General Characteristics of Gametes
Fertilization involves not a fully mature egg, but a secondary oocyte arrested at metaphase of the second meiotic division.
Gamete meeting is facilitated by their directed movement:
- The oocyte moves passively due to fluid currents and oviduct peristalsis.
- Spermatozoa initially move passively (via muscular contractions of the female tract) and then, once activated, transition to active flagellar motion. To locate the oocyte, they utilize negative rheotaxis (movement against fluid flow) and chemotaxis (response to oocyte gynogamones).
Phases of Fertilization
The process is classically divided into three main phases followed by a final preparation stage for cleavage.
1. Distant Interaction
At this stage, spermatozoa approach the oocyte. Within 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 stripped from the head surface, preparing the membrane for the acrosomal reaction.
2. Contact Interaction
Spermatozoa traverse the corona radiata and bind to receptors (Zp3) on the zona pellucida. This triggers the acrosomal reaction: the acrosomal membrane fuses with the sperm plasma lemma, releasing enzymes.
- Penetrase removes remaining follicular cells.
- Acrosin and hyaluronidase locally dissolve the zona pellucida, forming a channel for sperm passage.
3. Penetration
A single spermatozoon attaches to the oocyte plasma lemma (eliciting the formation of a fertilization cone). The enzyme acid phosphatase disrupts oocyte membrane phospholipids, and the head plasma lemma integrates into the egg. Only the nucleus and centrioles of the spermatozoon enter the interior.
Prevention of Polyspermy
Immediately following sperm penetration, the transmembrane potential of the oocyte changes, triggering a calcium ion efflux. This initiates the cortical reaction—exocytosis of cortical granules.
Their enzymes trigger a cascade of changes that block polyspermy:
- Receptors on the zona pellucida and plasma lemma are modified—other spermatozoa can no longer bind.
- Connections between the oocyte and the zona pellucida are severed, forming the perivitelline space.
- The zona pellucida hardens, transforming into an impermeable fertilization membrane.
- Gynogamones II are released, causing agglutination of the remaining spermatozoa.
Completion of Meiosis and Preparation for Cleavage
The entry of the sperm centrioles forces 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 synkaryon stage (approximately 12 hours):
- The sperm nucleus swells, and chromatin decondenses to form the male pronucleus.
- The pronuclei approach each other, but their nuclear envelopes do not fuse.
- DNA replication occurs, and chromosomes become two-chromatid structures.
During prophase of the first mitotic division, chromosomes condense in each pronucleus separately, and their envelopes break down. The genomes unite only at metaphase, forming a single metaphase plate.