Sea urchin sperm and calcium:
One of my childhood memories was when my mother’s college roommate and lifelong friend Jean Clark Dann would visit from Japan.  She showed us exquisite eight by ten prints of electron micrographs of sea urchin sperm that had been stimulated by exposing them to egg albumin. 

At the time it was heady stuff.  The electron microscope was a new technology and many were devoted to the idea that it should teach us something important.  You know, the Higgs boson of its day.  Don Faucet, head of the anatomy department at Harvard Medical School developed a lecture on the wonders to be seen in cytoplasm.  The nucleus did not yield its secrets to the electron microscope; that would have to await x-ray diffraction.  

Jean Dann demonstrated that the sea urchin sperm put out a little filament in response to a chemical clue that it had reached an egg.  I politely looked at the pictures but later asked my mother (always wait until the parade is over before you point out to anybody that the king has no clothes) who in the world could care.  She told me that there were couples that could not have children and anything that might help was important.  In particular she said that any problem with fertility in those days was always laid at the feet of the woman.  Her friend had proven that there are things the male had to accomplish, so that might equally well be the cause of problems. 

Would that I had been more enthusiastic.  It might be a different world. 

Professor Faucet did mention the finding in a lecture I attended many years later.  I suppose I am the only one in the class who remembers.  Nor have I heard that enigmatic filament mentioned elsewhere.

But the to-me-venerable sea urchin sperm has been mentioned again. (Sperm Steer with Calcium NATURE vol. 483. no. 7388 March 8, 2012 page 127 reviewing J. Cell Biol. They found that the little sperm swim in circles until they encounter an increase in the calcium concentration in the water.  It is not the level but the increase that they respond to.  Calcium is the way the egg signals the sperm, I suppose the sea urchin equivalent of expensive clothes and makeup. 

What happens then is that the sperm changes its search pattern.  Initially it swims in circles.  That makes sense.  It has no long distance to cover.  Sea urchins like every other animal are better off mating locally. 

When it picks up the calcium shift it stops swimming in circles and starts a series of random straight line runs.  Hmm.  I fail to see how that increases the chance of blundering into an egg.  Nor does it speed up.  It just starts covering about the same area with about the same speed.  I suppose that the caloric demands of the straight lines and bends are greater than the demands of lazy circles.  Else, if there were any advantage to search pattern two it would do it from the start.

But what the change might do is increase the chance of hitting the egg squarely, I think.  I’m not sure.  Your guess is as good as mine.  Probably it depends on the size of the circles the sperm is making compared with its rate of approach to the egg and compared with the diameter of the egg.   

Somewhere in the back of my mind I wonder whether that enigmatic filament has something to do with it. 

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