The Morphogenesis of the Notochord in Amphibia

Abstract

The cellular changes during the early development of the amphibian notochord have been described. They consist essentially in an increase in the area of contact between contiguous cells. After the cells become closely packed, the area of contact is still further increased by a change to a disk-like shape, which causes an augmentation in the proportion of cell surface to volume. The disks are arranged on top of one another like a pile of coins, forming a tissue in which all the cells have a similar (and very small) proportion of their surface in contact with the noncellular ambient medium. The later development of the notochord involves the conversion of the cell contents into cell membrane, the cells becoming large in volume but filled mainly by vacuoles containing a clear sap. At the time this process is beginning, a chordal sheath is being laid down around the cylindrical notochord, and it seems that this impedes an expansion in thickness and causes most of the enlargement to take the form of an increase in length. As development from late gastrula to neurula proceeds, isolated fragments of presumptive chorda become more resistant to the disaggregating influence of alkaline salines. Rather unexpectedly, Triturus tissue was always more easily disaggregated than Axolotl. The isolated cells exhibit the forms of movement already described by Holtfreter for other tissues of the amphibian embryo. Calcium ions and a detergent (sodium lauryl sulphonate) were without effect on the disaggregation. Single isolated presumptive chorda cells from late gastrulae or neurulae can proceed with their normal histogenesis, becoming fully differentiated vacuolated and polyhedral cells, although remaining somewhat smaller than normal.