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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 the 'quantity'. Sperm which do not properly 'swim', will not reach the egg in order to fertilize it. Sperm motility facilitates the passage of the sperm through the zona pellucida, which is a membrane that surrounds the plasma membrane of an 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.

Successful fertilization depends on the ability of sperm to penetrate extracellular matrix surrounding eggs. It also depends on the effectiveness of sperm motility in response to factors released from eggs. The migration of sperm through the female reproductive tract or in water to reach the egg is also important in fertilization.[1].

The sperm motility is activated by changes in intracellular ion concentration which leads to ejaculation. 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 acitvates ATPase which leads to decrease in potassium, thus induces membrane hyperpolarization. As a result, sperm motility is activated. [2]. 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.

In mammals, spermatozoa matures functionally which is known as capacitation as it moves through female reproductive tract. When spermatozoa reaches isthmic oviduct, its motility is reduced as it attaches to epithelium. Near the time of ovulation, hyperactivation occurs. During this process, the flagellar moves with high curvature and long wavelength.[3]. Hyperactivation is initiated by extracellular calcium; however, the factors that regulate calcium level is unknown.[4].

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.

Classifications

Sperm have been put into 3 different categories based on the evaluation of motility in most species. The three categories are non-motile, progressively motile or non-progressively motile. A non-progressively motile sperm swims but with abnormal paths, such as in tight circles rather than swimming forward in a more or less straight line as progressively motile sperm do.

"Total motility" is another term that is often used which refers to the fraction of sperm that display any type of movement. This concept is the norm for describing human sperm motility but rarely used for evaluating animal semen.[5]

References

  1. ^ Quill, A. T., Garbers, L. D. (2002). "Sperm Motility Activation and Chemoattraction". in Daniel M. Hardy. Fertilization. Carlifornia: Academic press. pp. 29. ISBN 0-12-311629-5.  
  2. ^ Darszon, A., labarca, P., Nishigaki, T., and Espinosa, F. (1999). Ion channels in sperm physiology. Physiol. Rev 79, 481-510
  3. ^ Mortimer, D., Aitken, R. J., Mortimer, S. T., and Pacey, A. A.(1995). Workshop report: Clinical CASA-the quest for consensus. Reprod. Fertil. Dev 7, 951-959
  4. ^ Yanagimachi, R. (1994). Mammalian fertilization. In "The Physiology of Reproduction" (E.Knobil and J. D. Neill, eds.), pp. 189-317. Raven Press, New York
  5. ^ http://www.vivo.colostate.edu/hbooks/pathphys/reprod/semeneval/motility.html

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