How Sperm and Egg Combine Forces to Create An Embryo

Now that the sperm is finally inside, the real magic happens. The egg literally swallows the sperm, merging the two cells’ membranes together. The moment the two meet, the outer part of the egg’s zona pellucida becomes totally rigid, forming a solid wall against any other sperm. If you looked under a microscope, you could actually see sperm trying to drill into the egg, but try as they might, they can’t get through. This is nature’s clever way of making sure that the fertilized egg develops the right number of chromosomes—if the egg is fertilized by more than one sperm, the embryo won’t be able to develop.

After the sperm and egg join, their genetic material begins mixing. The body of the sperm is actually an all-in-one transportation system for precious genetic cargo: the chromosomes, or genetic material, are packed tightly inside the sperm’s head. When the sperm is safely inside the egg, it breathes a sigh of relief, and its tightly packed genetic material unwinds. This expanded form is called the male pronucleus. The male pronucleus has 23 chromosomes, little packets of genetic material that will form half of the new baby’s DNA.

At the same time, the nucleus or center of the egg cell, which contains the mother’s DNA, goes through a process called “second meiotic division.” This means that the cell divides so that rather than having 46 chromosomes, which is the normal number for a human being, it has 23. This is called the female pronucleus.

It’s important for both the sperm and egg cells to have 23 chromosomes each, so that when they combine the embryo will have a total of 46 chromosomes, creating a full, completely unique, genetic code for the new baby.

Within 18 hours of first combining forces, the male and female pronuclei have swollen and are sitting on either side of the egg, waiting to come together. Ever so slowly, the two begin to move toward the center of the egg. When they meet, the two form one combined nucleus. This nucleus has 46 chromosomes, a complete human genetic code.

Once the egg and sperm have combined their genetic material, the egg is officially fertilized. Soon, the fertilized egg will begin splitting and growing, and will travel to the uterus where it will implant itself and become a pregnancy. In these few hours, something truly amazing has happened: you have created the genetic code of a brand new, totally unique, person.