time is telling

A few days ago Dr Fleury wrote me a quite short message, with just the subject line “time is telling” and a paper attached :

Who moves whom during primitive streak formation in the chick embryo, Manli Chuai and Cornelis J. Weijer, HFSP J. 2009 April; 3(2): 71–76. Published online 2009 March 31. doi: 10.2976/1.3103933.

Those interested by following the discussion on this post two more pieces will be helpful : The ECM Moves during Primitive Streak Formation—Computation of ECM Versus Cellular Motion, Evan A Zamir, Brenda J Rongish, and Charles D Little, PLoS Biol. 2008 October; 6(10): 1–9, doi: 10.1371/journal.pbio.0060247 and of course An elasto-plastic model of avian gastrulation. Fleury V., Organogenesis. 2005 Jan;2(1):6-16 [epmag]. The later being the first publication of Fleury’s model.

I was awaiting Fleury’s reaction since April 2009 and it came just a few days ago. We had earlier a discussion about Zamir et al. paper which contradicts Flery’s model, which itself contains contradictory elements.

Let’s see figure 4 of epmag, depicting an epiblastic cell, “embedded between extracellular membranes, and crawling towards the posterior pole“.

cpt.png

The traction from both sides would be responsible, according to Fleury, for the production of “a crack in the extra cellular matrix“.

Earlier, in fig 3 of epmag, Fleury presented the deformation of “a solid [which] is pushed between plates” as being a parabolic (Poiseuille) deformation, with the flow being oriented towards the midline of the blastodisk.

In fact epiblastic cells are not embedded between extracellular membranes. I used this Fleury’s error to make clear during a few exchanges that he was attached to the idea of the Poiseuille deformation, thus a tensile deformation which can’t produce a crack at the midline, but rather ripples of the ECM. When I illustrated the fact that according to this point of view crack would be expected not at the midline of the blastodisc but at the rear of the cells I didn’t get any answer except that I had to think about it ; I don’t call that an answer.

When Zamir et al. paper was published, and because the authors pointed what I had illustrated with a lazy experiment almost a year earlier, that is that :

At the very least, our data suggest that somehow epiblastic cells pull the matrix toward the midline, rather than crawl across it.

and they also nicely illustrated once more that there are just two vortices at the epiblast’s level during early gastrulation, I added a few comments at an ongoing discussion with Fleury.

That didn’t lead us to resolve the contradiction or explain the “crack” which propagated till the cteappv (first occurrence p24,c2,§1).

The rather laconic and quite sibylline message I received isn’t of any help. Let’s go back to Chuai &Weijer’s paper.

It would be more appropriate to formulate the title as a question: “Who moves whom during primitive streak formation in the chick embryo?

The conclusions of the paper leave the question open, after exposing what is known and what hypothesis may be tested to start getting answers and how to implement them:

These results [Zamir et al.] will need first of all confirmation by a series of independent experimental variations to test whether the labeled fibronectin molecules faithfully represent the dynamics of the matrix in vivo. If these results can be confirmed, then their thought provoking implications and some of the interpretations given by the Zamir et al. will need a lot more work to be substantiated and mechanistically understood. This will require the development and application of methods to identify cells that move actively and cells that are displaced passively, what forces are produced by cells in specific regions of the embryo, as well as methods to measure the mechanical characteristics of the extracellular matrix.

I “time is telling” something is that the model proposed by Fleury contains at least one major flaw (as two contradictory are proposed), maybe two as hypothetical solutions independent of Fleury’s model exist. And once more “time is telling” that there are two vortices on the epiblast, not four.

Recently, Fleury published on his website time-lapse animations supposed to show the two missing vortices, L2/R2. They time-travelled from early gastrulation to late neurulation. The latest animated gif is a detail without the somites but used to continuous and silent editing from Fleury according to remarks I kept a copy of the original. The description is:

Here especially, tracking of tissue flow by PIV technique, in vivo, in a chicken embryo (the blue cross is the last point, the green dots the subsequent positions superimposed on the final plate, with 5 minutes interval)

That’s a very special PIV technique. PIV stands for Particle Image Velocimetry and is usually performed seeding the observed flow with particles (not the case here) and using a LASER beam to illuminate a plane of the sample (not the case here) with a single camera (for 2D observations) or two cameras for stereoscopic 3D observations (not the case here, should be as the observed movements are complex due to neurulation). Anyway, the proposed cell trajectories have nothing to do with velocimetry!

On the other hand, you don’t set publicly available on the Net an image with EMBARGO on it, do you? Fleury do!

EBA4A692-6DE4-450B-AEE1-CB45042D9AD9.jpg

Reminder for Dr Fleury

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