Role of BMP-4 in the inducing ability of the head organizer in Xenopus laevis

Foregut endoderm is specified early in avian development through signal(s) emanating from Hensen's node or its derivatives

In this study, the initial specification of foregut endoderm in the chick embryo was analyzed. A fate map constructed for the area pellucida endoderm at definitive streak-stage showed centrally-located presumptive cells of foregut-derived organs around Hensen's node. Intracoelomic cultivation of the area pellucida endoderm at this stage combined with somatic mesoderm resulted in the differentiation predominantly into intestinal epithelium, suggesting that this endoderm may not yet be regionally specified. In vitro cultivation of this endoderm for 1-1.5 day combined with Hensen's node or its derivatives but not with other embryonic structures/tissues elicited endodermal expression of cSox2 but not of cHoxb9, which is characteristic of specified foregut endoderm. When the anteriormost or posteriormost part of the area pellucida endoderm at this stage, whose fate is extraembryonic, was combined with Hensen's node or its derivatives for 1 day, then enwrapped with somatic mesoderm and cultivated for a long period intracoelomically, differentiation of various foregut organ epithelia was observed. Such epithelia never appeared in the endoderm associated with other embryonic structures/tissues and cultured similarly. Thus, Hensen's node and its derivatives that lie centrally in the presumptive endodermal area of the foregut are likely to play an important role in the initial specification of the foregut. Chordin-expressing COS cells or noggin-producing CHO cells transplanted into the anteriormost area pellucida of the definitve streak-stage embryo could induce endodermal expression of cSox2 but not of cHoxb9, suggesting that chordin and noggin that emanate from Hensen's node and its derivatives, may be involved in this process.

XBtg2 is required for notochord differentiation during early Xenopus development

The notochord is essential for normal vertebrate development, serving as both a structural support for the embryo and a signaling source for the patterning of adjacent tissues. Previous studies on the notochord have mostly focused on its formation and function in early organogenesis but gene regulation in the differentiation of notochord cells itself remains poorly defined. In the course of screening for genes expressed in developing notochord, we have isolated Xenopus homolog of Btg2 (XBtg2). The mammalian Btg2 genes, Btg2/PC3/TIS21, have been reported to have multiple functions in the regulation of cell proliferation and differentiation but their roles in early development are still unclear. Here we characterized XBtg2 in early Xenopus laevis embryogenesis with focus on notochord development. Translational inhibition of XBtg2 resulted in a shortened and bent axis phenotype and the abnormal structures in the notochord tissue, which did not undergo vacuolation. The XBtg2-depleted notochord cells expressed early notochord markers such as chordin and Xnot at the early tailbud stage, but failed to express differentiation markers of notochord such as Tor70 and 5-D-4 antigens in the later stages. These results suggest that XBtg2 is required for the differentiation of notochord cells such as the process of vacuolar formation after determination of notochord cell fate.

In vitro induction and transplantation of eye during early Xenopus development

A vertebrate eye was induced via a series of coordinated inductive interactions. Here, we describe a novel in vitro system to induce eye formation at high frequency using Xenopus early gastrulae. The eye formed in vitro is morphologically similar to the normal eye. When the in vitro eye was transplanted into a stage-33 tadpole, the optic nerve was seen extending from the grafted eye to the tectum of the host brain and the in vitro eye graft was retained after metamorphosis. In addition, we transplanted the eye formed in vitro into a tadpole with both eyes removed. The resultant juvenile frogs could perceive brightness using the grafted eye and thereby control their skin color, suggesting that the eye formed in vitro could function normally.