Spermatozoa – a scientist’s favourite cell
People have preferences: favourite colours, favourite dishes, favourite authors, favourite this and that. Scientists, in addition, may have “favourite cells” as I have discovered early in my career. The American researcher Charles Brokaw in an article titled “My favourite Cell” had revealed in it that his was the sea-urchin spermatozoon. But he is not the only one who finds spermatozoans fascinating. For decades the Italian Baccio Baccetti and co-workers had been “at it” and were the first to report an in-depth study of backward swimming spermatozoans. To be precise, the sperm cells of the two species of fruit fly that were looked at did not exclusively swim backward (they use the forward gear when it comes to penetration of the egg cell), but the ability to reverse had only been reported once before.
That was in 1983, when the Swedish electron microscopist Björn A. Afzelius had observed an ability to switch from forward to backward swimming in sperm cells of the annelid worm Myzostomum. The change in swimming direction appears to be solely due to the direction of the flagellar beating: in backward motion the flagellum is leading, but when swimming forward the flagellum is in the rear position and propelling the head of the sperm cell is in front. To penetrate the egg cell membrane and enter the egg’s cytoplasm, forward motion is essential. However, Baccetti and co-workers believe that in order to escape from the common envelope that embeds the heads of maturing sperm cells in the male genital duct, the backward movement is advantageous. Whether the same reason could be advanced for backward motion of the annelid worm sperm, discovered by Afzelius earlier, is unknown.
Still stranger modes of spermatozoan locomotion have been known for a long time from roundworms (Nematodes): here cytoplasmic pseudopodia (little leglike cell projections that can change their shapes and sizes), appear and disappear like ghost fingers and move the male gamete, i.e., the sperm, along. The resulting creeping movement, slowly but steadily, takes the most “dedicated” sperm to its final destination: the egg cell. But crustaceans have even more, namely an enormous variety of sperm types to offer: in lobsters and crabs (as in other crustaceans with few exceptions) spermatozoa have no flagellum but the nucleus of a sperm can be pushed through the egg by an explosive action, which is the reason why such sperm are said to have an “explosion apparatus”. Although the longest sperm cells of any animal are those of the fruit fly Drosophila bifurca, which may be 20 times as long as the entire body of the fly, only the short head of the sperm and not its enormously long tail enter the egg cell. Where, however, an entire sperm 4 or 5 times the total length of the animal enters the egg, that is found in some mussel shrimps (Ostracoda), which are not mussels at all but small crustaceans with a shell that makes them resemble a swimming bivalve, i.e., a mussel.
And while we are at it: I, too, have a publication on sperm cells (together with another sperm cell enthusiast, the late Professor Gerd Alberti). The sperm we looked at under the electron microscope is that of an Antarctic marine mite and it is so incredibly beautiful that I can fully understand why for some scientists the spermatozoon is their favourite cell.
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2018.
Unauthorized use and/or duplication of this material without express and written permission from this site’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to V.B Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com with appropriate and specific direction to the original content.