How many sperm does it take to fertilize an egg?
It might only take one sperm to fertilize an egg, but that sperm is not alone. Hundreds of millions of sperm can be released during sexual intercourse.
Sperm and Sperm Production
Sperm (Figure below), the male reproductive cells, are tiny. In fact, they are the smallest cells in the human body. What do you think a sperm cell looks like? Some people think that it looks like a tadpole. Do you agree?
This drawing of a sperm shows its main parts. What is the role of each part? How do you think the shape of the sperm might help it swim?
A sperm has three main parts:
- The head of the sperm contains the nucleus. The nucleus holds the DNA of the cell. The head also contains enzymes that help the sperm break through the cell membrane of an egg.
- The midpiece of the sperm is packed with mitochondria. Mitochondria are organelles in cells that produce energy. Sperm use the energy in the midpiece to move.
- The tail of the sperm moves like a propeller, around and around. This pushes the sperm forward. A sperm can travel about 30 inches per hour. This may not sound very fast, but don’t forget how small a sperm is. For its size, a sperm moves about as fast as you do when you walk briskly.
To make sperm, cells start in the testes and end in the epididymis. It takes up to two months to make sperm. The steps are explained below:
- Special cells in the testes go through mitosis (cell division) to make identical copies of themselves.
- The copies of the original cells divide by meiosis, producing cells called spermatids. The spermatids have half the number of chromosomes as the original cell. The spermatids are immature and cannot move on their own.
- The spermatids are transported from the testes to the epididymis. Involuntary muscular contraction moves the spermatids along.
- In the epididymis, spermatids slowly grow older and mature. They grow a tail. They also lose some of the cytoplasm from the head.
- When sperm are mature, they can “swim.” The mature sperm are stored in the epididymis until it is time for them to leave the body.
Sperm leave the epididymis through the vas deferens. As they travel through the vas deferens, they pass by the prostate and other glands. The sperm mix with liquids from these glands, forming semen. The semen travels through the urethra and leaves the body through the penis. A teaspoon of semen may contain as many as 500 million sperm!
- epididymis: Male reproductive organs where sperm mature and are stored until they leave the body.
- meiosis: Process in cell division during which chromosome number is halved in order to produce gametes.
- mitochondria: Organelle of the cell in which energy is generated.
- mitosis: Division of the nucleus.
- nucleus: Eukayrotic cell structure that contains the genetic material, DNA.
- sperm: Male gamete, or reproductive cell.
- spermatid: Non-motile cell in the sperm maturation process that has half the number of chromosomes of the original cell.
- vas deferens: Tube that carries sperm from the epididymis to the urethra.
- semen: Fluid containing the sperm.
- Sperm are male gametes that form in the testes and mature in the epididymis.
- The head of the sperm contains the nucleus, the midpiece is packed with mitochondria, and the tail moves like a propeller.
Use the resource below to answer the questions that follow.
- Spermatogenesis at http://www.youtube.com/watch?v=POpbN6RHOO0 (0:45)
- What produces primary spermatocytes? How many chromosomes do primary spermatocytes have?
- What produces secondary spermatocytes? How many chromosomes do secondary spermatocytes have?
- What makes spermatids? How many chromosomes do spermatids have?
- What is the function of the tail of a sperm?
- Explain why sperm production is not completed when spermatids have been produced.