Structure of a Sperm Cell
The sperm cell is a reproductive cell. The structure of a sperm cell or a spermatozoon is ideally suited to its functions. The function of a spermatozoon is to reach up to the ovum, fuse with it, spermatozoa initiate early embryonic development in the egg and transfer paternal genes to the developing ovum. To accomplish its function the spermatozoon must be highly motile.
In reality, the structure of a typical spermatozoon is comparable to an efficient, active and highly motile body.
A typical sperm cell or spermatozoon is divisible into three distinct regions; head, middle piece and tail (or flagellum).
The anterior tip of the round (or oval) head of a sperm cell is modified into a structure called the acrosome. The function of acrosome is to penetrate the egg membranes and allow the sperm to establish contact with the egg cytoplasm during fertilization. A large part of the remainder of the head is occupied by the nucleus. Since the nucleus contains genes, it is responsible for the transfer of heritable paternal characters to the developing egg.
In the posterior region of the head, centrioles are present. The first centriole is arranged at right angles to the longitudinal axis of the sperm. It is called the proximal centriole and it is essential for the induction of cell division in the fertilized egg. Immediately behind, the second or distal centriole is located. From the distal centriole arises the axis filament of the tail and thus it forms the basal granule of the axial filament.
In the middle piece of the sperm cell the base of the tail or flagellum is located. The base of the tail is formed by distal centriole which lies just below the proximal centriole but in the middle piece. The distal centriole is placed in the longitudinal axis of the sperm and from it originates the axial filament of the flagellum (tail). Therefore, it forms the basal granule of the axial filament. In the mid-piece, both the distal centriole and the axial filament of the flagellum are surrounded by mitochondria which are fused together to a greater or lesser degree.
In mammals, the mitochondria fuse together to form a helical coil around the axial filament. In the other groups of animals the fused mitochondria may be present in the mid-piece as one or more lobes which are known as mitochondrial bodies.
The mitochondria contain oxidative enzymes and enzymes for oxidative phosphorylation which are responsible for providing the necessary energy to the highly motile active flagellum or tail of the spermatozoon.
The cytoplasm surrounds the middle piece as a thin layer and is known as manchette. Manchette also surrounds the posterior part of the head of the spermatozoon.
In the spermatozoa of some animals, a dark ring is present at the posterior end of the middle piece. It is sometimes referred to as the ring centriole and it forms the boundary between the middle piece and the tail. Electron microscopic studies have shown that in structure the ring does not resemble the centriole. The function of this body is also not known.
Tail or flagellum is the longest part of the spermatozoon. With the help of the tail, the spermatozoon actively swims about. The most important part of the tail is the axial filament. The microscopic structure of the axial filament is similar to the cilium and flagellum of other forms.
The entire spermatozoon is covered by an outer cytoplasmic layer or plasmalemma. Different animals exhibit different structural types of spermatozoa or sperm cell.
If you are interested in biology also read this notes – Reproductive cycle of male and female Easy Notes