Embryonic mesoderm in amphioxus, shown by serial block-face scanning electron microscopy, includes likely lateral plate

The mesoderm of amphioxus embryos has lately been examined by single-cell sequencing to elucidate gene regulatory programs directing the emergence of the differentiating tissues. One possible problem with this approach is that the molecular data risk being analyzed in the context of antiquated developmental anatomy, some aspects of which by now are considered contentious or even incorrect. To evaluate this potential difficulty, we examined early neurulae embryos of amphioxus with serial block-face scanning electron microscopy (SBSEM) to reconstruct the three-dimensional fine structure of all the mesoderm cells. We examined all the mesodermal zones (the major ones being the notochord, somitic mesoderm, and likely lateral plate). The results for the notochord strongly support the majority view that it arises as an up-folding along the dorsal midline of the archenteron without any contribution from cell involution over the dorsal lip of the blastopore. For the somitic mesoderm, we unexpectedly found minute accumulations of extracellular laminin-rich material embedded in the most posterior mesoderm. They evidently represent the earliest morphological boundaries of the newest somites. Finally, and most consequentially, our SBSEM indicates that lateral plate mesoderm, often considered to be lacking in amphioxus, is likely present, as it is in vertebrates.