The presomitic mesoderm (PSM) is an embryonic tissue that elongates, giving rise to dermis, cartilage, and muscle. Defects can cause caudal regression syndromes that have no cure. We know a tissue-level gradient of random cellular motility directs PSM elongation, but it is unknown how individual PSM cells form this gradient.
We segmented PSM from 44-hour chicken embryos into anterior to posterior sections, dissociated each section into single cells, and sparsely plated the cells onto glass-bottomed chamber slides for overnight culture and time-lapse imaging. We analyzed the resulting videos for appropriate motility properties, such as total motile cells and mean speed of motile cells.
Motile cells had multiple protrusions and non-motile cells were round. By this morphology, posterior sections contained more motile cells and anterior sections contained more non-motile cells. Motile cells, regardless of region, all moved with a comparable speed; posterior motile cells did not move faster than anterior motile cells.
This suggests that the tissue-level motility gradient along the PSM is formed by the graded distribution of motile (versus non-motile) cells with more motile cells existing in the posterior, rather than by the graded distribution of motile ability (i.e. speed).