Short notes on the structure of the human sperm
The sperm in human beings is a highly specialised structure. Also called male gamete, several thousand of them are produced and contained in a fluid called semen before it is transfered to the female for the process of fertilization. A mature sperm (also called spermatozoa) consists of the following four parts –
The head is almost concial in shape and is formed of acrosome and nucleus.
This is found at the anterior tip of the sperm (Gr.akron = extremity; soma = body). The acrosome forms a cap like structure called the head cap. This occupies the space between anterior half of the nucleus and the plasma membrane of the sperm tip. In its origin (during spermatogenesis), the acrosome is formed from the golig complex.
The acrosome itself is bounded by a unit membrane. It consists of a number of hydrolytic enzymes such as acid phosphatase, hyaluronidase and others. These enzymes help in tissue lysis (dissolving) and this facilitates the penetration of the sperm into the egg membrane. The enzymes are protolytic and help in dissolving the egg membrane.
The nucleus occupies most of the available space of the sperm head. It is the shape of the nucleus that ultimately decides the shape of the sperm head. Structurally it is enveloped by a nuclear membrane. Sometimes however the posterior part of nuclear membrane (towards the body of the sperm) is somewhat depressed to accommodate the proximal centriole. The nucleus consists of DNA as well as basic proteins. There is no nucleolus or any fluid contents.
The head is followed by a short neck to separate the middle piece of the sperm. The neck consists of just two granules (centrioles). These granules are called the proximal centriole and the distal centriole. Both these granules are situated very close and lie in the posterior depression of the sperm neck. The two centrioles enter the egg at the time of fertilization along with the nucleus. These two centrioles are necessary to initiate division in the zygote.
It is known that the centrioles help the zygotic division by forming the first mitotic spindle. The posterior or the distal centriole is responsible for the formation of the microtubules of the sperm tail.
3. Middle piece:
The middle piece of the sperm consists of the upper portion of the axial filament and in its structure it has the same organisation as the axial filament of any flagellum. It has a pair of longitudinal fibres called beta fibres surrounded by a ring of nine pairs of longitudinal fibres called alpha fibres. In human sperms, the alpha fibres of axial filament are accompanied on the outside by 9, much thicker fibres called gamma fibres or coarse fibres. The alpha, beta and gamma fibres are the sites of various enzymes.
For instance alpha fibres have ATPase enzyme, while beta fibres have acetylchosuccinic dehydrogenese. These fibres are anchored to the distal centrioles. The fibres are surrounded by the mitochondria. Very often the mitochondria are fused together and form a spiral sheet that surrounds the axonemal fibres. Around the periphery of mid piece of the sperm is found a thin sheet of cytoplasm mainly composed of microtubules. This layer is called manchettee.
The tail usually is the longest part in the sperm. In human beings it is about 55JJ. long. It consists of two main parts – the principal piece and the end piece. The principal piece which constitutes most of the length of tail consists of the central core made up of axial filaments with a 9+2 arrangement (2 central, 9 peripheral).
Surrounding this core is a fibrous tail sheath which often appears as semicircular ribs oriented at right angles to the long axis of the filament. Sometimes they appear as helical coils. In human beings two of the gamma fibres are fused with the surrounding ribs to form anterior and posterior columns extending throughout the length of the principle piece.
This arrangement divides the principal piece into two functional compartments – one having three gamma fibres and the other containing four. This symmetry is thought to help in a more powerful stroke of the tail in one direction. This is called the power stroke. The end piece is a small tapering portion of the tail containing only the axial filament covered with cytoplasm and plasma membrane. There is not stored food in the sperm. It also does not have cytoplasmic organelles such as ribosome’s and endoplasmic reticulum.