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The role of Nkx genes, transcription factors, in the ventral spinal cord as it relates to neural patterning is discussed in this perspective article. Three central questions can be seen from the general model. One deals with how the Sonic hedgehog (Shh) concentration gradient is generated, another with how the gradient is translated into distinct transcriptional responses, and the third with what the transcriptional regulators that lock in a particular neuronal or glial fate are. Evidence indicates cell type diversity in the dorsal and ventral central nervous system depends on the starting action of qualitatively distinct signals that act in a concentration-dependent way to set up autonomous subdomains of homeodomain protein activity along the D-V axis of the neural tube.

The essential function of Wnt-4 in mammary gland development is discussed. It is downstream of progesterone signaling. If the progesterone receptor is not present in mammary epithelium, ductal sidebranching does not take place. Ectopic expression of the photooncogene Wnt-1 overcomes this lack and its effects.

The Hedgehog signaling pathway is discussed in this review article which tells that new discoveries in the pathway shed light on how shaky understanding of the moleular and cellular mechanisms underlying this vital signaling pathway is. The signaling regulates a great variety of developmental events in the fly embryo and the vertebrate one and is central for several cancers. The model now widely accepted is that Hedgehog signaling leads to ligand-dependent receptor inactivation, the likely receptor encoded by the segment polarity gene, patched. The important role of cholesterol in this signaling is related to Patched, with sequestering and binding and the difference between the two of interest.

Results show that the transcriptional factor T (Brachyury) is involved in the Wnt signaling pathway and it modulates a balance between mesodermal and neural cell during gastrulation of mouse embryo.

The secreted bone-morphogenetic-protein (BMP) antagonist Gremlin, is shown to relay the sonic hedgehog (SHH) signal from the polarizing area to the apical ectodermal ridge (AER). Mesenchymal Gremlin expression is lost in limb buds of mouse embryos homozygous for the limb deformity mutation. If Gremlin-expressing cells are grafted into ld mutant limb buds, Fgf4 expression is rescued and the SHH/FGF4 feedback loop is restored.

Differentiation and proliferation of chondrocytes regulated by Indian hedgehog (Ihh) signaling, which is necessary for bone formation, is discussed. Analysis has been carried out in an Ihh null mutant, and a role for an Ihh/PTHrP feedback mechanism in control of chondrocyte maturation is indicated. Ihh signaling seems to be pivotal in coordinating several cellular processes essential for vertebrate skeleton morphogenesis.

Studies on negative feedback mechanisms are anticipated to advance novel information concerning the pattern formation of multicellular organisms. Two types of mechanisms can be determined based on the way they function in a cell-autonomous or non-cell autonomous way. One of the major issues in understanding the biological roles of negative feedback mechanism is the characterization of their thresholds for inhibitor induction.

The Hedgehog signalling pathway is required for tissue development during embryogenesis. Hedgehog-interacting protein (Hip) is a novel component identified in the vertebrate signalling pathway. It can bind Hedgehog proteins by encoding a membrane glycoprotein. A model where Hip attenuates Hedgehog signalling due to binding to Hedgehog proteins is supported by the findings.

Wnt-7a is a glycoprotein functioning as a signalling molecule in certain developmental contexts. The Wnt-7a associated sterility has been studied in postnatal mice. It was found that the Mullerian duct derivatives of homozygous Wnt-7a mutant females failed to differentiate properly, as compared to wild-type of heterozygous littermates. Wnt-7a-deficient females are infertile due to abnormal oviduct and uterus development, which are Mullerian duct derivatives.

The study of spontaneous mutations has aided the understanding of developmental processes. A large collection of spontaneous or 'classical' mouse mutations has been accumulated over many decades. One of the mutations causes the postaxial hemimelia (px) phenotype, which consists of limb patterning defects accompanied by Mullerian duct-associated sterility in both sexes. We were intrigued that both the limb and the Mullerian duct px phenotypes are similar to those caused by mutations in the gene encoding the Wnt 7a signaling molecule. In this paper, we investigate the nature of the px mutation. Morphological analysis and breeding experiments demonstrate that the px phenotype indeed results from a mutation in the Wnt 7a gene. Molecular analysis demonstrates that px results from a 515-bp deletion in the Writ 7a gene. This generates an abnormal splicing event, which ultimately produces a truncated Wnt 7a protein of half the normal size. Thus, the px mutation is predicted to be a likely null allele of the Writ 7a gene. Our results provide another interesting example of a classical mutation that disrupts an important patterning gene in development. Key Words: Writ 7a; limb/reproductive patterning; px mutation.

Epithelial-mesenchymal interactions govern the development of epidermal organs such as teeth. During the early stages of tooth development, a local ectodermal thickening which expresses several signaling molecules appears. It is believed that these in turn signal to the underlying mesenchyme triggering mesenchymal condensation and tooth development. For example, epithelially expressed Bmp4 induces Msx1 and Lef1 as well as itself in the underlying mesenchyme. In this paper we have investigated the role of four epithelial signaling molecules, Bmp2, Shh, Wnt10a, and Wnt10b, in the early inductive cascades that govern tooth development. We show that all four genes are specifically expressed in the epithelium between E11.0 and E12.0 when tooth morphogenesis is first apparent. Although Shh, Bmp2, and Wnt10b have similar, if not identical, expression patterns, each signal has a distinct molecular action on the jaw mesenchyme. Whereas Shh and Wnt10b can induce general Hedgehog and Wnt targets, Ptc and Gli for Shh and Lef1 for Wnt10b, only Bmp2 is able to induce tooth-specific expression of Msx1. Thus, there are distinct targets for all three pathways. Interestingly, both Bmp and Wnt signaling activate Lef1, making it a candidate for integrating the two distinct signaling pathways. Key Words: epithelial-mesenchymal interactions; tooth development; Wnt signaling; Shh signaling; Bmp signaling.

A ventral-to-dorsal gradient determines the pituitary cell types producing different hormones. An experiment produces in vivo and in vitro proof of the stages involved in the pituitary cell development. The signals that herald the different stages are listed. These cells play a critical role in enabling the pituitary gland to maintain homeostasis.

The mouse gene Noggin appears to play a role in patterning of the neural tube and somite. Noggin encodes an antagonist of bone morphogenetic protein (BMP). Noggin does not appear to be required for neural induction, but plays an essential role in neural tube growth and development. Differentiation of the somite is also deficient in null mutants.

Writ-1 encodes a secreted signaling molecule which is required for development of the midbrain and anterior hindbrain in the mouse. Wnt-1 expression is initiated early in the development of the central nervous system in a region predicted to give rise to the midbrain. Later in development Wnt-1 expression is restricted to regions of the forebrain, midbrain, and spinal cord. Previous studies identified a 5.5-kb enhancer in the Wnt-1 locus which is sufficient to activate transcription of a reporter gene in a pattern very similar to that of the endogenous Wnt-1 gene. Here we have assessed if this enhancer is an important component of the endogenous regulatory sequences of Wnt-1 by gene targeting and by testing if it is able to express Wnt-1 in a pattern which is sufficient to rescue the phenotype of loss of Wnt-1. Our results show that the 5.5-kb enhancer is both necessary and sufficient for Wnt-1 expression in vivo.

The secreted signaling molecule GDNF is expressed in the metanephric mesenchyme and has recently been implicated as a factor necessary for development of the metanephric kidney. We have examined the effects of GDNF on mouse kidney explants. We show that GDNF increases cell proliferation in ureter tips. There is an increase in the number of ureter tips and expansion and fusion of adjacent tips and some tips appear to grow toward the source of GDNF. These events are accompanied by transcriptional upregulation of several genes localized to the tips, including its own receptor, c-ret, the transcription factor Sox9, and the signal Wnt-11. These results support a model in which GDNF supplied by the mesenchyme regulates growth and branching in the metanephric kidney through the local regulation of ureter tip-specific factors.

The absence of both Wnt-1 and Wnt-3a genes is shown to lead to a marked deficiency in neural crest derivatives originating from the dorsal neural tube. The results are consistent with a model whereby local Wnt signalling controls the expansion of dorsal neural precursors. This could represent a model for Wnt signalling in the developing central nervous system.

Studies on the mouse Splotch (Sp) mutation, a deletion in the transcription factor Pax-3, have revealed that Pax-3 is essential for normal development of the neural crest. We have investigated the defect in neural crest development using a Wnt-1::LacZ reporter construct to mark neural crest cells. Staining embryos for [Beta]-galactosidase activity at different developmental stages revealed a severe reduction in the number of neural crest cells which emigrated from the neural tube at the vagal and rostral trunk levels. At the caudal thoracic, lumbar, and sacral levels there was a complete loss of neural crest cell emigration. In contrast to previous work in culture, we saw no evidence for any delay in the onset of neural crest cell migration at anterior levels. Pax-3 is expressed in the dorsal neural tube, where the neural crest cells originate, in migrating neural crest cells, and in somitic cells along the migratory pathway. Hence, it is not clear which aspect of the Pax-3 expression accounts for the observed phenotype. We addressed this problem by transplanting neural tissue between mouse and chick embryos. Our studies indicate that the defect in the Splotch mutation is not intrinsic to the neural crest cells themselves, but appears to reflect inappropriate cell interactions either within the neural tube or between the neural tube and the somite.

Wnt-3a mutant embryos show defects caudal to the forelimb level; somites are absent, the notochord is disrupted, and the central nervous system has a pronounced dysmorphology. Previous studies revealed that the primary defects of the mutant embryos are likely to be in the process of paraxial mesoderm formation. In this study, we analyzed the phenotype of Wnt-3a mutant embryos at early somite stages (8.0 days post coitum), when somite formation is initiated. In Wnt-3a mutants, cells which have ingressed through the primitive streak do not migrate laterally but remain under the streak and form an ectopic tubular structure. Several neural-specific molecular markers, but no paraxial mesoderm markers, are expressed in this structure, suggesting that the ectopic tube is an additional neural tube. In normal embryos, Wnt-3a is expressed in the primitive ectoderm, including the cells which are fated to give rise to the paraxial mesoderm and neurectoderm, but expression is absent in migrating mesoderm cells. These results suggest that Wnt-3a signaling may play a role in regulating paraxial mesodermal fates, at the expense of neurectodermal fates, within the primitive ectoderm of the gastrulating mouse embryo.

Mice with a homozygous disruption of the AP-2 gene exhibit multiple congenital defects and die at birth. The mice show anencephaly, craniofacial defects and thoraco-abdominoschisis. Bone deformations and their absence are seen in the head and trunk region. Early stages of embryogenesis indicate a failure of cranial neural-tube closure and defects in cranial ganglia development. A study on mice indicates that AP-2 regulates mammalian craniofacial development.

Genes from mouse embryos were hybridized with various probes to determine the role of mouse hedgehog genes in late embryogenesis. The genes were expressed in unique locations and showed no overlap in functions. These results suggest that hedgehog genes have their own signaling activity and play a role in various morphogenetic processes within the cell. Evidence also suggests that hedgehog genes are conserved in vertebrate development.

A vertebrate from the hedgehog family known as the Sonic hedgehog (Shh) is processed to generate the peptides Mr 19K and 26/27K. In vitro experiments conducted on various vertebrate embryos and neural explants show that Shh is capable of mediating floor-plate induction. Recombinant Shh peptides are applied to neural explants in serum-free conditions. Shh is proposed to display morphogenic qualities to induce different ventral cell types in the central nervous system of the vertebrate.

The study of the formation of vertebrate limbs reveals that the normal limb development requires the interaction between the signalling system of the axes. The signalling is performed by signalling molecules which mediate the patterning activities. The ectodermally derived signals control the Dorso-ventral identity and facilitate the formation of suitable limb structures.

Gene targeting of mouse embryonic stem cells reveals that the expression of Wnt-4, which codes for a secreted glycoprotein, is crucial for kidney tubulogenesis. Absence of Wnt-4 activity prevents the formation of pretubular cell aggregates, while other stages of mesenchymal and ureteric development are not influenced by this absence. Wnt-4 regulates the epithelial transformation of metanephric mesenchyme during nephron development.

Analysis of DNA sites for Msx-1 in kilobase DNA substrates, using DNA affinity splicing, reveals the existence of a Msx-1 specific DNA site within the transcriptional regulatory domain of the developmental control gene Wnt-1. Amino acid 10 of the Msx-1 homeodomain is localized in the vicinity of the 5' end of the 5'-(C/G)TAATTG-3' motif of the Msx-1-DNA complex. Inserting an ortho-phenanthroline-copper molecule at the amino acid 10 of the homeodomain aids the formation of a strategy to estimate the inclination of a homedomain and characterize the homeodomain consensus DNA sites in genomic DNA.