Hypochord, an enigmatic embryonic structure: study of the axolotl embryo.

The hypochord of the axolotl embryo is first visible at an early tailbud stage, forming a rod-like structure, situated immediately under the notochord. A profusion of extracellular matrix fibrils is attached to the dorsolateral regions of the hypochord, linking it with the somites. A basal lamina develops around the hypochord, indicating an epithelial type of cell differentiation. Abundant rough endoplasmic reticula in the hypochord cells suggest lively synthetic activity. Prospective endoderm cells were vitally labeled with the lipophilic dye 1,1-dioctadecyl-3,3,3',3'-tetramethylindodicarbocyanine perchlorate (DiD) at the gastrula stage. Cells labeled with the dye were later found in the hypochord as well as in the gut endoderm. This shows that the hypochord is of endodermal origin, contrary to recent suggestions that the hypochord is of mesodermal origin, but consistent with histological data. After about 8 days of existence, the hypochord disappears. Experimental results, using an apoptosis detection kit, indicate that the hypochord cells may disintegrate by a type of apoptotic cell death. The close association between the hypochord and developing dorsal aorta suggests that the hypochord could be involved in the positioning of the dorsal aorta, which forms under it.