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Notes: The fine structural localization of fibres immunoreactive for the adrenocorticotrope hormone (ACTH) was studied in the mesencephalic central grey substance (MCG) of the male Wistar rat. Light microscopically, varicose ACTH-immunoreactive fibres were found throughout the MCG in a dorsal, lateral and ventral, periventricular position. Electron microscopically, the immunoreactivity was most prominent in the direct vicinity of electron-dense secretory granules in axonal varicosities, and, although to a lower degree, around other cytoplasmic organelles such as electron-lucent synaptic vesicles, mitochondria and microtubules. With serial section analysis two types of ACTH-immunoreactive varicosity were discerned. The first type is large, contains many, small electron-lucent synaptic vesicles, that are located in the vicinity of a morphologically well-defined synaptic contact. In this type of varicosity, large dense-core secretory granules are scarce. Immunoreactivity is low or absent, particularly near the active zone. The second type is strongly immunoreactive. It always contains many large, dense-core secretory granules; electron-lucent vesicles are rare. The smaller varicosities of this type never make synaptic contacts, but a few of the larger varicosities have synaptic contacts with dendrites of MCG cells.

Notes: [Auszug] The relationships of the fibre tracts3'4 suggest that the hypertrophy of the valvula in the Mormyrids is related to the high degree of development of the lateral line system, which to a large extent has become transformed into an electroreceptor organ5'6. Microscopically both corpus and valvula ...

Notes: Abstract By way of introduction, an outline is presented of the origin and evolutionary development of the neocortex. A cortical formation is lacking in amphibians, but a simple three-layered cortex is present throughout the pallium of reptiles. In mammals, two three-layered cortical structures, i.e. the prepiriform cortex and the hippocampus, are separated from each other by a six-layered neocortex. Still small in marsupials and insectivores, this “new” structure attains amazing dimensions in anthropoids and cetaceans. Neocortical neurons can be allocated to one of two basic categories: pyramidal and nonpyramidal cells. The pyramidal neurons form the principal elements in neocortical circuitry, accounting for at least 70% of the total neorcortical population. The evolutionary development of the pyramidal neurons can be traced from simple, “extraverted” neurons in the amphibian pallium, via pyramid-like neurons in the reptilian cortex to the fully developed neocortical elements designated by Cajal as “psychic cells”. Typical mammalian pyramidal neurons have the following eight features in common: (1) spiny dendrites, (2) a stout radially oriented apical dendrite, forming (3) a terminal bouquet in the most superficial cortical layer, (4) a set of basal dendrites, (5) an axon descending to the subcortical white matter, (6) a number of intracortical axon collaterals, (7) terminals establishing synaptic contacts of the round vesicle/asymmetric variety, and (8) the use of the excitatory aminoacids glutamate and/or aspartate as their neurotransmitter. The pyramidal neurons constitute the sole output and the largest input system of the neocortex. They form the principal targets of the axon collaterals of other pyramidal neurons, as well as of the endings of the main axons of cortico-cortical neurons. Indeed, the pyramidal neurons constitute together a continuous network extending over the entire neocortex, justifying the generalization: the neocortex communicates first and foremost within itself. The typical pyramidal neurons represent the end stage of a progressive evolutionary process. During further development many of these elements have become transformed by reduction into various kinds of atypical or aberrant pyramidal neurons. Interestingly, none of the six morphological characteristics, mentioned above under 1–6, has appeared to be unassailable; pyramidal neurons lacking spines, apical dendrites, long axons and intracortical axon collaterals etc. have all been described. From an evolutionary point of view the typical pyramidal neurons represent not only the principal neocortical elements, but also the source of various excitatory local circuit neurons. The spiny stellate cells, which are abundant in highly specialized primary sensory areas, form a remarkable case in point. In these elements only two of the six original pyramidal attributes, i.e. spiny dendrites and an intracortical axonal arbor, are retained. The nonpyramidal neurons display a diverse morphology, but share a number of important morphological and functional features: (1) their dendrites bear only a few spines or none, (2) their axons do not leave the cortex, (3) their terminals make synapses of the flat vesicle/symmetric variety, (4) they use the inhibitory neurotransmitter GABA, and (5) almost all types make synaptic contacts with pyramidal neurons. Several subclasses of nonpyramidal neurons are selectively immunoreactive for particular calcium-binding proteins. The widely held notion that the pyramidal neurons constitute the relatively constant basic framework of the cortex, whereas the local circuit neurons are variable and increase during phylogenetic development in number as well as in diversity is untenable. A survey is presented of the structure, synaptology and chemodifferentiation of the various neocortical cell types, allocating them to three groups: pyramidal neurons, excitatory interneurons and inhibitory interneurons. The synaptic relations of the various neocortical neurons are pictorially summarized in two microcircuitry diagrams, which together form the pièce de résistance of the present treatise. The various approaches to the structure of the neocortex are discussed. It is emphasized that correlative structural, ultrastructural and electrophysiological studies of pyramidal neurons known to project to a given cortical or subcortical target form a promising field of interdisciplinary research.

Notes: In this paper the telencephalon of Latimeria chalumnae, the only surviving crossopterygian, is described and compared to that of other lower vertebrates. It is concluded that Latimeria cannot be related to a particular group of vertebrates, but stands intermediate between the dipnoans and the actinopterygians in its forebrain structure. With respect to the shape of the subpallium, the structure of the telencephalon medium, and the arrangement of its fiber systems, the latimerian forebrain closely approaches the dipnoan condition. The pallium and membranous structures of the telencephalon of Latimeria, on the contrary, are reminiscent in gross form and histological structure of their actinopterygian homologues.However, not all the structural features of the latimerian forebrain can be related to either the actinopterygian or the dipnoan plan. The subpallium, for instance, is more primitive than that of either group mentioned; in fact, it is more simply organized than that of any other living gnathostome.The forebrain of Latimeria appears to display no special structural affinities to the amphibian forebrain. This is not too surprising, since the Coelacanths, among which Latimeria is classified, represent only a side branch of the Crossopterygii, and are not in the main line of evolution to higher forms. It is known that members of the same class of lower vertebrates may vary considerably in their forebrain structure. Hence, the Rhipidistia, totally extinct Crossopterygii which are believed to have given rise to the terrestrial vertebrates, may have possessed a forebrain quite different from that of Latimeria.

Notes: The diencephalon of Polypterus can be divided into an epithalamus, thalamus, and hypothalamus. The habenulae, the nervous parts of the epithalamus, are comparable to their homologues in other lower vertebrates with respect to sulcal boundaries, cellular structure, and fiber connections. The thalamus of Polypterus is not divisible into a pars dorsalis and pars ventralis by the sulcus medius; rather this sulcus is in the middle of a uniform, laminated cytoarchitectonic field. In this respect Polypterus differs from other species in whom the sulcus medius divides the thalamus into dorsal and ventral parts. There are six migrated nuclei in the thalamus of Polypterus. There is only one circumscribed projection into the thalamus, i.e., the optic tract, but there are numerous diffuse fibers terminating in this region of the brain. The hypothalamus, except for a partially migrated nucleus, has retained the periventricular arrangement of cells. It has large fiber connections with the forebrain and brainstem.The literature on the diencephalon of lower forms has been reviewed with special emphasis on the question of how homologies are established in this brainpart. It appeared that three different criteria, either singly or in combination, have been employed as a clue to identification of structures in the diencephalon. These are, (1) ventricular grooves, (2) nuclear boundaries, and (3) fiber connections. In order to test the practical validity of these criteria the diencephalon of Polypterus was compared to that of five related species, i.e., the actinopterygians Acipenser and Polyodon, the dipnoans Protopterus and Neoceratodus, and the crossopterygian Latimeria. In addition three amphibians, Necturus, Ambystoma and Rana, were involved in our comparative considerations. It was concluded that, within the confines of the diencephalon of the species mentioned, cytoarchitectural differences form the most valid criterion for establishing homologies. The drawback and restrictions connected with the use of ventricular sulci and fiber connections, as a clue to identification have been evaluated and discussed.