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unopenablebox · 3 years

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ok here i am blogging about a paper (“The evolution of a new cell type was associated with competition for a signaling ligand”) because i needed to read it for journal club. i’ve included a long introductory section where i explain why anyone would do sea urchin biology, because that was the part i spent a bunch of time doing background reading about and i wanted to record my thoughts in educational blog format, but this really makes no effort not to be obscure and weird. please enjoy

WHAT’S UP WITH [THE DEVELOPMENTAL AND EVOLUTIONARY HISTORY OF] SEA URCHIN SKELETONS?

a preliminary report[1], as told by someone who saw a sea urchin at an aquarium once

Part 1: introduction

Echinoderms (lit. “bad to touch”) are very interesting to developmental biologists, because they have radial rather than bilaterial symmetry which is fun and cool, and also because a lot of developmental biology is founded on which animals it was relatively easy to put under a microscope and dye blue in 1929. Starfish and sea urchins are the echinoderms you’ve definitely heard of; feel free to google some others.

Early echinoderm development is pretty well-characterized. By this I mean that, as the echinoderm embryo grows from a single cell, you could pick out any particular cell and predict where it will go next and what organ(s) its descendants will be part of. We have a ... somewhat thorough understanding of what signals these cells use to create those tissues in the right patterns & orientations. Thorough relative to a lot of other animals, for sure! Developmental signaling in general is an active field with a lot of apparently correct yet competing theories about how things work, which is why people such as myself still study it. This paper is primarily about the signals that control the location and timing of skeleton development.

The only principle of developmental signaling you need to understand for this paper is that many signals work through a secreted ligand, often a protein but sometimes a lipid or other chemical. A group of cells secretes the ligand into the general area around them; other cells, which have a receptor on the surface in a corresponding shape, scoop the ligand up into the receptor. Cells have various clever tricks for how receptors report receiving the signal, including counting how much signal is received based on how many receptors bind a ligand and how often they pick one up. This signal makes a bunch of other genes turn on or off, ultimately changing which proteins the receiving cells produce and what kinds of cells they can divide into in the future. One kind of cells that you can tell things to turn into is bone! This eventually makes a skeleton.

In most echinoderms, the skeleton forms at the very end of embryonic development, when most of the body structures are already in place and right before the animal becomes able to move around and eat and stuff. Sea urchins, however, are weird and do not do this. Instead they make a really visually distinct set of cells, called “micromeres” because they are extra little[2], very very early on, when there are still only like thirty cells total in the whole embryo. Micromeres hang around at the top end of the embryo dividing for a little while, then all migrate to the middle of the animal to become mesenchyme (lit. “middle stuff”), way before any of the other cells start to do that. They[3] then begin to form a skeleton while the animal is still gastrulating[4]. This is fucking wildly early, since the sea urchin barely even has an “inside” and “outside” at this point, making it a very weird and interesting event from a dev bio perspective. It also forms a very interesting evolutionary question: “Hey, where’d you get those early embryonic skeletogenic micromeres? None of your siblings have those!”

Part 2: this paper

Ettensohn et al. 2019 looks at exactly what dictates that micromeres specifically turn into bone.

The micromere-derived bonemaking cells (hereafter called “PMCs” (see footnote 3 and/or don’t worry about it)) are the entire and only set of cells that make the skeleton. As such, you would probably expect them to be running some very special and unique skeleton programs that make only them able to become spooky, scary, etc.

However, we know that if you very carefully surgically remove all the PMCs from a sea urchin embryo, you… still get a skeleton. A group of other nearby cells, the BC cells[5], will helpfully step up right away to make a skeleton instead! This will eventually cause some problems but the skeleton they make is pretty much 100% normal and correct. BCs, it appears, have all the same skeleton-making capacity that the PMCs do, they just can’t turn it on unless the PMCs are gone! That is very exciting and suggestive of some kind of interesting interactions between the PMCs, the BCs, and whatever external cues/signals dictate the timing and specificity of skeleton development.

What follows is a series of very fun classical developmental biology experiments:

The authors prevent the sea urchins from making an important (previously identified) skeleton-inducing signal, VEGF, or the VEGF receptor, and show that this prevents skeleton formation by BCs even when PMCs have been removed. No VEGF, or no ability to grab & identify VEGF, means no turning on the emergency backup skeleton.

If they surgically replace all of the normal PMCs with VEGF-receptor-free PMCs, then it’s as if the PMCs aren’t there at all; a BC-based skeleton gets made instead.

If they add lots and lots of extra VEGF, then even though normal, VEGF-receiving PMCs are present, the BCs start making a skeleton anyway.

The authors conclude that normally, PMCs somehow prevent BCs from receiving the VEGF signal that would activate their skeleton formation. The authors specifically conclude that the reason PMCs need VEGF receptor for this is because the PMCs are acting as VEGF sponges, soaking up all the VEGF on their receptors so none of it can reach and activate the BCs. The reason extra VEGF turns on BC skeletogenesis anyway is because it’s a high enough dose that the PMCs couldn’t collect all the VEGF on their receptors and some of it made it through to the BCs. This is made more plausible because the PMCs are physically right next to the cells that make VEGF, so they could imaginably collect all the VEGF as soon as it’s produced without letting it circulate.

The authors don’t really rule out the possibility that some other signal the PMCs turn on in response to VEGF indirectly leads them to prevent BC skeletons, rather than just acting as a direct physical barrier, but their hypothesis is a pretty neat and appealing explanation of their results.

This is a fun conclusion to think about because we tend to think of these things in terms of signals activating other signals, rather than a physical competition for signaling molecules, and it’s an interesting possible answer to have in your toolkit.

Part 3: Takes

The thing about this paper is that it’s a totally functional, persuasive next step in the interpretation of developmental signaling mechanisms in sea urchins; I think the experiments are well-presented and logical and the conclusions directly derived from those experiments make sense and are interesting. However, absolutely none of this directly addresses the claim made in the title or seems to justify a specific evolutionary interpretation of the results. As far as I can tell this paper contains absolutely no experiments that directly test any evolutionary hypotheses, at all, whatsoever.

In particular: the hypothesis the paper claims to support is that there’s an ancestral set of skeletogenic signals that were evolutionarily redirected/preempted from specifically a later, adult-stage activation to the PMC’s super-early-stage skeleton-making. They further claim that the reason the BCs have a functional skeleton-making program is because they contain that ancestral program, now preempted by PMCs soaking up all the VEGF signal. But the BC cells also make a skeleton at basically the exact same time the PMCs do, in the exact same way! There’s no strong reason to think that they form some adult-stage skeleton mechanism, because they don’t act later than the PMCs do in sea urchins!

Okay, they are arguing that the VEGF signal is also earlier in sea urchins than it was in their ancestors, which could answer my objection. However: if they’re right, then the skeletons in all the other echinoderms without the “recently evolved” PMCs should make skeletons out of BCs/a BC-descended lineage in a VEGF-mediated way. You should be able to check if starfish make their skeletons out of BC cells, and if they have a VEGF pulse late in development that’s required for their skeletons to form. It does not appear to be known whether this is the case, and the authors don’t check. They should!

In conclusion, there’s nothing wrong with just doing developmental biology, you don’t need to pretend you’ve actually done evolutionary biology instead.

footnotes

1. “a summary of a paper i am in the middle of reading”

2. Again, these were named in like 1929 or something, relative bigness was most of the information they had.

3. Well, technically the micromeres’ descendants, which are now called “primary mesenchyme cells”, or PMCs; since this is to my knowledge the only kind of tissue that micromeres produce, the distinction is imo not important except that PMC is faster to type.

4. To understand gastrulation, imagine you have a spherical blob of something, and then you kind of stretch the blob out just enough to fold it in half. Now you have a blob with different stuff in the middle than there used to be, and also kind of a hollow inside bit, that you can make internal organs in! Stretching it out means it can have a front and back end now! You could put eyes on that thing! This blob-stretching movement is the basic organizational principle by which pretty much all animals go from “sphere” to “shaped”.

5. Short for “blastocoel”, a structure you don’t need to worry about.

#box opener #doctor worm #dev bio #long post about a paper i thought was basically fine but hyped itself wrong #u can really tell that i did the bg b4 readng the rest and realizing they never actually address the title at all #homeobox

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orphicsalon1 · 4 years

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