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G Q Fox

Publications and source records attributed to G Q Fox.

29 records · Page 2Linked to original sources
PubMed1984

Torpedo electromotor system development: a quantitative analysis of synaptogenesis.

Synaptogenesis in the electric organ of Torpedo marmorato has been studied quantitatively at the ultrastructural level of observation. In addition to establishing the normal developmental time course for this event we were interested in determining whether a gradient of synaptogenesis might be present because the electric organ produces several morphologically recognizable spatiotemporal gradients during its early ontogeny. These gradients genesis of electrocyte columns, both gradients of which are operative for periods of weeks. No gradient of synaptogenesis was found, indicating this to be a synchronous process. The idea is advanced that synaptogenesis in the electric organ is modulated by extrinsic influences, many of which may originate from the target electrocytes which, by this time, have become synchronized in their development.

Acetylcholinesterase↗
PubMed1982

The developmental morphology of Torpedo marmorata: the electric lobes.

The development of the electric lobes of Torpedo marmorata has been investigated using light and electron microscopical techniques. The lobe Anlagen become visible in the rhombencephalon along the floor of the 4th ventricle at the 10-mm stage. Many of the neuroepithelial cells in the Anlagen differentiate, Becoming postmitotic and axonic by the 24 mm stage. Proliferative zones of neuroepithelial cells disappear from the electric lobes by the 30-mm stage. After their initial, early differentiation the electromotor neurons remain monopolar until the 40-mm stage when dendrite formation begins. The differentiation of the electromotor neuron from a mono- to an immature multi polar form occurs between the 40- and 55-mm stages and involves, in addition to dendrite formation, a change from a pear-shaped to a spherical cell body, a dramatic increase in cytoplasmic volume, a centralization of the nucleus, an enlargement of the nucleolus and its migration away from the nuclear membrane, and differentiation of the axon hillock. The electric lobes are invaded by sinusoids at the 24-mm stage but formation of the capillary network by sprouting cords of endothelial cells begins later at the 40-mm stage. Neuronal cell death (26-74-mm stages) appears to be mainly an autolytic process and the debris is removed by immature glial cells. Afferent fiber growth cones are first recognized in the lobes at the 60-mm stage but synapses are not observed until the 78-mm stage. Myelination begins in the electric lobes concomitantly with the onset of synaptogenesis. A twofold increase in dendrite length occurs over the period when synapses begin to form in the lobes but dendritic maturation is not complete until the neonatal (120-mm) stage. The results are discussed in relation to the development of the electric organs.

Animals↗
PubMed1982

The developmental morphology of Torpedo marmorata: electric lobe-electromotoneuron proliferation and cell death.

Electromotoneuron proliferation and cell death have been quantitatively studied in the electric lobe of Torpedo marmorata from an embryonic body-length stage of 26-mm to adult animals. These neurons project to the electric organ and form synapses with electrocytes which possess a remarkably large postsynaptic target surface. For this reason cell death would not be predicted to occur if synaptic competition were to be hypothesized as the cause. Isolated observations at the ultrastructural level suggested, however, that cell death was indeed taking place and therefore it seemed appropriate to examine this question in detail. Our findings show first that neuron production appears to be a continuous process throughout the period studied, generating totals of over 70,000 electromotoneurons per lobe by adulthood. Second, two waves of cell death were identified, one occurring early in embryogenesis (stage 30 mm), well before the onset of synaptogenesis, and a second coincident with the onset of synaptogenesis (stages 55--74 mm). It is difficult to reconcile this latter wave with the hypothesis of synaptic competition as the postsynaptic surface at this time of development is largely devoid of synaptic contacts. We conclude that in the electromotor system of Torpedo, synaptic competition is probably not the mechanism of cell death.

Animals↗
PubMed1981

Immunohistochemical localization of cholinergic nerve terminals.

Most of the published light-microscopic methods for the localization of cholinergic nerve pathways present various difficulties of interpretation. The production and characterization of an antiserum that binds specifically to cholinergic terminals is described. The antiserum was raised to small synaptosomes prepared from the purely cholinergic electric organ of Torpedo marmorata. It was shown to lyse cholinergic synaptosomes in a mixed population derived from guinea-pig cortex. After partial purification by adsorption onto nonspecific antigens, It was used to label nerve endings in several tissues of Torpedo, rat and guinea pigs using indirect immunofluorescence histochemistry. The antiserum appears to provide a highly specific means of localizing cholinergic nerve endings in these tissues.

Acetylcholinesterase↗
PubMed1979

The developmental morphology of Torpedo marmorata: electric organ--electrogenic phase.

The electrogenic developmental phase of the electric organ of Torpedo marmorata begins at 40 mm of embryo length and is characterized by a horizontal flattening of the vertically orientated myotubes. The first sign of this process is a rounding up of the ventral poles of the myotubes and a disassembly of the myofibrils located therein. Occurring concomitantly with this is a migration of the nuclei to the cell center which results in a horizontal plane of nuclei. Filament bundles are then found within the ventral cytoplasm often projecting upwards from the ventral plasma membrane. The filaments of the bundles are dimensionally similar to the myofilaments of muscle and it is suggested that the bundles play a role in cellular transformation. In contrast the dorsal pole of the cell appears to be integrated "passively" with the final cell shape as no morphological correlates of a retraction process have been found. A canalicular system, composed of a complex network of irregular tubules and vacuoles, appears just below the dorsal plasma membrane characterizing this region of the cell. A mononucleated satellite cell population lies in close proximity to the dorsal surface of the differentiating cell and fusion between the two cell types occurs throughout development. Cell shape transformation is complete by 55 mm of embryo length and the intercolumnar nerves begin to invade the interelectrocyte space. The ingrowing neurites preferentially course along the ventral electrocyte surface establishing junctions similar to motor endplates.

Animals↗
PubMed1978

The developmental morphology of Torpedo marmorata: electric organ--myogenic phase.

The early development of the electric organ of Torpedo marmorata has been examined by light and electron microscopy to the 40-mm stage of embryo growth. The myogenic nature of this tissue is confirmed ultrastructurally by the presence of myoblasts and myotubes both containing myofibrils cross striated with Z,A and I bands. Fusion between these cells is also found taking place. A scheme is presented to explain the development of the overall structural plan of the organ and specifically the formation of the future electrocyte columns. AT 40 mm, a series of morphological transformations signals the onset of a divergent developmental pattern ultimately leading to the establishment of mature electrocyte columns. These features include rounding up of myotubes, dissolution of myofibrils and the appearance of intermediate size filaments (11 nm) and perhaps a non-muscular actin (5.5 nm). This early myogenic phase of development occurs in the absence of any specific nervous contact even though electromotor nerves are always in close proximity.

Animals↗
PubMed1977

The morphology of the oval nuclei of neonatal Torpedo marmorata.

The morphology of the oval nucleus of neonatal Torpedo marmorata is described at the light and electron microscopic level of examination. The nucleus is unique relative to other central electromotor centers of electric fish so far described being bilaterally symmetrical, composed of two nerve cell types, and possessing no gap junctions between neurons and their processes. This particular structural plan presents difficulties in accounting for presumed synchronous discharge since it has been strongly argued that electrotonic coupling by means of gap junctions is the primary process by which synchronization is accomplished. Close membrane apposition and dendritic bundling, common features within the nucleus, are discussed as possible alternative structural correlates.

Animals↗
PubMed1976

Morphology and fine structure of the feline neonatal medullary raphe nuclei.

A light and electron microscopic study of the caudal medullary raphe nuclei of neonatal kittens reveals that these nuclei are composed of three size classes of neurons with several possible subclasses. Internuclearly projecting dendritic arborizations in the transverse plane and intranuclear projections in the sagittal plane are common features of large and medium size class neurons of raphe nuclei magnus and obscurus though not for the cells of nucleus raphe pallidus. A positive correlation exists between neuron size and density of axosomatic and axodendritic synapses, which suggests that the large class neurons are the first to receive input in synaptogenesis, which is occurring at this time. A wide variety of synaptic forms and integration is also a characteristic feature within these nuclei, though it is not clear whether this morphological variance represents a phylogenic and/or ontogenic trend or just an expression of the multifunctional nature of this region.

Age Factors↗
PubMed1976

Fine structure of growth cones in medullary raphe nuclei in the postnatal cat.

Morphological aspects of the dynamic processes of growth cone formation and synaptogenesis have been studied in neonatal kitten (2-17 days) medullary raphe nuclei. The formation and elaboration of dendritic growth cones and primary dendritic trunks is actively taking place on the medium size class neurons (stellates) of these nuclei. The dendritic growth cones are morphologically distinctive due to their population of large dense-core vesicles and postsynaptic position. Another growth cone morphology, interpreted as axonal, is also described. This growth cone is typically found in close association or synaptic contact with the dendritic growth cones and contains, in addition to synaptic vesicles, a dense-core vesicle population distinguishable from that of the dendritic growth cone by the presence of a variety of vesicles containing an eccentrically positioned dense particle. No evidence of axo-axonic or dendrodendritic synapses has been found. Synaptogenesis was found to be occurring on somas, dendrites and dendritic growth cones throughout the medullary raphe nuclei, though this phenomenon was more apparent in indistinctly localized subnuclear spaces termed synaptogenic zones. Within these zones large class neurons are found to have greater densities of both axodendritic and axosomatic synapses than medium and small class neurons respectively. Axodendritic synaptic densities on primary and secondary dendrites of large and medium class neurons are greater than their respective axosomatic synapse densities, which may suggest that the latter forms at a later period of development.

Animals↗