Currently, many of our cellular definitions are based almost entirely on their visual appearance. Why is a neuron a neuron? Because it looks like a neuron. This tautological understanding of nervous system cells (it’s a neuron or not a neuron) over the past 100 years has carried increasing amounts of weight (and disconnect from actual mechanics) as more and more functions were assigned to neurons because of circular reasoning built on top of that tautology.
The assumption that the neuron is the primary unit of calculation in nervous systems, because the we defined the neuron as the primary unit of calculation of nervous systems is largely why any cell not a neuron has been largely ignored in the study of nervous systems. And why nervous systems have remained such a mystery despite such intense effort to understand them.
We still largely define cells by how they look rather than what they do, an astrocyte is an astrocyte because it “looks like a star”, a oligos and schawnns are oligos and schwanns because they look like other cells previously defined by their morphology.
While it’s fair to say that there are quite a number of useful properties we can infer from morphology, we often improperly assume that cells that look different must have different function, or that cells which are similar in appearance have similar function.
The most prevalent example of this on a much more macro scale is looking at the cerebellum and cerebrum. Cortical areas all mostly perform the same function, as extra storage or calculation space for “nuclei” style clusters. But because the cells in these regions appear so radically different, this correlation has been largely missed/ignored.
The example that came to mind however thinking about this post were mostly related to astrocyte heterogeneity, and the amount of hand wringing a lot of recent work seems to be going through over it. As an interesting counter-example of sort astrocytes actually look like how they work, that is their morphology is adaptive because they are adaptive.
Just as the idiom warns us that the appearance of the cover may not accurately reflect the contents of a book, or the genre of the book may not necessarily match the genre of the writing, relying on a single property to define cells probably hampers more than helps our understanding of nervous systems.
Edit: Okay, an actual thought experiment – Imagine you are presented with three glasses full of liquid, each liquid a different color. It is your task to ascertain the properties of these glasses and explain what their properties.
We come up with a few different tests and note that each glass has a slightly different flavor. We test the pH and each has a slightly different pH. Testing the electrical conductivity of the solutions reveals different properties as well. When we look around for similar instances, we notices that there are similar fluids out there in other places, with similar properties.
And so we start to determine each color fluid (e.g. “red”) can be differentiated by their color based on these tested properties.
In fact, all of the glasses are filled with exactly the same water and largely have exactly the same properties. The slight difference in properties are determined by which additive is in the solution.
Ultimately, what we think we are measuring (the fluid and it’s properties as a whole) isn’t actually what we are measuring (the properties of the additive in solution).