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Biomedical subjects

A Reichenbach

Publications and source records attributed to A Reichenbach.

At least 127 records · Page 7Linked to original sources
PubMed1990

Primary neurulation in teleosts--evidence for epithelial genesis of central nervous tissue as in other vertebrates.

Early teleostian embryos were studied by scanning electron microscopy. Transversal and longitudinal views of the neural anlage clearly demonstrate that it has an ordinary epithelial organization consisting of parallel columnar cells. Unless it has been supposed earlier, there is no solid thickened neural plate but the neural ectoderm is tightly folded forming a very narrow neural groove. Thus, primary neurulation in teleosts is shown to occur similar to that in other vertebrates viz. by folding of the neural plate to get the neural tube. That means that the neuroepithelial cells retain their polarity instead to become organized from a randomly oriented mass of unpolarized cells, as it is thought to occur in secondary neurulation.

Animals↗
PubMed1990

Radial glial cells are present in the velum medullare of adult monkeys.

As a rule, in thin mammalian CNS tissues such as the median eminence and the retina, radial glia remains the dominating macroglia in adulthood, whereas in most other regions of the brain radial glial is substituted by multipolar macroglia i.e. astroglia. The Velum medullare is another thin CNS tissue but there are no reports on the dominating macroglia forms of this structure. Thus, Golgi-impregnated sections of adult monkey brains were studied for the presence of radial glial cells. Indeed, this structure was found to be transversed by many radial glial fibres terminating with pial endfeet whereas in adjacent thick brain tissues the glia limitans was formed by marginal astrocytes. It is concluded that fibrous radial glia may dominate in adult mammalian and even primate CNS tissues with a thickness of up to 1 mm.

Animals↗
PubMed1989

Cell type-specific distribution of cathepsin B and D immunoreactivity within the rabbit retina.

The cellular localization of cathepsin B and D immunoreactivity was demonstrated at the light microscopic level in the retina of adult rabbits by use of the peroxidase-antiperoxidase technique. Antisera were raised against rat liver enzymes. Whereas cathepsin D immunoreactivity was confined to Müller (glial) cells, cathepsin B was demonstrated in some, but not all, neuronal cell types. It is proposed that the two enzymes might carry different functions within the neuronal versus glial compartment.

Animals↗
PubMed1989

Glia:neuron index: review and hypothesis to account for different values in various mammals.

The present paper proposes a hypothesis to account for different values of the glia:neuron index in comparable central nervous system tissues of various mammals. This hypothesis assumes that K+ ions released by active neurons are a mitogenic signal for glial cells. The thicker the tissue (for example, the brain wall), the more difficult is efficient K+ clearance, and more perinatal glial cell proliferation should occur. Thus, this hypothesis accounts for higher glia:neuron indices in mammals with thicker brain walls.

Aging↗
PubMed1989

Size and density of glial and neuronal cells within the cerebral neocortex of various insectivorian species.

Morphometric measurements were done on frontal sections through the somatosensory neocortex of various insectivorian species. All measured parameters varied with the size of animals; there was a better correlation with the ventriculartopial brain wall thickness than with the brain weight. The following rules were evaluated: with increasing brain wall thickness, 1) lamina I becomes thinner; 2) the nuclei of both neuronal and glial cells become larger; 3) the volume density of neuronal cells decreases greatly; 4) the volume density of glial cells increases slightly; and 5) as a result, the glia:neuron index increases markedly. There was no equal number of neurons under a unit surface area in the cortices of any species studied. Developmental processes that might account for the above-mentioned rules are discussed in this report.

Animals↗
PubMed1989

Attempt to classify glial cells by means of their process specialization using the rabbit retinal Müller cell as an example of cytotopographic specialization of glial cells.

The rabbit retinal Müller cell is one of the most widely studied glial cell types, and it has all forms of contacts that a glial cell can express, viz. 1) to a (ventricular) fluid space, 2) to a mesenchymal borderline (basal lamina), and 3) to neuronal compartments. This cell demonstrates the local adaptation of cell processes to the microenvironment with which they are in contact. Summarizing available data on Müller cells and other glial cell types, it is concluded that the structure with which the process is in contact determines the type of glial cell process that develops. The type I process has microvilli, desmosome-like junctions, and high Na+,K+-ATPase activity; this type of process is in direct contact with a fluid such as cerebrospinal fluid. The type II endfoot-bearing process contains gliofilaments and has a high K+ conductivity; this type of process is covered by a basal lamina and is in contact with mesenchyme. The type III sheath-bearing process insulates neuronal compartments and expresses suitable membrane properties for glia-neuronal communication. Since structurally similar processes have been shown to have similar physiological properties, a new systematic classification of glial cells is proposed, based on the presence or absence of defined types of cell processes. This approach is believed to provide new insights into the function of neuroglia in both the central and peripheral nervous systems, in vertebrates and invertebrates, and even during ontogenetic development.

Animals↗
PubMed1989

The structure of rabbit retinal Müller (glial) cells is adapted to the surrounding retinal layers.

Radial glial (Müller) cells of the rabbit retina were studied by various techniques including Golgi impregnation, scanning electron microscopy, horseradish peroxidase application, and staining of enzymatically isolated cells. This combination of methods produced detailed information on the specialized morphology of the Müller cells within the different topographical regions of the retina, and of the Müller cell processes within the various retinal layers. As a general rule, the retinal periphery contains short thick Müller cells with big endfeet, whereas the thick central retina is occupied by long slender cells with small endfeet. Independent of their location within the retina, Müller cell processes were found to be adapted to the structure of the surrounding retinal layers. Within the outer and inner nuclear layers, Müller cell processes (and somata) extend thin cytoplasmic "bubbles" ensheathing the neuronal somata, as do the "velate" astrocytes in the brain. In the plexiform layers, Müller cells extend many fine side branches between the neuropil, comparable to the protoplasmic astrocytes of the brain. In the thick myelinated nerve fibre layer of the central retina the Müller cell processes are rather smooth, similar to those of fibrous astrocytes. It is concluded that the neuronal microenvironment determines the morphology of a given glial process, or even of a part of a glial process running through a specialized neuronal compartment.

Animals↗
PubMed1989

Potassium as a signal for both proliferation and differentiation of rabbit retinal (Müller) glia growing in cell culture.

Retinal glial (Müller) cells were grown from explants of early postnatal rabbit retinae. The resulting monolayers of flat cells were exposed to control media (containing 5.85 mM K+), and to media with enhanced K+ concentrations (10 and 20 mM) or arginine-vasopressin (AVP, 20 micrograms/ml) or epithelial growth factor (EGF, 10 ng/ml). Autoradiographically, protein synthesis was quantified as L-[3H]-lysine incorporation, and DNA synthesis as [3H]-thymidine incorporation. Furthermore, the activity of Na+,K(+)-ATPase was measured radiochemically. Short exposure to either moderately enhanced K+ concentrations (10 mM) or to AVP, stimulated L-[3H]-lysine incorporation into the cells. Long-lasting exposure to either high K+ concentrations (20 mM) or to EGF stimulated [3H]-uptake. The Na+,K(+)-ATPase activity of cell cultures increased with increasing K+ concentration of the media. It is suggested that release of K+ by active neuronal compartments stimulates local protein synthesis of glial cells, resulting in the formation of glial sheaths with active K+ uptake capacity. Strong K+ release may even induce glial proliferation.

Animals↗
PubMed1989

Organelle-free cytoplasmic volume fraction of rabbit retinal Müller (glial) cells.

Retinal Müller (glial) cells are thought to act as "cables" carrying spatial buffering K+ currents from the sites of neuronal release into the reservoir of the vitrous body. In order to calculate the amplitude of such currents it is necessary to know the intracellular volume fraction which is able to carry these currents. Thus, this organelle-free volume fraction was measured in transmission electron microscopic photograms of rabbit Müller cells. This volume fraction was found to vary between 0.7 and more than 0.9 in various retinal layers except at the "external limiting membrane" where it was reduced to 0.24 by the accumulation of mitochondria. In enzymatically isolated cells all values are slightly increased by cell swelling.

Animals↗
PubMed1989

The course of axons within the postnatal rabbit retina.

The course of optic axons within the postnatal rabbit retina was studied by scanning electron microscopy (SEM) of the nerve fibre layer exposed by HCl and collagenase treatment, and by freeze-fracture electron microscopy. In the first days after birth, axon bundles were found to run along a rather tortuous way in between the developing Müller cell endfeet. Later on and in adults, the axon bundles show a more straight course. Single axons running into a bundle show a very irregular course before they assume their path in parallel to the other fibres of the bundle. These results support the idea that the course of growing axons is somehow determined by free "channels" in between the endfeet of glial cells.

Animals↗
PubMed1988

Cytotopographical specialization of enzymatically isolated rabbit retinal Müller (glial) cells: K+ conductivity of the cell membrane.

Müller (radial glial) cells were isolated from rabbit retinae by means of papaine and mechanical dissociation. Regional membrane properties of these cells were studied by intracellular microelectrode recordings of potential responses to local application of high K+ solutions. When different parts of the cell membrane were exposed to high K+, the amplitude of the depolarizing responses varied greatly, indicating a strong regional specialization of the membrane properties. Using morphometrical data of isolated rabbit Müller cells, and a simple circuit model, we calculated the endfoot membrane to constitute more than 80% of the total K+ conductance of the cell; the specific resistivity of the endfoot membrane was about 400 omega cm2, i.e., more than 40 times less than that of the membrane of the vitread process, which is immediately adjacent. This kind of regional membrane specialization seems to be optimized in respect to the Müller cells' ability to carry spatial buffering K+ currents.

Animals↗
PubMed1988

Efficient K+ buffering by mammalian retinal glial cells is due to cooperation of specialized ion channels.

Radial glial (Müller) cells were isolated from rabbit retinae by papaine and mechanical dissociation. Regional membrane properties of these cells were studied by using the patch-clamp technique. In the course of our experiments, we found three distinct types of large K+ conducting channels. The vitread process membrane was dominated by high conductance inwardly rectifying (HCR) channels which carried, in the open state, inward currents along a conductance of about 105 pS (symmetrical solutions with 140 mM K+) but almost no outward currents. In the membrane of the soma and the proximal distal process, we found low conductance inwardly rectifying (LCR) channels which had an open state-conductance of about 60 pS and showed rather weak rectification. The endfoot membrane, on the other hand, was found to contain non-rectifying very high conductance (VHC) channels with an open state-conductance of about 360 pS (same solutions). These results suggest that mammalian Müller cells express regional membrane specializations which are optimized to carry spatial buffering currents of excess K+ ions.

Animals↗
PubMed1988

Ultrastructure of rabbit retinal nerve fibre layer--neuro-glial relationships, myelination, and nerve fibre spectrum.

The ultrastructure of the rabbit retinal nerve fibre layer was studied both in retinal centre and periphery. The central nerve fibre layer was found to contain large masses of--mostly myelinated--nerve fibres, somata and processes of astrocytes and oligodendrocytes, vitreal processes of Müller cells, and blood vessels. Astrocyte and Müller cell processes could be discriminated both by their direction and by the thickness of their intermediate filaments which was about 7 nm in Müller cells and about 10 nm in astrocytes. Some peculiarities of nuclei and cytoplasmic organelles of rabbit retinal astrocytes and oligodendrocytes are described. Myelin sheaths are demonstrated to be derived from oligodendrocytes; in some cases, two axons were found within a common myelin sheath. In the retinal periphery, only sparse thin bundles of unmyelinated axons were found in between a thick row of big Müller cell endfeet; astrocytes, oligodendrocytes, and blood vessels were missing here. In both retinal regions, node-like membrane specializations of optic axons were found; these were always surrounded by a corona of fine glial processes arising from astrocytes as well as from Müller cells. The features of myelination within the rabbit nerve fibre layer were quantified, and compared with recent literature data. A hypothesis is offered relating the production of myelin to the release of diffusable substance(s) by active axons. This hypotheses allows to account for the striking finding that relatively thick axons remain unmyelinated in the nerve fibre layer of most mammalian retinae like in the rabbit retinal periphery whereas they become myelinated in the central rabbit retina like in central nervous system in general.

Animals↗
PubMed1987

High Na+ affinity of the Na+,K+ pump in isolated rabbit retinal Müller (glial) cells.

Rabbit retinal Müller (glial) cells were isolated by means of papain and mechanical dissociation. In a special perfusion chamber, the cells were penetrated with a recording microelectrode. Membrane potential changes were recorded in response to extracellular application of both high-K+ solutions and of ouabain, and that during perfusion with normal and Na+-free solutions, respectively. In other Müller cell preparations, Na+,K+-adenosine triphosphatase (ATPase) activity was measured using a radiochemical method, and its Na+ dependence was determined. All results strongly suggest that the Müller cell's Na+,K+ pump can be activated in the presence of extremely low amounts of Na+. This provides additional evidence for significant differences between the glial and the neuronal enzyme.

Animals↗
PubMed1987

Cell length to diameter relation of rat fetal radial glia--does impaired K+ transport capacity of long thin cells cause their perinatal transformation into multipolar astrocytes?

In thick sections of Golgi-impregnated late fetal rat brains, radial glial cells were measured for both length and diameter of their main (basal) processes. The process diameter was found to decrease proportionally to the square root of cell length; thus, the cytoplasm volume remained fairly constant for cells in the range of lengths studied (100-2500 microns). The measured data were used for calculation of the cell's space constant lambda in order to estimate their capability to carry spatial buffering K+ currents. These calculations show that long and slender cells are unable to perform sufficient K+ clearance by such currents. This supports the hypothesis that perinatally when the maturing neurons release enhanced K+ during electrical activity, such long thin cells are subject to long-lasting depolarizations and, thereby, forced to undergo mitotic cell division transforming them into multipolar astrocytes.

Animals↗
PubMed1987

Quantitative-morphometric aspects of Bergmann glial (Golgi epithelial) cell development in rats. A Golgi study.

Bergmann glial (Golgi epithelial) cells in the cerebella of rats of various ages were stained by the rapid Golgi technique, and their radial stem processes were measured for length and diameter. Additionally, the average number of such processes per cell was counted, and the development of bushy lateral protrusions was quantified. The length of radial processes--depending on the thickness of the molecular layer--was found to increase up to the end of the 2nd year of life. This elongation was accompanied by a reduction of the mean process diameter which was, however, not sufficient to prevent an increase in the cytoplasmic volume of the elongating cells. A marked outgrowth of lateral protrusions was observed up to at least the 5th month of life. These data are compared with earlier findings on the development of rat brain stem fetal radial glia, and of rabbit retinal Müller cells. Common mechanisms of glial cell development are discussed.

Age Factors↗