Fertilization

Recall that with one ejaculation, a male releases hundreds of millions of sperm into the vagina of a female. Once sperm are released, they swim through the vagina, cervix, and uterus and, finally, up the fallopian tubes. If ovulation occurs anytime from 72 hours before to 48 hours after ejaculation, sperm may encounter an egg in one of the fallopian tubes. Fertilization occurs when a sperm and an egg fuse and form a zygote. From this point, human development takes about nine months—a period known as gestation (jes-TAY-shuhn).

An egg in a fallopian tube is encased in a jellylike substance and surrounded by a layer of cells from the follicle of the ovary. As shown in Figure 51-9, several sperm may attach to an egg and attempt to penetrate its outer layers. Recall that the head of a sperm contains digestive enzymes. These enzymes break down an egg's outer layers and enable the cell membrane that surrounds the head of the sperm to fuse with the egg's cell membrane. The sperm's nucleus and midpiece then enter the cytoplasm of the egg. The tail of the sperm remains outside the egg. Usually, only one sperm is successful in penetrating an egg. Electrical changes that occur in an egg's cell membrane after a sperm enters the egg help keep other sperm from penetrating the egg.

After a sperm enters an egg, the egg completes meiosis II, and the sperm's nucleus fuses with the egg's nucleus. The diploid cell that results from this fusion is called a zygote. Recall that each gamete contains 23 chromosomes, the haploid (1n) number. Thus, fusion of a sperm nucleus and an egg nucleus causes a zygote to have 46 chromosomes, thus restoring the diploid (2n) number.

Cleavage and Implantation

Immediately following fertilization and while still in the fallopian tube, the zygote begins a series of mitotic divisions known as cleavage. The resulting cells do not increase in size during these cell divisions. Cleavage produces a ball of cells called a morula (MAWR-yoo-luh), which is not much larger than the zygote. Cells of the morula divide and release a fluid, resulting in a blastocyst. A blastocyst (BLAS-toh-SIST) is a ball of cells with a large, fluid-filled cavity.

figure 51-9

Several sperm surround this ovum, but only one will be able to fertilize it. (SEM 1165 x)

2-cell stage

4-cell stage Morula

Fallopian tube

2-cell stage

4-cell stage Morula

Fallopian tube

Implantation

' Developing follicle

Ovulation figure 51-10

' Developing follicle

Ovulation

As shown in Figure 51-10, the morula has become a blastocyst by the time it reaches the uterus. In the uterus, the blastocyst attaches to the thickened uterine lining. The blastocyst then releases an enzyme that breaks down the epithelial tissue that lines the uterus and burrows into the thickened lining. The process in which the blastocyst burrows and embeds itself into the lining of the uterus is called implantation (iM-plan-TAY-shuhn). Pregnancy begins at implantation, which occurs about a week after fertilization.

Implantation figure 51-10

The earliest stages of development occur within a fallopian tube as a zygote travels toward the uterus. It takes about a week for the zygote to travel from the fallopian tube to the uterine lining.

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