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Sperm motility describes the ability of sperm to move properly towards an egg. This can also be thought of as the quality of the sperm, which is a factor in successful pregnancies, as opposed to its quantity. Sperm that do not properly "swim" will not reach the egg in order to fertilize it. Sperm motility in mammals also facilitates the passage of the sperm through the cumulus oophorus and zona pellucida, which are a series of layers that surround the ovulated egg-cell plasma membrane of a mammalian oocyte. For example, in the wood mouse Apodemus sylvaticus, sperm aggregates form mobile trains that possess an enhanced fertilization capability because they are better suited to navigate the viscous environment of the female reproductive tract. The trains move in a sinusoidal motion, individual spermatozoa have impaired fertilization capacity.
It may also depend upon the effectiveness of sperm motility in response to certain factors released from eggs. The migration of sperm through the female reproductive tract (internal fertilization) or in water (external fertilization) to reach the egg is key to successful fertilization. Successful fertilization also depends on the ability of sperm to penetrate extracellular matrix surrounding eggs.
The sperm motility is activated by changes in intracellular ion concentration. The change in concentration that signals the mechanism is different among species. In marine invertebrates and sea urchins, the rise in pH to about 7.2-7.6 activates ATPase which leads to decrease in potassium, thus induces membrane hyperpolarization. As a result, sperm motility is activated. The change in cell volume which alters intracellular ion concentration can also contribute to the activation of sperm motility. In some mammals, sperm motility is activated by increase in pH, calcium ion and cAMP, yet it is suppressed by low pH in the epididymis.
Tail of the sperm (flagellum) which aids in its motility has 3 principal components:
- A central skeleton constructed of 11 microtubules collectively termed axoneme, similar to general cilia.
- A thin cell membrane covering the axoneme.
- Mitochondria arranged spirally around it.
Back and forth movement results from a rhythmical longitudinal sliding motion between the anterior and posterior tubules that make up the axoneme. The energy for this process is supplied by ATP produced by mitochondria. Velocity of a sperm in fluid medium is usually 1-4mm/min. This allows the sperm to move through in quest for ovum.
In mammals, spermatozoa mature functionally through a process which is known as capacitation. When spermatozoa reach isthmic oviduct, their motility have been reported to be reduced as they attach to epithelium. Near the time of ovulation, hyperactivation occurs. During this process, the flagellar moves with high curvature and long wavelength. Hyperactivation is initiated by extracellular calcium; however, the factors that regulate calcium level is unknown.
Without technological intervention, a non-motile or abnormally-motile sperm is not going to fertilize. Therefore assessing the fraction of a sperm population that is motile is perhaps the most widely used measure of semen quality making sperm motility an important factor of it. Insufficient sperm motility is a common cause of subfertility or infertility. There are several measures available in market today to improve the quality of sperms.
- Straight Moving,
- Zig-zag Moving,
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- Yanagimachi, R. (1994). Mammalian fertilization. In "The Physiology of Reproduction" (E.Knobil and J. D. Neill, eds.), pp. 189-317. Raven Press, New York
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