Secondary Oocyte Transport
The female gamete (secondary oocyte) lacks intrinsic motility structures, so its transport is exclusively passive. The oocyte moves from the infundibulum of the uterine tube (fallopian tube) toward the uterine cavity.
Progression is driven by a directed mucus current created by two main mechanisms:
- Ciliated epithelium: epithelial cilia in the uterine tube beat continuously, generating a fluid current in the correct direction.
- Tonic contractions: the muscular wall of the uterine tube contracts rhythmically, propelling the contents.
Both processes are under strict hormonal control. Progesterone stimulates mucus secretion to create a transport medium, whereas estrogens enhance tonic contractions of the smooth muscle in the tube wall.
Initial Stages of Sperm Transport
The progression of male gametes varies significantly depending on their location within the reproductive tract.
- Vagina. The vaginal environment is acidic. Under these conditions, sperm motility remains extremely low.
- Uterus and Uterine Tubes. At this stage, transport is predominantly passive. Sperm progression deeper into the tract is facilitated by suction-like tonic contractions of the female reproductive organs. Prostaglandins—specific hormonal components of seminal fluid secreted by the prostate gland—play an important role here. They act locally on the uterine and vaginal musculature, significantly increasing contractions.
As sperm advance from the vagina toward the openings of the uterine tubes, their total number decreases sharply.
Capacitation: Preparation for Fertilization
Capacitation is a crucial activation step for male germ cells that occurs within the female reproductive tract. This process takes place primarily in the uterine cavity and lasts approximately 7 hours.
Under the influence of the chemical environment of the female reproductive tract, spermatozoa undergo several critical changes:
- Cellular metabolism increases sharply, along with enhanced motility.
- Surface glycoproteins and various carbohydrate components are removed from the sperm head membrane.
As a result of capacitation, the sperm membrane becomes highly labile, which is necessary to prepare for the subsequent acrosome reaction. The main outcome of this process is that the spermatozoon acquires the ability to bind firmly to the zona pellucida of the oocyte.
Active Movement and Guidance of Male Gametes
Following capacitation, spermatozoa transition to independent, directed movement driven by active flagellar beating, producing forward progression combined with rotation around the long axis.
To locate the oocyte within the uterine tube, spermatozoa use two main orientation mechanisms (taxis):
- Rheotaxis — directed movement of the spermatozoon against the physiological mucus flow in the uterine tubes.
- Chemotaxis — movement along an increasing concentration gradient of specific chemical attractants (peptides, gynogamones I). These substances are actively released by the oocyte and its surrounding follicular cells.
After a complex journey and rigorous selection, only about 200 of the most active and viable spermatozoa reach the ampulla of the uterine tube, where the secondary oocyte awaits fertilization.