Sperm swimming towards an egg
The life of a sea urchin sperm is a difficult one. Once ejaculated, the cells have to navigate turbulent seas, with their eddies and currents, to fertilise a sea urchin egg. So how do they know where to go? They follow their chemical ‘noses’, so to speak.
In a series of recent papers, Adán Guerrero, from the Instituto de Biotechnología at the Universidad Nacional Autónoma de México, and colleagues describe the biology behind the mechanisms directing the sperm.
It’s all down to the process of chemotaxis: following the concentration gradient of a molecule, known as a chemoattractant.
In the sperm’s case, they orientate themselves by following the trail of a chemoattractant produced by the egg: the higher the concentration of the molecule, the closer they are to their goal.
The chemoattractants are known as Spermatozoa-activating peptides (SAPs), the most studied of which is speract, from the Californian purple sea urchin (Strongylocentrotus purpuratus). Speract is able to alter the movement pattern of a sperm by changing the behaviour of its tail.
Without SAP, the tail of a sperm beats asymmetrically, resulting in a circular swimming pattern. But when speract is around, it binds to receptors in the sperm’s tail, changing the concentration of calcium within it. This leads to a strong bend in the tail, initiating a turn, followed by a period of symmetrical tail movement to ensure straight swimming.
You can see for yourself in the video below.
This shows a sperm cell (labelled with a fluorescent marker to track calcium levels) from the painted sea urchin Lytechinus pictus in artificial seawater. After three seconds, the researchers activate speract using UV light (indicated by the red flash). The dot in the centre represents the highest concentration of speract.
When the sperm is moving away from the highest concentration of speract there is a sudden change in calcium ion levels and the sperm changes direction to swim towards where it perceives the egg to be.
L. pictus sperm constantly sample their environment to assess where they are in the concentration gradient. When swimming towards speract the sperm is able to suppress the fluctuations in calcium concentration, ensuring they continue on the correct course.
It’s fascinating stuff, and not just of interest to sea urchins. Though the majority of research into sperm chemotaxis is done on marine invertebrates (primarily due to their use of external fertilisation), the phenomenon has also been demonstrated in a variety of invertebrates and vertebrates including humans.
Benjamin Thompson
Benjamin Thompson is undertaking a work experience placement at the Wellcome Trust.
Image credit: Bill McConkey, Wellcome Images
Filed under: Biomedical Sciences Tagged: Sperm
