Imaging an Axon: The Squid Giant Axon of D. pealeii

The longfin squid (Doryteuthis pealeii), often called the “Woods Hole squid” by locals, has been helping scientists reveal the mysteries of neuroscience for decades. 

Squid, like most other cephalopods, are quick. They move by sucking water into their mantle, squeezing the mantle and pushing the water out — basically, they’re jet propulsion powered cephalopods. Partly controlling this action is a very large axon in the squid’s nervous system called the “squid giant axon,” which are long cable-like motor neurons that run from a specialized nerve bundle called a ganglion to the muscles in the mantle.  Unlike the more complicated nervous systems in vertebrates, which are usually made up of billions of small neurons and axons, the squid giant axon can be up to 1.5 mm in diameter (though they’re typically around 0.5mm), making them an ideal way to study cellular neuroscience. 

This 3D reconstruction of the giant axon in D. pealeii was taken at the Imaging Innovation Lab at the MBL. It is labeled for sialic acid, which localized to cell membranes. The “tree trunk” in the center  is the giant axon and the “vines” wrapping around are the vasculature. Squid giant axons are visible with the naked eye and can be kept alive for days after dissection, allowing scientists to answer fundamental questions about how neurons function, transmit signals, and more.

Studies on D. pealeii have led to major advances in neurobiology, including description of the fundamental mechanisms of neurotransmission. Alan Hodgkin and Andrew Huxley performed experiments on the giant axon of D. pealeii in the 1950s to help them understand nerve pulses, work that would go on to win them a Nobel Prize. 

At the MBL, it is used as a research organism to understand adaptive coloration, development, vision, and behavior. Like other cephalopods, Doryteuthis pealeii has the ability to extensively recode its own genetic information within messenger RNA. In summer of 2020, scientists at the MBL used CRISPR Cas-9 to knock out a target gene in D. pealeii — a first for any cephalopod. Learn more about that work. 

About the Microscope

The stage-scanning line confocal was developed at the MBL. It is a novel microscope that is able to capture large fields of view at high resolution much faster than conventional, point-scanning confocal systems. It is also much "gentler" than a conventional confocal, meaning that the light that it uses to excite fluorescence in the specimen is less intense and thus less likely to cause photobleaching, or photodamage to live samples.

The stage-scanning line confocal possesses two identical light paths that make it compatible with a wide range of specimen types and enables simultaneous dual-view acquisition. It is ideally suited to image specimens that require the high magnification and resolution of confocal microscopy, but which are prohibitively large or sensitive for conventional point-scanning systems.