Introduction 

Sperm is the male gamete or the reproductive cell of a male. The word sperm comes from the Greek word sperma, which means “seed”. The sperm cell has a function similar to a seed, which is giving rise to a new similar organism. Usually, the sperm cells of animals are motile and sperm cells of plants are non-motile but there are some ferns that have motile sperms. The human sperm cell contains only 23 chromosomes which means that it is haploid. The sperm cells are formed in the seminiferous tubules of the male testes. This process is called spermatogenesis. 

Sperm cells have a very shorter life span. They cannot divide but when they get fused with the ovum, a zygote is made which then divides and becomes a complete organism. The epididymis is the place where these sperms are stored in humans. They are released during ejaculation through the penis. The fluid that contains sperm cells along with water is called semen. According to a study, seminiferous tubules produce about 300 million spermatozoa in a single day, but only 100 million survive and become mature, viable sperms.

Spermatogenesis

The process by which a haploid sperm cell develops from a germ cell is known as spermatogenesis. This process takes place in the seminiferous tubules of the testes of a male organism. The process starts from the spermatogonial cells which are present near the basement membrane of the seminiferous tubules. These cells divide and produce two types of cells. One type of cells replenishes the stem cells while the remaining cells differentiate into the spermatocytes. There are three stages of spermatogenesis which are discussed below. Spermatogenesis is a long process and take usually 70 days to complete.

Spermatocytogenesis

In the process of spermatocytogenesis, the stem cell or the spermatogonium divides and produces diploid primary spermatocytes. These two primary spermatocytes duplicate their DNAs before going into meiosis I. The process of duplication happens in the specific compartment of seminiferous tubules called “the adluminal compartment”. After that, when the DNA is duplicated, the primary spermatocyte divides into two secondary spermatocytes. These cells are haploid i.e. they contain only one set of chromosomes. 

The important thing to be noted here is that not all the spermatogonia develop into primary spermatocytes. Instead, these spermatogonia keep on dividing and making copies of themselves. This process helps in maintaining the continuous supply of spermatogonia for spermatogenesis. 

Spermatidogenesis

Spermatidogenesis is the process in which secondary spermatocytes change into spermatids. This happens when the secondary spermatocytes produce during meiosis I rapidly enter into meiosis II and divide. Four haploid spermatids are produced from two secondary spermatocytes. This all happens in a very short process. It is so rapid that secondary spermatocytes are not usually seen in histological studies.

Spermiogenesis

The spermatids produced during spermatidogenesis change into mature spermatozoa in a process called spermiogenesis. The process starts when one of the centrioles grow microtubules. These microtubules then take the shape of a tail. The anterior part of the tail is thick due to the arrangement of mitochondria around the microtubules. Mitochondria ensure the supply of energy to sperm cells. 

The DNA of the spermatid changes into a highly condensed form. This tightly packed and condensed chromatin is surrounded by the Golgi apparatus. This structure is then called the acrosome. The packed chromatin is transcriptionally inactive. 

The process of maturation includes removing unnecessary cytoplasm and organelles. The level of testosterone greatly affects this maturation process. Sertoli cells remove excess cytoplasm from non-motile spermatozoa and release them into the seminiferous tubules. This process is called spermiation. The mature spermatozoa gain motility in the epididymis and become capable of fertilization.

Structure

The sperm cell is usually divided into three parts. These are the head, the midpiece, and the tail. Some scientists divide it into only two parts; the head and the tail. 

Head

The head is the most important structure of the sperm cell as it contains the nucleus where the genetic information is stored. The head is oval in shape. It has a tapering apex and a broad base due to which it resembles an egg. The sperm head has a cap-like structure over its interior half. This cap is produced by the Golgi apparatus and it is called the acrosome. 

The acrosome has two membranes. It contains several important digestive enzymes such as acrosin enzyme and hyaluronidase in the acrosomal matrix. Mannose, hexosamine, galactose, and other polysaccharides may also be present in the acrosome. The enzymes play an important role in fertilization as they break down the outer membrane i.e. zona pellucida of the ovum. 

The head also contains some vacuoles. These vacuoles were initially thought of as nuclear holes but now they are described as nuclear vacuoles. These vacuoles are associated with the failure of chromatin condensation. They may also increase the susceptibility to DNA damage. The nucleus of the human sperm cell is haploid which means that it contains only 23 chromosomes. The chromatin fibers present in the nucleus are densely coiled. During the fertilization process, the nucleus of the sperm cell fuses with the ovum and produces a diploid zygote which then continues to divide and eventually become a complete organism. 

Midpiece

The central thick part of the sperm cell between the head and the tail is called the body or the midpiece. It is a relatively short structure and makes up only 10 percent of the total length of the sperm cell. It is sometimes described as part of the tail of the sperm cell. 

The main function of the midpiece is to provide energy to the sperm cell as it contains tightly packed mitochondria. The mitochondria use glucose molecules and produce energy in the form of adenosine triphosphate, ATP. This energy is required for the swimming of the sperm cell in the reproductive system of a female. 

Between the midpiece and the head of the sperm cell, there lies a structure called centriole. The centriole plays an important role in the formation of sperm aster and zygote aster. Moreover, the mitotic apparatus produced by the centriole plays an important part in the cell division. 

Tail

The tail of the sperm cell is also called the flagellum. It is the longest part of the sperm cell and makes up about 80 percent of the entire length of the cell. The function of the tail is to propel the sperm cell for swimming in the female cervix, uterus, and uterine tubes.  

Previously, scientists used to say that it moves in a helical shape. But according to a new study, the movement of the tail is a mixture of asymmetrical standing, rotating, and traveling waves. This combination of different movement styles helps the sperm cell to achieve symmetry. The tail is made up of protein fibers. The long axial filament of the tail is called the principal piece. The principal piece along with the end piece of the tail help in generating the wavelike motion which is necessary for the swimming of the sperm cell. 

Importance of the Structure

The sperm cell has a streamlined structure which is helpful in swimming in the female reproductive cell. Due to its structure, the sperm cell has a high speed and it moves rapidly to reach the ovum of the female. The tapering apex of the head of the sperm cell helps in reducing the drag force. 

The female vaginal canal is acidic so there is a chance of the denaturation of the sperm cell. But nature has given the sperm cell some basic amines which help in the protection of the sperm cell. These basic amines such as cadaverine and spermine are important for successful fertilization.  The acrosome is also an important structure that helps in the chemotaxis to identify female gamete. It also contains the enzymes for the breakdown of the outer layer of the ovum. 

Acrosome Reaction

The anterior cap-like structure of the sperm cell is called the acrosome. When the sperm reaches the ovum, the enzymes in the acrosome break down the thick membrane or the zona pellucida of the ovum. Before this process, the sperm must go through a reaction known as the acrosome reaction. 

The reaction takes place in the ampulla of the fallopian tube of a female organism. When the sperm reaches the secondary oocyte, the plasma membrane of the head and the membrane surrounding the acrosome fuses. This process results in the release of acrosomal enzymes such as hyaluronidase and acrosin. Acrosin enzyme is present in its inactive form i.e. proacrosin, but when it comes in contact with the glycoproteins of the ovum membrane, it becomes active and starts breaking down the zona pellucida.  

Sperm Quality

Sperm quality along with sperm quantity are basically the parameters of semen quality. Semen quality refers to the ability of semen to accomplish fertilization. It depends on various factors like age, heat, and chemicals, etc. According to various studies, the genetic quality and the volume of the sperm cells decrease with the age.

The size of the sperm is also important in measuring sperm quality. Only the sperm cells having longer lengths can displace their competitors and reach the female secondary oocyte. Shorter sperm cells are considered bad as for as sperm quality is concerned. That’s why healthy males who can produce longer sperm cells in sufficient quantities are able to accomplish successful fertilization.

Sperm Donation

Sperm donation is a process in which a man provides his sperms. These sperms may be donated for the ‘fertility treatment’ of a woman or for artificial insemination, a process in which sperms are introduced in the female’s cervix by means other than sexual intercourse. The remaining course of pregnancy is the same as in the normal pregnancy achieved by sexual intercourse.

The man who donates his sperms is the biological father but not the legal father of the child as he gives up all the legal rights to any child produced. The sperms may be donated either directly or through a sperm bank. Sperm banks store the sperms and use them when they are required. Some sperm banks are so big that they may hold up to 170 litters of sperm. 

Summary

Sperm is the male gamete or reproductive cell which contains one set of chromosomes. It is released from the male reproductive organ into the female reproductive organ to fuse with the ovum and produce a diploid zygote. The human sperm cells are produced in the seminiferous tubules of the testes. The formation of the sperm cells is known as spermatogenesis. 

Spermatogenesis is divided into three phases. In the first phase, the stem cells are differentiated first into primary spermatocytes and then into secondary spermatocyte. In the second and third phases, these spermatocytes are changed into spermatids and then mature sperms in a process known as spermiation. These mature sperms are stored in the epididymis from where they are released through the penis in a process called ejaculation.

The structure of the sperm cell is unique and consists of a head, a midpiece, and a tail. The head houses the haploid nucleus and it also contains a cap-like structure called an acrosome. The acrosome contains enzymes such as acrosin which are necessary for the fertilization process as they breakdown the outer thick membrane of the ovum. The oval shape of the head reduces the drag force and helps the sperm cell in reaching the female gamete. 

The midpiece and the tail of the sperm help in the wave-like motion of the sperm in female reproductive organs. The tail is made up of the protein fibers and it is also called flagellum. The mitochondria present in the midpiece provide energy to the sperm cell by breaking down the glucose molecules which help in the movement of the sperm cell in the female’s cervix and uterus.

The sperm quality is a measure of the ability of the sperm cell to accomplish fertilization. It depends on many factors including age and temperature. The longer is the size of sperm cells, the more will be the sperm quality. 

References

1.  “Animal reproductive system – Male systems”. Encyclopedia Britannica. Retrieved 2020-02-20.
2. Boitrelle, F; Guthauser, B; Alter, L; Bailly, M; Wainer, R; Vialard, F; Albert, M; Selva, J (2013). “The nature of human sperm head vacuoles: a systematic literature review”. Basic Clin Androl. 23: 3. doi:10.1186/2051-4190-23-3. PMC 4346294. PMID 25780567.
3. Fawcett, D. W. (1981) Sperm Flagellum. In: The Cell. D. W. Fawcett. Philadelphia, W. B. Saunders Company. 14: pp. 604-640.
4. Lehti, M. S. and A. Sironen (2017). “Formation and function of sperm tail structures in association with sperm motility defects.” Bi

Ishijima, Sumio; Oshio, Shigeru; Mohri, Hideo (1986). “Flagellar movement of human spermatozoa”. Gamete Research. 13 (3): 185–197. doi:10.1002/mrd.1120130302