Life on earth is vastly different and full of unique creatures but deep down in the very beginning of organism creation there is one thing similar among most life forms. That is fertilization – a cell to cell process which involves a small motile cell (the sperm cell) recognizing a far bigger nonmotile egg cell.
Fertilization can be divided into four stages which we will discuss one by one here:
- Preparation Of The Sperm Cell
- Recognition of The Egg Cell And Binding
- Zygote Activation
The fusion between the membranes of these two cells isn’t entirely explained yet, but proteins such as the ADAM proteins are a possible key to understanding the whole nature of the process.
How Things Start
Egg and sperm cells have to travel in opposing directions in order to meet each other. This all happens in the women’s fallopian tubes after the male ejaculation (if it’s during menstruation otherwise there won’t be any egg cell). During women’s ovulation a single egg cell is released in one of the fallopian tubes to embark on its journey to the uterus. There, it is being prepared for sperm cell penetration and implantation.
During this part the estrogen and LH levels are drastically rising. Both have their specific roles in the process. LH tells the ovaries to release the egg, and estrogen stimulates the vaginal walls to excrete glycogen which metabolizes to lactate effectively lowering the surrounding pH to around 4. This crease a very hostile environment for any pathogens especially the ones causing sexually transmitted diseases. This environment can also be toxic to sperm. Thankfully the semen buffers the acidity and keeps the sperm cells healthy. Once the semen mixes with the vaginal fluids the pH gets to a point where it triggers a few reactions in sperm cells that increase their motility.
Curious Fact: Only one in a million sperm cells will ultimately reach the site of fertilization.
The estrogen hormone also helps the sperm cells in a very bizarre way. It relaxes the muscles in the vagina and even stimulates micro contractions which help the sperm cells navigate through the female’s reproductive system.
The relaxed cervix also helps sperm go from the vagina into the uterus to meet their final goal. The cervical mucus usually prevents sperm cells from entering the uterus but during ovulation its pH levels are lower and the mucus layer is thinner therefore easier to go through.
At the end of the day, all the changes that the vagina undergoes are all working in favor of the newly arrived sperm cells and a great medium is created for their journey. The contractions of the uterus are usually targeted at the correct fallopian tube thus leading the sperms into the right direction. Another fun fact is that uterine contractions are actually more helpful for the sperm movement than their own propulsion.
By now it is fairly obvious that the destiny of the sperm cells entirely depends on the female menstrual cycle and its current stage. If ejaculation happens at the appropriate time, it is highly unlikely that anything will ever happen. In other words, the closer ovulation is, the easier it is for sperm cells to move through the female’s reproductive system.
We will also hit on a well-known issue later – polyspermy. Now, let’s take a closer look at the four stages starting from the first one.
If you want to learn more about the process of storing embryonic cells, head out to our Main Page.
Stage 1 – Sperm Preparation
Contrary to common belief, ejaculated sperm cells aren’t fully ready to fertilize a female egg when they first enter the vagina. In fact, they undergo a lot of changes which are described with the term “capacitation”.
It all starts with the intracellular calcium levels. The increase of Ca2+ ions leads to activation of the sperm cells’ mobility and their tails change their speed with which they spin.
The last thing that happens to the sperm cell is that it loses its surface proteins. This results in the sperm cells being more susceptible to fertilizing the egg.
Stage 2 – Sperm-Egg Recognition And Binding
After finding the egg, the sperm cell attaches directly to the egg’s outer surface. This triggers a chain of acrosome reactions which leads to an intake of Na+ and excretion of H+. This increases the pH which on its own dissociates the egg layers ultimately allowing egg-sperm fusion. The only part which can prove to be a challenge for the sperm cell is the so-called “zona pellucida”. That zone triggers the acrosome reaction of the sperm. This includes the outer layer of the sperm to bind with the outer egg layers and the content of the sperm’s head will be released causing damage to the wall, making penetration easier.
When it comes to how the sperm cell actually finds the wall of the egg cell, it is proven that it all happens thanks to a chain of enzyme reactions. If one of the enzyme substances isn’t present in the sperm cell (a defect in the sperm) binding won’t happen.
Stage 3 – Sperm-Egg Fusion
Up until recently this stage was a total enigma to scientists. It is now known that a sperm protein is responsible for the binding. It only activates when the zona pellucida has been disintegrated, though.
Stage 4 – Egg’s Activation
The first and most important reaction of the egg cell after the fertilization is to prevent further sperm cells from entering, on in other words, prevent polyspermy. This means not to be fertilized by more than one sperm cell, which will sadly result in embryos with more than a normal amount of the parent’s DNA which causes defects and a total halt of the embryo’s development during the early stages of pregnancy.
The reaction that prevents polyspermy is the egg’s cortical reaction. Under the plasma membrane of all egg cells there are vesicles called granules. They secreted Ca2+ ions on the surface (above the zona pellucida) which prevents any sperm cells from entering.