Fertilization of the EggFertilization is believed to occur in the middle part of the tube, called the ampulla, not inside the uterus. Sperm travel through the uterine cavity and out into the tube to meet the egg.
When the sperm and egg join, the sperm must pass through the outer layer of the ovum, the corona radiata. The sperm must then digest its way through another layer of the ovum, the zona pelludda. Although several sperm may penetrate the outer layers of the ovum, usually only one sperm enters the ovum and fertilizes it.
After the sperm penetrates the ovum, the sperm head attaches to its surface. The membranes of the sperm and ovum unite, enclosing them in the same membrane or sac. The ovum reacts to this contact with the sperm by making changes in the outer layers so no other sperm can enter.
Once the sperm gets inside the ovum, it loses its tail. The head of the sperm enlarges and is called the male pronudeus; the ovum is called the female pronudeus. The chromosomes of the male and female pronuclei intermingle. When this happens, extremely small bits of information and characteristics from each partner unite. This chromosomal information gives each of us our particular characteristics. The usual number of chromosomes in each human is 46. Each parent supplies 23 chromosomes. Your baby is a combination of chromosomal information from you and your partner.
Embryonic Development Begins
The developing ball of cells is called a zygote. The zygote passes through the uterine tube on its way to the uterus as the division of cells continues. These cells are called a blastomere. As the blastomere continues to divide, a solid ball of cells is formed, called a morula. The gradual accumulation of fluid within the morula results in the formation of a blastocyst, which is tiny.
During the next week, the blastocyst travels through the uterine tube to the cavity of the uterus (3 to 7 days after fertilization in the tube). The blastocyst lies free in the uterine cavity as it continues to grow and to develop. About a week after fertilization, it attaches to the uterine cavity (implantation). The cells burrow into the lining of the uterus.
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