Hypochord, an enigmatic embryonic structure: Study of the axolotl embryo

Abstract

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.

The anuran Bauplan: a review of the adaptive, developmental, and genetic underpinnings of frog and tadpole morphology

“…3 for depiction of urostyle development in Hymenochirus boettgeri ). A comparable structure to the hypochord has also been found in lancelets, lampreys, fishes and urodeles, but is curiously absent from amniotes, suggesting secondary loss (see Löfberg & Collazo, 1997 and references cited therein). The embryonic (non‐ossified) hypochord has been implicated in regulating the medial migration of angioblasts to form the dorsal aorta in vertebrates (Cleaver & Krieg, 1998).…”

Ciona intestinalis: Chordate development made simple

“…The hypochord was believed to be a structure of mesodermal origin until 1997, when Lofberg and Collazo showed its endodermal origin by injection of vital dyes in axolotl embryos; the same authors have suggested that the hypochord might play a role in positioning the dorsal aorta (Lofberg and Collazo, 1997). Little is known about molecular markers of this structure in either model system: among the genes expressed in the zebrafish hypochord are some of the forkheadrelated genes (Odenthal and Nusslein-Volhard, 1998); in C. intestinalis, expression of Ci-fkh has been reported in trunk endoderm and endodermal strand (Di Gregorio et al, 2001).…”

The zebrafish tailbud contains two independent populations of midline progenitor cells that maintain long-term germ layer plasticity and differentiate based on local signaling cues

“…The mesodermal or endodermal origin of the hypochord has been a matter of debate, but a consensus view between disparate anamniote species (including Xenopus laevis, axolotl and zebrafish) has emerged ascribing an endodermal character to this tissue (Cleaver et al, 2000;Eriksson and Lofberg, 2000;Lofberg and Collazo, 1997). Amniotes lack a hypochord but its function in patterning blood vessels may have been taken over by dorsal gut endoderm.…”