beta-Catenin is a central component of both the cadherin-catenin cell adhesion complex and the Wnt signaling pathway. We have investigated the role of beta-catenin during brain morphogenesis, by specifically inactivating the beta-catenin gene in the region of Wnt1 expression. To achieve this, mice with a conditional ('floxed') allele of beta-catenin with required exons flanked by loxP recombination sequences were intercrossed with transgenic mice that expressed Cre recombinase under control of Wnt1 regulatory sequences. beta-Catenin gene deletion resulted in dramatic brain malformation and failure of craniofacial development. Absence of part of the midbrain and all of the cerebellum is reminiscent of the conventional Wnt1 knockout (Wnt1(-/-)), suggesting that Wnt1 acts through beta-catenin in controlling midbrain-hindbrain development. The craniofacial phenotype, not observed in embryos that lack Wnt1, indicates a role for beta-catenin in the fate of neural crest cells. Analysis of neural tube explants shows that (beta-catenin is efficiently deleted in migrating neural crest cell precursors. This, together with an increased apoptosis in cells migrating to the cranial ganglia and in areas of prechondrogenic condensations, suggests that removal of beta-catenin affects neural crest cell survival and/or differentiation. Our results demonstrate the pivotal role of beta-catenin in morphogenetic processes during brain and craniofacial development.

Inactivation of the beta-catenin gene by Wnt1-Cre-mediated deletion results in dramatic brain malformation and failure of craniofacial development.

Topographic organization of embryonic motor neurons defined by expression of LIM homeobox genes

The generation of distinct neuronal cell types in appropriate numbers and at precise positions underlies the assembly of neural circuits that encode animal behavior. Despite the complexity of the vertebrate central nervous system, advances have been made in defining the principles that control the diversification and patterning of its component cells. A combination of molecular genetic, biochemical, and embryological assays has begun to reveal the identity and mechanism of action of molecules that induce and pattern neural tissue and the role of transcription factors in establishing generic and specific neuronal fates. Some of these advances are discussed here, focusing on the spinal cord as a model system for analyzing the molecular control of central nervous system development in vertebrates.

https://science.sciencemag.org/content/sci/274/5290/1115.full.pdf

Diversity and Pattern in the Developing Spinal Cord

VERTEBRATE midbrain development depends on an organizing centre located at the isthmus, a constriction in the embryonic mid/hindbrain region1–3,28. Isthmic tissue grafts transform chick caudal forebrain into an ectopic midbrain that is the mirror image of the normal midbrain4. Here we report that FGF8 protein has the same midbrain-inducing and polarizing effect as isthmic tissue. Moreover, FGF8 induces ectopic expression in the forebrain of genes normally expressed in the isthmus, suggesting that the ectopic midbrain forms under the influence of signals from a new 'isthmus-like' organizing centre induced in the forebrain. Because Fgf8 itself is expressed in the isthmus, our results identify FGF8 as an important signalling molecule in normal midbrain development.

Midbrain development induced by FGF8 in the chick embryo

Cell cycle progression and exit must be precisely patterned during development to generate tissues of the correct size, shape and symmetry. Here we present evidence that dorsal-ventral growth of the developing spinal cord is regulated by a Wnt mitogen gradient. Wnt signaling through the beta-catenin/TCF pathway positively regulates cell cycle progression and negatively regulates cell cycle exit of spinal neural precursors in part through transcriptional regulation of cyclin D1 and cyclin D2. Wnts expressed at the dorsal midline of the spinal cord, Wnt1 and Wnt3a, have mitogenic activity while more broadly expressed Wnts do not. We present several lines of evidence suggesting that dorsal midline Wnts form a dorsal to ventral concentration gradient. A growth gradient that correlates with the predicted gradient of mitogenic Wnts emerges as the neural tube grows with the proliferation rate highest dorsally and the differentiation rate highest ventrally. These data are rationalized in a 'mitogen gradient model' that explains how proliferation and differentiation can be patterned across a growing field of cells. Computer modeling demonstrates this model is a robust and self-regulating mechanism for patterning cell cycle regulation in a growing tissue. Supplemental data available on-line

A mitogen gradient of dorsal midline Wnts organizes growth in the CNS

Wnt expression patterns in chick embryo nervous system.

Organization of motor pools supplying axial muscles in the chicken

Neurogenesis in the vertebrate neural tube.

The location and distribution of neural crest-derived Schwann cells in developing peripheral nerves in the chick forelimb.

Anterograde transport of horseradish peroxidase was used to map the initial projection patterns of motor and sensory axons innervating the wing of the chick embryo. Injections which resulted in labeling large numbers of motor and sensory axons, separately or in combination, were used to define the time course of innervation and to visualize the progressive morphogenesis of the peripheral nerve pattern. Motor axons emerged from the spinal cord and accumulated near the ventromedial border of the myotome where they remained for up to 16 hours before growing into the plexus region and limb bud. Despite the known later time of sensory neuron production, the first sensory axons projected to the wing at the same time as motor axons. When axons first entered the wing bud, they were distributed in two loosely organized sheets of axon fascicles, one projecting to dorsal muscle mass, the other to ventral muscle mass. The width of the sheets was between one-third to one-half the width of the wing bud, and this distance was more than twice the diameter of the proximal nerve trunks measured at stage 28. In the proximal limb the basic pattern of peripheral nerves emerged gradually from stages 26 to 28. During these stages, the loosely organized sheets of axonal fascicles seen at younger stages were progressively transformed into several coherent nerve trunks and muscle nerves extended from common nerve trunks. The implication of these observations is that many outgrowing axons appear not to follow preformed pathways corresponding to the mature peripheral nerve branching pattern. This pattern may instead result from axonal recognition of cues within a largely undifferentiated limb bud, and from the subsequent bundling together of loosely organized axon fascicles. These events occur concurrently with limb growth and differentiation.

Margaret Hollyday

Papers

Wnt expression patterns in chick embryo nervous system.

Organization of motor pools supplying axial muscles in the chicken

Neurogenesis in the vertebrate neural tube.

The location and distribution of neural crest-derived Schwann cells in developing peripheral nerves in the chick forelimb.

Chick wing innervation. I. Time course of innervation and early differentiation of the peripheral nerve pattern