Scientists at the University of York have shown that a sperm tail utilises interconnected elastic springs to transmit mechanical information to distant parts of the tail, helping it to bend and ultimately swim toward an egg.
Previous studies, from approximately 50 years ago, showed that the sperm tail, or flagellum, was made up of a complex system of filaments, connected by elastic springs resembling a cylinder-like structure. For many years scientists believed that this system provided the sperm tail with a scaffold, allowing it to swim in a hostile environment toward an egg.
New research at the University of York, however, has shown, through a mathematical model, that this system is not only needed to maintain the structure of the tail, but it is also vital to how it transmits information to very distant parts of the tail, allowing it to bend and move in its own unique way.
Distinct motion
Dr Hermes Gadêlha, mathematical biologist at the University’s Department of Mathematics, said: “Sperm flagella with this sort of internal structure can be seen in almost all forms of life. Interestingly, although the sperm tail has an internal structure that is conserved across most species – animal and human - they all create slightly different movements in order to reach an egg.
“This suggests that the tail’s structure is not the whole story to how they make their distinct tail-bending motion.”
Dr Gadêlha and collaborators had previously developed a mathematical formula for the way in which sperm move rhythmically through fluid, creating distinct fluid patterns, but scientists now needed to understand what was going on inside the sperm tail that allowed them to move in this way.
Dead sperm
To understand the structure of the tail, scientists examined how different parts of the tail bent by moving the tail of a dead sperm. Surprisingly a movement that started near the head of the sperm, resulted in an opposite-direction bend at the tip of the tail, called the ‘counterbend phenomenon’, suggesting that mechanical information is transmitted along the interconnected elastic bands in order to create movement along the full length of the tail.
Dr Gadêlha calculated these bending movements to form a mathematical model that would help hypothesise the triggers needed within the tail to make these distinct movements.
Complex 'boat'
Dr Gadêlha said: “If we imagine that the communication to distant parts of the tail is a bit like the communication between blindfolded rowers in a canoe boat. Blindfolded rowers can’t see each other’s motion to communicate what movement to make, and in the absence of shouting to each other, they must instead feel the mechanics of the boat and the movement that each rower is making in order to synchronise their motion.
“It seems that the molecular motors - the ‘rowers’ inside the sperm tail - are doing a similar thing, but in a much more complex ‘boat’."
"The mechanism of a sperm tail first creates a sliding motion between filaments, inside this cylindrically arranged structure, finally resulting in a tail bending, a bit like the piston that converts back and forth motion in to rotation of the wheel on a train." Any one movement in this complex sequence appears to be able to trigger motion right through to the distant parts of the tail.
“The big question now is, are particular springs in the tail coupled-up to transmit specific biomechanical information, and just are these ‘rowers’ self-organise?
Structure of Sperm
Sperm are smaller than most cells in the body; in fact, the volume of a sperm cell is 85,000 times less than that of the female gamete. Approximately 100 to 300 million sperm are produced each day, whereas women typically ovulate only one oocyte per month as is true for most cells in the body, the structure of sperm cells speaks to their function. Sperm have a distinctive head, mid-piece, and tail region.
The head of the sperm contains the extremely compact haploid nucleus with very little cytoplasm. These qualities contribute to the overall small size of the sperm (the head is only 5 μm long).
A structure called the acrosome covers most of the head of the sperm cell as a “cap” that is filled with lysosomal enzymes important for preparing sperm to participate in fertilization. Tightly packed mitochondria fill the mid-piece of the sperm. ATP produced by these mitochondria will power the flagellum, which extends from the neck and the mid-piece through the tail of the sperm, enabling it to move the entire sperm cell. The central strand of the flagellum, the axial filament, is formed from one centriole inside the maturing sperm cell during the final stages of spermatogesis.
Sources: University of York