Observations on the axon initial segment and other structures in the neocortex using conventional staining and ethanolic phosphotungstic acid

Neuronal diversity in the caudate nucleus: A comparative study between camel and human brains

Caudate nucleus (CN) neurons in camels and humans were examined using modified Golgi impregnation methods. Neurons were classified based on soma morphology, dendritic characteristics, and spine distribution. Three primary neuron types were identified in both species: rich-spiny (Type I), sparsely-spiny (Type II), and aspiny (Type III), each comprising subtypes with specific features. Comparative analysis revealed significant differences in soma size, dendritic morphology, and spine distribution between camels and humans. The study contributes to our understanding of structural diversity in CN neurons and provides insights into evolutionary neural adaptations.

Cytoskeletal organization of axons in vertebrates and invertebrates

The maintenance of axons for the lifetime of an organism requires an axonal cytoskeleton that is robust but also flexible to adapt to mechanical challenges and to support plastic changes of axon morphology. Furthermore, cytoskeletal organization has to adapt to axons of dramatically different dimensions, and to their compartment-specific requirements in the axon initial segment, in the axon shaft, at synapses or in growth cones. To understand how the cytoskeleton caters to these different demands, this review summarizes five decades of electron microscopic studies. It focuses on the organization of microtubules and neurofilaments in axon shafts in both vertebrate and invertebrate neurons, as well as the axon initial segments of vertebrate motor- and interneurons. Findings from these ultrastructural studies are being interpreted here on the basis of our contemporary molecular understanding. They strongly suggest that axon architecture in animals as diverse as arthropods and vertebrates is dependent on loosely cross-linked bundles of microtubules running all along axons, with only minor roles played by neurofilaments.

Fast prenatal development of the NPY neuron system in the neocortex of the European wild boar, Sus scrofa

Knowledge on cortical development is based mainly on small rodents besides primates and carnivores, all being altricial nestlings. Ungulates are precocial and born with nearly mature sensory and motor systems. Almost no information is available on ungulate brain development. Here, we analyzed European wild boar cortex development, focusing on the neuropeptide Y immunoreactive (NPY-ir) neuron system in dorsoparietal cortex from E35 to P30. Transient NPY-ir neuron types including archaic cells of the cortical plate and axonal loop cells of the subplate which appear by E60 concurrent with the establishment of the ungulate brain basic sulcal pattern. From E70, NPY-ir axons have an axon initial segment which elongates and shifts closer towards the axon's point of origin until P30. From E85 onwards (birth at E114), NPY-ir neurons in cortical layers form basket cell-like local and Martinotti cell-like ascending axonal projections. The mature NPY-ir pattern is recognizable at E110. Together, morphologies are conserved across species, but timing is not: in pig, the adult pattern largely forms prenatally.

Excitable domains of myelinated nerves: axon initial segments and nodes of Ranvier

Neurons are highly polarized cells. They can be subdivided into at least two structurally and functionally distinct domains: somatodendritic and axonal domains. The somatodendritic domain receives and integrates upstream input signals, and the axonal domain generates and relays outputs in the form of action potentials to the downstream target. Demand for quick response to the harsh surroundings prompted evolution to equip vertebrates' neurons with a remarkable glia-derived structure called myelin. Not only Insulating the axon, myelinating glia also rearrange the axonal components and elaborate functional subdomains along the axon. Proper functioning of all theses domains and subdomains is vital for a normal, efficient nervous system.

Localization of the 5-HT1A receptors in the brain of opossum Monodelphis domestica

This paper describes the distribution of 5-HT1A receptors in the brain of opossum Monodelphis domestica. They were visualized by immunohistological staining with an antibody against the amino acid sequence (170-186) of this receptor that was previously successfully used in the rat and monkey. As in Eutherians, high levels of immunostaining were present in the septum, hippocampus, raphe nuclei and some other brain stem nuclei. Neocortex, several thalamic nuclei and hypothalamus showed moderate density of the labeled structures. Moderate levels of 5-HT1A receptors were also observed in the caudate nucleus and putamen, unlike in the rat, in which labeling in these nuclei was almost absent. Another difference with the rat was observed in the neocortex: in the opossum immunostaining was absent in the layer 4 of many cortical areas. In general, distribution and density of this important receptor in the opossum is very similar to that described in the rat and monkey and therefore it follows a general mammalian pattern.

Making brain connections: neuroanatomy and the work of TPS Powell, 1923-1996

Dr. Thomas PS Powell was one of the founders of modern neuroanatomy. His career spanned an era that saw techniques for analyzing connections in the central nervous system dramatically increase in number and resolving power. In tracing the history of his research, one can see how the introduction of each new technique provided an incremental step in analytical capacity although eventually revealing its own limitations. Also evident is the extent to which prejudices born in the days of applying earlier techniques could continue to influence the interpretation of results obtained with new ones. Powell's contributions to neuroscience were extremely wide-ranging, encompassing investigations of the circuitry of the basal ganglia, corticofugal connections, topographic maps in sensory systems, central olfactory pathways, corticocortical and commissural connections, and pathways for sensory convergence in the cerebral cortex. From these investigations, made with tract tracing techniques, much existing knowledge of forebrain organization is derived. He was also one of the earliest investigators to use electron microscopy in the investigation of the central nervous system, and his electron microscopic studies on the olfactory bulb, thalamus, cerebral cortex, and basal ganglia laid, to a large extent, the foundations for all modern research on the synaptic circuitry of these structures. He was given to synthesizing data across systems in order to arrive at common principles of brain organization. A number of these syntheses have been sources of great interest and, occasionally, controversy.

Electron microscopy of Golgi-impregnated interneurons: notes on the intrinsic connectivity of the cerebral cortex

The Golgi-electron microscope technique has opened new avenues to explore the synaptic organization of the brain. In this article, we shall discuss basic methodological principles necessary to analyze axonal arborizations with this combined technique. To illustrate the applications of the method, we shall review the forms and distribution of the synapses in which the axonal arborizations of local cortical interneurons engage.

Microtubule bundles in fish cerebellar Purkinje cells

The initial axon segments and the cell bodies of Purkinje cells were examined in electron microscopic serial sections and toluidine blue semithin sections of goldfish cerebellum. We observed two characteristic cytoplasmic features different from those of other vertebrate neurons, 1. The areas of Nissl substance and Golgi apparatus are sharply divided in the periphery and center of the cytoplasm, 2. Microtubules fasciculated by cross-bridges in the axon hillock and initial axon segment remain bundled in the perikaryon, pass near the eccentric nucleus, and enter into the Golgi area of the central cytoplasm, where they are surrounded by mitochondria. We suggest that the intracellular fasciculated microtubules may establish a prepared pathway for fast anterograde and retrograde transport to and from the Golgi area of the cell body.

Axon terminals of GABAergic chandelier cells are lost at epileptic foci

Axon terminals of chandelier cells were analyzed in monkeys with cortical focal epilepsy produced by alumina gel to determine if this type of GABAergic terminal is lost at epileptic foci. These terminals form a dense plexus with the axon initial segments of pyramidal neurons, especially those in layers II and III. Axon initial segments of pyramidal neurons were traced for at least 40 micron in serial thin sections and beyond this point were observed to become myelinated. In single sections, 10-15 axon terminals were found to form symmetric synapses throughout the entire length of the axon initial segments from nonepileptic preparations and were observed to synapse with only these structures and not adjacent dendrites or spines. In epileptic cortex, the axon initial segments of pyramidal neurons were apposed by glial profiles that contained clusters of filaments typical of reactive astrocytes. Only a few, small axon terminals were observed to form symmetric synapses with these axon initial segments. Thus, the chandelier cell axons appeared to degenerate in epileptic cortex. The highly strategic site of GABAergic inhibitory synapses on axon initial segments suggests that they exert a strong influence on the output of pyramidal cells. The near absence of these chandelier cell axons in epileptic foci most likely contributes to the hyperexcitability of neurons.

Neurons of the bed nucleus of the stria terminalis: a golgi study in the rat

Neuronal morphology in the bed nucleus of the stria terminalis (BST) was studied using Golgi techniques. The principal neurons of the lateral subdivision of BST have ovoid perikarya and 4-5 dendrites that branch several times and exhibit a dense covering of spines. Adjacent to the internal capsule is a small region, termed the "juxtacapsular subdivision" of BST, that consists of small, spiny cells. Neurons of the medial subdivision of BST have ovoid perikarya and 2-3 dendrites that branch sparingly. Dendritic spine density varies from sparse to moderate. Dendrites in the dorsocaudal portion of the medial subdivision extend into a cell-sparse zone adjacent to the lateral ventricle. Cells in the lateral portion of the preoptic continuation of BST have dendrites oriented perpendicular to fibers of the stria terminalis which traverse this area while medially located cells are oriented parallel to fibers of the stria. Axons of BST neurons emit collaterals that arborize modestly near the cell of origin. Neurons in the lateral and medial subdivisions of BST resemble, respectively, cells in the lateral and medial subdivisions of the central amygdaloid nucleus. Neurons in the juxtacapsular subdivision of BST are similar to neurons of the intercalated masses of the amygdala.

Cytoarchitecture of the central amygdaloid nucleus of the rat

Since recent studies indicate that distinct neuropeptides and projections are associated with discrete portions of the central amygdaloid nucleus (CN), a detailed investigation of the cytoarchitecture of CN should contribute to an understanding of its organization. Qualitative and quantitative analyses of the rat CN using Nissl, Klüver-Barrera, and Golgi techniques suggests that it consists of four subdivisions. The medial subdivision (CM), which is closely associated with the stria terminalis, is narrow caudally but enlarges near the rostral pole of CN. Most neurons in CM have long dendrites that branch sparingly and have a moderate number of dendritic spines. A smaller number of CM neurons have thick dendrites with virtually no spines. Lateral to CM is the lateral subdivision (CL) which appears round in coronal sections. Neurons of CL have a very dense covering of dendritic spines and resemble medium-size spiny neurons of the striatum. Area X of Hall contains spiny neurons similar to those of CL and spine-sparse neurons that resemble medium-size spine-sparse cells of the striatum. Since area X encapsulates the lateral aspect of CL, it is termed the lateral capsular subdivision (CLC) of CN. The lateral capsular subdivision enlarges rostrally and is divided into dorsal and ventral portions by a laminar extension of the putamen. Near the rostral pole of CN a small region of tightly packed, intensely stained neurons is interposed between CL and CM. Golgi preparations reveal that this intermediate subdivision (CI) of CN contains neurons similar to those of CM. The lateral subdivision, CLC, and CM correspond, in part, to subdivisions recognized in previous Nissl studies. The intermediate subdivision has not been recognized as a distinct subdivision in previous investigations. This is the first Golgi study to recognize differences in neuronal morphology in particular subdivisions of the rat CN. The correlation of Nissl and Golgi preparations has permitted a more accurate determination of the boundaries and total extent of each subdivision than the use of Nissl techniques alone.

Neurons of the basolateral amygdala: a Golgi study in the opossum (Didelphis virginiana)

The cytoarchitecture of the opossum basolateral amygdala was studied using Golgi techniques. The neuronal morphology was similar in all nuclei of the basolateral complex, an three distinct cell classes were recognized. Class I neurons, which vary in size in different nuclei, have spiny dendrites and long, projection axons. Axon hillocks and initial axonal segments often have spinous protrusions, while more distal portions of the axon give off several beaded collaterals that arborize primarily in the vicinity of the cell. Class II neurons are smaller, spine-sparse cells that are found in all nuclei of the basolateral amygdala but are greatly outnumbered by class I neurons. Axons branch and give off beaded collaterals which form a moderate to dense arborization within the dendritic field of the cell. Class II neurons exhibit considerable morphologic variability including one subtype that resembles the chandelier cell of the cerebral cortex. Varicosities (1.0 - 1.5 micrometers swellings) found along the axonal collaterals of these amygdaloid chandelier cells do not have a uniform distribution but tend to be aggregated. Segments of the collaterals displaying such clustered varicosities sometimes form nest-like entanglements. Clusters of varicosities have been observed forming multiple contacts with initial segments of class I axons. Class III neurons are neurogliaform cells which have many short, varicose dendritic branches that contact dendrites of class I neurons. Only the initial portions of their axons were impregnated. This study indicates that many of the cell types seen in the generalized, metatherian opossum are similar to those described in more specialized, placental mammals. This is the first description of amygdaloid chandelier cells and their contacts with the spiny initial segments of class I projection neurons.

The axon initial segment as a synaptic site: ultrastructure and synaptology of the initial segment of the pyramidal cell in the rat hippocampus (CA3 region)

The axon initial segments (ISs) of pyramidal cells in the rat hippocampus (CA3 region) were studied by means of light microscopy of Golgi-impregnated material and electron microscopy of random and serial thin sections. The ISs display three distinguishing characteristics; fascicles of microtubules, membrane undercoating and clusters of ribosomes. The ISs contain cisternal organelles which are often associated with synapses and are in continuity with smooth and rough endoplasmic reticulum. Small spines are recognized on the ISs both in the light and electron microscope. There are 10-25 on each IS and they are usually concentrated on the proximal 30 micrometers of the IS. Axonic spines contain spine apparatuses, clusters of ribosomes, multivesicular bodies and other organelles. Several collaterals are also recognized to originate from the axon proximal to the start of a myelin sheath. The IS receives many synapses both on its shaft and spines. Almost all of them are of the symmetrical type with flattened vesicles but a few asymmetrical synapses with spherical vesicles occur. Pyramidal cell ISs are very rarely presynaptic at asymmetrical synapses with spherical vesicles. Based on serial sectioning studies, the number of synapses on one IS is estimated at 100-200. These abundant synaptic contacts on the IS suggest that it is an important synaptic site. The possibility that there are two different inhibitory systems controlling the output of the pyramidal cell is discussed.

Ruffed cell: a new type of neuron with a distinctive initial unmyelinated portion of the axon in the olfactory bulb of the goldfish (Carassius auratus): II. Fine structure of the ruffed cell

The fine structure and synaptic features of the ruffed cell, especially those of the initial unmyelinated portion of its axon (IP), were investigated by means of electron microscopy. The round or oval cell body is 10 to 20 micrometers in diameter. Near the cell body, there is a specialized region of the IP, about 20 to 40 micromerters in diameter, which consists of branched protrusions from the IP. Many neuronal processes end on these protrusions, and the whole assembly reminds one of a bird's nest. The nest has a little higher electron density than the surrounding field. The nucleus of the cell itself is round or oval with irregular undulations, and measures 7 to 10 micrometers in diameter. The perikaryon contains the usual cell organelles; especially many clusters of free ribosomes. Two kinds of dendrites arise from the cell body: thick dendrites, about 4 micrometers thick, which appear to be extensions of the perikaryon; and thinner dendrites, about 1 to 2 micrometers in diameter. In addition to these two, processes resembling those of glia arise from the cell body or from the thick dendritic trunk. Some of the dendritic branches enter the glomerulus. The IP arises from the cell body as a protrusion-bearing process about 0.8 to 1.5 micrometer in diameter. The IP is divided into three parts: part 1, where many elaborate protrusions arise to constitute a nest; part 2, where several scattered protrusions arise; and part 3, which has no protrusions. Part 1 of the IP is further subdivided into three portions according to the fine structure of its shaft. The first portion, about 10 to 15 micrometers long, is rather straight and exhibits the two characteristics of the conventional initial segment, i.e., membrane undercoating and fascicles of microtubules. The second portion, about 10 to 20 micrometer long, shows the membrane undercoating, but no fascicles of microtubules. The third portion of part 1 as well as parts 2 and 3 exhibits neither of the two distinguishing characteristics of the initial segment. Synapses are encountered on the cell body, the dendrites, and the IP. Most of them are formed with granule cell dendrites. The ruffed cell is presynaptic in asymmetrical synapses whose postsynaptic elements are the granule cell dendrites and other kinds of neuronal processes of unknown origins. It is also postsynaptic in symmetrical synapses from the granule cell dendrites. Reciprocal pairs of these two types of synapses are also seen, both on the dendrite and the IP. The numbers of synapses on the dendrite and the cell body seem far less than on the IP. The number of synapses on one IP is roughly estimated to be 1,000 to 2,000. The ratios of the synaptic types are as follows: asymmetrical synapses from the IP, 63%; symmetrical synapses onto the IP, 12%; and reciprocal pairs of synapses, 25%. Gap junctions are also seen between protrusions of the ruffed cell IP and dendrites of the perinest cell, which is a small neuron located at the periphery of the nest...

Presence of the ruffed cell in the olfactory bulb of the catfish, Parasilurus asotus, and the sea eel, Conger myriaster

The ruffed cell is present in the olfactory bulb of the catfish, Parasilurus asotus, and of the sea eel, Conger myriaster. Its morphological features have been studied by light microscopy, high-voltage electron microscopy, and conventional electron microscopy. The ruffed cell of the catfish is very similar to that of the goldfish in its location and in its structural features. It has a spheroidal or ovoid cell body about 15 to 30 micrometer in diameter. Several dendrites arise from the cell body to form a rounded dendritic field about 100 micrometer in diameter near the cell body. The initial unmyelinated portion of its axon (IP) consists of a shaft and many protrusions arising from it. The shaft, about 1 micrometers in diameter, located extends for about 100 to 200 micrometer, where it acquires a myelin sheath. The protrusions intermingle with one another in a complex manner to form a rather discrete spheroidal field about 20 to 50 micrometers in diameter, located in the vicinity of the cell body. In contrast, the ruffed cell of the sea eel differs rather significantly from that of the goldfish in its morphological features. The ruffed cell of the sea eel is of a bipolar type. One thick dendrite arises from the cell body and extends for about 50 to 100 micrometers, where it gives rise to many thread-like dendritic branches. The IP arises from the cell body as a smooth thin process. However, about 30 to 70 micrometers distant from its origin, many elaborate protrusions arise from the axonal shaft. These intermingle with one another to form a spheroidal or ovoid field about 20 to 40 micrometers in diameter. Distal to this protrusion-bearing part, the axon continues as a smooth, thin process. In spite of these differences in structural features, the ruffed cell of the catfish and that of the sea eel are very similar in their synaptic features in the nest (the special synaptic field around the ruffed cell IP, composed of its protrusions, of granule cell dendrites, and of other neuronal processes); that is, synapses between the ruffed cell IP (its shaft and protrusions) and granule cell dendrites and serial synapses made by the ruffed cell IP, granule cell dendrites, and perinest cell dendrites. These results suggest that the ruffed cell is generally present in the teleostean olfactory bulb, although its detailed structural features may vary from species to species. Moreover, the neuronal organization of the olfactory bulb seems to be fundamentally similar in various species of teleosts.

Ruffed cell: a new type of neuron with a distinctive initial unmyelinated portion of the axon in the olfactory bulb of the goldfish (Carassius auratus) I. Golgi impregnation and serial thin sectioning studies

A new type of neuron was recognized in the olfactory bulb of the goldfish (Carassius auratus) by means of light and high voltage electron microscopy of Golgi-impregnated material, combined Golgi-electron microscopy, and electron microscopy of serial thin sections. The neuron is located in the layer between the olfactory nerve layer and the anterior olfactory nucleus. It has a spherical cell body, about 10--20 microns in diameter, and several dendrites which form a spherical dendritic field, about 70--100 microns in diameter, in the vicinity of the cell body. The most remarkable structural feature of this neuron is that its initial unmyelinated portion of the axon (IP) has elaborate protrusions with many synapses. The IP can be divided into three parts, parts 1, 2 and 3, based on its structural features. Part 1 is the initial part of the IP, about 20--40 microns in length. Many elaborate protrusions arise from the shaft and intermingle with one another to constitute a spherical field, about 20--40 microns in diameter, around the shaft. Part 2 is the middle part of the IP, about 10--20 microns in length. There are several collateral-like protrusions, which are scattered along the shaft and extended laterally about 5--15 microns. Part 3 is the last part of the IP, and is cylindrical without protrusions. The length of part 3 varies from 20 to more than 100 microns. The axon acquires a myelin sheath at distance of 70--250 microns from its origin. Protrusions make synaptic contacts mainly with granule cell dendrites. Some of them are of the reciprocal type. Protrusion are presynaptic in asymmetrical synapses, and postsynaptic in symmetrical synapses with granule cell dendrites. The shaft of the IP also has synapses similar to those on protrusions. The neuron described is a new type of neuron in the vertebrate central nervous system. We propose for it the name "ruffed cell."

E-PTA stains oligodendroglial surface membranes and microtubules in optic nerves during myelination

Aldehyde fixed Xenopus tadpole and frog optic nerves were stained en bloc with ethanolic phosphotungstic acid (E-PTA). During rapid myelination, intense staining was observed on cytoplasmic faces of paranodal terminal loops and loosely wrapped oligodendroglial membranes found along inner and outer surfaces of compact myelin sheaths. Oligodendroglial microtubules also were heavily stained. Where stained cytoplasmic faces fused to form a lamella of compact myelin, the intense staining was reduced to a thinner, fainter line. In optic nerves of adult frogs, the staining was less dense but the pattern was similar. The staining distribution and available histochemical evidence indicate that E-PTA stains positively charged proteins non specifically. Since myelin basic protein is found in oligodendroglia during myelination, we suggest that it is being stained by E-PTA while being transported along microtubules to sites where it is inserted into developing myelin lamellae.

Structural complexes in the squid giant axon membrane sensitive to ionic concentrations and cardiac glycosides

Giant nerve fibers of squid Sepioteuthis sepioidea were incubated for 10 min in artificial sea water (ASW) under control conditions, in the absence of various ions, and in the presence of cardiac glycosides. The nerve fibers were fixed in OsO4 and embedded in Epon, and structural complexes along the axolemma were studied.