Introduction
A gamete is a reproductive cell that fuses with another reproductive cell during sexual reproduction to form a zygote. Male gametes are called sperm and female gametes are called eggs or ova. Organisms that produce gametes are called gonads. In humans, the male gonad is the testis and the female gonad is the ovary. Gametes are produced through a type of cell division called meiosis. During meiosis, each parent’s set of chromosomes is reduced by half so that when fertilization occurs, the zygote has a full set of chromosomes from both parents.
Sperm
Sperm are produced in the testes and stored in the epididymis. They are released into the vas deferens during ejaculation. Sperm travel through the vas deferens to reach the seminal vesicles where they mix with seminal fluid to form semen. From there, they travel through the urethra and out of the penis during ejaculation (Figure 1). Semen contains around 5% sperm by volume but it only takes one sperm to fertilize an egg. The average human ejaculate contains about 300 million sperm cells but only about 0-100 million of these will actually reach the site of fertilization in the fallopian tube .
Eggs
Eggs, or ova, are produced in ovaries and stored in ovarian follicles (Figure 2). During ovulation, one or more eggs are released from mature follicles into nearby fallopian tubes where they may be fertilized by sperm cells travelling up from the uterus . If fertilization does not occur within 12-24 hours after ovulation,the egg(s) degenerate and die . An unfertilized egg cannot divide and grow on its own so it must be fertilized soon after being released from its follicle if pregnancy is to occur . A woman is born with all her eggs but she will only release 400-500 of them over her lifetime .
Fertilization
When a sperm cell enters an egg cell , their nuclei fuse to create a single nucleus for their offspring . This process is known as syngamy (Figure 3). The resulting diploid zygote then rapidly begins dividing via mitosis into many cells which go on to develop into an embryo . For mammals , development continues inside the mother’s uterus until birth . In contrast , most amphibians , reptiles , birds , fish , and invertebrates lay their eggs outside their bodies where development must occur without parental assistance . Some species such as sharks retain their young inside their bodies until they have developed enough to fend for themselves while others such as kangaroos give birth very early in development so that their young can climb into their mothers’ pouches for protection and further development