Formation and regeneration of rhombomere boundaries in the developing chick hindbrain.

Development in the chick hindbrain is founded on a segmented pattern. Groups of cells are allocated to particular segmental levels early in development, the cells of each segment (rhombomere) mixing freely with each other, but not with those of adjacent segments. After rhombomere formation, cells in the boundary regions become increasingly specialised. Rhombomeres are thus separate territories that will ultimately pursue different developmental fates. We are investigating the mechanisms that establish and maintain the pattern of rhombomeres and their boundaries. Donor-to-host transplantation experiments were used to confront tissue from different axial levels within the hindbrain. The frequency of boundary regeneration and patterning in the hindbrain was then assessed, based on gross morphology, arrangement of motor neurons and immunohistochemistry. We found that when rhombomeres from adjacent positions or positions three rhombomeres distant from one another were confronted, a normal boundary was invariably reconstructed. Juxtaposition of rhombomere 5 with 7 also yielded a new boundary. By contrast, donor and host tissue of the same positional origin combined without forming a boundary. The same result was obtained in combinations of rhombomeres 3 and 5. Confrontation of tissue from even-numbered rhombomeres 4 with 6 or 2 with 4 also failed to regenerate a boundary in the majority of cases. These results suggest that cell surface properties vary according to rhombomeric level in the hindbrain, and may support the idea of a two-segment periodicity.     

Characterization and linkage mapping often sheep microsatellite markers derived from a sheep x hamster cell hybrid

A cosmid library has been constructed from a sheep x hamster cell hybrid containing sheep chromosome t₁, rob (6;24). Clones containing sheep DNA were identified by hybridizing to a total sheep genomic DNA probe. Small fragments (<500 bp) containing (AC)_n microsatellites were subcloned and sequenced. Ten microsatellite markers were characterized and six were mapped back to chromosomes 6 and 24. The remaining microsatellites mapped to chromosome 26, which was shown to be present in a small proportion of cells of the cell line.     

Color categorization and color constancy in a neural network model of V4

We develop a neural network model that instantiates color constancy and color categorization in a single unified framework. Previous models achieve similar effects but ignore important biological constraints. Color constancy in this model is achieved by a new application of the double opponent cells found in the blobs of the visual cortex. Color categorization emerges naturally, as a consequence of processing chromatic stimuli as vectors in a four-dimensional color space. A computer simulation of this model is subjected to the classic psychophysical tests that first uncovered these phenomena, and its response matches psychophysical results very closely.     

Segmentation and the development of the vertebrate nervous system

1. Recent experiments on the development of neural segmentation in chick embryos are reviewed. 2. Segmentation of the spinal peripheral nerves is governed by a subdivision of the somite-derived sclerotome into anterior and posterior halves. Migrating neural crest cells and outgrowing motor axons are confined to the anterior sclerotome as a result, in part, of inhibitory interactions with posterior sclerotome cells. 3. The sclerotomal distribution of certain molecules known to influence growing nerve cells in vitro, namely laminin, fibronectin, N-CAM, N-Cadherin and J1/tenascin/cytotactin, suggest that these molecules play no critical role in determining the preference of nerve cells for anterior sclerotome. 4. Peanut agglutinin (PNA) recognises cell surface-associated components on posterior cells which, when incorporated into liposomes, cause the abrupt collapse of sensory growth cones in vitro. The PNA receptor(s) may be inhibitory for nerve cells in vivo. 5. The chick hindbrain epithelium is segmented early in its development. Each branchiomotor nucleus in the series of cranial nerves V, VII and IX derives from a pair of segments lying in register with an adjacent branchial arch. Neurogenesis of motor and reticular axons begins in alternate segments, suggesting parallels with insect pattern formation.     

Purification and physicochemical properties of superoxide dismutase from two photosynthetic microorganisms.

Superoxide dismutase (EC 1.15.1.1) has been isolated and characterised from the blue-green alga Spirulina platensis and from aerobically-grown Rhodopseudomonas spheroides, a purple, non-sulphur bacterium. The former enzyme contains 1 gatom of iron and the latter 1 gatom of manganese per mol; both enzymes have a molecular weight of 37 000-38 000, being composed of two non-covalently joined subunits of equal size. Various spectral studies have been carried out including absorbance, circular dichroism and electron spin resonance. Catalytic activity has been studied as a function of pH and shows a decrease at alkaline pH values. The manganoenzyme is generally more stable to various potentially denaturing conditions and is resistant to inactivation by hydrogen peroxide. Amino acid compositions and N-terminal residue determinations are presented.     

Patterns of cell division and interkinetic nuclear migration in the chick embryo hindbrain.

Early in its development, the chick embryo hindbrain manifests an axial series of bulges, termed rhombomeres. Rhombomeres are units of cell lineage restriction, and both they and their intervening boundaries form a series that reiterates various features of neuronal differentiation, cytoarchitecture, and molecular character. The segmented nature of hindbrain morphology and cellular development may be related to early patterns of cell division. These were explored by labeling with BrdU to reveal S-phase nuclei, and staining with basic fuchsin to visualise mitotic cells. Whereas within rhombomeres, S-phase nuclei were located predominantly toward the pial surface of the neuroepithelium, at rhombomere boundaries S-phase nuclei were significantly closer to the ventricular surface. The density of mitotic figures was greater toward the centres of rhombomeres than in boundary regions. Mitotic cells did not show any consistent bias in the orientation of division, either in the centres of rhombomeres, or near boundaries. Our results are consistent with the idea that rhombomeres are centres of cell proliferation, while boundaries contain populations of relatively static cells with reduced rates of cell division.     

Phaseshifting of the photoperiodic flowering response rhythm in Pharbitis nil by red-light pulses

The control by light of the flowering response rhythm in the short-day plant Pharbitis nil Choisy cv. Violet was examined by giving a single pulse of light at various times between 1 and 6 h after a 24-h light period. When the first circadian cycle of the rhythm was monitored, it was found that a pulse of red light given at 1, 2 or 3 h into a 72-dark period caused a 1-h delay of the phase of the response rhythm, while a pulse at 6 h caused a 2-h delay. These results support the hypothesis that, when red-light pulses are given at hourly intervals, they are as effective as continuous light in preventing the onset of dark timing because they repeatedly return the rhythm to the circadian time at which it is apparently suspended in continuous light. The perception of and response to continuous light and red-light pulses are also briefly discussed.     

Mortality event in freshwater drum Aplodinotus grunniens from Lake Ontario, Canada, associated with viral haemorrhagic septicemia virus, type IV

A mortality event primarily affecting freshwater drum Aplodinotus grunniens was noted during April and May 2005 in the Bay of Quinte, Lake Ontario, Canada. A conservative estimate of the number of dead drum was approximately 100 metric tonnes. Large numbers of dead round goby Neogobius melanostomus were also seen, as well as a few muskellunge Esox masquinongy. In the drum, there was a consistent histological pattern of variably severe panvasculitis, a necrotising myocarditis, meningoencephalitis and a segmental enteritis. Moderate numbers of bullet-shaped viral particles consistent with a rhabdovirus were identified by transmission electron microscopy (TEM) in affected heart tissue. Following primary isolation from pooled tissues on fathead minnow (FHM) cells, a morphologically similar virus, approximately 165 x 60 nm in size, was visualised. Identification of the isolate as viral haemorrhagic septicemia virus (VHSV) was confirmed by enzyme immunoassay and by polymerase chain reaction. An appropriately sized product (468 bp) of the G-glycoprotein gene (nucleotides [nt] 340 to 807) was generated with RNA extracted from FHM cell supernatant. Analysis of a 360 nt partial glycoprotein gene sequence (nt 360 to 720) indicated a 96.4 to 97.2% nucleotide identity with known strains of North American (NA) VHSV. Analysis using Neighbour-joining distance methods assigned the isolate to the same lineage as the NA and Japanese isolates (Genogroup IV). However, there was sufficient sequence divergence from known NA VHSV isolates to suggest that this isolate may represent a distinct subgroup. The effects of ongoing mortality in freshwater drum and in multiple species during spring 2006 suggest that this newly recognised virus in the Great Lakes will have continued impact in the near future.