Each of our cells contains 23 pairs of chromosomes, for a total of 46 chromosomes in a healthy body cell. But sperm cells, like the egg cells that fertilize them, have half as many chromosomes as normal cells.
Scientists recently discovered evidence that sperm cells arrange their chromosomes in a very specific way. This is called chromatin condensation.
The Head
The head of a sperm cell (or spermatozoon) contains the condensed haploid nucleus. It also has a cap-like structure over it, called an acrosome. The acrosome releases enzymes that break down the outer layer of an egg known as zona pellucida and help sperm penetrate it to fertilize it. The sperm head is oval in shape, which reduces drag on the sperm cell as it moves through seminal fluid and the body’s tubules.
The middle section of a sperm cell contains energy-producing mitochondria and is surrounded by a plasma membrane. This membrane protects the chromosomes inside from oxygen and other harmful chemicals. The tail of a sperm cell is the longest part of the cell, and it makes up about 80 percent of the sperm’s total length. The tail propels a sperm cell through the female reproductive tract to an egg for fertilization.
Sperm cells have 23 pairs of chromosomes, half as many as regular cells in the human body. When an egg and sperm meet, they swap a pair of their 23 chromosomes with each other. This process is called meiosis.
In a normal cell, chromosomes are lined up in tandem—head-to-tail—in a specific order. But in a sperm cell, the chromosomes are grouped together in a less precise manner, with some loosely arranged genes and some that are closer to each other. This allows the chromosomes to be more easily released by the acrosome when the sperm hits the egg.
The Midpiece
The head of the sperm cell contains the extremely compact haploid nucleus and also has a cap-like structure called an acrosome that contains enzymes that help it enter the egg. Its oval shape reduces the drag force on sperm and helps it to move through a woman’s cervix, uterus and tubes. A teaspoon of semen may contain up to 500 million sperm cells.
The midpiece is 1-1.5 times the length of the head and contains dense outer longitudinal fibers that encircle a central axoneme. The axoneme is lined with mitochondria, the organelles that produce energy, known as ATP. The mitochondria give sperm its ability to swim and propel itself through the female reproductive tract.
After fertilization, the egg and sperm swap their DNA with each other through a process called meiosis. This involves the two homologous chromosomes separating into two identical daughter cells, with each having 23 chromosomes (23 pairs). The newly separated chromosomes have the same genes, but with small variations in their DNA letters.
Then the sperm and egg fuse to form a single fertilized cell, which has 46 chromosomes (46 pairs). The new cell contains 23 autosomes, which come in pairs, as well as two sex chromosomes that determine whether a baby will be male or female. The sex chromosomes are labeled X and Y, with the X chromosome coming from the father and the Y chromosome coming from the mother.
The Tail
The image of sperm cells moving their long tails in search of an egg is well-known. But the tail’s role in sperm propulsion has remained mysterious until now. A team of scientists from the UK have used high-speed microscopy to track sperm cells and mathematical analyses to break their movements down into two components. The results suggest that a sperm cell’s tail may be powered by a wiggle of one side of the cell, rather than its entire length as previously thought.
Mature sperm, also known as spermatozoon, are male reproductive cells produced in the testes. They carry the genetic information of a man’s father and unites with (fertilizes) an egg in the female reproductive tract to produce offspring. A sperm cell contains 23 of a man’s chromosomes, and 23 of his mother’s. The two sets of chromosomes are then swapped around in the cell to give each new reproductive cell half of the mother’s chromosomes and half of the father’s.
This swapping process is called meiosis and occurs only in cells that are going to become sperm or eggs. The newly recombined homologous chromosome pairs, each with its own unique combination of DNA letters, then join together to make a single fertilized egg cell, or zygote. The egg and sperm cell both have 46 chromosomes (23 each), which is twice as many as regular human cells, and they are now ready to grow into a baby.
The Nucleus
The head (spermatozoon) of a human sperm cell contains a condensed haploid nucleus. It also houses a cap-like structure called the acrosome which is necessary for the fertilization process as it contains enzymes that will break down the outer layer of the ovum. The acrosome and the nucleus are the only parts of a sperm cell that are visible under a microscope.
All cells in the body contain 23 pairs of chromosomes, but only some cells carry all the information needed to create a baby: the egg in a woman and a man’s sperm cell. These are called haploid cells because they only have half the number of chromosomes found in non-sex cells – 23 from the mother and 23 from the father.
Each sperm cell also has an additional 23 chromosomes that are unique to it. These chromosomes determine if the child will be male or female. When the sperm cell joins with an egg in a woman’s body, the genes from both the sperm and the egg will be combined into one single cell. This cell will then go on to become a new, fertilized egg that will then create the next generation of humans.
Every living thing – including your muscles, skin, blood, nerves, and bones – are made of living cells. These cells are tiny, living building blocks that keep making more cells as they get older and die. Cells are the tiny, rod-shaped structures that carry all the instructions for how your body functions – the DNA that contains 23 pairs of chromosomes.
