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.
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.
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.