Geometric Clutch Hypothesis

The geometric clutch hypothesis is a brilliant paper that I read a few months back. Been wanting to write about it for quite some time now.

This paper explains in detail about the motility of the spermatozoa (eukaryotic axonemal function) Just when I thought I was done with all the physics in school, the principle behind the motility of the spermatozoa is completely physical. The functioning of the sperm motility is almost equally compared with the functioning of a motor. Honestly I have never enjoyed reading the laws of physics so much as I was reading this paper.

The male gamete is the only motile cell in a living organism that reproduces sexually. It has the following structures, Head, neck, mid piece and tail. The tail is the most prime part of the motile cell as it helps in the movement of the sperm to reach the oocyte (female gamete). The tail is also called the flagellum or axoneme, within which are the intricate structures that work together to propel the sperm forward. Looking at the bigger picture, The basic principle behind the whip like movement of the tail of the sperm is transverse force. A force that acts perpendicular to the body of the tail which results in the bending of the tail.

Internal make-up of the axoneme:

The axoneme consists of a 9 pairs micro tubule doublets arranged in a circular fashion with one doublet at the center. The doublets are connected to each other by nexin links and to the center doublet by radial spokes. These connections are of course mobile, to aid in the bending of the flagellum. The power needed for the bending for the flagellum is provided by a protein named dynein. These motor proteins (dynein arms) are present outside the micro tubule doublet. The doublet that has the dynein arm is called as sub-tubule A.

The flagellar beat:

The flagellar beat is driven passively by the transverse force (t- force) and actively by dynein motor proteins. The initiation of the beat frequency is passive which is proceeded by active beating of the flagella. Therefore, this results in the movement. During the movement, the internal micro tubule skeleton are propelled by the help of the dynein proteins. The movement of the micro tubules are overlapping and can be compared to the movement of the wheels of a train.

The micro tubular arrangement is such that, during the bending of the flagellum, the overlapping of the micro tubule doublets are alternating between the micro tubule doublets on the left (6-9) and right (1-4) and the lower middle (5&6) micro tubule pairs or doublets held in position.

Axoneme showing micro tubules

(A) shows the active formation of a new bend. The t- force acting on the left side of the flagellum is showed by the arrow. The dynein arms on the left side of the axoneme (6-9) are engaged and pulling, while the dynein arms on the right side (1-4) present on micro tubules are resting. As a result, the doublet 1 is pushed upward due to the tension caused by the t-force acting and the doublets 5-6 is pushed downward and is under compression.

(B) as the flagellar beat is initiated and bend is formed, the t-force increases large enough to disengage the micro tubule doublets on the left. The t-force is slowly passed onto the central doublet pair through the radial spokes. This central force now limits the flagellum from getting distorted. This now makes the micro tubule doublets on the right (1-4) to begin to engage. This constitutes one beat cycle and as it continues it propels the cell forward.

In a nutshell, the theory behind the geometric clutch hypothesis is that, the flagellar movement is built on the t-force but, also requires the axonemal complex to respond to the force in a manner that alters the micro tubule doublet spacing.