Walter K. Schwerdtfeger

Morphology of identified entorhinal neurons projecting to the hippocampus. A light microscopical study combining retrograde tracing and intracellular injection

Cells of origin of the entorhinohippocampal pathway were retrogradely labeled by injection of Fast Blue into the ipsilateral hippocampus. The cells, which were located in layers I, II and III of the lateral entorhinal cortex, were then intracellularly injected with Lucifer Yellow to reveal their complete morphology. We could thus establish that among the hippocampally projecting entorhinal cells there are pyramidal and pyramid-like cells, spiny stellate cells of various shapes, sparsely spinous horizontal and multipolar cells. The involvement of horizontal and multipolar neurons in projections has not previously been recognized although all of these cell types have already been described in Golgi studies. The results indicate that the organization of the perforant path is more complex than has been assumed. Finally, they are at variance with the classical concept which subdivides cortical neurons into projection neurons (pyramidal and spiny stellate) and interneurons (non-pyramidal, local circuit neurons).

Various types of non-pyramidal hippocampal neurons project to the septum and contralateral hippocampus

The morphology was studied of hippocampal neurons which had their somata in the hilus of the area dentata, and in stratum radiatum or stratum oriens of Ammons horn, and which sent projections to the septum and contralateral hippocampus, respectively. The fluorescent marker Fast Blue was injected into the septum or contralateral hippocampus. Somata were then identified by their fluorescent label in slices of perfused brains. After intracellular injection of these somata with Lucifer Yellow, it was found that contralaterally projecting neurons were pyramidal cells, inverted fusiform and multipolar cells in CA3c, and stellate, fusiform and multipolar cells in the hilus. After septal injections, we identified two groups of aspiny stellate cells in the hilus; pyramidal basket cells, polygonal basket cells, horizontal basket cells in stratum oriens; and stellate cells in stratum radiatum of CA1 and CA3, as well as pyramid-like aspiny cells in stratum radiatum of CA1. These cells also had short locally arborizing axons, thus probably contributing to local circuits. Such cells may constitute a third class of hippocampal neurons combining the properties of principal cells and interneurons. These results support the opinion that the simple concept of separating hippocampal cells into projection neurons and local-circuit neurons needs reconsideration.

Quantitative neuroanatomy of the brain of the La Plata dolphin, Pontoporia blainvillei

SummaryThe brain of the La Plata dolphin, Pontoporia blainvillei, was studied with methods of quantitative morphology. The volumes and the progression indices of the main brain structures were determined and compared with corresponding data of other Cetacea, Insectivora and Primates.In Pontoporia, encephalization and neocorticalization are clearly greater than in primitive (“basal”) Insectivora. The indices are in the lower part of the range for simian monkeys. The paleocortex is regressive in accordance with the total reduction of the olfactory bulb and olfactory tract. In contrast to the situation in primates, the septum, schizocortex and archicortex are not progressive in Pontoporia. The striatum and cerebellum are strongly progressive, corresponding to the efficiency and importance of the motor system in the three-dimensional habitat. The diencephalon, mesencephalon and medulla oblongata show considerable progression. Obviously, this is correlated with the extensive development of structures of the acoustic system.The superficial correspondence of the brains of dolphins and primates in relative size and in the degree of gyrencephaly is rather a rough morphological convergence than a sign of functional equivalence. It is coupled to a strongly divergent development of the various functional systems in the two mammalian orders according to their specific evolution.

GABAergic neurons in the cerebral cortex of the brain of a lizard (Podarcis hispanica).

To check the occurrence of GABAergic neurons in a reptilian species, sections from brains of the lizard Podarcis hispanica were immunoreacted with an antibody to GABA. GABA-immunoreactive somata were found predominantly in the external and internal plexiform layers, some also in the granular layer of the medial, dorsomedial and dorsal cortices. GABAergic boutons were also found in the 3 layers; however, their densest distribution occurred in the granular layer and in the outer zone of the external plexiform layer. The distribution of GABAergic somata and boutons supports the suggestion that some areas of the cerebral cortex of reptiles may be homologous to parts of the hippocampus of the mammalian brain.

Interbulbar axonal collateralization and morphology of anterior olfactory nucleus neurons in the rat.

The organizational patterns of the bilateral projections of the anterior olfactory nucleus (AON) to the main olfactory bulb (MOB) were defined in the rat with Golgi staining, HRP tracing-methods and fluorescent dyes. Three issues were addressed: (1) description of the morphology of the AON-neurons projecting to the MOB, (2) quantitative analysis of the bilateral pathways arising in different AON subdivisions and (3) ultrastructural identification of AON to MOB channels. The cytoarchitectural features of the AON as recognized in Golgi preparations were correlated with its neural architecture as revealed by retrograde HRP-tracing from the MOB. The following cell types were determined: (1) pyramidal like neurons typified by a lack of basal dendrites and a sparse covering with long spines (pars externa), (2) fusiform shaped cells with bipolar dendritic arborisations (pars medialis) and (3) densely spined fusiform, pyramidal, and polygonal neurons (pars ventroposterior, lateralis and dorsalis) with a tendency of radial orientation of their apical dendrites. In addition, in the more caudal parts of the pars ventroposterior there were neurons with tertiary dendritic processes oriented nearly parallel to the molecular layer. Quantitative analysis of AON neurons projecting to the MOB showed that the pars externa neurons project exclusively to the contralateral MOB while pars medialis neurons project almost exclusively to the ipsilateral MOB. All subdivisions of the AON which establish specific termination patterns within the MOB, participated in about equal portion in the ipsilateral projections to the MOB. The highest proportion of the bilaterally projecting neurons were found in the dorsal subdivision, followed by the lateral and ventroposterior subdivisions. The postsynaptic targets of the AON to MOB channel are the spinous processes and varicosities of the proximal and distal-most dendrites of granule cells. The boutons derived from AON projection neurons contained clear spherical vesicles and established exclusively asymmetric synaptic junctions.

Combining retrograde tracing, intracellular injection, anterograde degeneration and electron microscopy to reveal synaptic links

Synaptic circuitry was investigated by combining retrograde tracing, intracellular staining, anterograde degeneration and electron microscopy in the same piece of tissue. This methodological procedure was successfully applied to disentangle a disynaptic neuronal chain, which originated in the olfactory bulb, was synaptically relayed in the entorhinal cortex and terminated in the ipsilateral hippocampus. Presumed entorhinal relay cells were retrogradely labelled from their hippocampal termination site by means of a fluorescent tracer. Subsequently, the marked projection neurones were intracellularly injected with Lucifer yellow in fixed slice preparations. Following a simple photo-conversion procedure, dye filled cells were processed for electron microscopy. The origin of presynaptic afferents to identified relay cells was revealed ultrastructurally after lesion induced anterograde degeneration of olfactory mitral cell axons. Due to its reliability, technical simplicity and a high degree of selectivity the new approach is considered an appropriate tool for unravelling neuronal networks.

Transient occurrence of chloride cells in the abdominal epidermis of the guppy, Poecilia reticulata Peters, adapted to sea water

SummaryChloride cells are characterized by a great number of mitochondria and a uniquely proliferated agranular endoplasmic reticulum through which ions are transported to an apical pit for secretion to the environment. Chloride cells are extremely rare in the epidermis of the guppy, Poecilia reticulata Peters, adapted to fresh water. They are frequent in the epidermis of guppies adapted to sea water three weeks after adaptation, but have disappeared in fish that were kept in sea water for approximately one year. The fine structure of epidermal chloride cells and the influence of prolactin treatment on their number are described. The significance of these cells with respect to the possible roles of the epidermis and the pituitary hormone prolactin in teleost osmoregulation is discussed.

Interhemispheric connections through the pallial commissures in the brain of Podarcis hispanica and Gallotia stehlinii (Reptilia, Lacertidae)

The cells‐of‐origin and the mode and site of termination of the interhemispheric connections passing through the anterior and posterior pallial commissures in the telencephalon of two lizards (Podarcis hispanica and Gallotia stehlinii) were investigated by studying the anterograde and retrograde transport of unilaterally injected horseradish peroxidase. The commissural projections arise mainly from pyramidal cells in the medial, dorsomedial, and dorsal cortices (medial subfield). Additionally some non‐pyramidal neurons in the medial and dorsal cortices contribute to the commissural system. Medial cortex neurons project to the contralateral anterior septum through the anterior pallial commissure. The dorsomedial cortex projects contralaterally via the anterior pallial commissure to the dorsolateral septum and to the medial, dorsomedial, and dorsal cortices. The projection to the medial cortex terminates in two bands at the inner and outer border, respectively, of the cell layer; the projection to the dorsomedial and dorsal cortex ends in a zone in layer 1 which previously has been described to be Timm‐negative, and in a diffuse band in the inner half of layer 3. The medial subfield of the dorsal cortex projects through the anterior pallial commissure to the dorsomedial and dorsal cortices with a similar pattern of termination to that found for the dorsomedial cortex. The posterior pallial commissure contains only the projections from the ventral cortex to its contralateral counterpart and to the ventral part of the caudal medial cortex. The similarities found between this commissural system and the mammalian hippocampal interhemispheric connections are discussed.

Entorhinal and prepiriform cortices of the European hedgehog. A hisstochemical and densitometric study based on a comparison between Timm's sulphide silver method and the selenium method

The Timm and selenium staining techniques, based on silver amplification of endogenous zinc, produce an electron-dense precipitate in boutons. The staining characteristics of the two methods were compared by examining two allocortical regions, prepiriform cortex and entorhinal cortex, in the brain of the European hedgehog. In the 3 layers of prepiriform cortex and the 6 layers of entorhinal cortex the methods revealed sublayers, which allows a precise delimitation of areas 28M, 28L, a short transition zone, and the prepiriform cortex. The lamination of the entorhinal cortex of the hedgehog is similar to that found in the rat, but appears less distinct. This may point to a lower degree of afferent organization. For light microscopical investigations, the selenium technique appears superior to Timms method because it produces a more distinct zonation pattern.

GABAergic neurons in the septum of the lizard,Podarcis hispanica

With an antibody to gamma-aminobutyric acid (GABA), GABA-immunoreactive somata and boutons were identified in the septum of a lizard brain. GABA-positive somata occur at all rostrocaudal levels and in all septal nuclei. Parallel acetylcholinesterase (AChE) staining revealed a similar distribution of AChE-positive neurons. GABAergic boutons were seen throughout the septal nuclei, and were more densely concentrated in the outer zone of the dorsolateral septum. Axons of the anterior pallial commissure contained GABA- and AChE-reactivity.