli consist of?
- An inner Theca Interna
- An outer Theca Externa
What are the characteristics of the Theca Interna?
What is the Theca Externa?
A fibrous capsule
What do Theca and Granulosa cells collaborate to do?
What is the primary oocyte known as after the pre-antral stage?
What happens in the Antral stage of maturation?
Granulosa cells continue to proliferate, and a fluid appears between them
What does the appearance of fluid between the granulosa cells in the antral stage form?
What happens as more fluid forms in the Antral stage?
The secondary, or Graffian, follicle expands
How big does the secondary follicle expand to in the Antral stage?
2mm in diameter, without stimulation from reproductive hormones
What does continued development of the secondary follicle, past 2mm, depend on in the Antral stage?
What reproductive hormones are required to continue the development of the antrum in the Antral stage?
What does FSH bind to in the Antral stage?
The Granulosa cells only
What does LH bind to in the Antral stage?
Thecal cells only
What do Thecal cells do under the influence of LH?
What happens to the androgens secreted from the Thecal cells?
They are converted to oestrogens by Granulosa cells
What is the conversion of androgens to oestrogens by the Granulosa cells under the influence of?
When does the Pre-ovulatory phase begin?
37 hours before ovulation
What happens in the Pre-ovulatory stage?
Oestrogen causes receptors for LH to appear on outer Granulosa cells
What stimulates the LH receptors on the outside of Granulosa cells in the Pre-ovulatory stage?
What is the result of the stimulation of the receptors on the outer Granulosa cells by the LH surge?
Rapid changes in the follicle
What happens within 3 hours of the LH surge?
The follicle restarts meiosis, and the first meitoic division is complete
What is true of the first meiotic division in the pre-ovulatory stage?
It is asymmetric
What is meant by the first meiotic division in the pre-ovulatory stage being asymmetric?
Cytoplasm remains with one daughter cell, and the other forms a condensed polar body
What happens after the first meiotic division in the pre-ovulatory stage?
The secondary follicle enters meiosis II, and arrests again 3 hours prior to ovulation
What happens to the follicle in the pre-ovulatory stage?
It increases dramatically, to 25mm diameter
Why does the follicle size increase dramatically in the pre-ovulatory stage?
Due to increase in antral fluid volume
What happens once the follicle has increased in size in the pre-ovulatory stage?
The structure begins to weaken
What happens to the weakened structure in the pre-ovulatory stage?
LH stimulates collagease activity, leading to follicle rupture
What happens to the ruptured follicle?
The ovum is carried out in the fluid, and gathered up into the fallopian tube by the fimbria
When is meiosis completed in the pre-ovulatory phase?
When the ovum is fertilised
Not completed unless the ovum is fertilised
What happens to unfertilised cells?
They degenerate 24 hours after ovulation
What promotes the development of the corpeus luteum?
What happens in the development of the corpeus luteum?
- Remaining granulosa and theca interna cells become vascularised
- Develop yellowish pigment and change into lutein cells
What does the corpeus luteum do?
Secretes oestrogen and progesterone
What is the purpose of the secretion of oestrogen and progesterone by the corpus luteum?
Stimulates uterine mucosa to enter secretory stage
What is the purpose of the stimulation of the uterine mucosa to enter the secretory stage?
It prepares for embyro implantation
What happens to the corpeus luteum if no fertilisation occurs?
Degenerates after 14 days
What happens in the degeneration of the corpus luteum?
- Forms corpus albicans
- Progesterone production decreases
What is the corpus albicans?
A mass of fibrotic scar tissue
What does a decrease in progesterone production precipitate?
What happens to the corpus luteum if fertilisation occurs?
- Degeneration of CL prevented by human chorionic gonadotrophin
- Corpus luteum continues to grow
- Cells continue to secrete progrestone until about 4th month
What secretes human chorionic gonadotrophin?
The developing embryo
What does the continuation of growth of the corpus luteum when fertilisation occurs form?
The corpus luteum of pregnancy, the corpus luteum graviditatis
Why does the corpus luteum stop secreting progesterone at about the 4th month?
The secretion of progesterone by the placenta then becomes adequate
What is the role of LH and FSH in ovulation?
They stimulate the rapid growth of follicles several days before ovulation
What is the mature follicle called?
What does the LH surge cause?
- Increase in collagenase activity
- Prostaglandin increase
What do prostaglandins cause in ovulation?
Local muscular contractions in ovarian wall, leading to the oocyte being extruded and breaking free from ovary
What happens in oocyte transport?
- Shortly before ovulation, fimbriae sweep over surface of ovary, and uterine tubes begin to contract rhymthically
- Oocyte is carried into tube by sweeping movements of fimbriae, and by motion of cilia on epithelial linings
- Oocyte then propelled by peristaltic muscular contractions of tube and cila in the mucosa
How long does it take for the oocyte to reach the uterine lumen if fertilised?
3 to 4 days