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, in biology, the male gamete (sex cell), corresponding to the female ovum in organisms that reproduce sexually. In higher animals the sperm is produced in the testis of the male; it is much smaller than the ovum and consists primarily of a head,
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the male sexual cell in animals.
In all vertebrates and most invertebrates a flagellum, or tail, enables the spermatozoa to move forward. The spermatozoa of some worms, myriopods, crustaceans, mites, and ticks are tailless and are characterized by a wide variation in structure and by amoeboid movement. Spermatozoa range in length, according to species, from tens to hundreds of micrometers; the spermatozoa of insects may reach several millimeters in length.
Spermatozoa with tails have a comparatively short head that may be spherical, conical, or hooked in shape. A nucleus containing condensed chromatin is found in the head. At the head’s anterior end there is an acrosome containing lysins. Below the head there is a thinner middle piece that ends in a threadlike tail. In most animal species the middle piece is short and encloses a basal granule (centriole) consisting of nine triplets of short microtubules arranged in a ring and surrounded by a halo of four to ten large mitochondria.
Extending from the basal granule is an axial filament, or axo-neme, which by contracting enables the flagellum to beat and the spermatozoon to move. The ultrastructure of the axial filament is virtually the same in all spermatozoa and is similar to that of cilia. A filament consists of fibrils (microtubules 200–250 angstroms in diameter), two of which lie in the center surrounded by a ring of nine peripheral fibril doublets (9 + 2). Sometimes nine additional fibrils are found outside the doublets (9 + 9 + 2). Flagellate peripheral fibrils are transformed directly into microtubules of the basal granule, and the central fibrils remain free. Proteins similar to actin and myosin (found in skeletal muscles) and capable of splitting ATP are found in flagellate axial filaments in fibrils and in structures directly associated with fibrils.
In insects the head of a spermatozoon is very elongated and sometimes spirally coiled, extending almost imperceptibly into a long tail. Much of the tail contains a distinctive structure of mitochondrial origin called a mitochondrial helix.
In many animals several types of spermatozoa are formed in the testes. Most are atypical, resulting from spermatogenic disturbances primarily occurring during meiosis. These spermatozoa apparently cannot fertilize ova.
REFERENCESWilson, E. Kletka i ee rol’ v razvitii i nasledstvennosti, vol. 1. Moscow-Leningrad, 1936. (Translated from English.)
Rukovodstvo po tsitologii, vol. 2. Moscow-Leningrad, 1966.
Hancock, J. L. “The Ultrastructure of Mammalian Spermatozoa.” In Advances in Reproductive Physiology. London, 1966.
L. V. DANILOVA