Endoscope-assisted microsurgery of large vestibular schwannomas.

AIMS: The application of endoscope-assisted microsurgery in the treatment of small or medium-sized vestibular schwannomas is of proven value. The goal of our study is to evaluate its usefulness in cases of large schwannomas. PATIENTS AND METHODS: Eighteen patients were included in this prospective study. Their average tumor diameter was 3.9 cm. The retrosigmoid approach was used in all cases. The endoscope was applied during all stages of tumor removal. RESULTS: The facial nerve was visualized endoscopically at early stages of surgery in 9 patients and the abducent nerve in 7 patients. The source of bleeding was identified in 1 case. Tumor remnants in the region of the fundus of the internal auditory canal after apparently total removal were identified in 2 cases. Exposed and unobliterated temporal bone air cells were not observed. DISCUSSION: Even in cases of large schwannomas, the location of the facial nerve can be determined endoscopically early in the procedure. The application of endoscope-assisted microsurgery increases the rate of cranial nerve preservation and of total tumor removal. Although the application of the endoscope did not provide useful information in some cases, it is a safe procedure that did not lead to any complications and/or to considerable prolongation of the operative time. Its application is justified in all cases.

The fine structure of the subthalamic nucleus in the cat. II. Synaptic organization. Comparisons with the synaptology and afferent connections of the pallidal complex and the substantia nigra.

The synaptic organization of the subthalamic nucleus (Sth) of the cat has been investigated by means of electron microscopy. On the basis of the following criteria: the size and the shape of the synaptic boutons, their origin, the size and the shape of the synaptic vesicles, the distribution and density of the vesicular population, and the characteristics of the active synaptic zones, several types of synaptic boutons have been discriminated: F1, F2, SR, LR1, LR2, d.c.v., and "d" profiles. The F1 and F2 types have pleomorphic vesicles and form symmetrical synapses with the neuronal perikarya, the proximal dendrites and their spines, as well as with the initial axonal segments. The SR, LR1 and LR2 types contain round or oval vesicles and form asymmetrical synapses mainly with middle sized and small dendrites, and their spines. The d.c.v. boutons contain a mixed population of clear synaptic vesicles and dense core vesicles. The d.c.v. type forms asymmetrical synapses. The "d" profiles share identical features with the vesicle containing dendrites. The F2, SR, LR1, LR2, and "d" profiles take part in synaptic diads and/or triads, and occasionally participate in the synaptic glomeruli. The LR1 and LR2 take part in glomeruluslike formations. The Sth has a distinct synaptic pattern that permits its discrimination as a separate ultrastructural entity. The Sth seems to share some common ultrastructural features with the thalamic nuclei. On the other hand, the ultrastructural aspects of the Sth are much more different from its closest embryological allies: the both pallidal segments, and the zona reticulata of the substantia nigra.

The fine structure of the subthalamic nucleus in the cat. I. Neuronal perikarya.

The fine structure of the neurons in the subthalamic nucleus (Sth) of the cat has been studied. On the basis of the following criteria: the size and the shape of the neuronal perikarya, the ultrastructural characteristics of their organelles, the presence and quantity of somatic spines and their afferentation, three categories of neuronal perikarya were discriminated: large, medium and small neurons. The large neurons have a diameter above 18 microns and nucleocytoplasmic ratio above 1:1,8. These neurons are the richest in organelles, possess the most numerous spines, and are most afferented. According to their shape, the large neurons were subdivided in multipolar, fusiform, and irregular types. The medium sized neurons have a mean diameter between 16 and 13 microns and nucleocytoplasmic ratio 1:1,5-1:1,3. Their cytoplasm contains a lesser number of organelles that are more loosely distributed. The medium neurons possess a small number of somatic spines and are less afferented. The small neurons have a mean diameter below 12 microns and their nucleocytoplasmic ratio is below 1:1,2. Their cytoplasm is poor in organelles and they usually lack spines. The small neurons are remarkably oligoafferented. The large neurons constitute approximately 25%, the medium sized - about 50%, and the small neurons - less than 5% of the neuronal population. The remaining neuronal perikarya do not offer reliable electron microscopical criteria that could allow an unequivocal discrimination.

Electron microscopic identification of reticulo-subthalamic axon terminals in the cat.

Synaptic boutons emanating from axons of nucleus tegmenti pedunculopontinus origin were identified by electron microscopy in the neuropil of the subthalamic nucleus. Such boutons measure 1.5-3 microns, contain round synaptic vesicles and make asymmetrical axodendritic and axosomatic synaptic contacts with large subthalamic neurons. Very few contacts with vesicle-containing dendrites, and no contacts with the perikarya of the small neurons were observed. The present findings, in keeping with the relevant light microscopic and electrophysiologic data, furnish evidence for a substantial bilateral tegmenti pedunculopontinofugal projection that excites monosynaptically the relay subthalamic neurons.

Electron microscopic evidence for the existence of a corticonigral tract in the cat.

Young adult cats were subjected to unilateral ablation of the frontal and prefrontal cerebral cortex and were allowed to survive for 4 to 5 days. Routine electron microscopic technique was employed to examine orthograde degenerative changes in the ipsilateral and contralateral substantia nigra (Ni). A moderate number of degenerating synaptic boutons (d. s. b.) were observed in the ipsilateral Ni-pars comacta, and only very few d.s.b. were observed in the ipsilateral Ni-pars reticulata. The d. s. b. exhibited features attributed to the dark degeneration type, and might be referred to two main categories; "small-round-vesicle bouton" and "large-round-vesicle bouton" (Hajdu et al., 1973; Hassler et al. 1975). The d. s. b. performed asymmetrical synapses with dendrites of varying size, much more rarely -- with the neuronal perikarya, and extremely rarely -- with initial axonal segments. No degeneration was observed in the contralateral Ni.

Electron microscopy of the subthalamic nucleus in the baboon. I. Synaptic organization of the subthalamic nucleus in the baboon.

Our study of the synaptic organization of the baboon subthalamic nucleus has revealed at least nine different types of synapses. Most frequently encountered is the type I (F) axo-dendritic or axo-somatic synapse. It is characterized by scattered flat vesicles and many large mitochondria. The axon terminal is either elongated, tubule shaped (type IF en passant), and undergoes many short button-like en passant junctions with a parallel running dendrite, or it is spindle shaped with similar synaptic contacts (type IFa). In other cases a more compact bouton forms many short junctions at the same time with a dendrite and a beak-shaped spine springing from it. A smaller bouton with flat vesicles, which show a tendency to fuse together (type IIF) usually undergoes slightly asymmetric contacts with two vesicles-free dendrites between which they frequently appear wedged. Three types of boutons with small, round vesicles have extended asymmetric contacts forming elongated (type III), star-shaped (type IV), or oval (type V) synapses. The latter form contacts mostly with dendritic terminals or spines. Another type of synapses contain larger, pale, pleomorphic vesicles (VI (SO)): The bouton is more compact and often in contact with a dendritic terminal. Type VII (LO) shows a looser arrangement of vesicles intermingled with more dense core vesicles. Type VIII (F) is a dendritic terminal with loosely arranged, flat vesicles and is in contact either with a type VI or with a type VII P bouton. Finally, there is an axo-spinous microsynapse type IX, which partly degenerates after contralateral pallidum externum coagulation. A few axon preterminals filled with dense core vesicles do not undergo synaptic contacts.

Electron microscopy of the subthalamic nucleus in the baboon. II. Experimental demonstration of pallido-subthalamic synapses.

The present study demonstrates the existence in the baboon of a powerful ipsilateral pallido-subthalamic projection, composed of myelinated fibers. These axons give rise to type I (F) terminals, including all subtypes identified by Hassler et al. (1982), and terminate on the perikarya of the subthalamic relay neurons, on proximal dendrites, and on somatic and dendritic spines. Endings on subthalamic interneurons could not be found. Following experimental lesions in the pallidum externum, these pallido-subthalamic neurons undergo distinctive changes characterized as "pale", "intermediate" and "dark" degeneration, the form of the degeneration depending on the survival times of 3, 4 and 5 days. The ratios between the individual types of degeneration may vary. The ultrastructural features of the pallido-subthalamic terminals, which are believed to be GABA-ergic (Fonnum et al., 1978), are different from those of the GABA-ergic striato-nigral connections (Kim et al., 1971). After pallidum externum lesions, the pale form of degeneration is found in a few instances in the contralateral subthalamic nucleus in the type IX mini-synapses and the type I (F) beak-shaped synapses. There is a considerable convergence of different afferents with specialized synapses on the somata, stem dendrites, and dendritic spines of the neurons of the subthalamic nucleus.

Modified Nauta and Fink-Heimer procedures with special reference to the demonstration of monoaminergic pathways in the central nervous system.

Modifications of the selective silver impregnation methods of Nauta and Fink-Heimer are described. Both procedures were tested in wide variety of species: from reptiles to carnivores, as well as in human autoptic cases. The modified Nauta procedure is recommended as a method of choice for tracing axonal trajectories due to an improved impregnation selectivity of the degenerating axoplasm. Its use for the demonstration of monoaminergic (NA, DA, 5-HT) pathways, however, remains limited. The modified Fink-Heimer procedure demonstrates successfully the degeneration of axons, and especially--the thin preterminal arborizations, as well as the terminal boutons in many fiber systems studied, including the nigrostriatal, raphe-nigral, and coerulocortical tracts. Both methods provide an additional cytoarchitectonic orientation due to a counterstain of nerve cell bodies with cresylechtviolett.

Vesicle containing dendrites in the subthalamic nucleus of the cat.

During the ultrastructural investigation of the cat's subthalamic nucleus (STN), profiles exhibiting features attributed to dendrites and simultaneously containing synaptic vesicles, were encountered. Most of them take part in synaptic triads, usually as an intermediate component. On the basis of their electron microscopical appearance, and synaptic relationships, they might be discriminated, as a distinct ultrastructural entity. Although not conspicuous in number, they might have their own contribution to the synaptic events taking place in the STN.

Pallidosubthalamic projection in the cat. Electron microscopic study.

The degenerative changes within the cat's subthalamic nucleus (Sth) following lesions of the external pallidum were studied by electron microscopy. Four to five days following pallidal lesions a great number of terminals undergoing degenerative changes were encountered in the ipsilateral Sth. The contralateral Sth was free of degeneration. The degenerating terminals show predominantly the light degenerative type, less frequently the dark degenerative pattern, and occasionally exhibit signs of filamentous hyperplasia. The degenerated boutons usually insert on perikarya of the large Sth neurons, on proximal dendrites, and more rarely contact dendritic spines. They were observed neither to perform synaptic contacts with the perikarya of the small Sth neurons nor with other vesicle-containing profiles. On the basis of the ultrastructural aspect of the degenerating terminals, they were identified as F1 terminals, discriminated in a previous study (Romansky et al., 1978). The normal appearance, the synaptic relationships, and the degenerative features of the F1 terminials in the Sth closely resemble the entopeduncular terminals in the thalamus described by Rinvik and Grofová (1974a), and Grofová and Rinvik (1974). The possible contribution of the interrupted passing fibers to the observed degeneration is discussed. The present findings corroborate the relevant morphological, physiological, neurochemical, and neuropharmacological data in the literature.