Theodor W. Blackstad

Mapping of experimental axon degeneration by electron microscopy of golgi preparations

SummaryFor the mapping of the terminal area of transected axons within the central nervous system, electron microscopy has recently been adopted. A greater accuracy is thereby obtained than with silver impregnation and light microscopy, since it becomes possible to determine the kinds of structure (soma, dendrites, spines) with which the degenerating boutons establish synaptic contact. In the present study this technique was extended by Golgi impregnation of such material with the aim of making possible classification of the postsynaptic neuron. A few days after transection of a pathway (commissural fibres to the hippocampus being used as a model in this study) the brain was fixed by perfusion with phosphate buffered formalin with sucrose. This was followed by immersion in an osmium tetroxide-potassium dichromate mixture (Daltons fixative without sodium chloride) later replaced by a solution of silver nitrate. Satisfactory Golgi impregnation of nerve cells and processes was obtained. By careful trimming, and reembedding of selected areas, blocks for ultramicrotomy could be obtained which contained only one type of impregnated cell, e.g., hippocampal pyramidal cells.The relation of basal dendrites of such cells to degenerating boutons of commissural fibres was studied. Numerous examples of contact between degenerating boutons and spines belonging to the basal dendrites were seen. Although the Golgi precipitate obscured the postsynaptic substance in the spines, a number of features at the sites of contact were considered strong indication that many of the contacts were synapses and not merely the result of random juxtaposition. This combined procedure is supposed to be applicable to other problems and to other parts of the nervous system as well.

Distribution of mitochondria in pyramidal cells and boutons in hippocampal cortex

SummaryThe amount of mitochondria has been recorded in various parts of neurons. This was done in electron micrographs of cerebral cortex from the hippocampal region. The outlines of boutons, somata and dendrites of varying diameters were transferred to tracing paper together with the outlines of the contained mitochondria. The same was done for whole tissue for comparison. After cutting out and weighing the outlined areas, the fraction of the various tissue constituents, or of whole tissue, occupied by mitochondria was determined. The absolute values are shown in the illustrations (Figs. 4–9). The dendritic shafts of pyramidal cells, coursing through stratum radiatum of regio superior (CA 1), are particularly poor in mitochondria (about 2%). In the branches, the amount as a rule increases with decreasing diameter (to nearly 13% in stratum moleculare).Boutons were the structures richest in mitochondria, but the amount varied with location.

Oxidative enzymes (tetrazolium reductases) in the hippocampal region of the rat

SummaryThis study deals with the activity and distribution of the following tetrazolium (Nitro-BT) reductases in the hippocampal region of the rat: NADH-,NADPH-, α-glycerophosphate-, succinate-, the NAD+-linked lactate-, malate-, isocitrate-, glutamate-, β-HO-butyrate-, the NADP+-linked glucose-6-phosphate-, malate-, and isocitrate tetrazolium reductases. Several architectonic pictures of enzyme distribution were obtained, the number of which was further increased by adding to the incubation media phenazine methosulfate, which is supposed to function as an electron transport agent.The laminar patterns of the various enzymes are described and commented on for each cortical area (p. 173). Then the patterns of parcellation resulting from the changes of laminar patterns are presented (p. 198). There is an excellent agreement between enzyme distribution and previously established architectural landmarks.Finally some aspects of the observations are discussed and correlated to earlier enzyme histochemical and neurobiochemical studies.

Tract Tracing by Electron Microscopy of Golgi Preparations

The unique contribution of the Golgi methods is their well-known ability to demonstrate soma, axons, and dendrites together and thereby to permit classification of neurons at the light microscopic level. Chapter 9 deals with various aspects of this subject. With the aid of electron microscopy (EM), on the other hand, details of synaptology and other ultrastructural features can be demonstrated. A combined Golgi-EM strategy makes possible ultrastructural analysis of synaptic relations where one of the components of a synapse is classified by means of Golgi impregnation. The impregnated component may be either post- or presynaptic. A few such situations suitable for Golgi-EM analysis will be mentioned (see Fig. 1).

Computer methods in neuroanatomy: determining mutual orientation of whole neuronal arbors.

In most neurons orientation can be recognized because their arbors are more or less polarized and/or flattened. These are morphological characteristics of great functional importance. This paper deals with three-dimensional display and mathematical definition of orientation planes and vectors in whole arbors. An orientation plane can be derived from the flattest rectangular prism with which it is possible to enclose the arbor, or may be found by best-fit least square determination (based on all digitized points of the arbor). Both approaches allow description and comparison, in quantitative terms, of the orientation of neurons under various normal, pathological or experimental conditions.

Laminar Specificity of Dendritic Morphology: Examples from the Guinea Pig Hippocampal Region

Since the function of neuronal structures everywhere depends in part on spatial, tridimensional features, most neuroanatomical projects have, to some degree, a quantitative dimension. The present note will, essentially, be based on the type of quantitative study consisting of a recording of the tridimensional characteristics of dendrites in Golgi-impregnated specimens. Same aspects of sampling and actual and potential use of data will be illustrated by considering branching pattern and surface morphology in relation to the environment of the dendrites, more specifically the cortical laminae in which they lie. Three different types of cell from the hippocampal region will be mentioned.

Observations and comments

In the following, a number of illustrated examples will be given of specific regions or pathways in which we have studied axonal degeneration. Our presentation is primarily an attempt to demonstrate the potentialities of the principle as well as its practical application. No exhaustive description of the degenerative process itself or of the pathways mentioned is intended. Our examples are partly from the hippocampal region in the rat, partly from the brain stem of the cat.

Observations on hippocampal mossy cells in mink (Neovison vison) with special reference to dendrites ascending to the granular and molecular layers

Detailed knowledge about the neural circuitry connecting the hippocampus and entorhinal cortex is necessary to understand how this system contributes to spatial navigation and episodic memory. The two principal cell types of the dentate gyrus, mossy cells and granule cells, are interconnected in a positive feedback loop, by which mossy cells can influence information passing from the entorhinal cortex via granule cells to hippocampal pyramidal cells. Mossy cells, like CA3 pyramidal cells, are characterized by thorny excrescences on their proximal dendrites, postsynaptic to giant terminals of granule cell axons. In addition to disynaptic input from the entorhinal cortex and perforant path via granule cells, mossy cells may also receive monosynaptic input from the perforant path via special dendrites ascending to the molecular layer. We here report qualitative and quantitative descriptions of Golgi‐stained hippocampal mossy cells in mink, based on light microscopic observations and three‐dimensional reconstructions. The main focus is on the location, branching pattern, and length of dendrites, particularly those ascending to the granular and molecular layers. In mink, the latter dendrites are more numerous than in rat, but fewer than in primates. They form on average 12% (and up to 29%) of the total dendritic length, and appear to cover the terminal fields of both the lateral and medial perforant paths. In further contrast to rat, the main mossy cell dendrites in mink branch more extensively with distal dendrites encroaching upon the CA3 field. The dendritic arbors extend both along and across the septotemporal axis of the dentate gyrus, not conforming to the lamellar pattern of the hippocampus. The findings suggest that the afferent input to the mossy cells becomes more complex in species closer to primates.