Search

Synaptic organization of the internal plexiform layer (IPL) of the retina has been studied electron microscopically in two aquatic and two land Chelonia species. Certain signs in common of the IPL structure have been revealed, they are similar with those in other Vertebrata, as well as common peculiarities inherent in Chelonia of all the four species studied. Any species-specific differences are not detected. In the land Chelonia, unlike the aquatic Chelonia, a relative content of the amacrinal cell synapses is higher in serial combinations. The ratio of the amacrinal cell synapses to all bipolar neurocyte synapses is within the limits 3.1-4.1.

An ultrastructural study is performed with a quantitative analysis of some parameters of the synaptic organization of the superficial plexiform layer of the three basic cortical zones (hippocampal, dorsal and pyriform) of the forebrain in two aquatic and two land Chelonia species. On the basis of our own and literature data it is inferred that all the basic elements of the axo-spino synaptic complexes (dendritic spines--active synaptic zones--nerve endings) exhibit both stability, ensuring the structural durability and reliability of the contacts' functioning, on the one hand, and plasticity responsible for homeostasis and adequate transformations, on the other. Normally, functional shifts in the fine structure of synaptic apparatus are presumably too slight to conceal the specific features of the nerve centres' synaptic organization. This made feasible a comparative study of the structure of the axo-spinal complexes in functionally dissimilar zones, which revealed the following three categories of characters: (1) stable, common of all the Chelonia species under study and characterizing a particular level of the integration of nervous processes, and (2) variable, ecologically-specific and (3) species-specific, ensuring adaptation changes within a given level of interneuronal organization. It has been revealed that ecological differences in the synaptic organization of the cortical zones in the aquatic and land Chelonia are most pronounced in the pyriform and hippocampal cortices, i.e. zones of exclusively and predominantly olfactory projections respectively, and are considerably less pronounced in the dorsal cortex, the zone of predominantly visual afferentation.

Electron microscopic study of the neuronal composition of Emys orbicularis cortical zones as compared with the light microscopy data has revealed that the distinctions in the cell cytoplasm ultrastructure with respect to the dendrite branching pattern and the peculiarities of their localization in the cortical plate are suggestive of a possible metabolic heterogeneity and can serve as a foundation of the morpho-functional characteristics of the neurons. A few neuronal types of neurons have been revealed: 1. light neurons distinguished by the cytoplasm matrix devoid of any electron density, a poorly developed endoplasmic reticulum and a few cytoplasmatic organells and inclusions; 2. light neurons having slight osmophility of the cytoplasm matrix, a more developed endoplasmic reticulum with elongated cisternae, a great number of organells including neurofilaments; 3. dark neurons having a high degree of cytoplasm matrix and the nucleus, a large number of the nuclei, cytoplasm, dendrites and axons in contrast to the above described patterns.

The morpho-functional characteristics of the optic nerve and tectum opticum were studied electronmicroscopically and electrophysiologically after unilateral enucleation in 4 Chelonia species. Different types of degeneration of the myelinated and unmyelinated optic fibers and their endings in the tectum are revealed. Comparison of the types and times of the degeneration of retinal axons and their terminals with changes of the tectum evoked potentials at different times after enucleation gives ground to conclude that myelinated fibers produce nerve endings degenerating on the dark pattern and unmyelinated, all the remaining endings with the light, neurofilamentous, glycogenous, vesicular and other degeneration patterns. A stratal arrangement of the retino-tectal projections of various type fibers is demonstrated. Asynchrony of the process of degeneration of different groups of fibers and the relationship between the rate of degeneration and the diameter of myelinated fibers, as well as the absence of a relationship between the change pattern and the diameters of myelinated and unmyelinated axons are revealed. In land tortoises and water turtles differences are revealed in the set of major degeneration patterns and their endings also in the patterns of stratal distribution of various categories of the retinal fibers and tectum. A possible relationship is proposed between the above differences and ecological properties of the Chelonia groups under study. Also revealed are species-specific features of some substantial aspects of the morpho-functional organization of the levels of the visual analyzer in Chelonia.

A study of ultrastructural changes in the neuronal terminals of optic fibres at the level of tectum opticum (TO) in different periods after unilateral enucleation has revealed the process of degeneration in the optic system of Testudo horsfieldii as well as in Emysorbicularis L. to take a considerable time: from two weeks to six months. The character of destructive changes and the mode of utilization of desintegrated elements are dissimilar in different groups of retinal axons in the optic nerve and their terminals in TO. A comparison of the degeneration periods and the character of the changes in the optic fibres has made it possible to establish the following approximate correlations: all the nerve terminals of myelinated fibres degenerate after the "dark" type, whereas the terminals of non-myelinated axons are subjected to other kinds of transformations ("light", vesicular", neurofilamentous", "glycogen"). The analysis of the distribution of degenerating nerve terminals in the TO layers has demonstrated that there is a regularity in localization of different nerve terminals at certain levels of TO which is indicative of the layered organization of retino-tectile connections. Considerable differences in the neuronal and synaptic organization of TO have been revealed as well as in the character and the time of destructive changes of retino-tectile fibres after enucleation in T. horsfieldii and E. orbicularis.

A comparative study was made of cytoarchitectonics, neuronal composition and ultrastructural organization of the three basic cortical zones (hippocampal, dorsal and pyriform) of the forebrain of four members of Chelonia (two aquatic and two land species). The features under study fall into three major groups as follows: (1) common to all the Chelonia under investigation and possibly of the entire order; (2) ecologically specific, i.e. common to animals with similar ecology and different in animals with different ecology; (3) species-specific, i.e. differing in individual species. The common structural features of the forebrain cortical plate in Chelonia underlie the morphological substrate responsible for specific functions in different cortical zones. It is precisely with respect to the common features that the dorsomedial dorsal cortex exhibits a more complex neuronal and synaptic organization as compared with other cortical zones. This part of Chelonia cortex is assumed to be homologous to mammalian neocortex. The more plastic ecologically specific and species specific features appear to provide adaptive specializations within a particular level of the nervous system organization.

An electron microscopical investigation of synaptic organization of the superficial plexiform layer of three forebrain cortical zones (hippocampal, dorsal and piriform) in two aquatic and two land Chelonia species has been performed. As demonstrate the data obtained and those from the literature, the main elements of the axo-spinal complexes (spines--synapses--neural terminals) possess both stability properties, contributing to structural firmness of the contacts and their reliability during activities, and plastisity ensuring the maintenance of dynamic equilibrium and a possibility for adequate rearrangements. Under normal conditions, the functional shifts in ultrastructure of the synaptic apparatuses are, possibly, so negligible that they cannot mask certain specific fractures of the synaptic organization of the neural centers. When comparing structures of the axo-spinal complexes in functionally dissimilar cortical zones, more constant, common for all species of Chelonia studied signs are definitely revealed. They characterize a certain level of the neural processes integration. There are signs more variable, evidently, ensuring adaptive rearrangements within limits of the given level of the interneuronal connections organization which can be considered as ecological and species-specific peculiarities.

The neuronal and synaptic organization of tectum opticum was studied in two species of tortoises (Clemmys caspica Gmel, and Emys orbicularis L.) by means of Nissl, Golgi and electron microscopical methods. The Clemmys caspica tectum differs from that of the Emys orbicularis in a greater diversity of neuronal composition; a greater variety and quantity of cells with short axons, the presence of neurons with relatively short dendrites, in a greater relative thickness of the upper and deep systems of horizontal neurons, in a small number of axo-axonal synapses in sublayer Ia, a more diverse composition of synaptic zones in sublayers IIa and IIb, the presence of axons in layers Ib and IIb with certain species specific features of degeneration, the pattern of localization of tectal ganglionic cell projections. The above mentioned specific features appear to be connected with the species differences of these tortoises.

Electron microscopic investigation indicates that in T. graeca the optic nerve consists of about 216,000 fibers of which 11% are myelinated and 89%--unmyelinated ones. The diameter of myelinated fibers varies within 0.2-2.5 mu, the main bulk of fibers (42%) having the diameter 0.5-0.7 mu. The diameter of unmyelinated fibers varies between 0.1 and 0.35 mu. With respect to their diameter, 8% of the fibers of both groups overlap each other. During electrical stimulation of the optic nerve, action potential in the latter is recorded which consists of two high amplitude components. The first component (propagation velocity 1.5 m/sec) presumably corresponds to conduction of excitation along myelinated fibers, the second one (propagation velocity 0.6 m/sec)--along unmyelinated fibers.

A layer-by-layer study of ultrastructural synaptic organization of mesencephalic optic centre (tectum opticum) of T. horsfieldi Gray was made. The predominating type of synaptic contacts is axo-dendritic, however, there are some axo-axonic synapses which are met most frequently in the regions where polysynaptic complexes and glomeneruli (sublayer Ib, sublayer IIa and the upper zone of layer III) are disposed. Axo-somatic contacts are localized virtually in all the neuronic types. In the external layers of the tectum opticum where the terminals of optic afferents are disposed, a considerable morphological variety of nerve terminals is revealed which are distinguished not only by their ultrastructural parameters but also by the character of organization in synaptic complexes at different levels of the cortical plates of the investigated centre.

The description of cytoarchitectonics and topography of the three principle cortical zones of the forebrain of Emys orbicularis reveals that the cortical structures in the species are precisely distinguished with respect to their neuronal characteristics, density and the situation of cellular elements. Emys orbicularis occupies an intermediate position among the species of the order Chelonia described in the literature as regards the level of development of the forebrain cellular structures. Emys orbicularis is less progressive in respect to the structure of telencephalic cortical formations than Testudo carolina and has a number of progressive properties as compared with Testudo geometrica. Testudo graeca seems to be close to Emys orbicularis in the development of forebrain cellular structures.

A comparative study of dendritic spines in the principal cortical regions of the forebrain was performed for two turtle species by the Golgi and electron-microscopy methods. Common features of spines were revealed for every cortical region of both turtle species. Distribution peculiarities in the proximal, middle and distal sections of the spines, as well as their density, shape, length and ultrastructure were taken into account. Apart from similar features for two turtle species, there are some differences between spines of the same cortical region in Testudo horsfieldi and Emys orbicularis, which might be associated with ecological peculiarities of both species.

A comparative investigation of dendritic spines has been performed in three cortical regions (dorsal, pyriform and hippocampal) of the forebrain cortical plate in the tortoise and turtle by means of light and electron microscopy. There are differences both between various cortical zones and at various levels of dendrites (proximal, middle and distal) within the limits of one cortical formation. The dorsal cortex has a number of signs demonstrating its more complex formation in comparison with the hippocampal and pyriform cortex. A possibility on homologicity of this zone with some mammalia is supposed. Owing to the data obtained, three groups of signs can be defined in the spinal organization of the cortical zones studied: common for all four species investigated (perhaps they are connected with a certain evolutional level in the neural system development), ecologically specific, those make difference between the tortoise and the turtle, and species-specific.

Several methods (light optic, electron microscope and electrophysiological) were used to study the optic nerve and the tectum opticum of water turtles (two species) and of land tortoises (two species). As the result of study of the two levels of the visual analyser in four species of Chelonia three groups of signs of the morpho-functional organization were distinguished: signs common for the Chelonia under study; ecologically specific signs--common for water turtles and species-specific traits which are--different in species of the same ecological group. Specific features of structural-functional organization of the visually directed behaviour in Chelonia at the neuronal populations level were shown to correlate with the ecological features to a greater degree than with the systematic position of the species.

Studies have been made on.morpho-functional changes in the optic nerve and midbrain tectum after enucleation of the tortoise T. horsfieldi. It was found that intensive degeneration of terminals which follows "dark" type (3--31/2 months after enucleation) corresponds to degeneration of myelinated fibers. Terminals degenerating accordingly to "light" type (2--3 months) presumably belong to unmyelinated fibers, which degenerate at this period in the nerve. Terminals which follow "vesicular" degeneration, survive up to 6--61/2 months, probably due to survival of some unmyelinated fibers in the nerve. During degeneration of the optic fibers, corresponding changes in amplitude-temporal parameters of the evoked potentials in the midbrain tectum were observed during electrical stimulation of the optic nerve. These changes include the increase of the interval between the stimulus and the peak of both components, as well as the decrease of the amplitude of the responses. These changes were originally observed 11/2--2 months after enucleation, becoming more evident at later stages; 6--61/2 months after enucleation, both components of the evoked potential disappeared almost simultaneously.

The stratification, neuronal structure and synaptic organization of tectum opticum have been studied in two species of tortoises (Testudo graeca and Testudo horsfieldi, Gray) by means of Golgi methods and electron microscopy. The midbrain tectum in these closely related animals has some common properties, but at the same time it possesses certain species specific features. Their properties in common appear to be connected with the common ecology and closely systematic place of Testudo graeca and Testudo horsfieldi. The specific features appear to be connected with the species differences in visual behavior of these species.

Along with the commonly known patterns of terminal destruction ("dark", "light", "neurofilamentous"), new types of changes have been revealed, referred to as "vesicular" and "glycogen" trasformation types. Vesicular transformation which may develop according to the dark or light type is characterized by a sharp deformation of synaptic vesicles; glycogen transformation is distinguished by agglutination and disappearance of synaptic vesicles, the inside of the terminal being filled up with glycogen granules. Ultrastructural differences of the terminal degeneration process in different groups of nerve fibers reflect the morpho-functional heterogeneity of the retina ganglion cells and of their axons.

Electrophysiological and electron microscopic studies have been made on degeneration in the optic nerve and midbrain tectum after enucleation of the tortoise T. graeca. It was found that intensive degeneration of terminals which follows "dark" type (1/2--1 month after enucleation) corresponds to degeneration of myelinated fibers. Mass degeneration of the "light" type is associated with degeneration of unmyelinated fibers, being observed 1 1/2--2 months after the enucleation. The main bulk of fibers and terminals degenerates at the period of 4--4 1/2 months. Changes in functional characteristics of the evoked potentials (EP) follow morphological ones and consist in an increase of the interval between the stimulus and the peak of EP components, as well as of the decrease of the amplitude of the EP. Within 4--4 1/2 months, both EP components disappear almost simultaneously.

In two tortoise (Agrionemys horsfieldi Gray, Testudo graeca L.) and three turtle (Emys orbicularis L., Clemmys (Mauremys) caspica Gmel, Tryonix sinensis Wieg) species, contacts of basal processes (synaptic peduncles of the cone cells) have been studied electron microscopically. Their ultrastructure and peculiarities in organization of specialized junctions between each other and with other neural elements are described. The interreceptory contacts revealed are considered to be of one of the gap junction type responsible for electrotonic interactions in the nervous system. The basal processes are shown for the first time to possess an ability to form connections on the body and on the fine branches of the tubular type horizontal cells. It correlates with a view of some physiologists on a mediated interaction of photoreceptors via the horizontal neurons.

A comparative study of the dendritic spines in three principal cortical zones of the turtle forebrain reveals certain patterns of spine distribution, length, shape and other specific morphological characters in every cortical region. The dorsal cortex spines show the highest density, the greatest length, the smallest head diameter, the largest number of mitochondria and invaginated spines. The above features markedly distinguish the dorsal cortex spines from the those of the medial and lateral cortex, which is associated with a more complex morpho-functional organization of this cortical region.

A layer-to-layer study of the neuronal ultrastructure in the mesencephalic optic centre of T. horsfieldi has been made, the results being compared to those of similar experments with Emys orbicularis L. Neurons of the efferent type (layers IV, V), in contrast to short axon elements (layers, I, 22) are characterized by a greater amount of cytoplasm and by a variety of cytoplasmatic organelles. A few morphological types of neurons are distinguished: light, dark and neurons with a light nucleus and a dark cytoplasm. In two systems of horizontal neurons dark cells predominate to be assumed as being an independent morphofunctional neuronal type. In contrast to E. orbicularis, the neurons of T. horsfieldi show a large amount of axo-somatic synapses on the surface, and a deficiency in spiny outgrowths, both on the apical and basal dendrites.

The optic nerve and mesencephalic optic centre (tectum opticum, TO) of the tortoise, Testudo horsfieldi, Gray, in the norm and after the enucleation have been studies using Golgi's method, electron microscopy and the electrophysiological technique. The optic nerve comprises about 400,000 fibres represented by two classes of axons: myelinated (10%) and unmyelinated (90%). The diameters of the former vary within 0.3-3 mum, of the latter within 0.3-1.1 mum. The neurogramms of the optic nerve contain two components corresponding to the two rates of conduction through different groups of fibres (1.3 musec and 0.5 musec). The stratified synaptic organization of the TO has been investigated. The optic terminals are shown to join in complex synaptic formations at certain levels of the upper layers of the TO (Strata I, II and III). The process of degeneration in the optic system of Testudo is drastically stretched in time. It was demonstrated by the electrophysiological control that the TO potentials induced in response to the nerve stimulation were not registered any longer only 6-6.5 months after the enucleation. Certain groups of retinal fibres of the optic nerve and their terminals in the TO are shown to degenerate differently and asynchronously. Earlier unknown types of destructive changes of the terminals (vesicular, neurofilamentous, glycogen and others) are described. It is suggested that the different types of degeneration of the optic fibres and their terminals are the result of possible biochemical heterogeneity of the retinal ganglious cells. The comparison of the terms and types of degeneration of different groups of nervous fibres in the optic nerve and their terminals in the TO has revealed some correlations suggesting that the myelinated optic fibres produce nervous terminals degenerating by the "dark" type, whereas the unmyelinated optic axons may be subdivided into several groups, each producing nervous terminals subjected to a definite type of destruction ("clear", "vesicular", "neurofilamentous" etc.). The localization of certain groups of therminals at certain levels of the cortical plate of the TO confirms the electrophysiological data on the stratified organization of the retino-tectal projections in the TO of lower vertebrates. It has been shown by the comparison of the results obtained with those reported for Emys orbicularis, L. that the two species have marked differences in the morpho-functional characteristics of the retino-tectal system, neuronic composition of the TO, the character of destructive changes of the terminals after the enucleation and the component composition of polysynaptic complexes and glomerules. Probably these differences may come from the peculiarities of the perception and transformation of visual information in the species.

The optic nerve of the tortoise Agrionemys horsfieldi contains about 400,000 fibers (90% unmyelinated and 10% myelinated ones). the diameter of unmyelinated fibers varies from 0.3 to 1.1 mu, mean value being equal to 0.5 mu; fibers with a diameter 0.4-0.7 comprise 77%. The diameter of myelinated fibers varies within 0.3-3.0 mu, average value being 0.5-0.8 nu; fibers with a diameter 0.5-0.9 mu amount to 62%. Electrogram of the optic nerve consists of two components which are equal in their amplitudes. These components presumably reflect summary firings of modal groups of unmyelinated and myelinated fibers. The velocity of propagation of excitation along the fibers producing the first component is equal to 1.3 m/sec, wheras that in fibers producing the second component - to 0.5 m/sec. The data obtained are compared with those related for the other tortoise - Emys orbicularis.

Morpho-functional changes in the mesencephalic centre of the turtle (Emys orbicularis L.) were studied during optic nerve degeneration. It was observed that synaptic terminals in this centre undergoing the "dark" degeneration are associated with myelinated optic fibres and terminals undergoing the "light" one are associated with unmyelinated fibres. The first low amplitude component of the evoked potential disappears with degeneration of myelinated fibres of large diameter during the "dark" degeneration (4-5 months after enucleation). The first high amplitude component disappears with degeneration of the middle-sized myelinated fibres and the end of the "dark" degeneration (7 months after enucleation). The characteristics of the second high amplitude component which reflects the excitation of both thin myelinated and all unmyelinated fibres, begin to change with degeneration of unmyelinated fibres during "light" degeneration (2.5-3.5 months). This component disappears with degeneration of the thin myelinated fibres (7 months).