4.3 The path of the sperm cells to the oocyte - capacitation

Sperm cell capacitation


Quiz 17

After the ejaculation the sperm cells go through several essential physiological changes during their time in the female genital tract before they, at the end, are able to penetrate the oocyte membrane.
The first change in this cascade is capacitation. The sperm cells accomplish this during the ascension through the female genital tract (in contact with its secretions). It has to do with a physiological maturation process of the sperm cell membranes, which is seen as the precondition for the next step to follow, namely the acrosome reaction.

Capacitation is a functional maturation of the spermatozoon. The changes take place via the sperm cell membrane in which it may be that receptors are made available through the removal of a glycoprotein layer. The area of the acrosomal cap is also so altered thereby that the acrosome reaction becomes possible.
Through the membrane alterations, the motile properties of the spermatozoon also change. Discharging whipping movements of the tail together with larger sideways swinging movements of the head take place. This type of motility is designated as hyperactivity. One can therefore say that the visible consequences of capacitation consist in hyperactivity of the spermatozoon.


Quiz 18

Capacitation is what one calls the changes that lead to hyperactivity of the spermatozoon and which later allow the spermatozoon to go through the acrosome reaction.

Fig. 29 - Before capacitation Fig. 30 - After capacitation  Legend

Fig. 29, Fig. 30
These are monitor screen images from an instrument which records the movement paths of the sperm cells heads (white points) during a certain time span and displays them with a green line. Before capacitation the majority of the lines are straight. After capacitation almost all the sperm cells have now gone over to swinging their heads strongly as indicated by the jagged lines.

Since it cannot be determined ahead of time when the exact moment is that the oocyte and spermatozoon will meet, the maturation mechanisms are so configured that various groups of sperm cells are able to keep their chances of fertilization upright over a relatively long time after cohabitation. For this purpose the ejaculated sperm cells do not all end their capacitation at the same time, thus creating heterogenous groups of sperm cells.

The groups have differing histories in terms of their age and their storage conditions. Thus, in what concerns maturation/capacitation, they are not all on the same level. This makes it possible to keep fertilization-able sperm cells, since they mature in such a staggered sequence. One can imagine that at any given time a small population of spermatozoa may have ended their capacitation and are thus ready to fertilize an oocyte that has possibly made its way to the ampullary part of the uterine tube.

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