The acrosome of a sperm

Sperm Acrosome | function of acrosome

What is acrosome?

Structure of Sperm

The sperm is composed of a head, middle piece, and tail. The plasma membrane encloses the whole body of sperm. The acrosome is present in the head of the sperm.

Head – (contains acrosome)

• The head of the sperm contains the DNA, which when combined with the egg’s DNA, will create a new individual. The head also contains many enzymes. These help the sperm to break through the cell membrane of an egg. The tip of the sperm head is the portion called the acrosome, which enables the sperm to penetrate the egg.
• The head of the sperm varies in shape for each animal species. In humans, it is flattened and almond-shaped, four to five micrometers long and two to three micrometers wide (there are about 25,000 micrometers in an inch).
• The head portion is mainly a cell nucleus; it consists of genetic substances, called chromosomes, and these genetic substances are responsible for transmitting specific characteristics of an individual, such as their eye color, hair, and skin.

1. Acrosome

• The acrosome is a cap that covers the head of the sperm and it contain enzymes that break down the outer membrane of an egg cell, allowing the sperm to fertilize the egg. Only one sperm fertilizes each egg, even though 300,000,000 to 400,000,000 sperm are contained in an average ejaculation.
• Each egg and sperm produced has slightly different genetic information carried in the chromosomes; this accounts for the differences and similarities between children of the same parents.

2. Plasma Membrane

• The plasma membrane of sperm is rich in polyunsaturated fatty acids. It is also rich in a variety of proteins, and its function is associated with sperm capacitation, acrosome reaction, and sperm-egg fusion.
• The plasma membrane of sperm is designed for a number of adhesion and fusion events leading to fertilization at the end.

3. Nucleus

• The sperm head is the part of the cell that contains the nucleus. The nucleus takes up 65 percent of the head and consists of 23 chromosomes. The nucleus holds the DNA of the cell.
• After the sperm cell enters the egg, the chromosomes combine with the female gamete and makeup 46 chromosomes. These 46 chromosomes together determine the characteristics of the new organism (fetus, etc).
  You can read about How many chromosomes do people have? in this linked article.
  
• During the process of fertilization, the sperm enters the ovum and the sperm cell becomes a pronucleus. A pronucleus is the nucleus of a sperm or an egg cell.

Mid Piece

• The midpiece is the central part of the sperm cell and is present between the head and the tail of the sperm. This midpiece of the sperm is packed with mitochondria.

1. Centriole

• The centriole is part of the sperm cell located between the head and the midpiece. In a complex referred to as the centriole-centrosome complex, the centriole is involved in the formation of sperm aster and zygote aster.
  Recommended Read – The Role of Sperm Centrioles in Human Reproduction

2. Mitochondria

• Mitochondria are organelles in cells that produce energy. Sperm use the energy in the midpiece to move. Mitochondria supplies the energy which the tail needs to move. In fact, it provides the energy required for swimming.

Tail

• The sperm tail is a thin, elongated structure that makes up about 80 percent of the entire length of the sperm. The tail helps in the sperm cell movement.
• By beating their tails, sperm cells swim to the female reproductive cell (oocyte – an immature egg) and fertilize it.

1. Flagellum

• The tail of the sperm, sometimes called the flagellum, is a hairlike bundle of filaments that connects to the head and middle portion.

• The flagellum of the sperm provides a whip-like movement in order to move it forward. The sperm cannot swim backward due to the nature of the flagellar movement and propulsion. The sperm consists of a flagellum of around 50 µm in length.
  
  Must Read – Hydrodynamic propulsion of human sperm.
  

Acrosome Reaction

• Sperm cells go through a process known as the acrosome reaction, which is the reaction that occurs in the acrosome of the sperm as it approaches the egg.

• The acrosome is a cap-like structure over the anterior half of the sperm’s head. As the sperm approaches the zona pellucida of the egg, which is necessary for initiating the acrosome reaction, the membrane surrounding the acrosome fuses with the plasma membrane of the oocyte, exposing the contents of the acrosome. The contents include surface antigens and numerous enzymes which are responsible for breaking through the egg’s tough coating and allowing fertilization to occur. This reaction is triggered by the release of lysin which is a protein that creates a hole in the envelope of the egg thereby allowing the sperm to pass through the envelope and fuse with the egg.

• Acrosome reaction has two main outcomes.
  a. Release of enzymes that degrade the zona pellucida surrounding the plasma membrane of the female gamete (at the zona pellucida)
  b. Exposing the sperm head (the nucleus) to the egg.

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