Development of the hypochord and dorsal aorta in the zebrafish embryo Danio rerio

Abstract

The hypochord of the zebrafish embryo (Danio rerio) emerges at the 9-somite stage as a single row of cells in the dorsomedial endoderm immediately ventral to the notochord. It is recognizable from the 2(nd) or 3(rd) somite and extends along the trunk to the same extent as the somites. A basal lamina surrounds the hypochord and its cells are slightly larger than the nearby endoderm cells. TEM studies have shown that the hypochord cells contain, in addition to mitochondria, well-developed rough endoplasmic reticula and Golgi networks, indicating synthetic activity. Once formed, the hypochord will stay in close association with the notochord, and this axial complex gradually moves dorsally, separating the hypochord from the endoderm as a one-cell-wide, rod-like structure that is bean-shaped in transverse section. This is the situation in the 15-somite embryo, at the level of the 4-5(th) somites. In the gap between the hypochord and the endoderm, angioblast cells aggregate and start to form the dorsal aorta, which becomes intimately associated with the hypochord. In the 17-somite embryo the aortic rudiment is established just ventral to the hypochord as a tube with a lumen. As development proceeds, the size of the hypochord decreases. In the pec fin embryo the hypochord is still recognizable in the posterior trunk, but has apparently vanished in anterior regions. The temporal correlation between the appearance of the hypochord and the formation of the dorsal aorta, coupled with the intimate relationship between these structures, suggest that the hypochord may play a role in the positioning of the dorsal aorta.

Expression of the fras1/frem gene family during zebrafish development and fin morphogenesis

“…This suggests that frem3 expression is activated only when cells of the hypochord become significantly differentiated from a midline precursor population; an observation suggesting characteristic changes in ECM complement when cells of the hypochord differentiate. Similar expression of other ECM components at this stage have previously been noted in zebrafish (Yan et al, 1995;Higashijima et al, 1997) and morphological studies of the hypochord have shown that it develops ultrastructural characteristics of an epithelia, including a well-defined basal lamina (Eriksson and Lofberg, 2000). Mammalian studies suggest that many Fras/Frem interactions also take place in the context of a basement membrane (reviewed in Short et al, 2007).…”

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.…”

Loss of Function of Parathyroid Hormone Receptor 1 Induces Notch-Dependent Aortic Defects During Zebrafish Vascular Development

“…The hypochord is a transient structure sitting immediately between the notochord and the developing aorta, and is believed to contribute to aortic development. 31,32 Compared with control morphants, pthr1 morphant embryos ( Figure 4B) frequently demonstrated defects in Notch signaling in the hypochord (thin arrow). When we quantified the percentage of embryos with such a defect in a blinded manner, we found that defective Notch signaling could be identified in 58% of PTHR1 morphants (n=40), whereas only 5% (1 embryo from 20) of control morphants had any interruption in hypochordal Notch signaling ( Figure 4C).…”