Axon guidance in the vertebrate central nervous system

The development of connections in the central nervous system depends on the ability of the tips of growing axons to find their appropriate, often distant, target field. Factors that regulate axon outgrowth may be distinct from those that influence direction finding. Tissue culture methods have helped to distinguish between possible in vivo mechanisms and, in some cases, have identified candidate molecules.     

Liquid fermentation to produce biomass of mycoherbicidal strains of Fusarium oxysporum

Conditions for optimizing spore production, especially chlamydospores, by host-specific mycoherbicidal strains of Fusarium oxysporum causing vascular wilts in coca (Erythroxylum coca) and poppy (Papaver somniferum) were studied in 2.5-1 fermentors. The fermentor dissolved oxygen and pH had significant effects on the growth characteristics of F. oxysporum strains. The effect of the fungal strain, however was not significant for most of the variables studied except for chlamydospore formation. After 14 days of fermentation, the spore types produced were microconidia and chlamydospores, with very little production of macroconidia. While the total viable counts were significantly higher under high than under low dissolved O₂, the chlamydospore counts were significantly higher under low than under high dissolved O₂. The percentage of chlamydospores obtained, as a proportion of total viable was significantly higher when the fermentor pH was increased, than when it was not. Scaling-up the liquid fermentation to 20 l, yielded log₁₀ c = 6.8 (where c = chlamydospores ml⁻¹) after 14 days' fermentation, with biomass viable counts of log₁₀ v∼8.0 (where v = viable counts g⁻¹ air-dried biomass). A single-step liquid fermentation reported in this study increased chlamydospore yields and reduced the time required for their production with techniques currently available from 5 weeks to less than 2 weeks. Received: 24 April 1997 / Received revision: 6 August 1997 / Accepted: 29 August 1997     

Dynamic domains of gene expression in the early avian forebrain.

The expression domains of genes implicated in forebrain patterning often share borders at specific anteroposterior positions. This observation lies at the heart of the prosomeric model, which proposes that such shared borders coincide with proposed compartment boundaries and that specific combinations of genes expressed within each compartment are responsible for its patterning. Thus, genes such as Emx1, Emx2, Pax6, and qin (Bf1) are seen as being responsible for specifying different regions in the forebrain (diencephalon and telencephalon). However, the early expression of these genes, before the appearance of putative compartment boundaries, has not been characterized. In order to determine whether they have stable expression domains before this stage, we have compared mRNA expression of each of the above genes, relative both to one another and to morphological landmarks, in closely staged chick embryos. We find that, between HH stage 8 and HH stage 13, each of the genes has a dynamic spatial and temporal expression pattern. To test for autonomy of gene expression in the prosencephalon, we grafted tissue from this region to more caudal positions in the neural tube and analyzed for expression of Emx1, Emx2, qin, or Pax6. We find that gene expression is autonomous in prosencephalic tissue from as early as HH stage 8. In the case of Emx1, our data suggest that, from as early stage 8, presumptive telencephalic tissue also is committed to express this gene. We propose that early patterning along the anteroposterior axis of the presumptive telencephalon occurs across a field that is subdivided by different combinations of genes, with some overlapping areas, but without either sharp boundaries or stable interfaces between expression domains.     

Specification of distinct motor neuron identities by the singular activities of individual Hox genes.

Hox genes have been implicated in specifying positional values along the anteroposterior axis of the caudal central nervous system, but their nested and overlapping expression has complicated the understanding of how they confer specific neural identity. We have employed a direct gain-of-function approach using retroviral vectors to misexpress Hoxa2 and Hoxb1 outside of the normal Hox expression domains, thereby avoiding complications resulting from possible interactions with endogenous Hox genes. Misexpression of either Hoxa2 or Hoxb1 in the anteriormost hindbrain (rhombomere1, r1) leads to the generation of motor neurons in this territory, even though it is normally devoid of this cell type. These ectopic neurons have the specific identity of branchiomotor neurons and, in the case of Hoxb1-induced cells, their axons leave the hindbrain either by fasciculating with the resident cranial motor axons at isthmic (trochlear) or r2 (trigeminal) levels of the axis or via novel ectopic exit points in r1. Next, we have attempted to identify the precise branchiomotor subtypes that are generated after misexpression and our results suggest that the ectopic motor neurons generated following Hoxa2 misexpression are trigeminal-like, while those generated following Hoxb1 misexpression are facial-like. Our data demonstrate, therefore, that at least to a certain extent and for certain cell types, the singular activities of individual Hox genes (compared to a combinatorial mode of action, for example) are sufficient to impose on neuronal precursor cells the competence to generate distinctly specified cell types. Moreover, as these particular motor neuron subtypes are normally generated in the most anterior domains of Hoxa2 and Hoxb1 expression, respectively, our data support the idea that the main site of individual Hox gene action is in the anteriormost subdomain of their expression, consistent with the phenomenon of posterior dominance.