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J A Markham

Publications and source records attributed to J A Markham.

11 recordsLinked to original sources
PubMed1991

Development of rostrocaudal dendritic bundles in rat thoracic spinal cord: analysis of cholinergic sympathetic preganglionic neurons.

Using monoclonal antibodies to choline acetyltransferase (ChAT) and glial fibrillary acidic protein (GFAP), we have analyzed the development of the dendritic bundles formed by cholinergic sympathetic preganglionic neurons (SPNs) in relationship to changes in the organization of glial fibers. In adult rat thoracic spinal cord, SPNs in the intermediolateral (IML) and central autonomic (CA) regions extend dendrites in both the mediolateral and rostrocaudal directions, forming a ladder-like pattern in horizontal sections of thoracic spinal cord. We report that, while the mediolateral dendrites form prenatally, the rostrocaudal dendritic bundles are not detected until at least a week later, during early postnatal life. The rostrocaudal dendrites develop rapidly during the first postnatal week, and achieve an adult-like pattern by postnatal day 14. The observed ontogenetic arrangements of dendritic bundles were correlated with the developing organization of astroglial processes with which they are intimately associated. While the appearance of mediolateral dendrites is consistent with the radial organization of glial in the embryonic spinal cord, the developmental time course of the rostrocaudal dendritic bundles coincides with the transformation of glial cells from this predominantly radial or transverse orientation to the randomly-oriented, stellate pattern of mature astrocytes. This temporal association suggests that ontogenetic changes in the organization of glial cells may contribute to the differential development of mediolateral and rostrocaudal dendritic patterns in the spinal cord.

Aging↗
PubMed1991

Migration patterns of sympathetic preganglionic neurons in embryonic rat spinal cord.

The displacement of immature neurons from their place of origin in the germinal epithelium toward their adult positions in the nervous system appears to involve migratory pathways or guides. While the importance of radial glial fibers in this process has long been recognized, data from recent investigations have suggested that other mechanisms might also play a role in directing the movement of young neurons. We have labeled autonomic preganglionic cells by microinjections of horseradish peroxidase (HRP) into the sympathetic chain ganglia of embryonic rats in order to study the migration and differentiation of these spinal cord neurons. Our results, in conjunction with previous observations, suggest that the migration pattern of preganglionic neurons can be divided into three distinct phases. In the first phase, the autonomic motor neurons arise in the ventral ventricular zone and migrate radially into the ventral horn of the developing spinal cord, where, together with somatic motor neurons, they form a single, primitive motor column (Phelps P. E., Barber R. P., and Vaughn J. E. (1991). J. Comp. Neurol. 307:77-86). During the second phase, the autonomic motor neurons separate from the somatic motor neurons and are displaced dorsally toward the intermediate spinal cord. When the preganglionic neurons reach the intermediolateral (IML) region, they become progressively more multipolar, and many of them undergo a change in alignment, from a dorsoventral to a mediolateral orientation. In the third phase of autonomic motor neuron development, some of these cells are displaced medially, and occupy sites between the IML and central canal. The primary and tertiary movements of the preganglionic neurons are in alignment with radial glial processes in the embryonic spinal cord, an arrangement that is consistent with a hypothesis that glial elements might guide autonomic motor neurons during these periods of development. In contrast, during the second phase, the dorsal translocation of preganglionic neurons occurs in an orientation perpendicular to radial glial fibers, indicating that glial elements are not involved in the secondary migration of these cells. The results of previous investigations have provided evidence that, in addition to glial processes, axonal pathways might provide a substrate for neuronal migration. Logically, therefore, it is possible that the secondary dorsolateral translocation of autonomic preganglionic neurons could be directed along early forming circumferential axons of spinal association interneurons, and this hypothesis is supported by the fact that such fibers are appropriately arrayed in both developmental time and space to guide this movement.

Animals↗
PubMed1990

Ultrastructural analysis of choline acetyltransferase-immunoreactive sympathetic preganglionic neurons and their dendritic bundles in rat thoracic spinal cord.

We have used a monoclonal antibody against choline acetyltransferase (ChAT) to aid in the identification of sympathetic preganglionic neurons (SPNs) and to examine their ultrastructure in rat thoracic spinal cord. The clusters of ChAT-immunoreactive (ChAT-IR) preganglionic cell bodies and their distinctive bundles of dendrites give rise to a ladder-like appearance in horizontal light microscopic sections. This organization also produced a characteristic appearance of preganglionic neuropil when viewed electron microscopically. The intermediolateral (IML) nucleus contained numerous rostrocaudally oriented ChAT-IR dendrites. In addition, mediolaterally oriented ChAT-IR dendrites extended between the IML and the central autonomic region. Both the ChAT-IR dendrites and somata of preganglionic neurons were intimately associated with astroglial processes. The cell bodies were typically covered over a large proportion of their surface by a thin astrocytic sheath, and this was associated with a paucity of axon terminals forming axosomatic synapses. Instead, the vast majority of synapses upon SPNs were of the axodendritic type. The most frequently observed type of axon terminal contained numerous round, clear vesicles along with several dense-core vesicles (DCVs). In addition, some boutons contained a predominance of DCVs. Serial section analysis revealed that these apparently diverse morphological classes of synaptic boutons may simply represent variability of structure throughout a single terminal, with a greater proportion of DCVs being located distal to the synaptic specialization and a greater number of round, clear vesicles being present adjacent to the synapse. Analysis of the dendritic bundles revealed that individual dendrites, like the cell bodies, were often isolated from each other and the surrounding neuropil by astrocytic processes. This arrangement usually was interrupted only at regions of synaptic contact where astrocytic processes surrounded the synaptic complex as a whole. Thus, astroglial ensheathment of SPNs seems designed to minimize cross-talk between the bundled dendrites, as well as to isolate or segregate the diverse afferent inputs known to impinge on these cells.

Adrenergic Fibers↗
PubMed1988

Astrocyte proliferation precedes a decrease in basket cells in the dentate fascia following chronic ethanol treatment in mice.

The effect of chronic ethanol administration on the density of basket cells in the dentate gyrus of mice selectively bred for their sensitivity to acute ethanol exposure (long-sleep, LS and short-sleep, SS) was assessed in two experiments. In addition, the effect of chronic ethanol on the density of dentate granule cells and astrocytes was examined. In the first experiment, mice received 3 weeks of a liquid ethanol diet with 35% of their calories derived from ethanol (EDC). In this experiment, LS mice did not demonstrate a change in the density of granule cells or in the density of basket cells. There was, however, a significant increase in the density of astrocytes as a result of this treatment for the LS mice. The SS mice were unaffected on all measures. In the second experiment, portions of which have been reported previously, mice received a diet with 23% EDC for 3 months. As a result of this exposure, LS mice showed a significant decrease in the density of basket cells, but there was no change in the density of granule cells or astrocytes. There was no difference between controls and experimental mice from the SS group on any of these parameters. These results suggest that at least in the dentate gyrus, chronic ethanol treatment selectively reduces the density of basket cells but only in mice that are more sensitive to the hypnotic effects of acute ethanol exposure. Furthermore, this effect seems to be preceded by an apparent increase in the density of astrocytes.(ABSTRACT TRUNCATED AT 250 WORDS)

Alcoholism↗
PubMed1987

The effect of chronic ethanol consumption on the fine structure of the CA1 stratum oriens in short-sleep and long-sleep mice: short-term and long-term exposure.

The effect of chronic ethanol administration on the fine structure of the hippocampal CA1 stratum oriens was examined in two lines of mice selectively bred for their differential sensitivity to acute ethanol exposure (long-sleep, LS and short-sleep, SS mice). Two experiments were performed. In the first experiment, mice received a liquid diet for 3 weeks with the final amount of ethanol being 35% ethanol-derived calories. In the second experiment, mice received 23.5% ethanol-derived calories for 3 months. Quantitative electron microscopy of the dendritic spines and synaptic appositions in the stratum oriens of CA1 revealed an interaction between diet and line of mice, but only in the 3-month exposure condition. This difference was due to a significant decrease in the density of spines and synaptic appositions in the LS mice receiving ethanol. Additionally, baseline differences between lines indicate that the lines are differing in the density of spine synapses in the absence of ethanol treatment. The possible interaction between acute sensitivity to ethanol and differences in fine structure are examined.

Animals↗
PubMed1987

Changes in the frequency of basket cells in the dentate fascia following chronic ethanol administration in mice.

The frequency of basket cells in the granule cell layer of the dentate fascia of Short Sleep (SS) and Long Sleep (LS) mice was determined following 3 months of ethanol exposure. These mice were bred for their differential susceptibility to the narcotic effects of acute doses of ethanol. The ethanol-insensitive SS mice were unaffected by the treatment while the ethanol-sensitive LS mice that received ethanol showed a significant decrease in basket cell frequency over their control group counterparts. These basket cells are thought to control the tonic level of activity of the granule cells. Thus, a decrease in basket cell frequency might lead to higher granule cell activity following chronic ethanol exposure. This effect could counteract the assumed stronger depressant effect of ethanol in the relatively ethanol-sensitive LS mice.

Animals↗
PubMed1987

Effect of chronic ethanol consumption on the fine structure of the dentate gyrus in long-sleep and short-sleep mice.

The effect of short- and long-term chronic ethanol consumption on the fine structure of the dentate gyrus was examined in two lines of mice selected for their differential sensitivity to acute ethanol administration. Quantitative electron microscopic analysis of dendritic spines, axon terminals, and synaptic appositions revealed significant differences between the long-sleep and short-sleep mice. In control preparations, long-sleep mice were found to have larger spine areas and perimeters, larger axon terminals, and longer synaptic appositions than short-sleep mice. In addition, the shape of dendritic spines in the long-sleep mice was significantly more complex than those of short-sleep mice. Ethanol tended to increase this complexity in long-sleep mice only. Ethanol had only a limited effect on the other anatomical measures. The results provide evidence for ultrastructural differences between the nervous systems of these lines of mice which may have a role in their differential sensitivity to acute ethanol administration.

Alcoholism↗
PubMed1986

Semen collection and spermatozoa characteristics in budgerigars (Melopsittacus undulatus).

Semen samples were obtained from budgerigars by applying gentle pressure to both sides of the cloaca. The semen appeared to be stored in the seminal glomera, two structures on either side of the proctodeum. These structures have been described before in Passeriformes. The spermatozoon in budgerigars is similar to the spermatozoon of the domestic fowl, but differs from the spermatozoon of Passeriformes.

Animals↗
PubMed1986

Actin filament organization within dendrites and dendritic spines during development.

The myosin S-1 subfragment was used to label actin filaments in the developing rat brain. The results show actin filaments present throughout the dendritic region with highest concentrations within growth cones and regions of spine development. Between 6 and 25 days postnatal, spines became more complex and actin filaments within them increased in number and formed a complex network. The observed organization of actin supports the hypothesis that actin has a role in the protrusion of spines from the dendrite during development.

Actin Cytoskeleton↗