General characteristics of gametes
Fertilization involves not a fully matured ovum, but a secondary oocyte arrested at metaphase of the second meiotic division.
The meeting of gametes is ensured by their directional movement:
- The ovum moves passively via fluid flow and oviduct peristalsis.
- Spermatozoa initially move passively (due to muscular contractions of the female reproductive tract) and then, upon activation, switch to active flagellar movement. To locate the ovum, they use negative rheotaxis (movement against fluid flow) and chemotaxis (response to gynogamones of the ovum).
Phases of fertilization
The process is classically divided into three main phases and a final stage of preparation for cleavage.
1. Distant interaction
At this stage, spermatozoa approach the ovum. In the female reproductive tract, they undergo capacitation — functional maturation. Under the influence of oocyte peptides (gynogamones II), their metabolism and motility sharply increase, and glycoproteins are removed from the head surface, preparing the membrane for the acrosome reaction.
2. Contact interaction
Spermatozoa cross the corona radiata (corona radiata) and bind to receptors (Zp3) of the zona pellucida (zona pellucida). This triggers the acrosome reaction: the acrosomal membrane fuses with the sperm plasmalemma, releasing enzymes.
- Penetrase removes the remnants of follicular cells.
- Acrosin and hyaluronidase locally dissolve the zona pellucida, forming a channel for the passage of the spermatozoon.
3. Penetration
A single spermatozoon attaches to the plasmalemma of the oocyte (prompting the formation of a fertilization cone in response). The enzyme acid phosphatase degrades the phospholipids of the oocyte membrane, and the plasmalemma of the sperm head integrates into the egg. Only the nucleus and centrioles of the spermatozoon penetrate inside.
Prevention of polyspermy
Immediately after sperm penetration, the transmembrane potential of the oocyte changes, causing a release of calcium ions. This initiates the cortical reaction — exocytosis of cortical granules.
Their enzymes trigger a cascade of changes that block polyspermy:
- Receptors of the zona pellucida and plasmalemma are modified — other spermatozoa can no longer attach.
- The bonds between the oocyte and the zona pellucida are broken, forming the perivitelline space.
- The zona pellucida condenses, turning into the impermeable fertilization membrane.
- Gynogamones II are released, causing agglutination (clumping) of the remaining spermatozoa.
Completion of meiosis and preparation for cleavage
The penetration of the sperm centrioles prompts the oocyte to complete the second meiotic division. A haploid female pronucleus is formed, and the second polar body is extruded. The cell becomes a zygote.
Next comes the syndiploidy (synkaryon) stage (about 12 hours):
- The sperm nucleus swells, and chromatin decondenses, forming the male pronucleus.
- The pronuclei approach each other, but their membranes do not fuse.
- DNA replication occurs, and chromosomes become two-chromatid.
During prophase of the first mitotic division, chromosomes condense separately in each pronucleus, and their membranes break down. Genomes merge only in metaphase, forming a single equatorial plate ("metaphase plate").
Briefly: answers to topical questions
In which specific part of the uterine tube does fertilization normally occur in humans?
Normally, fertilization in humans occurs in the ampullary region of the uterine tube. The ampullary part is the longest section of the tube, accounting for about half of its length; the meeting of the sperm and oocyte takes place precisely in this region.
Which classes of gamones (gynogamones and androgamones) participate in the fertilization process?
The fertilization process involves gamones that ensure the chemical interaction of gametes:
- Androgamones — secreted by spermatozoa.
- Gynogamones — secreted by ova.
Gynogamones are involved in sperm chemotaxis, and gynogamones II (oocyte peptides) are listed among the initiators of sperm capacitation.
Which glycoprotein fractions make up the zona pellucida?
The zona pellucida (zona pellucida) is composed of specific glycoprotein fractions Zp1, Zp2, and Zp3.
- Zp2, following the cortical reaction, participates in blocking the penetration of extra spermatozoa.
- Zp3 acts as species-specific receptors for spermatozoa and ensures gamete binding.
Reaction logic: Capacitation → Acrosome reaction → Penetration → Cortical reaction.
Class and exam questions
At what stage of meiosis is the oocyte at the moment of fertilization?
At the moment of fertilization, it is a secondary oocyte arrested at metaphase of the second meiotic division.
Do the nuclei of the human sperm and ovum fuse?
No, classical fusion of nuclear envelopes (syngamy) does not occur. The pronuclei approach each other (synkaryon), their membranes break down, and chromosomes merge only at the metaphase plate stage of the first mitosis.
What exactly from the spermatozoon enters the oocyte?
Only the nucleus and centrioles penetrate into the oocyte cytoplasm.
What else to study on this topic
- Mechanisms of gamete taxis (electrotaxis, chemotaxis)
- Molecular basis of binding to Zp3
- Role of calcium ions in the cortical reaction
- First mitotic division of the zygote
- Acrosomal enzymes and their specific action
Master the entire topic in the "Histology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on "Fertilization" and 3,485 more pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 137 video lessons on the subject
Other topics in the section "Germ cells, fertilization, cleavage, blastula"
The entire «Germ cells, fertilization, cleavage, blastula» sectionFree video lessons: histologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.