Limulus Sperm Dynamics

Elaine Bearer, Drew Kennedy, Niema Pahlevan

Actin is a small, globular protein that is the monemeric subunit of microfilaments. These microfilaments are a major cytoskeleton component as they are essential to cell function. Muscle contraction, cell motility, vesicle and organelle movement are a few of the many cellular processes in which microfilaments are essential. Since actin is highly conserved, differing in less than 5% in diverse organisms, we can study actin's action in simpler organisms and apply the characterization to that of higher organisms. The acrosomal reaction of male horseshoe, or limulus sperm is one of the actin related process in lower has been investigated in lower animal species. As shown in the diagram below, the acrosomal process forms at the apex of the sperm head, supported by a core of actin microfilaments. The membrane at the tip of the acrosomal process fuses with the egg plasma membrane.

The false discharge starts at the basal end of the bundle and acrosomal process has a curly shape. The true discharge starts with the opening of the acrosomal vesicle and is followed immediately by a complete uncoiling of the bundle which generates a straight acrosomal process.

This spring-force like physical reaction mechanism has been reported to occur only in presence of calcium, but the exact dependence is not known.

Using state-of-the-art imaging capabilities, we successfully captured for the first time on film, under the microscope, the complete acrosomal process: true and false discharges. The first picture shown below depicts our field of view which is crowded with sperm. The second picture shows a false discharge while the third one shows unreacted and true discharge.

Our field of view is crowded with sperm

False discharge of acrosomal process

Unreacted and true discharge of acrosomal process

From these observations we have discovered that, contrary to what current literature in the field suggests, the portion of the acrosomal reaction in which the acrosomal process 'shoots out' occurs in less than 1 s. Additionally, the true discharge is straight, approximately 60 um in length. The false discharge is helical and 50 um. Flagellar retraction was found to vary in speed and relative timing.

With the ability to quantifiably observe the acrosomal reaction, it is reasonable that furture work may be able to answer questions about where calcium ion density affects the speed of the discharge. Also, what does the variability in speed of the flagellar retraction mean. Finally, these quantitative observations maybe used to calculate the energy stored in the coil .

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