Vertebrate floor-plate specification: variations on common themes

Situated at the ventral-most part of the vertebrate neural tube, the floor plate (FP) is an important signalling centre that controls the regional differentiation of neurons in the nervous system. It secretes guidance molecules that direct ventrally navigating axons crucial for the correct wiring of neuronal circuits. Although the function of the FP is well-conserved from fish to humans, discrepancies exists with respect to both the signalling system involved in FP induction, and the origin of the FP in various vertebrate species. Recent findings from the embryos of zebrafish, chicken and mouse provide insights that reconcile previous results and suggest common themes in vertebrate FP specification.     

Vertebrate eye development.

Vertebrate eye determination is mediated by a series of inductive interactions that have now been more precisely defined with the use of regional markers. Analyses of the genes responsible for eye mutations and the cloning of genes delimiting spatial domains within the developing eye have begun to elucidate the molecular basis of this process.     

Vertebrate development: taming the nodal waves

Recent results have provided evidence that regulatory loops operating in vertebrate embryos between the developmental signalling factors Nodal and Lefty may provide a real example of the kind of reaction-diffusion process long predicted to be a mechanism of pattern formation.     

Vertebrate myotome development

The embryonic myotome generates both the axial musculature and the appendicular muscle of the fins and limbs. Early in embryo development the mesoderm is segmented into somites, and within these the primary myotome forms by a complex series of cellular movements and migrations. A new model of primary myotome formation in amniotes has emerged recently. The myotome also includes the muscle progenitor cells that are known to contribute to the secondary formation of the myotome. The adult myotome contains satellite cells that play an important role in adult muscle regeneration. Recent studies have shed light on how the growth and patterning of the myotome occurs.     

Vertebrate development: Et in Arkadia

The similarities in organiser formation in Xenopus and mouse embryos have remained elusive. Recent evidence suggests a common mechanism, in which an intracellular protein, Arkadia, is required for formation of the mouse organiser and potentiates the effects of the signalling protein Nodal.     

Vertebrate development: wnt signals at the crest

Neural crest cells, a defining feature of vertebrate embryos, form at the neural plate border in response to inductive signals from the ectoderm. Recent studies have shown that Wnt signals are essential mediators of this induction.     

Vertebrate craniofacial development: novel approaches and new dilemmas.

Unexpected patterns of neuroblast, angioblast and myoblast movement and tissue organization have been defined using lineage tracing and transplantation methods. The most novel and enigmatic new data derive from analyses of genes in the Hox- and Pax-gene families. In addition to the characterization of expression patterns, the effects of Hox-gene knock-out and retinoic acid treatment have been assessed. These basic studies are complemented by the identification of correlations between inherited craniofacial anomalies, for example Waardenburg's syndrome, and the function of specific genes.     

Collective specification of cellular development

Studies of chimeras and in vivo development demonstrate that cell lineages are often quite variable, apparently in response to chance perturbations. This points to an apparent contradiction: although individual cells are the units of genetic information and differentiation, not all cellular events need be precise for the development of functional organisms. The social organization of ants can serve as a metaphor that helps understand the mechanisms that underlie such development. Ants suggest that continued cellular interactions and environmental conditions could specify the proportion and general location of specialized units. Leaf venation is used as a concrete example of this general principle. A signal produced continuously by all cells specifies a requirement for vein differentiation. The cells that respond by differentiation then transport the signal away from the leaf; this removal acting as a feedback indicating that the requirement is being met. Because transport increases during vein differentiation, early initiation occurs in excess and vein 'competition' for the signal assures an acceptable outcome. Such specification would be robust since it does not depend on events in any single cell, and chance events, rather than being corrected or reversed, may be built upon in reaching an expected, collective phenotype. The absence of detailed information preceding development distinguishes this hypothesis from the common alternatives of a program or blueprint. Collective specification would have important implications for developmental plasticity and evolution.     

Vertebrate head development: Segmentation,novelties, and homology

Vertebrate head development is a classical topic lately invigorated by methodological as well as conceptual advances. In contrast to the classical segmentalist views going back to idealistic morphology, the head is now seen not as simply an extension of the trunk, but as a structure patterned by different mechanisms and tissues. Whereas the trunk paraxial mesoderm imposes its segmental pattern on adjacent tissues such as the neural crest derivatives, in the head the neural crest cells carry pattern information needed for proper morphogenesis of mesodermal derivatives, such as the cranial muscles. Neural crest cells make connective tissue components which attach the muscle fiber to the skeletal elements. These crest cells take their origin from the same visceral arch as the muscle cells, even when the skeletal elements to which the muscle attaches are from another arch. The neural crest itself receives important patterning influences from the pharyngeal endoderm. The origin of jaws can be seen as an exaptation in which a heterotopic shift of the expression domains of regulatory genes was a necessary step that enabled this key innovation. The jaws are patterned by Dlx genes expressed in a nested pattern along the proximo-distal axis, analogous to the anterior–posterior specification governed by Hox genes. Knocking out Dlx 5 and 6 transforms the lower jaw homeotically into an upper jaw. New data indicate that both upper and lower jaw cartilages are derived from one, common anlage traditionally labelled the “mandibular” condensation, and that the “maxillary” condensation gives rise to other structures such as the trabecula. We propose that the main contribution from evolutionary developmental biology to solving homology questions lies in deepening our biological understanding of characters and character states.     

Cortical development: the art of generating cell diversity

The fascinating question of how the enormous diversity of neuronal and glial cells in the cerebral cortex is generated during development was recently discussed at a meeting on cortical development and stem cells in Greece. What emerged from this meeting is an equally fascinating answer, namely that precursor diversity at rather early stages of development anticipates later cell type diversity.     

Embryonic development: a new SPN on cell fate specification

The recent identification and characterization of the Caenorhabditis elegans gene spn-4 has shed new light on the mechanisms that link embryonic polarity to the specification of cell fates.