How squid and octopus get their big brains
Overview: Neural stem cells of cephalopods work in the same way as those of vertebrates during nervous system development.
Cephalopods – including octopus, squid and their cuttlefish cousins - are capable of truly charismatic behavior. They can quickly process information to transform shape, color and even texture and blend in with their environment. They can also communicate, show signs of spatial learning, and use tools to solve problems. They are so smart that they can even get bored.
It’s no secret what makes it possible: cephalopods have the most complex brains of any invertebrates on Earth. What remains mysterious, however, is the process. In short, scientists have long wondered how cephalopods even get their large brains?
A Harvard lab studying the visual system of these soft-bodied creatures — on which two-thirds of their central processing tissue focuses — thinks they’ve almost got it figured out. The process, they say, looks surprisingly familiar.
Researchers at the FAS Center for Systems Biology describe how they used a new live-imaging technique to watch near real-time how neurons are created in the embryo. They were then able to track those cells through the development of the nervous system in the retina. What they saw surprised them.
The neural stem cells they tracked behaved eerily similar to the way these cells behaved in vertebrates during the development of their nervous systems.
It suggests that vertebrates and cephalopods, despite differing from each other 500 million years ago, not only use similar mechanisms to make their cerebrums, but that this process and the way the cells work, divide and form are essentially form the required blueprint. develop this type of nervous system.
“Our conclusions were surprising because much of what we know about nervous system development in vertebrates has long been considered special to that lineage,” said Kristen Koenig, a John Harvard Distinguished Fellow and senior author of the study.
“By observing the fact that the process is very similar, it suggested to us that these two independently evolved, very large nervous systems use the same mechanisms to build them. What that suggests is that those mechanisms – those tools – that the animals use during development, may be important for building large nervous systems.”
The scientists at the Koenig Lab focused on the retina of a squid called Doryteuthis pealei, better known as a type of longfin squid. The squid grows to about a foot in length and is abundant in the northwestern Atlantic Ocean. As embryos they look very cute with a big head and big eyes.
The researchers used similar techniques to those made popular to study model organisms, such as fruit flies and zebrafish. They created special tools and used advanced microscopes that could take high-resolution images every ten minutes for hours to see how individual cells behave. The researchers used fluorescent dyes to mark the cells so they could map and track them.
This live-imaging technique allowed the team to observe stem cells called neural progenitor cells and how they are organized. The cells form a special kind of structure called a pseudostratified epithelium. The main feature is that the cells are elongated so that they can be packed closely together.
The researchers also saw the core of these structures move up and down before and after dividing. This movement is important for keeping tissue organized and continuing growth, they said.
This type of structure is universal in how vertebrate species develop their brains and eyes. Historically, it was considered one of the reasons why the nervous system of vertebrates could grow so large and complex. Scientists have observed examples of this type of neural epithelium in other animals, but the squid tissue they looked at in this case looked unusually like vertebrate tissues in size, organization and the way the nucleus moved.
The research was led by Francesca R. Napoli and Christina M. Daly, research assistants at the Koenig Lab.
Next, the lab plans to look at how different cell types arise in the brains of cephalopods. Koenig wants to determine whether they are expressed at different times, how they decide to become one type of neuron versus another, and whether this action is similar across species.
Koenig is excited about the possible discoveries ahead.
“One of the key points of this type of work is how valuable it is to study the diversity of life,” Koenig said. “By studying that diversity, you can really come back to fundamental ideas about even our own development and our own biomedically relevant questions. You can really talk to those questions.”
About this neuroscience research news
Author: John Siliezar
Contact: Juan Siliezar – Harvard
Image: The image is in the public domain
Original research: Closed access.
“The development of the cephalopod retina demonstrates vertebrate mechanisms of neurogenesisby Kristen Koenig et al. Current Biology
The development of the cephalopod retina demonstrates vertebrate mechanisms of neurogenesis
- Retinal progenitor cells in the squid undergo interkinetic nuclear migration
- Progenitor, post-mitotic and differentiated cells are transcriptionally defined
- Notch signaling can regulate both retinal cell cycle and cell fate in squid
Coleoid cephalopods, including cuttlefish, cuttlefish, and octopuses, have large and complex nervous systems and camera-like eyes with high acuity. These features are comparable only to features that evolved independently in the lineage of vertebrates.
The size of animal nervous systems and the diversity of their constituent cell types is the result of the tight regulation of cellular proliferation and developmental differentiation.
Developmental changes during evolution that result in a diversity of neural cell types and variable size of the nervous system are not well understood.
Here we pioneered live imaging techniques and conducted functional interrogations to demonstrate that the squid Doryteuthis pealei exploits mechanisms during retinal neurogenesis that are characteristic of vertebrate processes.
We find that retinal progenitor cells in the squid undergo nuclear migration until they exit the cell cycle. We identify the retinal organization corresponding to progenitor, post-mitotic and differentiated cells.
Finally, we find that Notch signaling can regulate both the retinal cell cycle and cell fate. Given the convergent evolution of comprehensive visual systems in cephalopods and vertebrates, these results reveal common mechanisms underlying the growth of highly proliferative neurogenic primordial.
This work highlights mechanisms that can alter ontogenetic allometry and contribute to the evolution of complexity and growth in animal nervous systems.
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