Functions of the Spermatozoon | Gametogenesis | Embryology

The function of the spermatozoon, for which its whole organization is adapted, is to reach the egg and, by fusing with the egg, to cause the egg to start developing and to transmit to the developing embryo the paternal genes. In most cases this means that the spermatozoon must have a high degree of mobility, and in fact, the organization of typical spermatozoa is largely determined by the presence of a highly developed locomotory mechanism.

The spermatids, though possessing a haploid set of chromosomes, are still not ca­pable of functioning as male gametes. They have to undergo a process of differentiation to become the spermatozoa.

A typical spermatozoon consists of the following main parts – the head, the middle piece, and the tail or flagellum. The anterior tip of the head is differentiated as the acrosome, the function of which is to enable the spermatozoon to penetrate through the egg envelopes and to establish connection with the egg cytoplasm.

The major part of the head is occupied by the nucleus containing the genes and is thus responsible for the transmission of hereditary characters from the male parent. The posterior part of the head also contains the centriole of the spermatozoon, which will serve during the cell division in the fertilized egg. The middle piece of the spermatozoon contains the base of the flagellum and—around it—the mitochondria.

The latter, being carriers of oxidative enzymes and the enzymes responsible for oxida­tive phosphorylation, are the “power plant,” supplying the flagellum with energy in a suitable form to be used for the propulsion of the spermatozoon. The tail or flagellum, usually by far the longest part of the spermatozoon, by its movements causes the spermatozoon to swim with the head (acrosome) foremost.

The changes which transform the spermatid into a spermatozoon are of a most radical nature. The nucleus of the spermatid, after the telophase of the second meiotic division, assumes the typical structure of an interphase nucleus with finely dispersed chromatin and a nuclear membrane.

During transformation of the spermatid into a spermatozoon, the nucleus shrinks by losing water from the nuclear sap, and the chro­mosomes become closely packed into a small volume. This is necessary in order to re­duce the dead weight to be carried by the locomotory apparatus of the spermatozoon and to enhance its motility.

It appears that everything superfluous is removed from the nucleus, everything not directly concerned with transmission of hereditary characters, leaving only the actual material of the genes. All ribonucleic acid, which is abundant in – the functioning nucleus, especially in the nucleolus, is eliminated, leaving only the deoxyribonucleoproteins present—the material of the genes.

The shape of the nucleus also changes – instead of the usual spherical form the nucleus becomes elongated and narrow, an obvious adaptation for propulsion in water. In different animals the shape of the head of the spermatozoon (largely dependent on the shape of the nucleus) varies considerably.

It may be ovoid and flattened from the sides (in man and bull), drawn out into a scimitar shape with a pointed tip (in rodents and amphibia), or spirally twisted like a corkscrew (in birds and some molluscs); occasionally it may be almost round (bivalve, molluscs). Some of these shapes may also be interpreted as an adaptation for propulsion in water.

The acrosome of the spermatozoon is derived from the Golgi bodies. The Golgi body in an early spermatid consists of a series of membranes arranged concentrically around an aggregation of small vacuoles. In the next stage one or more of the vacuoles start enlarging, and inside the vacuole a small dense body, the proacrosomal granule, appears.

If more than one vacuole and granule are found, as sometimes happens, they fuse together so that eventually one big vacuole remains, containing a single large dense granule. The contents of the vacuole and the granule yield a positive staining reaction for mucopolysaccharides—the so-called periodic acid-Schiff reaction. The vacuole with its granule now becomes closely applied to the tip of the elongating nucleus.

The granule increases further and be­comes the acrosomal granule, which forms the core of the acrosome. The vacuole loses its liquid content, and its walls become spread out over the acrosomal granule and the front half of the nucleus, covering them with a double sheath known as the cap of the spermatozoon. The remainder of the Golgi body undergoes a gradual re­gression and eventually is discarded as the “Golgi rest,” together with some of the spermatid’s cytoplasm.

The foregoing description of acrosome development is derived from a study of spermatogenesis in the cat. In other animals the develop­ment may be complicated by the formation of a second main component which appears between the acrosomal granule and the nucleus, protruding itself from behind into the acrosomal granule in the direction of the main axis of the spermatozoon.

This axial body or acrosomal cone may perhaps be the rudiment of the acrosomal filament developing in the spermatozoon during its approach to the egg. As for the acrosomal granule, there is evidence that it contains a supply of enzymes which are used to dissolve the egg envelopes during fertilization.

The centrosome of a spermatid after the second meiotic division consists of two centrioles, which can be shown with the aid of an electron microscope to have the struc­ture of two cylindrical bodies, lying at right angles to each other.

At an early stage in the differentiation of the spermatozoon, the two centrioles move to a position just be­hind the nucleus. A depression is formed in the posterior surface of the nucleus, and one of the two centrioles becomes placed in this depression with its axis approximately at a right angle to the main axis of the spermatozoon.

This is the proximal centriole of the spermatozoon; the other centriole, the distal centriole, takes up a position behind the proximal centriole with its axis coinciding with the longitudinal axis of the spermato­zoon. The distal centriole then gives rise to the axial filaments of the flagellum of the spermatozoon for which it serves as a starting point or basal body.

The axial filament of the spermatozoon has the same organization as the axial filaments of the flagella and kinocilia present in other animal (and plant) cells; that is, it has a pair of longitudinal fibers along its middle and a ring of nine pairs of longitudinal fibers surrounding it. These fibers are anchored in the distal centriole in the same way that the fibers of a cilium or flagellum are connected to their basal bodies.

The distal centriole and the proximal part of the axial filament lie in the middle piece of the spermatozoon. They are surrounded by the mitochondria, which become concentrated in this region from other parts of the cell. In the middle piece, the mito­chondria lose their individuality to a certain degree by fusing together to a greater or lesser extent. In many animals (mammals in particular) the mitochondria join in one continuous body which becomes twisted spirally around the axial filament and the proximal centrosome.

However, in other animals the spiral arrangement of the mitochondria is lacking, and they are joined in one or more massive clumps (mito­chondrial bodies), forming the bulk of the middle piece of the spermatozoon. Around the periphery of the middle piece the cytoplasm forms a condensed layer known as the manchette which also surrounds the posterior part of the head of the spermatozoon, where it is not covered by the cap.

A dark ring is sometimes seen at the posterior end of the middle piece, forming the boundary between the middle piece and the tail. This structure has been known as the “ring centriole,” but it has been shown by electron microscopy that it does not in any way resemble a centriole in its structure, and that therefore it must have a completely different origin. The function of the ring is not known.

The axial filament is the main part of the tail or flagellum of the spermatozoon, and its structure is essentially similar to that found in cilia and flagella of other cells. The axial filament is surrounded in the simpler cases by a very thin layer of cytoplasm and by the plasmalemma. Some other structures may also be present. In mammals the nine peripheral pairs of longitudinal fibers are ac­companied on the outside by nine much thicker fibers that are wedge-shaped in cross section.

These fibers start in the middle piece of the spermatozoon but do not quite reach the tip of the flagellum. Another set of threads—or, actually, flattened bands—surrounds the longi­tudinal fibers of the spermatozoon tail in mammals.

For a long time the threads were thought to be coiled spirally around the central core of the tail, but careful electron microscopic studies have shown that the fiber does not form a continuous spiral but is composed of semicircular ribs articulating with each other on the opposite sides of the sperm tail.

The tip of the mammalian sperm tail (the “end piece”) lacks the additional elements and consists of only the axial filament covered with cytoplasm and plasmalemma—a structure similar to the structure of the whole tail of less elaborately differentiated invertebrate spermatozoa.

A different kind of complication is present in some fishes and amphibians. In sper­matozoa of these animals there is an undulating membrane which stretches along most of the length of the tail and presumably takes an active part in the locomotory activity of the sperm.

Much of the cytoplasm of the spermatid becomes redundant in the spermatozoon, and it is simply discarded. As the acrosome is being formed at the anterior end of the spermatid nucleus, the cytoplasm flows away from it in the opposite direction, leaving only an extremely thin layer with the plasmalemma covering the acrosome and the nu­cleus. The bulk of the cytoplasm is then attached to what will be the middle piece of the spermatozoon, while the tail is growing out at the posterior end.

After the mitochondria have arranged themselves around the base of the axial filament of the flagellum, the remainder of the cytoplasm (containing also the “Golgi rest”) is simply pinched off from the spermatozoon, leaving only a fairly narrow sleeve of cytoplasm surrounding the mitochondria in the middle piece. The detached part of the cytoplasm disintegrates.

In a few groups of animals, such as the nematodes and the decapod crustaceans, the spermatozoa do not have flagella and are therefore incapable of swimming. These spermatozoa are also in other respects profoundly different from those in other animals.

When the spermatozoa of a decapod were examined with the electron microscope, it was found that they do not possess centrioles and are completely devoid of mito­chondria—which makes sense, as they do not require large amounts of energy for movement. The mechanism of penetration of the non-flagellate sper­matozoa into the egg must be very different from that of flagellate spermatozoa.