| A typical Macrostomum sperm can be divided into three parts. From top to bottom in the picture on the right: the "feeler", a very motile extension (note the waves at the tip); the "head", broader and apparently filled with mitochondria: and the "tail", which is also very motile but thicker than the feeler, and (in the species studied so far) contains the nucleus. In some species a brush-like structure (the "brush") is present at the end of the tail, and often a pair of bristles project between the head and the tail (as can be seen in the picture). |
Sperm from M. spirale. |
| Macrostomum sperm are very motile, progressing forward (feeler-direction) in a snake-like fashion. Once it finds a surface, it seems to probe it with the feeler and, often, "drills" its way through tissue (see picture on the right). In the female antrum, the sperm attaches itself to the epithelium with the feeler. It thus seems that this structure may serve as both a drill and an anchor. The possible roles of the other appendages are less clear. We currently think that the bristles may hinder the attachment of other sperm, and/or prevent the recipient worm from removing the sperm from the female antrum (through a sucking behaviour). |
Sperm from M. spirale finding its way into tissue |
| The nucleus, which is thought in many cases to help give stability to the sperm, is arranged, in the species so far studied, in a string of round packed DNA clumps. This string is found not in the head, but towards the end of the tail. The arrangement of the nucleus varies. For instance, in M. lignano is long and with relatively large clumps (top picture), while in M. spirale the clumps are small, arranged in a compact spiral (bottom picture). |
Sperm from M. lignano stained with Hoechst. n: nucleus, m: mitochondria 
Sperm from M. spirale stained with Hoechst. n: nucleus, m: mitochondria |
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