Ilya I. Glezer

Cellular distribution of the calcium-binding proteins parvalbumin, calbindin, and calretinin in the neocortex of mammals: phylogenetic and developmental patterns

The three calcium-binding proteins parvalbumin, calbindin, and calretinin are found in morphologically distinct classes of inhibitory interneurons as well as in some pyramidal neurons in the mammalian neocortex. Although there is a wide variability in the qualitative and quantitative characteristics of the neocortical subpopulations of calcium-binding protein-immunoreactive neurons in mammals, most of the available data show that there is a fundamental similarity among the mammalian species investigated so far, in terms of the distribution of parvalbumin, calbindin, and calretinin across the depth of the neocortex. Thus, calbindin- and calretinin-immunoreactive neurons are predominant in layers II and III, but are present across all cortical layers, whereas parvalbumin-immunoreactive neurons are more prevalent in the middle and lower cortical layers. These different neuronal populations have well defined regional and laminar distribution, neurochemical characteristics and synaptic connections, and each of these cell types displays a particular developmental sequence. Most of the available data on the development, distribution and morphological characteristics of these calcium-binding proteins are from studies in common laboratory animals such as the rat, mouse, cat, macaque monkey, as well as from postmortem analyses in humans, but there are virtually no data on other species aside of a few incidental reports. In the context of the evolution of mammalian neocortex, the distribution and morphological characteristics of calcium-binding protein-immunoreactive neurons may help defining taxon-specific patterns that may be used as reliable phylogenetic traits. It would be interesting to extend such neurochemical analyses of neuronal subpopulations to other species to assess the degree to which neurochemical specialization of particular neuronal subtypes, as well as their regional and laminar distribution in the cerebral cortex, may represent sets of derived features in any given mammalian order. This could be particularly interesting in view of the consistent differences in neurochemical typology observed in considerably divergent orders such as cetaceans and certain families of insectivores and metatherians, as well as in monotremes. The present article provides an overview of calcium-binding protein distribution across a large number of representative mammalian species and a review of their developmental patterns in the species where data are available. This analysis demonstrates that while it is likely that the developmental patterns are quite consistent across species, at least based on the limited number of species for which ontogenetic data exist, the distribution and morphology of calcium-binding protein-containingneurons varies substantially among mammalian orders and that certain species show highly divergent patterns compared to closely related taxa. Interestingly, primates, carnivores, rodents and tree shrews appear closely related on the basis of the observed patterns, marsupials show some affinities with that group, whereas prototherians have unique patterns. Our findings also support the relationships of cetaceans and ungulates, and demonstrates possible affinities between carnivores and ungulates, as well as the existence of common, probably primitive, traits in cetaceans and insectivores.

Implications of the “initial brain” concept for brain evolution in Cetacea

We review the evidence for the concept of the “initial” or prototype brain. We outline four possible modes of brain evolution suggested by our new findings on the evolutionary status of the dolphin brain. The four modes involve various forms of deviation from and conformity to the hypothesized initial brain type. These include examples of conservative evolution, progressive evolution, and combinations of the two in which features of one or the other become dominant. The four types of neocortical organization in extant mammals may be the result of selective pressures on sensory/motor systems resulting in divergent patterns of brain phylogenesis. A modular “modification/multiplication” hypothesis is proposed as a mechanism of neocortical evolution in eutherians. Representative models of the initial ancestral group of mammals include not only extant basal Insectivora but also Chiroptera; we have found that dolphins and large whales have also retained many features of the archetypal or initial brain. This group evolved from the initial mammalian stock and returned to the aquatic environment some 50 million years ago. This unique experiment of nature shows the effects of radical changes in environment on brain-body adaptations and specializations. Although the dolphin brain has certain quantitative characteristics of the evolutionary changes seen in the higher terrestrial mammals, it has also retained many of the conservative structural features of the initial brain. Its neocortical organization is accordingly different, largely in a quantitative sense, from that of terrestrial models of the initial brain such as the hedgehog.

Calretinin-immunoreactive neurons in the primary visual cortex of dolphin and human brains

A new class of gamma-aminobutyric acid (GABA)ergic neurons immunoreactive to the calcium-binding protein calretinin (CR) was demonstrated in primary visual cortices of the bottlenose dolphin (Tursiops truncatus) and humans (Homo sapiens). Comparative analysis revealed several differences between dolphin and human visual cortex in the laminar distribution of CR-positive perikarya, although general typology of the immunoreactive CR-positive neurons was similar in both species. Thus, in both human and dolphin primary visual cortex almost all CR-positive neurons are non-pyramidal, either fusiform or bipolar cells, oriented with their long axis along the radial axis of the cortex. Large multipolar stellate cells were also observed in layers I and VI. The CR-positive neurons in the dolphin visual cortex are concentrated almost exclusively in layer I and, to a lesser extent, in layer II. In all other layers (IIIa, b, IIIc/V and VI) of the dolphin visual cortex CR-positive neurons were only rarely seen. In the human primary visual cortex CR-positive neurons are located mainly in layers II, III and IVa, b, c, with considerably lower densities of these cells observed in layers V and VI. CR-positive neurons in layer I of the human visual cortex are represented by Cajal-Retzius horizontal cells, whereas no such cells were seen in layer I of the dolphin neocortex. The numerical density of CR-positive neurons in the dolphin primary visual cortex is significantly lower than in the area of cortex in humans.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurochemical and Cellular Specializations in the Mammalian Neocortex Reflect Phylogenetic Relationships: Evidence from Primates, Cetaceans, and Artiodactyls

Most of the available data on the cytoarchitecture of the cerebral cortex in mammals rely on Nissl, Golgi, and myelin stains and few studies have explored the differential morphologic and neurochemical phenotypes of neuronal populations. In addition, the majority of studies addressing the distribution and morphology of identified neuronal subtypes have been performed in common laboratory animals such as the rat, mouse, cat, and macaque monkey, as well as in postmortem analyses in humans. Several neuronal markers, such as neurotransmitters or structural proteins, display a restricted cellular distribution in the mammalian brain, and recently, certain cytoskeletal proteins and calcium-binding proteins have emerged as reliable markers for morphologically distinct subpopulations of neurons in a large number of mammalian species. In this article, we review the morphologic characteristics and distribution of three calcium-binding proteins, parvalbumin, calbindin, and calretinin, and of the neurofilament protein triplet, a component of the neuronal cytoskeleton, to provide an overview of the presence and cellular typology of these proteins in the neocortex of various mammalian taxa. Considering the remarkable diversity in gross morphological patterns and neuronal organization that occurred during the evolution of mammalian neocortex, the distribution of these neurochemical markers may help define taxon-specific patterns. In turn, such patterns can be used as reliable phylogenetic traits to assess the degree to which neurochemical specialization of neurons, as well as their regional and laminar distribution in the neocortex, represent derived or ancestral features, and differ in certain taxa from the laboratory species that are most commonly studied.

Comparative analysis of calcium-binding protein-immunoreactive neuronal populations in the auditory and visual systems of the bottlenose dolphin (Tursiops truncatus) and the macaque monkey (Macaca fascicularis)

This study compares the distribution of three calcium-binding protein-immunoreactive (CaBP-immunoreactive) neuronal populations (calretinin-, calbindin- and parvalbumin-immunoreactive) in the visual and auditory systems in two mammalian species which are fundamentally different in their evolutionary traits and ecology, the aquatic toothed whale Tursiops truncatus (bottlenose dolphin) and the terrestrial Old World primate, Macaca fascicularis (long-tailed macaque). Immunocytochemical analyses, combined with computerized morphometry revealed that in the visual and auditory systems of the bottlenose dolphin, calretinin and calbindin are the prevalent calcium-binding proteins, whereas parvalbumin is present in very few neurons. The prevalence of calretinin and calbindin-immunoreactive neurons is especially obvious in the auditory system of this species. In both auditory and visual systems of the macaque monkey, the parvalbumin-immunoreactive neurons are present in comparable or higher densities than the calretinin and calbindin-immunoreactive neurons. In some structures of the visual and auditory systems of the macaque monkey, the calretinin- and calbindin-immunoreactive neurons are nearly absent. The prevalence of parvalbumin-immunoreactive over calretinin- and calbindin-immunoreactive neurons is particularly prominent in the visual system of primates. Thus, the dominant sensory systems in both aquatic and terrestrial mammals are enriched in specific phenotypes of calcium-binding protein-immunoreactive neurons.

Ultrastructure of synapses and golgi analysis of neurons in neocortex of the lateral gyrus (visual cortex) of the dolphin and pilot whale.

Qualitative and computerized quantitative analyses of ultrastructural features of synapses in different layers of the primary visual cortex in the dolphin (Stenella coeruleoalba) and the pilot whale (Globicephala melaena) were carried out. Also, Golgi and cytoarchitectonic analyses were performed in the same species of cetaceans and, additionally, in Tursiops truncatus and Phocaena phocaena. It was found that on a synaptic level, as well as in cytoarchitectonic and Golgi features, the neocortex of cetaceans combines evolutionary progressive features and conservative features with a marked prevalence of the latter. Thus, the total number of synapses in visual neocortex in cetaceans is closer to this value in higher Primates. On the other hand, the laminar density of synapses per mm3 is generally the same in all layers in cetacean visual cortex and numerically is close to values found in small lissencephalic brains. Also, the synapse/neuron ratio in the dolphin visual cortex is of the same order as in cortices of rodents and lagomorphs and much higher than in cortices of advanced terrestrial mammals. Layers I and II contain approximately 70% of the total synapses in the cortical slab through visual cortex. Layer I also contains the extraverted dendrites of neurons of layer II and thus these two layers resemble a paleoarchicortical type of organization superimposed on a more typical neocortical organization of the lower cortical layers. In this respect the convexity neocortex of cetaceans is generally similar to the neocortices of phylogenetically ancient extant mammals such as basal Insectivora and Chiroptera.

The primary auditory cortex in cetacean and human brain: A comparative analysis of neurofilament protein-containing pyramidal neurons

To extend our investigation of the anatomy of sensory systems in highly adapted aquatic and terrestrial mammals, we have analyzed the distribution of a particular population of efferent neurons in the cetacean and human primary auditory cortex using an antibody to non-phosphorylated neurofilament protein (SMI32). The neurofilament protein triplet is differentially distributed within neuronal subpopulations in the primate and cetacean neocortex. In primates, it appears that the somatodendritic domain of a subset of pyramidal neurons furnishing specific corticocortical connections contains high concentrations of neurofilament protein. In the human primary auditory cortex these neurons are located in layers III, V and VI, whereas in cetaceans they are concentrated almost exclusively in the cortical efferent layer IIIc/V. Previous analyses have shown that SMI32 immunoreactivity in the cetacean neocortex is uniformly distributed among functionally different areas, while in human neocortex, the distribution of SMI32-positive neurons exhibit a high degree of regional and laminar specialization that is correlated with the functional and anatomical diversity of the cortical areas. In addition, the overall distribution of SMI32-immunoreactive neurons in the cetacean neocortex is comparable to that observed in paralimbic areas of the human, suggesting that the cetacean neocortex has retained many features of phylogenetically older cortical regions.

Ultrastructure of the blood-brain barrier in the dolphin (Stenella coeruleoalba)

Light and electron microscopic methods were used for investigation of angioarchitectonics, glioarchitectonics and the structural basis of the blood-brain barrier in the dolphin Stenella coeruleoalba. It was shown that the cortical plate of the dolphin brain is extremely rich in capillaries and small arteries that are organized into a complicated net of continuous loops surrounding neuronal groups. The density of the capillary loops is related to the cytoarchitectural density of the cortex. It was also found that the neuronal microenvironment in the dolphin cortex is characterized by the presence of a large number of the astroglia-like cells that make a multi-layered investment surrounding capillaries and small arteries. These glial cells, unlike typical astrocytes of terrestrial mammals, have a large number of different organelles and their nuclei are similar to those of the oligocytes. The ultrastructure of the blood-brain barrier in the dolphin is characterized by the presence of extremely long tight junctions between endothelial cells and by specialized junctions between pericapillary astroglia-like cells. A belt of the glial end-feet interlocked with different types of junctions such as zonulae adherentes, maculae adherentes and gap junctions was found around all investigated capillaries. This system of specialized interendothelial and glio-glial junctions is tentatively hypothesized to be a feature of adaptation of the dolphin to the aquatic environment.

Distribution of dopaminergic fibers and neurons in visual and auditory cortices of the harbor porpoise and pilot whale

The distribution of putative dopaminergic fibers in two sensory cortical areas in the brain of the harbor porpoise (Phocoena phocoena) and pilot whale (Globicephala melaena) was analyzed at the light and electron microscopic levels using tyrosine hydroxylase (TH) immunohistochemistry. The quantitative analysis of the distribution of labeled fibers demonstrates that the primary visual cortex located in the lateral gyrus and entolateral sulcus contains a denser dopaminergic innervation than the auditory cortex within the posterior portion of the presylvian gyrus. In both areas, TH-immunoreactive fibers are densest in layer I, while layers IIIab and VI have intermediate densities and layers II and IIIc-V have the lowest fiber counts. Layer I is characterized by the presence of very thick TH-immunoreactive fiber populations, in addition to the thin and varicose fiber plexus observed throughout the cortical layers. Electron microscopic analyses demonstrated that some of these thick fibers represent the dendrites of TH-immunoreactive neurons located in the deep portion of layer I. The patterns observed in the present study suggest that the dopaminergic projections to the neocortex in whales have a different organization than in terrestrial mammals, particularly rodents and primates. These differences may reflect the fact that during evolution, the cetacean neocortex has retained many of the cytoarchitectonic features that are usually observed only in proisocortical regions in progressive terrestrial mammals.

Right and Left Accessory Renal Arteries Arising from a Common Trunk Associated with Unrotated Kidneys

We describe a rare anomaly of the kidneys and its vessels, which was found in a white adult male cadaver. The anomaly consisted of unrotated kidneys with partially extrarenal calices and pelves. In addition to the normal vessels, each kidney received a branch from a common trunk, originating from the inferior end of the aorta.