This one is quite short but certainly the most important points. Fleury presents:
We assume that the blastodisc is a composite structure, namely a “sandwich”: Extra-Cellular Matrix/ cellular layers/Extra-Cellular Matrix. (p8, c1)
I suppose that here what is described is not the blastodisk but the epiblast. But this is not clear as there are several cellular layers. But later on it seems that the epiblast is considered to be the “sandwich”, the cell movements described being those of cells in epiblast, at least until ingression through the PS starts. What is bothering is that the “sandwich” is described as ECM/cellular layers/ECM.
If we assume that the “sandwich” is the epiblast, we must consider a layer of cells. Organized much like a columnar epithelium, producing a basal membrane. The BM may be the ventral ECM of Fleury’s “sandwich”. So, what about the dorsal ECM?
Data describing the BM don’t show it’s main constituents present dorsally. Thus it’s more correct to differentiate the two ECMs: ECM(?)/epiblastic cells/BM
The BM is a product composed by molecules synthesized by both epiblastic and hypoblastic cells, and isn’t complete ’till stage XIII. The few depictions of the BM at the KS I have seen, present a discontinuity of the BM in the vicinity of and at the KS, with closer contacts between the epiblastic and underlying cells at the sickle’s level. Well, probably there is nothing to « crack » there. And maybe that’ the reason why epiblastic cells don’t seem to move over this area. That’s pure speculation.
On the other hand, the BM under the PS is largely intact up to early stage 3 and is dissolved at stage 3+1.
There seem not to be a correlation between some « crack » of the BM and PS formation.
This is clearly antinomic with Fleury’s modeling.
I hope that what is considered to be the « top plate or membrane » in fig 3B corresponding to one of the « ECM » of the “sandwich” is not the vitellin membrane (just an idea). Else the model should be revisited, starting with the “sandwich”’s definition and using some other analogy, as between the epiblast and the vitellin membrane there is no connexions.
1 http://sternlab.anat.ucl.ac.uk/Tutorial.htm, page 8 of 37 (relative position):
The early triangular streak (stage 2) is made up of a dense accumulation of middle layer cells between epiblast and endoblast (Fig. 3); however it rapidly straightens, to become a mesenchymal rod of parallel sides (stage 3). At this stage there is still no groove in the overlying epiblast and the basement
membrane is largely intact.