Larry J. Stensaas

Astrocytic neuroglial cells, oligodendrocytes and microgliacytes in the spinal cord of the toad

SummaryThree types of glial cells corresponding to astrocytes, oligodendrocytes, and microgliacytes were found in the toad spinal cord stained with a modification of the Golgi-Río Hortega technique. Each can be correlated with a characteristic type of nucleus stained with toluidine blue.Astrocytic neuroglial cells are common near the central canal and have large nuclei with lightly stained nucleoplasm and finely granular chromatin. In silver impregnations, astrocytic neuroglial cells are characterized by many fine, spinose or lamellate excrescences which arise from cell somata and from the long peripherally directed processes that extend to the pia. No cells having the stellate form of mammalian astrocytes were seen, and large end-feet have only been seen near the surface of the brain suggesting that the primary relation of this cell is with the pia rather than with the capillaries.Oligodendrocytes are common in the white matter and near capillaries, but do not occur as neuronal satellites. Nuclei are characterized by large aggregates of chromatin and deep membrane invaginations. A spectrum of oligodendrocytes has been seen with the Golgi technique similar to the four types described in mammals by del Río Hortega. Small stellate cells of type I and II are most common and are seen in both white and gray matter. Tubular reticulate structures typical of type IV oligodendrocytes are identical to Golgi impregnated Schwann cells of peripheral nerves and are most abundant in the white matter. The absence of an identifiable soma raises the question of whether the reticulum is located on the outer or the inner surface of myelin.Microgliacytes are most common in areas of dense neuropile and do not form satellites. Nuclei are small, dark, and elongate often with irregular protuberances. In Golgi impregnations two or more long processes arise from a small, irregularly shaped soma. They are covered by spinous or thorn-like processes similar to those of the primitive mammalian pseudopodial variety.

Chronically implanted intrafascicular recording electrodes

A newly designed intrafascicular electrode for chronic neural recording was studied by implanting 12 electrodes in the radial nerves of 6 cats for 6 months. Action potentials were monitored at specified intervals throughout the experiment. The number and size of the signals recorded suggest that this type of electrode provides information that is appropriate for feedback control in functional electrical stimulation (FES) systems. Histology of the nerve revealed that the implants are biocompatible and that little damage is caused by the presence of the electrode.

Histopathological evaluation of materials implanted in the cerebral cortex.

SummaryHistopathological changes of the cerebral cortex in response to small, penetrating metal and non-metal implants were analyzed by means of light and electron microscopy. The needle-shaped implants were left in place during all stages of histological preparation and embedded in plastic together with the cortex. Changes of the brain-implant boundary were classified as non-reactive, reactive, or toxic, according to the reactive cellular constituents. Among the non-reactive materials were several plastics and metals such as aluminum, gold, platinum, and tungsten. The boundary of these implants displayed little or no gliosis and normal neuropile with synapses within 5 μm of the implants surface. The boundary of reactive materials such as tantalum or silicon dioxide was marked by multinucleate giant cells and a thin layer (10 μm) of connective tissue. Toxic materials such as iron and copper were separated from the cortical neuropile by a capsule of cellular connective tissue and a zone of astrocytosis. Cobalt, a highly toxic material, produced more extensive changes in the zones of connective tissue and astrocytes. These results indicate that a variety of materials are well tolerated by the brain and could be used in the fabrication of neuroprosthetic devices.

Ultrastructure of the human vomeronasal organ.

Virtually all vertebrates have a vomeronasal system whose involvement in pheromone detection plays a crucial role in reproduction. In humans, the vomeronasal organ has been assumed to be vestigial or absent and without functional significance. In the present study involving over 400 subjects, vomeronasal pits were observed in all individuals except those with pathological conditions affecting the septum. Electron microscopy of the adult human vomeronasal organ indicates the presence of two potential receptor elements in the pseudostratified epithelial lining: microvillar cells, and unmyelinated, intraepithelial axons. In addition, unmyelinated axons are common in the lamina propria surrounding the organ. They appear to constitute the components essential for a functional chemosensory system, and may thus provide the basis for a pheromone detection system as in other animals.

An electron microscope study of cells in the matrix and intermediate laminae of the cerebral hemisphere of the 45 mm rabbit embryo

SummaryThe morphology and intercellular relations of cells in the matrix, lower intermediate, and upper intermediate laminae of the cerebral hemisphere of rabbit embryos was studied with the electron microscope. Models of cells reconstructed from serial sections confirm previous observations made with the Golgi technique. Most cells in the matrix lamina appear to be spongioblasts; there are relatively few neuroblasts and columnar epithelial cells. Neuroblasts predominate in the intermediate lamina. Their short processes are intercalated among axons and spongioblast processes in the lower part. A large process, the preapex, distinguishes nerve cells in the upper part of the intermediate lamina, and its orientation in the direction of movement suggests that it may actively participate in the migration of neuroblasts.Serial section analysis confirms the fact that mitotic cells in the matrix lamina are spherical and have no processes. Assuming that neuroblasts are incapable of further division, it seems probable that intermitotic germinal cells have the form of spongioblasts and columnar epithelial cells and that they give rise to neuroblasts and other spongioblasts.

Patterning in the regeneration of type I cutaneous receptors

1. Type I sensory fibres in cat hairy skin innervate structures characterized by twenty to fifty specialized epithelial (Merkel) cells aggregated in a small dome‐shaped elevation. Only one fibre enters each dome and it branches repeatedly to supply at least one terminal to each Merkel cell. After the nerve is cut, the Merkel cells and the dome ultimately disappear.

Light and electron microscopy of motoneurons and neuropile in the amphibian spinal cord

Summary Neuropile of the amphibian spinal cord was studied by light and electron microscopy to determine whether there are intercellular relations involving dendrites which would provide support for electrical interaction between montoneuron dendrites. Golgi studies show dendrites of monotoneurons to extend for long distances in a rostro-caudal direction, and electron micrographs reveal dendrodendritic contacts to be common in the vicinity of motoneuron somata. However, no membrane specializations were seen, and the percentage of the dendrite surface apposed to other dendrites is quite small except in small, circumscribed areas referred to as dendrodendritic ‘thickets’ where approximately 15% of the surface of large dendrites involves such contacts. Measurements were also made showing appositional relations of axons, astrocytes, and other elements to neuron somata and dendrites.

Pericytes and perivascular microglial cells in the basal forebrain of the neonatal rabbit

SummaryThree types of pericytes outline the vascular bed in Golgi preparations of the newborn rabbit brain. Elongate cells (Type I) are restricted to capillaries, elements resembling smooth muscle cells (Type II) surround vessels of intermediate size, and large flat forms (Type III) cover the surface of arterioles and venules. Electron microscopy shows all types to be located within a well defined perivascular basement membrane. It also reveals the presence of filaments in the cytoplasm of some pericytes resembling the myofilaments of smooth muscle cells. It suggests the possibility that some pericytes are capable of contraction and may participate in regulating blood flow in small vessels.Microglia cells bear no resemblance to pericytes in terms of their shape, distribution or staining characteristics. Microglia cells are located outside the vascular basement membrane (external basal lamina) in the brain parenchyma, and they vary in form according to their location and the character of the surrounding extracellular space. This study does not support the hypothesis that microglia cells arise from pericytes but indicates that they originate either by in situ division or from hematogenous elements that enter the brain by crossing the vessel wall.

Inhibition of regeneration: the ultrastructure of reactive astrocytes and abortive axon terminals in the transition zone of the dorsal root

Publisher Summary This chapter presents a study in which regenerating sensory axons have been shown to form abortive terminals in the transition zone of the dorsal roots of the spinal cord after a lesion to the nerve at a distant site. The axons vigorously regenerated in the peripheral nerve, penetrated the basal lamina, and then abruptly ceased to grow as soon as they entered central nervous tissue and met processes of reactive astrocytes. These abortive terminals contained little cytoskeletal material and an abundance of vesicles, endoplasmic reticulum, and abnormal organelles. Membrane specializations indicative of cell–cell interaction were observed. Mechanisms whereby the astrocytic processes might inhibit axonal outgrowth are discussed in the chapter. Astrocytes have long been known to play a major role in the reactive processes that occur following an injury to the central nervous system. One particularly advantageous model for studying the interaction between astrocytes and regenerating sensory axons is provided by the transition zone between peripheral and central portions of the nervous system in the dorsal root of the spinal cord.

Axon regeneration following a lesion of the carotid nerve: Electrophysiological and ultrastructural observations

Carotid nerves of the cat were crushed and allowed to regenerate in order to study the properties of regerating fibers and the role of carotid body parenchymal cells (glomus or type I, and sustentacular or type II) in the transduction of chemosensory activity. Such activity is reinitiated 6 days after the nerves are crushed close (1-2 mm) to the carotid body. The process of recovery is delayed when a crush is made at successively greater distances (5-6 and 10-12 mm) from the carotid body. Ultrastructural studies show that the reappearance of nerve endings on the glomus-sustentacular cell complex coincides in time with the onset of chemosensory activity. The regenerated nerve endings increase in size and number and appear normal by 48 days. Some barosensory activity can be elicited 6 days after a nerve crush close to the carotid sinus, but rhythmic barosensory discharges only occur after the 21st day when myelinated axons reappear in the carotid sinus adventitia. Results suggest that recovery of chemosensory function depends on the reestablishment of apposition between regenerating carotid nerve fibers and parenchymal cells of the carotid body.