Experimental and modeling study of C5H10O2 ethyl and methyl esters.

Due to the world's over-reliance on fossil fuels there has been a developing interest in the production of renewable biofuels such as methyl and ethyl esters derived from vegetable oils and animal fats. To increase our understanding of the combustion chemistry of esters, the oxidation of methyl butanoate and ethyl propanoate, both with a molecular formula of C5H10O2, have been studied in a series of high-temperature shock tube experiments. Ignition delay times for a series of mixtures, of varying fuel/oxygen equivalence ratios (phi = 0.25-1.5), were measured behind reflected shock waves over the temperature range 1100-1670 K, and at pressures of 1.0, and 4.0 atm. It was found that ethyl propanoate was consistently faster to ignite than methyl butanoate, particularly at lower temperatures. Detailed chemical kinetic mechanisms have been assembled and used to simulate these experiments with good agreement observed. Rate of production analyses using the detailed mechanisms shows that the faster reactivity of ethyl propanoate can be explained by a six-centered unimolecular decomposition reaction with a relatively low activation energy barrier producing propanoic acid and ethylene. The elimination reaction itself is not responsible for the increased reactivity; it is the faster reactivity of the two products, propanoic acid and ethylene that leads to this behavior.

Primary neurons that express the L2/HNK-1 carbohydrate during early development in the zebrafish.

In zebrafish, many nerve pathways in both the CNS and periphery are pioneered by a small and relatively simple set of 'primary' neurons that arise in the early embryo. We now have used monoclonal antibodies to show that, as they develop, primary neurons of several functional classes express on their surfaces the L2/HNK-1 tetrasaccharide that is associated with a variety of cell surface adhesion molecules. We have studied the early labeling patterns of these neurons, as well as some non-neural cells, and found that the time of onset and intensity of immunolabeling vary specifically according to cell type. The first neuronal expression is by Rohon-Beard and trigeminal ganglion neurons, both of which are primary sensory neurons that mediate touch sensitivity. These cells express the epitope very strongly on their growth cones and axons, permitting study of their development unobscured by labeling in other cells. Both types initiate axogenesis at the same early time, and appear to be the first neurons in the embryo to do so. Their peripheral neurites display similar branching patterns and have similar distinctive growth cone morphologies. Their central axons grow at the same rate along the same longitudinal fiber pathway, but in opposite directions, and where they meet they appear to fasciculate with one another. The similarities suggest that Rohon-Beard and trigeminal ganglion neurons, despite their different positions, share a common program of early development. Immunolabeling is also specifically present on a region of the brain surface where the newly arriving trigeminal sensory axons will enter the brain. Further, the trigeminal expression of the antigen persists in growth cones during the time that they contact an individually identified central target neuron, the Mauthner cell, which also expresses the epitope. These findings provide descriptive evidence for possible roles of L2/HNK-1 immunoreactive molecules in axonal growth and synaptogenesis.

Early axonal contacts during development of an identified dendrite in the brain of the zebrafish.

We have identified the initial synaptic contacts made onto the Mauthner (M) cell, an identified neuron that arises during early development of the zebrafish hindbrain. The contacts are made by a small bundle of pioneering trigeminal sensory axons onto the M cell soma before it forms dendrites. The sensory bundle is then partially enveloped by the M cell. The lateral dendrite appears at about the site of the contact, and eventually the trigeminal inputs are shifted to its trunk. As the dendrite elongates, other sensory contacts are made on its distal regions, sequentially from the acoustico-vestibular nerve and the lateral line nerves. To learn whether the earliest inputs induce the initial outgrowth of the M cell dendrite, we ablated the trigeminal neurons by laser irradiation before they contacted the M cell. Morphogenesis of the M cell, including its dendrite, appeared normal.

Segmental pattern of development of the hindbrain and spinal cord of the zebrafish embryo.

In the ventral hindbrain and spinal cord of zebrafish embryos, the first neurones that can be identified appear as single cells or small clusters of cells, distributed periodically at intervals equal to the length of a somite. In the hindbrain, a series of neuromeres of corresponding length is present, and the earliest neurones are located in the centres of each neuromere. Young neurones within both the hindbrain and spinal cord were identified in live embryos using Nomarski optics, and histochemically by labelling for acetylcholinesterase activity and expression of an antigen recognized by the monoclonal antibody zn-1. Among them are individually identified hindbrain reticulospinal neurones and spinal motoneurones. These observations suggest that early development in these regions of the CNS reflects a common segmental pattern. Subsequently, as more neurones differentiate, the initially similar patterning of the cells in these two regions diverges. A continuous longitudinal column of developing neurones appears in the spinal cord, whereas an alternating series of large and small clusters of neurones is present in the hindbrain.

Segmental homologies among reticulospinal neurons in the hindbrain of the zebrafish larva.

We have examined the morphology of identified reticulospinal neurons in larval zebrafish by retrogradely labeling them with horseradish peroxidase. We described the morphology of 27 different types of reticulospinal neurons found in the hindbrain 5 days after fertilization. Nineteen of these types are present as single identified neurons on each side of the brain; the others are present as pairs or small groups of cells. The hindbrain reticulospinal neurons are present in seven bilateral clusters that are spaced periodically along the neuraxis. Each cluster contains two to five different types of reticulospinal neurons. Cells with similar morphological features are found in adjacent clusters. By considering cell position within the cluster and axon pathway, nearly all of the cells can be assigned to one of about seven serially repeated classes. Independent morphological features of the cells support the same classification. We propose that the clusters represent hindbrain segments and that the neurons of the same class that are present in the different clusters are segmental homologues. Assuming that this series evolved by successive duplications and divergence of the primitive segments, we have analyzed the changes that may have occurred during the evolution of each new segment. Changes between ipsilaterally and contralaterally projecting axons may have occurred several times during the evolution of the series. In addition, cells may have been added or deleted.

Sensory neuron growth cones comigrate with posterior lateral line primordial cells in zebrafish.

The development of neuromasts and sensory neurons of the posterior lateral line was studied in zebrafish (Brachydanio rerio) in order to determine the relationship between growing axons of sensory neurons and the migratory cellular primordium of midbody line neuromasts. Scanning electron microscopy revealed that a primary system of six neuromasts develops during the second day after fertilization and evidence is presented that these arise from cells of a migratory primordium. The primordium is first detected in the postauditory region immediately adjacent to the developing sensory ganglion. Growth cones of posterior lateral line sensory neurons are found within the premigratory primordium when it is adjacent to the ganglion. At later times growth cones of these sensory neurons are found within the primordium as it migrates caudally along the midbody line. These results demonstrate that although the growth cones of the sensory neurons grow over a considerable distance to their final destination, they are never very far from their target cells (or target cell precursors), which migrate with them and may even lead them.

T reticular interneurons: a class of serially repeating cells in the zebrafish hindbrain.

We describe a class of reticular neurons, named T interneurons after the branching pattern of their axons, in young larvae of the zebrafish Brachydanio rerio. The cells were identified by filling them with HRP from application sites within the CNS. A serially repeating set of about ten such neurons is present in a longitudinal column on each side of the caudal hindbrain. The T axons project across the midline, and branches course both rostrally and caudally within the medial longitudinal fascicle (mlf). The cells receive synaptic input from the Mauthner neurons, from unidentified axons in the mlf, and perhaps from trigeminal sensory fibers. They project to cranial and pectoral motor nuclei. T interneurons appear to be homologous to giant fiber neurons in the hatchetfish and to some of the cranial relay neurons in the goldfish. We discuss a possible functional role and comparative implications of their distribution in the hindbrain.

Anatomy of the posterior lateral line system in young larvae of the zebrafish.

We studied the anatomy of neuromasts, afferent sensory neurons, and efferent neurons of the midbody branch of the posterior lateral line in larvae of the zebrafish (Brachydanio rerio), 5 days after fertilization. This simple sensory system consists of ten or 11 neuromasts, 15-20 sensory neurons, and about nine efferent neurons. The neuromasts are typical free neuromasts and both afferent and efferent synapses are present on hair cells within them. The sensory neurons project into a single longitudinal column of neuropil in the hindbrain. The sensory terminals appear by light microscopy to contact the dorsolateral dendrite of the ipsilateral Mauthner cell. Three types of efferent neurons are present; two types in the hindbrain and one type in the diencephalon. We provide several lines of evidence that demonstrate that these central neurons are efferent to the lateral line. We conclude from this morphology that the larval system includes all of the components of the adult system and is probably functional at this early stage. We also found that larvae have all of the efferent neurons found in adult zebrafish, while the number of neuromasts and sensory neurons will increase during subsequent development.

Noninvasive recording of the Mauthner neurone action potential in larval zebrafish.

We describe the identification of Mauthner (M-) cell action potentials in an intact zebrafish larva, utilizing recording electrodes located outside the fish: 1. The externally recorded spike occurs at approximately the same time, and its waveform changes with recording site in the same way, as the extracellular M-spike recorded within the central nervous system. 2. The externally recorded M-spike may be readily distinguished from other forms of neural activity. 3. The M-spike can be identified in recordings from unrestrained larvae. This finding permits the direct study of M-cell function in the freely behaving animal.

Brain neurons which project to the spinal cord in young larvae of the zebrafish.

A small number of brain neurons project to caudal levels of the spinal cord in the larva of the teleost Brachydanio rerio. These cells were identified in animals 6 days after fertilization by backfilling with horseradish peroxidase following transection of the cord at the level of the cloaca. In preparations with the most labeled cells a total of 30-40 were present on each side of the midline. They were located within three regions of the brainstem: the midbrain nucleus of origin of the medial longitudinal fascicle (mlf), the hindbrain reticular formation, and the hindbrain vestibular nucleus. A total of 15 classes of cells could be distinguished by soma positions, dendritic fields, and axonal pathways. For some of these classes only one or two cells were usually present on each side of the brain. Most of the labeled cells contributed axons to the mlf ipsilateral to the soma; however, the Mauthner cells and three new types of hindbrain reticulospinal reticulospinal cells have decussating axons that enter the contralateral mlf. The observed distribution of labeled reticulospinal cells is similar to that previously described for large reticular cells in adult teleosts and to the system of identified Mauthner and Müller cells in the lamprey.