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Biomedical subjects

C A Mason

Publications and source records attributed to C A Mason.

At least 55 records · Page 3Linked to original sources
PubMed1995

Optical microscopy. 3. Tracking fluorescently labeled neurons in developing brain.

For decades, time-lapse microscopy has been used to track dynamic events associated with biological phenomena. Time-lapse studies of the developing nervous system have been restricted to analysis of dissociated cell cultures or of a series of static images from living organisms. The advent of new fluorescent dyes and video imaging technology has produced novel views of the behavior of neurons in the context of the developing nervous tissue, such as migrations within and away from proliferative zones and navigation of axonal processes to synaptic targets. After fixation of the tissue preparation, time-lapse monitoring can be followed by other analytical techniques and forms of microscopy, e.g., immunocytochemistry or electron microscopy, producing information on the interactions of individual cells whose behavioral histories are known. The power of video time-lapse microscopy of living brain tissue lies in the firsthand documentation of developmental patterning, which in turn can serve as an experimental assay.

Animals↗
PubMed1995

Two-Dimensional Gel Electrophoresis Analysis of the Response of Pseudomonas putida KT2442 to 2-Chlorophenol.

The effects of exposure of Pseudomonas putida KT2442 to 2-chlorophenol as a model for the chemical stress response were examined by two-dimensional polyacrylamide gel electrophoresis. Individual protein concentrations were determined at 45, 65, and 95 min following the addition of 2-chlorophenol at a concentration of 1.63 mM to exponentially growing cultures of P. putida KT2442 by silver staining the separated proteins. The changes in the protein concentrations could be classified into four categories, namely those which increased continuously during exposure, those which decreased in concentration, those which showed a concentration peak at some point following exposure, and those which were essentially unaffected. Thirty proteins with isoelectric points between pH 4 and 6 increased in concentration, 27 decreased, and 90 had a concentration maximum or minimum between 45 and 95 min. Of those proteins with isoelectric points between 5.5 and 10, 68 increased in concentration, 39 decreased in concentration, and 47 showed a concentration peak in the middle of the sampling period. Thus, in the evaluation of the stress response, a functional description requires an understanding both of proteins which are required at higher concentrations and of those whose presence appears to be no longer essential.

Journal Article↗
PubMed1995

The first retinal axon growth in the mouse optic chiasm: axon patterning and the cellular environment.

The retinofugal pathway is a useful model for axon guidance because fibers from each eye project to targets on both sides of the brain. Studies using static and real time analyses in mice at E15-17 demonstrated that uncrossed axons from ventrotemporal retina diverge from crossed axons in the optic chiasm, where specialized resident cells may direct divergence. Other studies, however, suggest that pioneering uncrossed retinal axons derive from a different retinal region, take a different course, and enter the ipsilateral optic tract independent of fiber-fiber interactions. We examine these differences by dye-labeling the earliest optic axons and immunocytochemically identifying cells in their path. The first optic axons arising from dorsocentral retina, enter the diencephalon at E12.5. All axons initially grow caudally, lateral to a radial glial palisade. In contrast to later growing axons, early uncrossed axons enter the ipsilateral optic tract directly. Crossed axons enter the glial palisade and course medially, then anteriorly, in a pathway corresponding to the border of an early neuronal population that expresses SSEA-1, CD44, and beta-tubulin. Axon patterning occurs independent of fiber-fiber interactions from both eyes, as the first uncrossed axons enter the optic tract before crossed ones from opposite eye. These analysis, in conjunction with our previous studies during the principal period of retinal axon growth in the diencephalon, suggest that the adult visual projection arises from age-dependent variations in the types and relative contribution of cues along the path through the emerging optic chiasm.

Animals↗
PubMed1995

Developmental determinants at the mammalian optic chiasm.

The mammalian optic chiasm is widely and properly regarded as a region where axons from the temporal retina take an uncrossed course and separate from axons arising in the nasal retina that take a crossed course. However, this is but a rough approximation of the adult situation, and developmental studies must take account of several distinctive stages and axon rearrangements that characterize the region of the chiasm. At the early and late stages of development of nonprimate species the axons do not segregate in accordance with a strict naso-temporal rule at all, and their behavior at the chiasm is not relevant to the formation of the naso-temporal division. As the axons pass from the eye to the chiasm they tend to lose their retinotopic order, to gain a chronotopic order, and then, in the region of the chiasm, to regain some aspects of the retinotopic order before reaching their terminal sites. Molecular or cellular cues that allow the several distinct organizational steps to occur must be expected in the retina, on the axons themselves, and also along the pathway of the axons, prechiasmatically and at the chiasm. Some of these cues will be associated with local nerve cells, some with specialized glial elements and some with the retinofugal axons themselves. Several candidate molecules have been identified in the retina and along the path of the axons, but to date no clearly defined role in the specific events of the pathway determination have been identified. The sequence of developmental processes that characterizes the formation of the optic chiasm provides an interesting and useful challenge to experimentalists, because the advancing axons can now be observed in vitro and in the living brain. The pattern of growth changes as development proceeds, it shows distinctive properties in different species and in their genetic mutants, and it can be readily modified by simple experimental procedures. These all provide opportunities for investigating the function of proposed molecular cues that act in the development of the chiasm.

Animals↗
PubMed1995

Retinal axon divergence in the optic chiasm: uncrossed axons diverge from crossed axons within a midline glial specialization.

A long-standing question is how fiber pathways in the mammalian CNS project to both sides of the brain. Static and real-time analyses of dye-labeled retinal axons (Godement et al., 1990, 1994) have demonstrated that at embryonic day 15-17 in the mouse, crossed and uncrossed axons from each eye diverge in a zone 100-200 microns proximal to the midline of the optic chiasm. In this study, we identify cellular specializations in this zone that might serve as cues for retinal axon divergence. Second, using growth cone morphology as an indicator of growth cone destination, we analyzed how crossed and uncrossed retinal growth cones related to these cellular components. Monoclonal antibody RC2, a marker for radial glia in embryonic mouse CNS, revealed a palisade of radial glia straddling the midline. At the midline, a thin raphe of cells that appear morphologically distinct from the radial glia express a free carbohydrate epitope, stage-specific embryonic antigen 1 (SSEA-1). Sections containing Dil-labeled axons and immunolabeled cells indicated that all axons enter the radial glial palisade. Uncrossed axons turn within the palisade, but never beyond the raphe of SSEA-1-positive cells. In addition, ultrastructural analysis indicated that all growth cones contact radial glia, with projections of the growth cone interdigitating with glial fibers. These results demonstrate that retinal axons diverge within a cellular specialization centered around the midline of the developing optic chiasm, consistent with the hypothesis that cues for divergence are located in this zone.

Animals↗
PubMed1994

Adolescent problem behavior: the effect of peers and the moderating role of father absence and the mother-child relationship.

Examined the effect of peer problem behavior, the absence of a father or equivalent in the home, and the mother-adolescent relationship as predictors of adolescent problem behavior in a sample of 112 African American adolescents. Statistical analyses compared a moderator model to a mediational model and a cumulative risk model. As predicted, the moderator model was superior to the alternative models. Specifically, whereas the mediational model predicted that the effect of father absence and the mother-child relationship upon adolescent problem behavior would be mediated by peer problem behavior, neither effected peer problem behavior or adolescent problem behavior. Similarly, a cumulative risk index did not predict either child or parent reports of problem behavior and was not sensitive to specific contingencies that existed between the predictor variables. In contrast, an interactive, moderator model described the data quite well. This model suggested that father or equivalent absence magnifies the negative impact of peer problem behavior, while a positive mother-adolescent relationship attenuates this risk. A strong mother-adolescent relationship also served to protect adolescents in father-absent homes from the risk of peer problem behavior.

Adolescent↗
PubMed1994

Cell-cell interactions influence survival and differentiation of purified Purkinje cells in vitro.

To determine the role of cell-cell interactions in Purkinje cell survival and dendritic differentiation, perinatal mouse Purkinje cells were purified, and their development was analyzed in vitro. In isolation at low density, Purkinje cell survival was poor, improved by neuronal contacts, either with purified granule neurons or with Purkinje cells themselves. Moreover, coculture with specific cell populations led to widely different degrees of Purkinje cell differentiation. Purified Purkinje cells cultured alone or with an inappropriate afferent, the mossy fibers, did not progress beyond immature forms. With astroglia, Purkinje cells had thin smooth processes. Proper Purkinje cell differentiation was driven only by coculture with granule cells, resulting in dendrites with spines receiving synapses. These results suggest that Purkinje cell differentiation is regulated by local epigenetic factors, provided in large part by the granule neuron.

Afferent Pathways↗
PubMed1994

Retinal axon divergence in the optic chiasm: dynamics of growth cone behavior at the midline.

To study how retinal ganglion cell axons diverge in the optic chiasm, the behavior of dye-labeled fibers was monitored in real time with video microscopy in an isolated preparation of embryonic mouse brain, with a focus on embryonic day 15-16. These real-time studies have revealed the dynamics of the growth of individual retinal axons, especially the tempo of extension and growth cone behaviors during divergence in the chiasm, a model for "decision" regions in developing pathways. Within the chiasm, retinal growth cones extend by saltatory growth, consisting of bursts of rapid advance alternating with pauses in extension. During pauses, growth cone appendages remain motile, and develop asymmetries prior to a change in the axis of growth. In a zone straddling the midline, retinal fibers, irrespective of destination, display long pauses for up to several hours, making small advances and retractions with no net extension. While crossed fibers ultimately progress through the midline, uncrossed fibers from inferior temporal retina develop wide-ranging branched growth cones, and then turn back to the ipsilateral side. Turns are effected by the selective retraction or micropruning of asymmetric foci of motile activity, and by the transformation of a backward-directed filopodium into a new growth cone. The behavior of retinal axons at the midline supports the hypothesis that this locus contains cues important for retinal axon divergence. Moreover, the observations of growth cone kinetics in the chiasm elucidate which growth cone forms seen in static preparations mediate growth cone turning, and suggest a model of axon navigation in decision regions in the intact nervous system.

Animals↗
PubMed1994

Astroglial differentiation is required for support of neurite outgrowth.

Models of astrocyte differentiation stress a lineage program that involves a progressive loss of astroglial support of neuronal differentiation. These models predict that astroglial promotion of neurite extension declines with the "age" of the astrocyte. An alternative view is that astroglial support of neurite growth is regulated by epigenetic factors that induce the cells either to differentiate and support neuronal functions or to undergo cell proliferation and fail to support neurons. To compare the contribution of astroglial cell "age" to astroglial support of neurite extension, mouse cerebellar astroglia were maintained in vitro for 3-90 d, and assayed for their ability to support neurite formation. When cultured in isolation, astroglial support of neurite extension declined with time in vitro, as assayed by quantifying outgrowth from explants of pontine nuclei, falling from a robust level just after the astroglia were harvested to negligible levels 21-90 d later. Since previous studies have shown that neurons can change the state of astroglial cells (Hatten, 1985), we tested the neurite promoting activity of astroglia that were cultured for 21-90 d in vitro and subsequently induced to differentiate by the addition of neurons. When granule neurons were added to aged astroglia and pontine explants plated 2 d later, neurite growth from the explants was exuberant, regardless of the time astroglia spent in vitro prior to the addition of neurons. The state of astroglia that were growth promoting or growth inhibiting was examined by bromodeoxyuridine staining and with antisera to glial filament protein. Aged astroglia cultured alone and thus inhibitory to axon growth, proliferated at high rates and had polygonal shapes. In contrast, aged astroglia to which neurons had been added, proliferated at low rates and developed process-bearing stellate shapes. To test further whether proliferation levels related to the growth-supporting properties of astroglia, astroglia were plated alone in medium without serum, or with the addition of transforming growth factor-beta 1, each treatment known to arrest proliferation. In both cases, promotion of neurite growth was restored in aged astroglia, but the morphology of astroglia did not correlate with the ability to support neurite growth. Finally, the growth-inhibiting properties of aged astroglia do not appear to be mediated by diffusible factors, and require close apposition with living astroglial cells. We conclude that astroglial support of neurite extension depends on the state of differentiation of astroglial cells, and that these properties can be modified by coculture with neurons or conditions that arrest of astroglial proliferation, irrespective of astroglial "age".

Animals↗
PubMed1993

Dispersion of neural progenitors within the germinal zones of the forebrain.

One of the early events in the establishment of regional diversity in brain is the subdivision of the forebrain into the cerebral cortex and underlying basal ganglia. This subdivision is of special interest, owing to the striking difference in cellular patterning in these two regions. Whereas the dorsal aspect of the telencephalon gives rise to the laminar, cortical regions of brain, the basal aspect gives rise to nuclear, subcortical regions. To examine early events in the regionalization of the forebrain, we visualized cell movement within the ventricular zones of the dorsal and basal regions of the E15 murine telencephalon. Over an 8-24-hour observation period, labelled cells moved extensively in the plane of the cortical ventricular zone. Cell dispersion was restricted, however, at the border between the cortical ventricular zone and the lateral ganglionic eminence, the basal telencephalic ventricular zone. We suggest that this restriction of cell movements establishes a regional pattern of neurogenesis in the developing brain.

Animals↗
PubMed1993

Guidance of retinal fibers in the optic chiasm.

At the optic chiasm of vertebrates, there occurs a sharp divergence in the destination of retinal ganglion cell axons as they are on their way to more central targets. Recent studies in the mouse indicate that the growth cones of each set of fibers diverge close to the midline of the optic chiasm, at the borders of a palisade of immature radial glia. Static and dynamic views of retinal growth cones in this decision region reveal that extensive exploratory behavior and selective retraction of parts of the growing tips of uncrossed fibers, in response to cellular cues at the midline, is a major event in the guidance of these fibers. In vitro experiments further show that presumptive crossed and uncrossed fibers differ in their responses to contact with cells from the optic chiasm. As with other instances of selective guidance of fibers at midline structures, the divergence of crossed and uncrossed retinal fibers therefore involves a selective remodeling of their growing tips and transitory axon-cell contacts during growth at the optic chiasm, presumably due to biochemical heterogeneity among crossed and uncrossed ganglion cell fibers.

Animals↗
PubMed1992

Specificity of a target cell-derived stop signal for afferent axonal growth.

With a novel model culture system in which afferents are co-cultured with purified populations of target neurons, we have demonstrated that a target cell within the central nervous system (CNS), the cerebellar granule neuron, poses a "stop-growing signal" for its appropriate afferents, the mossy fibers. To ask whether this stop signal is afferent specific, we co-cultured granule neurons with another cerebellar afferent system, the climbing fibers from the inferior olivary nuclei, which normally contact Purkinje neurons, and with retinal ganglion cell afferents, which never enter the cerebellum. Granule neurons do not pose a stop signal to either of these afferents. In contrast to pontine mossy afferents that grow well on laminin and showed reduced outgrowth on granule neurons, both olivary and retinal fibers displayed similar growth on laminin alone or on granule neurons. In addition, each afferent showed different degrees of fasciculation and growth cone morphology on laminin. Thus, the growth arrest signal sent by granule neurons is specifically recognized by their appropriate afferents. Moreover, these three types of afferents exhibit varying growth patterns on the same noncellular and cellular substrates, implicating distinct molecular characteristics of growth regulation for different classes of neurons that would contribute to specificity of synapse formation.

Animals↗
PubMed1992

Heat shock gene expression in continuous cultures of Escherichia coli.

Temperature inducible systems for the controlled expression of recombinant genes are finding increasing industrial applications. These involve either short or long term exposure of the process culture to superoptimum temperatures. It is well known that bacteria respond to a sudden increase in their environmental temperature with an immediate transient increase in the synthesis rates of specific heat shock proteins. The use of continuous flow processes for the production of recombinant proteins would allow higher productivity and smaller scale bioreactors. However, the induction patterns of heat shock proteins in continuous culture after defined heat shocks are not well defined despite a large amount of information which is now available concerning heat shock protein induction in batch cultures. An overview of this information is presented to enable a better understanding of the response in continuous cultures. The latter was investigated by monitoring the transient expression of a representative heat shock gene, htpG, in E. coli in continuous culture. The relative magnitude of the response was found to be both temperature and exposure time dependent, but growth rate independent. Changing medium composition resulted in both different steady and transient state expression levels.

Bacteriological Techniques↗
PubMed1992

Cerebellar target neurons provide a stop signal for afferent neurite extension in vitro.

The contributions of cell-cell interactions to the establishment of specific patterns of innervation within target brain regions are not known. To provide an experimental analysis of the regulation of afferent axonal growth, we have developed an in vitro assay system, based on the developing mouse cerebellum, in which afferent axons from a brainstem source of mossy fiber afferents, the basilar pontine nuclei, were cocultured with astroglia or granule neurons purified from the cerebellum. In the absence of cells from the cerebellum, pontine explants produced axons that fasciculated and extended rapidly on a culture surface treated with poly-lysine or laminin. When pontine neurites grew onto cerebellar astroglial cells, outgrowth was more abundant than on substrates alone, suggesting that glial cells provide a positive signal for axon extension. Time-lapse video microscopy indicated that the rate of neurite extension increased from less than 50 microns/hr to more than 100 microns/hr when axonal growth cones moved from the culture substratum onto an astroglial-cell surface. Acceleration of neurite extension was also observed as pontine neurites grew onto other pontine neurites. By contrast, when pontine neurites grew on granule neurons, the appropriate targets of mossy fibers, the length of pontine neurites was greatly reduced. As growing axons terminated on granule neurons, the target cells appeared to provide a "stop-growing signal" for axon extension. The length of pontine neurites decreased with increasing granule neuron density. Two lines of evidence suggested that the stop signal was contact mediated. First, video microscopy showed that pontine growth cones stopped extending after contacting a granule neuron. Second, the length of afferent axons was not reduced when pontine neurites grew at a distance from granule neurons. Competition experiments where both astroglia and granule neurons were plated together suggested that the growth arrest signal provided by granule neurons could override the growth-promoting signal provided by astroglial cells. These results suggest that specific cell-cell interactions regulate the growth of pontine afferent axons within their cerebellar target, with axoaxonal and axoglial interactions promoting axon extension and axon-target cell interactions interrupting axon extension.

Afferent Pathways↗
PubMed1991

Astroglia in CNS injury.

The astroglial response to CNS injury is considered in the context of neuron-glial relationships. Although previous models suggested that astroglial cells present in "scars" impede axon regrowth owing to irreversible changes in the glial cell following injury, recent in vivo and in vitro studies indicate that astroglial cells exhibit considerable plasticity, elevating expression of the glial filament protein and altering expression of properties which support axons, including extracellular matrix components and cell surface adhesion systems. Both in vivo and in vitro studies on neuron-glia interactions in different brain regions suggest that glia express region-specific properties, including ion channels, neurotransmitter uptake and receptor systems, and cell surface adhesion systems. Together these findings suggest that a more detailed analysis of glial response to injury in different brain regions will lead to an appreciation of the diversity of the astroglial response to injury, and its regulation by neuron-glia relationships.

Animals↗
PubMed1991

Physiological aspects of growth and recombinant DNA stability in Saccharomyces cerevisiae.

Despite the fact that plasmid stability in the yeast Saccharomyces cerevisiae is influenced by both genetical and physiological parameters most attention has been focused on the former. Physiological factors affecting the stability of plasmids have been poorly characterized despite the need for such information in order to optimize the use of S. cerevisiae as a host for recombinant protein production processes. The physiology of wild type S. cerevisiae differs considerably when grown using different cultivation techniques. A limited amount of phenomenological data has been reported concerning plasmid instability effects under these different conditions and in this article these have been collected together with the intention of providing an overview to instability effects and to try and propose reasons as to how the physiological response to different growth conditions can be manifested as stability/instability effects.

Cell Cycle↗