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Fertilization

The female genital tract

The fertilization process takes place in the female genital tract.

Fig. 5 - The female genital tract
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  1. Ovary
  2. Infundibulum
  3. Fimbriae
  4. Fallopian or uterine tube
  5. Ampullary part of the tube
  6. Uterine musculature
  7. Uterine mucosa
  8. Cervix
  9. Portio
  10. Vagina
  11. Ligamentum ovarii proprium
  12. Suspensory ligament of the ovary
  13. Ovary cut open (follicles in various stages)

Legend
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
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  1. Tertiary follicle
  2. Ovary cortex
  3. Medulla of the ovary
  4. Theca with capillary network of the graafian follicle
  5. Oocyte within the cumulus oophorus
  6. Stratum granulosum (granulosa cells)
  7. Antrum folliculi with follicle fluid
  8. Peritoneal cavity

Legend
Fig. 6

Schematic diagram of a graafian follicle in the ovary.

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
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  1. Ovarian tissue
  2. Antrum folliculi
  3. Remains of the cumulus oophorus
  4. The diameter of the follicle is of 22 mm

Legend
Fig. 7

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

© Dr. L. Rayo, Inselspital Bern

 
Video

Ultrasound.

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
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E2
Oestradiol
Pr
Progesteron
LH
Luteinizing hormone
FSH
Follicle-stimulating hormone

Legend
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,

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.

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 interaktive popup

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

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