Transport of secondary oocyte
The female gamete (secondary oocyte) lacks its own structures for locomotion, so its transport is exclusively passive. The oocyte moves from the infundibulum of the uterine tube (fallopian tube) toward the uterine cavity.
Progression is ensured by a directed mucus flow created by two main mechanisms:
- Ciliated epithelium: cilia of the uterine tube epithelial cells beat continuously, creating a fluid flow in the required direction.
- Tonic contractions: the muscular layer of the uterine tube contracts rhythmically, propelling the contents forward.
Both of these processes are under strict hormonal control. Progesterone is responsible for stimulating mucus secretion, creating a medium for transport, whereas estrogens enhance tonic contractions of the smooth musculature of the tube wall.
Initial stages of sperm transport
The progression of male gametes varies significantly depending on which section of the reproductive tract they are in.
- Vagina. The vaginal environment is markedly acidic. Under such conditions, sperm motility remains extremely low.
- Uterus and uterine tubes. At this stage, transport is predominantly passive. The advancement of spermatozoa deeper into the tract is ensured by suction tonic contractions of the female reproductive tract itself. An important role in this is played by prostaglandins, specific hormonal components of seminal fluid secreted by the prostate gland. They act locally on the uterine and vaginal musculature, significantly enhancing their contractions.
As they progress from the vagina toward the ostia of the uterine tubes, a consistent dynamic is observed: the total number of spermatozoa decreases rapidly.
Capacitation: preparation for fertilization
Capacitation is a crucial stage of male germ cell activation that takes place inside the female body. This process occurs mainly in the uterine cavity and takes approximately 7 hours.
Under the influence of the chemical composition of the female reproductive tract environment, spermatozoa undergo a series of critical changes:
- There is a sharp increase in cellular metabolism and an enhancement of motility.
- Surface glycoproteins and various carbohydrate components are removed from the head membrane of the spermatozoon.
As a result of capacitation, the sperm membrane acquires high lability, which is a necessary preparation for the subsequent acrosomal reaction. The main outcome of this process is that the spermatozoon acquires the ability to tightly bind to the oocyte's zona pellucida (zona pellucida).
Active movement and navigation of male gametes
Upon completion of capacitation, spermatozoa transition to independent, directed movement. This is driven by active flagellar beating, creating forward locomotion accompanied by rotation of the cell around its axis.
To locate the oocyte within the uterine tube, spermatozoa use two main orientation (taxis) mechanisms:
- Rheotaxis is the directed movement of a spermatozoon against the physiological flow of mucus in the uterine tubes.
- Chemotaxis is movement along an increasing concentration gradient of specific chemical attractants (peptides, gynogamones I). These substances are actively secreted by the oocyte itself and the surrounding follicular cells.
As a result of a complex pathway and rigorous selection, only about 200 of the most active and viable spermatozoa reach the ampullary region of the uterine tube, where the secondary oocyte typically awaits fertilization.
Briefly: answers to topical questions
What is the average speed of active movement of a spermatozoon through the uterine tube?
The speed of human sperm movement is 30–50 µm/s. Active movement after capacitation is ensured by flagellar beating: forward movement is combined with rotation around the axis; tail beat frequency is 5 s⁻¹ (5 Hz).
To navigate through the uterine tube, spermatozoa use:
- Rheotaxis — movement against the fluid flow (uterine tube mucus).
- Chemotaxis — movement along the concentration gradient of attractants (peptides/gynogamones I) secreted by the oocyte and follicular cells.
Sperm movement speed is affected by temperature and environmental pH.
Which enzymes are released from the sperm acrosome to penetrate the corona radiata and zona pellucida?
To overcome the barriers surrounding the oocyte, enzymes are released from the sperm acrosome. The fusion of the sperm plasma lemma with the outer acrosomal membrane and subsequent vesiculation lead to the release of the acrosomal contents outward.
- Penetrase ensures the dissociation of residual follicular cells from the zona pellucida.
- Collagenase participates along with penetrase in the dissociation and removal of follicular cell remnants.
- Acrosin is a protease that locally dissolves the zona pellucida (zona pellucida).
- Hyaluronidase participates along with acrosin in the dissolution/local softening of the zona pellucida, resulting in the formation of a narrow channel for sperm passage.
Hormonal regulation of the tube: "Estrogens — Energy (muscle contraction), Progesterone — Swimming (mucus secretion)."
Class and exam questions
Why do spermatozoa move slowly in the vagina?
Their motility is suppressed due to the acidic environment of the vagina, which is unfavorable for the active movement of male gametes.
What is the process of capacitation?
This is the process of final maturation and activation of spermatozoa in the female tract. During this process, glycoproteins are stripped from the head membrane, metabolism increases, and the membrane prepares for the acrosomal reaction.
How does a spermatozoon know where to swim to reach the egg?
It is aided by rheotaxis (movement strictly against the mucus flow) and chemotaxis (movement toward the "scent" of chemical substances — gynogamones secreted by the oocyte).
What else to study on this topic
- Structure and chemical composition of the zona pellucida.
- Mechanism and stages of the acrosomal reaction.
- Ultrastructural structure of the sperm flagellum.
- Properties and functions of gynogamones I.
- Histology of the ciliated epithelium of the uterine tubes.
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