The establishment of spemann's organizer and patterning of the vertebrate embryo

Abstract

Molecular studies have begun to unravel the sequential cell-cell signalling events that establish the dorsal-ventral, or 'back-to-belly', axis of vertebrate animals. In Xenopus and zebrafish, these events start with the movement of membrane vesicles associated with dorsal determinants. This mediates the induction of mesoderm by generating gradients of growth factors. Dorsal mesoderm then becomes a signalling centre, the Spemann's organizer, which secretes several antagonists of growth-factor signalling. Recent studies have led to new models for the regulation of cell-cell signalling during development, which may also apply to the homeostasis of adult tissues.

Constructing the hindbrain: Insights from the zebrafish

“…The source of the first ("activation") signal is the embryonic organizer (the dorsal lip of the amphibian blastopore), which is required for specification of neural tissue as well as for dorsalization of mesoderm and endoderm through production of BMP antagonists (reviewed by De Robertis et al, 2000;Schier, 2001). The source and molecular identity of the second ("posteriorizing") signal is more controversial.…”

The ankyrin repeat protein Diversin recruits Casein kinase Iε to the β-catenin degradation complex and acts in both canonical Wnt and Wnt/JNK signaling

“…Components of the Wnt-signaling pathway are known to control axis formation in Xenopus embryos (De Robertis et al 2000). Ventral injection of Diversin mRNA into 4-cell stage embryos blocked the formation of secondary body axes that are induced by Dishevelled or CKI⑀ (Fig.…”

“…The establishment of the dorsal organizer and the subsequent formation of the embryonic axis is an early event in zebrafish embryogenesis and requires Wnt/␤-catenin signals (De Robertis et al 2000;Schier 2001). Interference with these early processes perturbs subsequent development.…”

“…Recent work shows that signals mediated by Wnt ligands branch into several intracellular pathways as follows: (1) the canonical Wnt pathway employs ␤-catenin/Tcf as transcription factors to activate target genes important in embryonic patterning and tumorigenesis (Behrens et al 1996;Molenaar et al 1996; for review, see Bienz and Clevers 2000;De Robertis et al 2000); (2) the planar cell polarity pathway (Wnt/JNK pathway) depends on JNK activity and controls cytoskeletal rearrangements (Mlodzik 2000;Sokol 2000); (3) the Wnt/Ca 2+ pathway controls PKC and CamKII and regulates cell adhesion and motility (Kuhl et al 2000;Winklbauer et al 2001); and (4) a not well-characterized novel pathway regulates spindle orientation and asymmetric cell division ; for review, see Huelsken and Birchmeier 2001). Much recent work has been directed toward deciphering the molecular aspects of cytoplasmic diversification of the Wnt-signaling pathway.…”

Cholesterol homeostasis in development: The role of Xenopus 7‐dehydrocholesterol reductase (Xdhcr7) in neural development

“…2Aa-e') is restricted to organizing centres (Spemann organizer and notochord). This suggests a need for de novo cholesterol biosynthesis for early patterning events in Xenopus, since many patterning signals are secreted from these structures during vertebrate development (Cleaver et al, 2000;De Robertis et al, 2000;Fleming et al, 2004). Furthermore, the highest expression of Xdhcr7 was detected by RT-PCR analysis from early gastrulation to early organogenesis stages, a time-window during which different tissue and organ identities are established.…”