Structure of sperms: It will help students understand the process of reproduction better. Do you know how a female egg cell in the fallopian tube gets fertilised by the sperm cell? Do you know how a sperm cell travels all along the female reproductive part to reach the egg cell? The sperm cell has a tail that shows flagellated movement, and it helps it travel all along the female reproductive tract to reach the egg cell. 

The sperm comprises the head, neck, middle piece and tail, where the whole body is enveloped in a plasma membrane. The question arises, where do sperm cells get the energy to move? How does it break the protective layers of the egg cell? Read this article to get the answer to all these questions.

Structure of Human Sperm- Definition

The human sperm cell is the male germ cell that contains a haploid set of chromosomes that forms a diploid zygote when fused with an egg cell.

Fig: Human Sperm

Spermatogenesis results in the formation of haploid sperms. Spermatids formed during spermatogenesis change spermiogenesis, where a specialised structure is formed called sperm. Two types of sperms are produced, angiosperms and gymnosperms, Y and X containing sperms, respectively.

Both are \(50-50\) per cent. If angiosperms, i.e., Y containing sperms, are fused with the egg, the resulting offspring would be a boy, while if gymnosperms, i.e., X containing sperms, fuse with the egg, then the resulting offspring would be a girl. The chances of getting a girl or boy offspring are always \(50\%.\) Because X containing sperms are \(50\%\) of the total sperm present and the same for Y containing sperm.

Explain the Structure of Human Sperm with Labelled Diagram

Fig: Structure of a sperm cell

What is the Structure of Sperm?

Human sperm is a microscopic structure whose shape is like a tadpole. It has flagella which make it motile. Its diameter is \(2 – 5{\rm{ \mu m}},\) and its length is \(60{\rm{ \mu m}}.\) It is surrounded by a plasma membrane. It has no nutritional material. It lacks most cell organelles like ribosomes, endoplasmic reticulum, etc. The seminal plasma, along with the sperms, constitutes the semen.
The sperm comprises four parts, namely, the head, neck, middle piece, and tail.

Head: It is composed of an acrosome and a nucleus. The shape, size and structure of sperm heads vary significantly in different groups of vertebrates.  The nucleus of the sperm occupies a significant part of the head, and its shape ultimately determines the shape of the head of the sperm a cap-like structure called acrosome is present at the anterior end of the sperm nucleus.

The acrosome, a flattened sac, cap-like structure, lies at the tip of the nucleus. It is formed from the part of the Golgi apparatus of the spermatid. Acrosomal membrane binds acrosome, and it contains certain acrosomal polysaccharides like galactose, mannose, fructose, and hexosamine (Kopency, \(1976\)).

Acrosome consists of hyaluronidase enzyme, corona penetrating enzyme and acrosin. These are collectively called sperm lysin, which has the capacity to dissolve the coverings or cell membranes of the ovum. Corona penetrating enzymes digest the cells of the corona radiata layer. These cells are held together by hyaluronic acid. So, hyaluronidase dissolves this acid and cells are released. Acrosin digests the zona pellucida, a layer of the ovum. Therefore, these enzymes are secreted during fertilisation so that sperm can release its haploid nucleus into the ovum. This dissolution of layers of the egg on the arrival of the sperm is called an acrosomal reaction.

The nucleus lies below the acrosome. In humans, it is flat and oval. It is formed by the condensation of chromatin material and loss of RNA, nucleolus and RNA proteins. The nucleus contains its genetic material in the form of DNA and its associated proteins. The nucleus has become pointed anteriorly and posteriorly it has a depression.
The plasma membrane surrounds the complete head part. The upper half of the nucleus and acrosome is covered by one more membrane inside the plasma membrane called galea. The lower half of the nucleus, the neck region and the middle piece are covered by an extra membrane called Manchette. The galea and manchette are the additional coverings around the sperm.

Neck: This region comprises two centrioles, one proximal centriole and other distal centrioles formed from centrosomes of the spermatid. The Neck region is a small region with a narrow depression present below the nucleus. Proximal centriole lies at the depression of the posterior part of the nucleus, which is perpendicular to the main axis of the sperm.

It is responsible for spindle formation during the first cleavage of the zygote. The distal centriole lies behind the proximal centriole, but its axis is parallel to the longitudinal axis of the sperm. It acts as a basal body and gives rise to the axial filament of the tail of the sperm. Each centriole has a triplet structure and a \(9+0\) arrangement.

Middle Piece: After the neck is the middle piece. In the middle piece, spiral ribbon-like mitochondria are arranged called Nebenkern around the axial filament. It is cylindrical and lies behind the neck, and as it is composed of mitochondria, and we know mitochondria are the powerhouse of the cell, so it provides energy to the sperm cell for its motility which is essential for fertilisation.

Tail: It is the longest part of the sperm. It has an axial filament formed by the distal centriole. It is in a \(9+2\) pattern. The flagellum with which sperm moves forward and reaches the egg for fertilisation. Axial filament makes the skeletal structure of the tail. The cytoplasm is almost lost, but very little cytoplasm is present in the tail region.

Viability: The sperms retain the power of fertilising the egg only for \(24\) hrs. However, it can live longer, but its capability to fertilise is only for \(24\) hrs. Human males ejaculate about \(200-300\) million sperm during coitus, and the average sperm count is between \(40\) million and \(300\) million sperm per millilitre.

Sperm: Function

Sperm has the following functions-
1. It provides a pronucleus containing the haploid genome to the egg cell, which fuses with the haploid genome of the egg cell and forms a diploid zygote.
2. It delivers centriole, which helps in the formation of centrosomes.
3. It acts as a stimulating factor that activates the dormant oocyte.

Summary

Sperm is the male germ cell formed by the process of spermatogenesis. Spermatids formed during spermatogenesis change spermiogenesis, where a specialised structure is formed called sperm. The sperm consists of the head, neck, middle piece and tail. The head has a nucleus and acrosome. The nucleus consists of genetic material, while the acrosome lying on the nucleus has lytic enzymes.

The Golgi body forms an acrosome. The neck comprises two centrioles, one proximal and the other distal. The middle piece consists of mitochondria in the form of a ribbon. The tail consists of the axial skeleton, and it helps in its movement. Sperm genetic material fuses with the egg genetic material to form a diploid zygote.

FAQs on the Structure of Human Sperm

Frequently asked questions related to the structure of human sperm are listed as follows:

Q.1. What are the four main parts of a sperm?
Ans: The four main parts of sperm are the Head, Neck, Middle piece and Tail.

Q.2. What is the structure and function of sperm cells?
Ans: Human sperm is a microscopic structure whose shape is like a tadpole. It is a flagellated, microscopic, motile structure that has a head, neck, middle piece and tail. Its main function is to provide genetic material to the egg cell to form a diploid zygote.

Q.3. What is the size of human sperm?
Ans: Sperm’s diameter is \(2 – 5{\rm{ \mu m}},\) and length is \(60{\rm{ \mu m}},\)

Q.4. How many days can sperm live?
Ans: Sperms retain the power of fertilising the egg only for \(24\) hrs. However, it can live longer, even up to \(5\) days.

Q.5. Do sperm have eyes?
Ans: No, sperms do not have eyes, but it moves towards the egg cell with their tail due to the chemical stimulation from the egg cell.

We hope this detailed article on the Structure of human sperm helps you prepare. If you get stuck, do let us know in the comments section below, and we will get back to you at the earliest.