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4
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Fertilization

The cervical canal

After the ejaculation the sperm cells are cloaked by a slightly alkaline, buffering seminal plasma that protects them from the acidic vaginal milieu. Nevertheless a large portion of the sperm cells meets there their end. The survivors are attracted by the alkaline, sperm-friendly milieu of the cervix.

At the time of ovulation the properties of the cervix mucus changes from a "sperm-hostile "environment to a very "sperm-friendly" one.

Before the ovulation (Fig. 26) the cervical canal is narrow and the cervix mucus is strongly meshed (it forms the so-called cervical barrier) that hinders the passage of sperm cells.

At the time of ovulation (Fig. 27) the cervix wall becomes looser and the canal wide. The folds of the mucosa (Fig. 28) increase in number and let deeper and branched crypts come into being; there are then also more cervix glands.
Under the influence of the estradiol that increases shortly before ovulation the cervix mucus is restructured and the mucus barrier becomes passable for sperm cells.

Fig. 26 - Cervix canal before ovulation
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  1. Sperm cells
  2. Mucus fibers (strongly meshed)
  3. Crypt of a cervix gland

Fig. 27 - Cervix canal during ovulation
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4
Mucus fibers (loosely meshed)
5
Portio entrance

Legend
Fig. 26

Before ovulation the cervix canal is narrow, with a few, small cervix glands, and the mucus is highly meshed. The sperm cells "jam up" at the portio entrance.

Fig. 27

At ovulation the cervix canal becomes a lot wider, with many, widely branching cervix glands, and the mucus becomes loosely meshed. Passages form therein and the sperm cells are now able to pass through.

With the restructuring of the cervical barrier the mucus becomes thinner and more fluid. Therein meandering passages coated with specific chemotactic molecules form that the sperm cells will prefer in order to pass through the cervix.

The passage through the cervical canal is an important step for the selection of the sperm cells. The cervical mucus barrier functions as a filter in which atypical sperm cells remain hanging. They are hindered in ascending by means of a hydrodynamic effect. Through this simple mechanism it is assured that only normally formed and highly mobile sperm cells are able to overcome the cervical mucus barrier.

Commentary
In assisted fertilization the procedure of the sperm cell filtration and selection process is achieved by centrifugation through a density gradient gel.
Fig. 28 - Histological cross section of the cervical canal
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  1. Epithelium (mucus-producing)
  2. Cervical canal
  3. Membrane folds
  4. Crypts (actually cervix glands)

Legend
Fig. 28

The epithelial cells that line the cervical canal produce mucus. Epithelial folds form deeply branched crypts in which the mucus collects. Therefore, one also calls these structures cervix glands. At the time of ovulation they widen and deepen, and are able to retain quiescent sperm cells for several days.