Sperm is a male reproductive cell responsible for fertilising the egg (female reproductive cell) to produce a zygote, further developing into a complete fetus. 

Multiple black thread-like structures around a black sphere

 Sperms surrounding an egg

A male heterogamete cell is called sperm in most animals, including humans, whereas in plants, they are known as pollen grains. The sperm is mobile, while the egg is stationary. Therefore, the sperm swim towards the egg cell to fuse and fertilise to form a zygote, finally developing into a young offspring. The sperm cells are developed and matured in the testes, a male reproductive organ in humans. In plants, the sperm cell or male gametes are formed in the pollen grain developed in the anther; part of the male reproductive organ called the stamen. The sperm cells in humans and plants can be distinguished based on their size and appearance, as they are morphologically dissimilar. However, the function of both the sperm cells remains the same. Humans are unisexual, while plants can be unisexual and bisexual across various plant species. 

Spermatogenesis in Humans 

Spermatogenesis is the formation and development of sperms in the testes (a male reproductive organ). The testes consist of thinly coiled seminiferous tubules where the sperms are produced. Sertoli cells present in the walls of seminiferous tubules provide nourishment for the immature sperm cells and assist in the transportation of these sperm cells to the central channel of the seminiferous tubule. Meanwhile, spermatogenesis takes place between the Sertoli cells. 

Flowchart of formation of sperm


 Spermatogenesis through meiotic cell division 

Spermatogonium (germ cell) is an early-stage cell in the formation of sperm or spermatozoa produced in the seminiferous tubules. The diploid spermatogonium undergoes mitotic cell division to additional diploid spermatogonia. Whereas a single spermatogonium transforms into a diploid primary spermatocyte containing 46 chromosomes duplicated into a total of 92 chromosomes. This spermatocyte undergoes first meiotic cell division to produce two diploid (46 chromosomes each) secondary spermatocytes, which divide by meiosis to make four haploid spermatids, containing 23 chromosomes in each spermatid. The spermatids further mature into a haploid spermatozoa or sperm. The sperms are then ejected through the urethra via vas deferens. A single ejaculation contains about 15 million sperm cells in one milliliter of semen in a healthy male. It takes around 74 days for the immature spermatogonium to mature into spermatozoa. 

Aspermia is a condition where some males fail to produce sperms. whereas oligospermia is a condition where the males have low count sperms (<15 million sperms per milliliter of semen). There are multiple causes leading to this condition, such as prostate cancer, diabetes, high blood pressure, etc. therefore; it is essential to have a healthy lifestyle to reduce any chance of probable cause of these conditions. 

Structure of sperm

The sperm is a haploid cell consisting of a head, mid-piece, and tail. The Head includes an acrosome and a nucleus. Acrosome contains enzymes that play a role in penetrating the cell membrane of a female egg, causing fertilisation. At the same time, the nucleus contains the genetic material that is transferred into the egg cell and combined to form a single zygote. The nucleus contains 22 autosomal chromosomes and a single sex chromosome. 

Blue coloured structure of a differentiated cell

 structure of a human sperm

The midpiece contains mitochondria responsible for generating ATP (adenosine triphosphate) energy. Since the sperm is motile, they require energy to travel towards the egg cell. Finally, the tail, also known as the flagellum, assists the sperm in driving forward. 

The sperms thrive under moist and warm conditions and therefore survive longer than in dry conditions. Sperm can survive for five days inside the body of a female. It has a milky appearance, increased density compared to water, and is viscous. Additionally, the pH of sperm ranges from 7.2 to 8. Further, the male sperm in humans are distinguished from the animals based on the shapes, sizes, and amounts produced, although the functions are the same.

Gametogenesis in plants

A pollen grain in plants contains two male gametes (sex cells). The male reproductive system of a flowering plant is called the stamen, which consists of a filament and an anther. The filament is responsible for supporting the anther. Anther sacs contain pollen grains that carry the male gametes. The primary function of the pollen grain is to transport male gametes into the ovule contained inside the ovary of the female reproductive system. 

A pollen grain falls into the sticky part of the female reproductive system (pistil)  called stigma through pollination. A pollen tube is formed through the stigma, and style finally reaches the ovary. The pollen grain then releases the two male gametes that travel through the pollen tube, thus entering inside the ovary. The two gametes then reach the ovule and fertilise with the egg cell inside the ovule to form a zygote that develops into a seed. 

Flowchart of development of pollen

Detailed developmental stages of a pollen grain from a microsporocyte

The structure of pollen grain consists of an exine and intine. The intine can be easily broken down as opposed to the exine. Exine contains sporopollenins that are very sustainable under harsh and adverse conditions and resistant to disintegration, therefore are responsible for preserving the male gametes. In contrast, the intine is the inner layer of the pollen grain. It consists of cellulose, cytoplasm, and two male gametes. 

Pollen grains are produced by meiotic cell division from a diploid microsporocyte. The diploid nucleus of the microsporocyte undergoes meiotic cell division twice to develop into a tetrad containing four haploid microspores, called microsporogenesis. Each microspore is the pollen that undergoes mitotic cell division twice to transform into tricellular pollen containing two male gametes and a vegetative nucleus. The process of transforming from a single microspore into a mature pollen grain is called microgametogenesis. 

Asha Rachel Thomas


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