Our new #zebrafish preprint is up at biorxiv! https://www.biorxiv.org/content/10.1101/783043v1
This was such a fun project to work on because we got to do a lot of cool imaging!

This is a story about this group of cells, called the lateral line primordium. They migrate along the embryo, between the skin and the muscle, depositing epithelial clusters that will go on to form sense organs. But... how do they move effectively, squished in that tight space?

We know that they use basal filopodia and cryptic lamellipodia like these on their basal surface to migrate along the horizontal myoseptum.

But what about up top, against the skin? Well the first clue came from high resolution isotropic DiSPIM imaging done by our collaborators at NIBIB. Here is a side view of the primordium, and notice that row of cells along the top, pressed against the skin? What are they doing?

We can reconstruct the whole primordium cell by cell in a 3D rendering and see that these cells wrap around the entire epithelial core, like a sheath.

So... What are these cells doing? Some high resolution imaging later, and we discover that they're sending out protrusions against the skin that look a whole lot like lamellipodia! This gives us a great opportunity to study how cells migrate in confinement in vivo.

Actin dynamics have traditionally been hard to observe in live embryos, but in this situation the imaging power of the Zebrafish embryo, combined with fast Airyscan from @zeiss_micro, allowed us to see retrograde actin flow in these protrusions. And it's fast (6-7um/min)!

It looks like these cells are using both the basal and apical surface to crawl along. So, what happens when we remove the skin? Well, a tungsten needle and a steady hand gave us the answer: They can't move without the skin, but they start moving as soon as the skin heals back!

What happens to these apical lamellipodia when you remove their substrate? Surprisingly, they turn into membrane blebs. You can see the rapid membrane expansion (magenta), followed by accumulation of cortical actin (green).

But, as soon as the skin heals back over, these blebs are converted back into lamellipodia, and the primordium resumes migration.

But, why can't they migrate using the basal lamellipodia alone? After all, cells in culture migrate just fine with only one side attached to a substrate.

Turns out that removing the skin also causes the basal lamellipodia to depolarize and start blebbing, even though they're not in direct contact with the skin. It seems like the compression from the skin itself might be required for effective migration.

There's a lot more in the paper, including some movies of Actin dynamics during the healing process, chemical inhibition experiments, and all the quantification of these observations. Also, for those going, I'll be giving a talk on this (and more!) at #ASCB2019 in December in DC!

Lastly, I want to mention that this whole project started as a summer project for an intern we had in the lab, who noticed something weird and decided to follow up on it. I think it's a good lesson for students to run with interesting observations, and for mentors to let them