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| Structure of Gametes - Spermatozoan |
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| A sperm is a haploid male gamete which fuses with the ovum to form the diploid zygote. |
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| Electron Microscopic Structure of Mammalian Sperm |
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| The sperms or the male gametes are solitary cells specialized for swimming and delivering themselves to the ovum to fertilise them. It consists of 4 parts - the head, neck, middle piece and a tail. |
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| The head is almost conical in the human sperm. It is formed of an acrosome and nucleus enclosed in a thin membrane. |
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| The acrosome forms a cap at the anterior end of the nucleus. It is formed from the golgi complex. It secrets tissue - dissolving lytic enzymes called hyaluronidase which help to penetrate the ovum. |
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| The nucleus contains densely packed DNA and proteins. The posterior margin is depressed to accommodate the proximal centriole. |
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| The head is followed by a short neck. It consists of 2 centrioles, the proximal and the distal centriole. The proximal centriole is introduced into the egg during fertilisation and it is necessary to initiate the cleavage of the zygote. The distal centriole provides attachment to the axial filament of the sperm tail. |
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| The middle piece consists of apical part of the axial filament surrounded by a tightly coiled spiral sheath of elongated mitochondria. The mitochondria contains oxidative enzymes and provides energy for sperm motility. |
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| It consists of a central axial filament, a thin layer of cytoplasm and an outer smooth plasma membrane. The axial filament is formed of 9 pairs of longitudinal fibres which extend upto the tip of the axial filament. |
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| In mammalian sperm, another set of nine thicker fibres is present outside the longitudinal fibres. These fibres do not reach the tip of the sperm tail. The free end of the sperm tail without additional fibres is called end piece. |
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| Viability (the period upto which the sperm is able to fertilise an ovum) is about 24 - 48 hours. |
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