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Read this article to get information about the Structure, Functions and Types of Mature Sperm!
The spermatozoa of different groups of animals exhibit great variety of form. Those of chordates, however, show a remarkable uniformity of structure.
The spermatozoon, which performs the function of carrying genetic material from the male to the oocyte, consists of two principal parts= head and tail.
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The tail consists of four components—neck, mid-piece, principal piece and end piece (Phillips, 1975).
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Both of these parts of spermatozoon i.e. head and tail are contained as in living cells, in a continuous plasma membrane. The whole cell of sperm is streamlined and paired down for action of a special sort and of limited duration, namely, to swim and to meet an egg, to fuse with the cortex of an egg, and to introduce sperms nucleus and Centriole in the egg interior.
Structure of head of sperm:
The sperm head consists of mainly the nucleus and acrosome. Its shape, size and structure vary greatly in different groups of vertebrates (Fig. 7.) The head of sperm performs two functions—genetic and activation.
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The genetic function is embodied in the sperm nucleus which consists almost entirely of DNA plus nuclear proteins and thus is responsible for the transmission of hereditary characters from the male. The nucleus of the sperm occupies major part of the head and its shape, ultimately, determines the shape of the head of sperm. At the anterior end of the sperm nucleus occurs a cap-like structure called acrosome.
The shape and size of the acrosome vary among different species. The acrosome is bounded by an acrosomal membrane and it contains certain acrosomal polysaccharides like galactose, mannose, fructose, and hexosmine (Kopency, 1976). A large number of enzymes especially hydrolases are also present in the acrosome. It also contains two most important enzymes such as hyaluronidase and zona Iysin or acrosin which functions during sperm entry into the ovum (Nelson, 1985).
In the sperm of some animals such as fowl, there occurs a cone-shaped structure called axial body or acrosomal cone in between acrosome and nucleus (Fig. 8 A). The acrosomal cone develops into an acrosomal filament at the time of fertilization. Very little cytoplasm occurs in between nucleus and plasma membrane of sperm head. The cytoplasm in between the membrane of acrosomal vesicle and sperm plasma membrane is called periacrosomal cytoplasm.
Structure of Tail of Spermatozoon:
The motor apparatus of the spermatozoon tail is the axoneme or axial filament complex which consists of the usual central pair or axial fibril (or microtubules) surrounded by an inner row of nine evenly spaced doublet microtubules, each with two rows of arms that project towards the adjacent double tubule, one row of radial spokes that radiate inwards towards the central pair of microtubules, with outer ring of nine coarse longitudinal fibres (Guraya, 1987).
Actually, all the structural components of the flagellum, which include the connecting piece, 9 + 2 axoneme, fibrous sheath and outer dense fibres, are structurally interlocked into one functional unit.
A central sheath, made up of projections has been described surrounding the two central tubules. It is connected by nine spokes, radial links to the nine doublets, which are also connected to each other by inter doublet links. (Fig. 8 d).
From morphological point of view tail consists of the following sub divisions:
1. Neck:
The neck is a short, slightly constricted segment made up of projections located between the base of the head and the first gyre of the mitochondrial helix of the middle piece. The neck differs clearly from the head and also from the rest part of the tail (i.e. midpiece, principal piece and end piece.) in certain morphological features of plasmalemma, a sharp demarcation of its upper limit by the posterior ring, and lack of continuity between the segmented columns and the outer dener’s fibers of the tail (Fig- 8 A). The two Centrioles lie at right angles to each other are proximal and distal Centriole.
The distal Centriole forms and gives attachment to the axial filament of the sperm tail; the proximal Centriole has no active function in the spermatozoon but is a potential activist within an egg during first cleavge division of the fertilized egg.
Two or three mitochondria are also present in the neck. These generally establish close relationship with either end of the proximal Centriole by wrapping around the lateral surface of the latter. These mitochondria are continuous with the uppermost mitochondria of the mid piece helix.
2. Middle piece:
Anatomically, the mitochondrial sheath and the outer ring of coarse fibres characterize the mammalian sperm mid piece. It is that part of the flagellum which lies between the neck and annulus and forms the most important site for various metabolic activities of the sperm.
The axoneme of the mammalian sperm is surrounded by nine outer dense fibers which are also called the coarse or accessary fibres. These run for the major part of its length, thus constituting a 9+9+2 cross-sectional pattern (Fig. 2.8 d).
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The mitochondria of the mid piece arranged end to end constitute a helix around the longitudinal fibrous elements of the tail. The end on junctions of mitochondria are generally seen at random along the course of the helix.
The mitochondrial sheath is believed to be the source of energy (ATP) for sperm motility. However, this energy is limited and once utilized cannot be renewed, except in mammals and in those animals where spermatozoa remain alive within maternal body because there are energy sources available to the spermatozoon.
At the junction of mid piece and principal piece is present the annulus which is also known as the ring Centriole or Jensen’s ring. The annulus is composed of the closely packed filamentous subunits, 3 to 4 nm in diameter. It develops in close association with the plasma membrane and remains firmly adhered to it. The functional significance of annulus is still not clear but according to some scientists the function of the annulus could be to prevent displacement of the mitochondria.
3. Principal piece:
The main piece or principal piece of mammalian spermatoza is surrounded by a fibrous sheath which shows a similar basic organization in different species of mammals. Fibrous sheath is composed of a series of circumferentially oriented ribs that extend half way around the tail end in two longitudinal columns which run along opposite sides of the sheath for its whole length. The sheath is not attached to the plasma membrane.
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The longitudinal columns extend in the principal piece along the whole length of the fibrous sheath in its dorsal and ventral surfaces. These are composed of 15 to 20 nm thick longitudinal subunits attached to the axoneme during sperm movement.
The plasma membrane is independent of this complex. Towards the end of the piece, the longitudinal columns progressively reduces in size. Meanwhile the ribs become slender. The abrupt ending of the fibrous sheath marks the junction of the principal and end piece.
4. End piece:
The end piece consists of a central pair of axial fibrils and ring of nine doublet fibres, which are surrounded by the plasma membrane. The 9+ 2 pattern of axial filament complex extends through most part of the tail including the end piece, but the arrangements of the fibres in the tip of the end piece is changed and decreasing number of fibres suggests a successive termination of the single subfibres (Fig. 8 F-H).
Functions of the spermatozoon:
The main function of the spermatozoon is to carry the paternal genetic dowry and to activate the ovum.
Types of sperm:
The type of the sperm produced varies from species to species. The size of the sperm may be as little as 0.018 mm in Amphioxus or as large as 2.25 mm or more in toad. The sperm head is, however, species specific. It may be spheroidal (teleosts), rod or lance-shaped (amphibians), spoon-shaped (man and many other mammals), or hooked (mouse and rat).
The sperm types are again divided into two main types found in animals-flagellate spermatozoa, which posses a flagellum or tail like biflagellate (in Opsanus, a toad fish) spermatozoa. The non-flagellate spermatozoa lack flagella and are found in Ascaris, crab etc.
