In humans, there are two types of gametes been produced: in females, non-motile egg and in males, motile sperm.
Millions of sperms race against each other to fertilize the egg. Out of millions of sperms, one sperm wins and succeeds in fertilizing the egg. When the nucleus of a human sperm fuses with egg, the process is termed as fertilization.
In this article, we will study the structure of Human Sperm in details and know about the functions each part of Human Sperm. We will also answer interesting facts related to Human Sperm and fertilization process such as:
- What is so unique about every sperm which makes it faster than the other sperms?
- Once a sperm fertilizes an egg, how does it prevent other sperms from entering the egg? How is polyspermy prevented?
- How is sex of a child determined?
- Is it correct to blame women for giving birth to only girl child?
STRUCTURE OF HUMAN SPERM
A human sperm consists of following parts:
- Head: Acrosome, Nucleus
- Middle Piece: Mitochondria
- Axial Filament
- Tail
Functions of main part of Human Sperm:
The functions of each part of Human Sperm are as follows:
- Head
Head of a sperm consists of two parts:
a) Acrosome b) Nucleus
a) Acrosome:
Acrosome is present at the head of the sperm. The special feature of acrosome is that it secretes an enzyme called Hyaluronidase which dissolves the ovum wall thus facilitating entry of the sperm into the egg.
b) Nucleus:
The nucleus of the sperm forms the genetic material consisting of 23 chromosomes (22 chromosomes + X or Y chromosome). When the sperm fertilizes egg, the nucleus of sperm fuses with nucleus of egg thus 23 chromosomes of sperms combines with 23 chromosomes of egg forming a zygote having 46 chromosomes.
Number of Chromosomes:
Sperm = 23 (22 + X chromosome or Y Chromosome)
Egg = 23
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Zygote = 46 chromosomes (0r 23 pairs of chromosomes)
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How is sex of a child determined? Is it correct to blame women for giving birth to only girl child?
If a sperm nucleus containing X chromosome fertilizes with the egg, then a girl child is born and if a sperm nucleus containing Y chromosome fertilizes with the egg, then a boy child is born. Hence sex of a child is determined by the type of sperm (X oY chromosome) which fertilizes the egg.
Biologically speaking, the sex of a child will be determined by the male sperm which fertilizes the egg where no role will be played by the female egg in terms of sex determination of a child. Hence blaming women for giving birth only to girl child is not correct. Moreover, even the man does not have any control as to which type of sperm (X or Y chromosome) will fertilize the egg, hence blaming any of the partner for gender determination of a child is wrong and baseless.
2. Mitochondria
The middle piece of human sperm consists of Mitochondria also known as "Power House of the cell".
Mitochondria provides energy (ATP - Adenosine Triphosphate) for the sperm to swim through the cervix upto the oviducts to fertilize the egg.
The mitochondria provide energy to the sperm's tail for its lashing movements.
3. Axial Filament or Flagellum
The core of the flagellum consists of axoneme surrounded by outer 9 dense fibers. Axoneme itself consists of 2 microtubules singlets in the center which are surrounded by 9 microtubules doublets (9+2 arrangement).
Flagellum is response for the flagellar movement of the tail of the sperm which helps sperm to move forward. The energy requirement for flagellar movement is derived from Mitochondria present the middle piece of the sperm through ATP hydrolysis.
*Note: Flagellar movement is driven by dynein motor protein.
4. Tail
Tail is present at the end of human sperm. The tail provides the propulsion effect to the sperm through its flagellar movements during sperm's swimming journey towards the egg.
How is polyspermy prevented in Humans? After one sperm has entered the egg, how other sperms are prevented from entering the egg?
There are two mechanisms which ensures that only one sperm fertilizes the egg:
a) Rapid depolarization of plasma membrane of the egg
b) Cortical reaction by Ca2+ wave or oscillations
When a sperm fuses with the egg, it causes rapid depolarization of egg's plasma membrane thereby preventing entry of other sperms, thus acting as the primary block to polyspermy. It is done through local increase in cytosolic Ca2+ which spreads through the cell in a wave.
The egg's cell membrane is repolarized after a while, thus a secondary process is needed to prevent polyspermy in long run which is achieved through Egg Cortical Reaction. The local increase in cytosolic Ca2+ is followed by a prolonged Ca2+ wave or oscillations.
The Ca2+ wave or oscillations activate the egg to begin development, and they initiate the cortical reaction, in which the cortical granules release their contents by exocytosis. The contents of the cortical granules include various enzymes that are released by the cortical reaction and change the structure of the zona pellucida. The altered zona becomes “hardened,” so that sperm no longer bind to it, and it therefore provides a block to polyspermy.
There is evidence that the Ca2+ wave or oscillations are induced by a protein that is introduced into the egg by the sperm, but the nature of the protein is unknown.
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