4.1 Ovulation


The female genital tract


The fertilization process takes place in the female genital tract.

Fig. 5 - The female genital tract  Legend

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 Ovary
Infundibulum
Fimbriae
Fallopian or uterine tube
Ampullary part of the tube
Uterine musculature
Uterine mucosa
Cervix
Portio
Vagina
Ligamentum ovarii proprium
Suspensory ligament of the ovary
Ovary cut open (follicles in various stages)

Fig. 5
Schematic drawing of the female sexual organs. Frontal view: vagina, uterus; right tube and right ovary are cut open.


The ovary and the dominant follicle


Roughly a week before the midpoint of the menstrual cycle the dominant follicle develops in one of the two ovaries. This grows faster than the other tertiary follicles and prepares itself for ovulation. It reaches a diameter of up to 25 mm and is also known then as the graafian follicle. Fig. 6 - Graafscher Follikel  Legend

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8		 Tertiary follicle
Ovary cortex
Medulla of the ovary
Theca with capillary network of
the graafian follicle
Oocyte within the cumulus
oophorus
Stratum granulosum
(granulosa cells)
Antrum folliculi with follicle fluid
Peritoneal cavity

Fig. 6
Schematic diagram of a graafian follicle in the ovary.






















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The ultrasound image in Fig. 7 shows the follicle fluid as a black surface. The recording was made shortly before ovulation. Since at this moment the cumulus oophorus has detached itself from the granulosa, the oocyte "swims" surrounded by its cumulus cells (so-called corona radiata) in the follicle fluid. This cannot be seen, though, in the figure.
The graafian follicle bulges out the ovary's surface. It is ready to tear open, i.e., its wall and the peritoneal covering will soon rupture and its watery contents will pour into the infundibulum of the tuba, together with the oocyte. The diameter of the follicle that is about to rupture is here 22 mm (corresponding to 5 ml of follicle fluid).		 Fig. 7 - Ultrasound image of a
graafian follicle		  Legend

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 Ovarian tissue
Follicle about to rupture
Remains of the cumulus oophorus
The diameter of the follicle is of 22 mm

Fig. 7
Single frame from a series of video images that shows the follicle about to rupture

© Dr. L. Rayo, Inselspital Bern


Video (150 kB)


A hormonal regulation is responsible for follicle maturation. The selection criteria for choosing the dominant follicle are still unknown, however. The following plot shows the course of the concentrations of the hormones that are involved during a monthly cycle.


Fig. 8 - Hormone concentrations during a cycle  Legend

LH
E2
Pr
FSH
 Luteinisierendes Hormon
Östradiol
Progesteron
Follikel stimulierendes Hormon

Fig. 8
The curves show the concentrations of the FSH, LH, estradiol, and progesterone hormones in the blood. They have been recorded for the average period duration of 28 days. Roughly 38 hours before ovulation, a LH peak occurs.


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Approximately one-and-a-half days before the midpoint of the cycle, the concentration of the luteinizing hormone (LH) rises steeply. The LH peak is responsible for a whole series of processes: triggering ovulation, creating an optimal milieu for a successful fertilization, and harmonizing the temporal sequence of the processes with one another.


With the LH peak the following maturation steps are now triggered in and around the oocyte - up to ovulation:

In the oocyte:
 
 
 
 Termination of the first meiosis with ejection of the first polar body
Begin of the second meiosis with arrest in the metaphase
Maturation of the oocyte cytoplasma by preparing molecules and structures that will be needed at the time of fertilization.

In the follicle:
 
 
 
 
 
 
 
 
 
 The granulosa cells that sit just outside on the pellucid zone withdraw their processes from the oocyte surface back into the pellucid zone. Those processes were in charge of transferring substances to the oocyte
The perivitelline space forms between the oocyte and the pellucid zone. This space is necessary for allowing division of the oocyte and for harboring the first polar body formed in the division.
Loosening of the granulosa cells in the vicinity of the cumulus oophorus and proliferation of the granulosa cells
Increasing the progesterone concentration in the follicle fluid via increased production in the granulosa cells.

The results of these processes are:

 
 
 
 
 
 The correct placement of the uterine tube infundibulum upon the ovarian surface
The rupture of the follicle wall and the flow of the follicle fluid with the oocyte into the infundibulum
The inhibition of the maturation of further follicles

Commentary

What causes the LH peak?
What are the hormonal relationships that lead to the LH peak? How do they influence the follicle in its maturation process?

More about this in an interactive popup. (620 kB!)



















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More detail about these effects is provided in the next pages.



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