Morphology of neurons in the dorsal lateral geniculate complex in turtles of the genera Pseudemys and Chrysemys.

The morphology of neurons in the dorsal lateral geniculate complex of pond turtles has been studied by extracellular filling with horseradish peroxidase. The dorsal lateral geniculate complex is a rostrocaudally elongate structure that includes the nucleus ovalis and dorsal lateral geniculate nucleus of Papez. It is divided into three cytoarchitecturally distinct subnuclei: the subnucleus ovalis, the dorsal subnucleus, and the ventral subnucleus. Each subnucleus consists of a neuropile immediately internal to the optic tract and a cell plate of densely packed somata forming the medial face of the complex. The cell plate contains medial (or parvicellular) and lateral (or magnocellular) sublaminae that are separated by a cell-poor zone in subnucleus ovalis and the ventral subnucleus. The density and size of somata vary between subnuclei. Geniculate neurons fall into two distinct groups. Cell plate neurons have somata in the cell plate and vary substantially in morphology. Those in the medial sublamina have fusiform somata and dendrites that extend either mediolaterally (subnucleus ovalis) or run rostrocaudally in the cell plate (dorsal and ventral subnuclei). Those in the lateral sublamina have some dendrites that extend medially within the cell plate and others that extend into the neuropile. The latter dendrites branch and bear arbors of fingerlike, varicose branchlets in the outer half of the neuropile. By contrast, neuropile cells have fusiform somata and dendrites extending concentrically with the optic tract in the neuropile. Both groups of geniculate neurons can be retrogradely labeled by horseradish peroxidase injections in the lateral forebrain bundle. These results lead to the recognition of two principles of geniculate organization in turtles. The first is that the geniculate complex is divided into three subnuclei that vary in size and density of their neurons. The second is that the complex has a form of laminar organization different than that seen in the geniculate complex of mammals.

Spatial distribution of individual medial lemniscal axons in the thalamic ventrobasal complex of the cat.

Neurons responding to deep or cutaneous stimuli are situated in different parts of the ventrobasal complex. Within the cutaneous region elongated clusters of cells with common place and modality properties project to single columns in the somatic sensory cortex. The present study sought to determine to what extent single lemniscal axons contribute terminals to different regions and to different cell clusters. Lemniscal axons, anterogradely labelled by horseradish peroxidase injected into the medial lemniscus of cats were examined light and electron microscopically. Labelled axons bore one or two, mainly anteroposteriorly oriented, terminal ramifications. These ramifications were relatively small when compared to the length of the complex. Some of the axons bore one or two collaterals that ascended towards the dorsal edge of the complex and formed an additional small ramification there. Electron microscopic analyses of labelled lemniscal axons provided further evidence to that already available that most of their boutons synapse on proximal dendrites of relay neurons and on presynaptic dendrites, presumably belonging to interneurons. A concurrent study of Golgi-impregnated ventrobasal neurons showed three morphological types all with dendritic fields of similar extent. From measurements of the lemniscal terminal ramifications and the counting of counterstained cells it was calculated that 50-120 neurons may receive input from a single terminal ramification. However, because of the restricted extent of the ramifications, the elongated clusters of cells projecting to a single cortical column probably receive input from multiple lemniscal axons and not all members of the cluster receive inputs from the same axons.

Organization of nucleus rotundus, a tectofugal thalamic nucleus in turtles. III. The tectorotundal projection.

Nucleus rotundus is the primary thalamic recipient of projections from the optic tectum in pond turtles. Although the projection of the retina to the optic tectum is known to be topographically organized, earlier studies suggest that the tectorotundal projection is not topographically organized. Three types of analyses are used in this paper to characterize the organization of the projection of the optic tectum to nucleus rotundus. First, large iontophoretic injections of horseradish peroxidase into the optic tectum anterogradely fill axons with reaction product after the use of a cobalt-enhanced diaminobenzidine procedure. These preparations show that shafts of axons in the tectothalamic tract give rise to thinner, primary collaterals that enter nucleus rotundus from its caudolateral aspect and form sparsely branching arbors within the nucleus. Very thin secondary collaterals branch from these collateral bear terminal collaterals with frequent varicosities. Although the total size of such arbors is unknown, the evidence suggests that each arbor is large in relation to the size of nucleus rotundus. Thus, injection sites restricted to central tectum label axons throughout nucleus rotundus. Second, subtotal lesions of the tectum produce degeneration throughout nucleus rotundus in silver degeneration preparations. Finally, analysis of electron microscopic degeneration material indicates that tectal boutons are distributed along the full lengths of the dendrites of rotundal neurons, but not on their somata. These boutons form asymmetric synaptic junctions and contain round synaptic vesicles. In view of the relatively large size of the dendritic fields of rotundal neurons, these data suggest that the tectorotundal projection is both strongly convergent on individual neurons and strongly divergent from single tectorotundal axons. This type of organization is consistent with physiological evidence that rotundal neurons have receptive fields that cover at least one-half of the contralateral visual field and often include the entire hemifield. It seems unlikely that nucleus rotundus can be involved in neuronal transactions that preserve detailed spatial information, but it may be involved in processing information on other visual parameters such as stimulus velocity or color.

Organization of nucleus rotundus, a tectofugal thalamic nucleus in turtles. II. Ultrastructural analyses.

Nucleus rotundus in a large, tectorecipient nucleus in the dorsal thalamus of the pond turtles Pseudemys scripta and Chrysemys picta. Rotundal neurons form a single, morphologically homogeneous population (Rainey, '79) that projects to the dorsal ventricular ridge in the telencephalon. The present paper examines the morphology of and the distribution of synapses upon rotundal neurons. Astrocytes, oligodendrocytes, and neurons can be identified in both 1-micrometer sections stained with toluidine blue and electron micrographs of nucleus rotundus. Rotundal neurons contain euchromatic nuclei and the usual complement of mitochondria, rough endoplasmic reticulum, and free ribosomes in their cytoplasm. They are morphologically homogeneous. Two types of terminal boutons can be defined in rotundus. RA boutons contain round synaptic vesicles and form asymmetric synaptic junctions with rotundal dendrites. FS boutons contain small, flattened or pleomorphic vesicles and form nearly symmetric synaptic junctions with rotundal dendrites and somata. RA boutons occasionally form clusters of contiguous boutons that are presynaptic to one or more thin, central profiles. These profiles are probably the dendritic appendages observed on peripheral dendrites in Golgi material. The distribution of RA and FS boutons along dendrites was investigated by a two-step procedure. First, rotundal neurons were retrogradely solid-filled with horseradish peroxidase reaction product. Dendritic diameters were measured at 20 micrometer intervals along dendritic shafts to produce a plot of dendritic diameter as a function of distance from the soma. Second, the percentage of membrane on dendritic profiles of different diameters that was contacted by RA and FS terminals was determined from electron micrographs. Comparison of the two plots indicates that both bouton types are distributed along the full extent of the dendritic tree, but RA boutons are much more common on the distal two-thirds of rotundal dendrites. This analysis suggests that rotundal neurons form a single population of cells that are morphologically homogeneous and project to the forebrain. There is no indication of interaction between neurons in nucleus rotundus, either via axonal collaterals or presynaptic dendrites. Boutons are distributed on rotundal neurons such that FS boutons are prevalent on the somata and most proximal segments of the dendritic shafts, while RA boutons are most common on the more distal dendritic shafts. RA boutons also contribute to synaptic clusters that may center around complex dendritic appendages.

Organization of nucleus rotundus, a tectofugal thalamic nucleus in turtles. I. Nissl and Golgi analyses.

This study consists of a detailed cytoarchitectonic and Golgi analysis of a major tectofugal thalamic nucleus in the red-eared turtle, Pseudemys scripta elegans. Neurons in nucleus rotundus have a unimodal soma size distribution and a common dendritic branching pattern. They have long dendrites which undergo sparse, dichotomous branchings and contribute to dendritic fields that cover a third to half the dimensions of the nucleus. Spicules, 1-2 mu long, and complex appendages, 5-20 mu long, are found with low density on many dendrites in Golgi-Kopsch material. A few cells have beaded dendritic processes. Three cytoarchitectural regions can be differentiated in nucleus rotundus: a shell, a cell-poor region and a core. The shell is a monolayer of somata forming the peripheral boundary of most of the nucleus. The cell-poor region forms a thin zone concentric with and internal to the shell. Shell cells send some of their dendrites concentrically within this zone and others radially into the core region. Core neurons are dispersed within the neuropil of the nucleus and usually have spherical dendritic fields. However, peripheral core neurons have asymmetrical fields, so their dendrites do not extend beyond the shell. Caudomedial and central subregions of the core can be defined on the basis of neuronal density and cytology. Somata in the caudomedial area of the core are densely packed and have slightly darker staining cytoplasm than those in the central subregion. However, their dendrites are similar to those of the central core neurons. There is extensive dendritic overlap between the two subregions.

Mass spectrometry of N-nitrosamines.

The preparation of a series of N-nitrosamines for carcinogenicity studies presented an opportunity to study mass spectral fragmentation schemes in detail. Condensed spectra are listed for 146 N-nitrosamines of widely differing structures, including nitroso derivatives of commercial drugs and insecticides. Aliphatic nitrosamines were generally characterized by molecular ions and loss of OH. Subsequent fragmentation via alpha-cleavage is similar to that of aliphatic amines. The loss of OH is believed to result in a cyclic ion. Subsituted aliphatic nitrosamines varied in fragmentation schemes with structure and position of the substitutent groups. However, most showed alpha-cleavage at some point in fragmentation. When substituted with aromatic groups prominent peaks due to the aromatic moiety were observed. The alicyclic nitrosamines showed losses of NO, NOH and OH and subsequent alpha-cleavages. Nitrosamides were characterized by rupture of the carbonyl to nitrogen bond. Spectra of substituted ureas usually showed charge retention by the carbonyl fragment, while carbamate esters showed ions from both fragments.

Mass spectral identification of 2-(O-acyl)hydroxy fatty acid esters in the white portion of the rabbit Harderian gland.

A major lipid component of white portion of the rabbit harderian gland has been shown to be a mixture of 2-(O-acyl)hydroxy fatty acid esters. The fatty acid moieties in this lipid class are exclusively saturated and range in chain length from C14:0 to C22:0, with C16:0 being the major component (65%). The fatty alcohols are also saturated and composed primarily of C20:0, C21:0, and C22:0 chains. The hydroxy fatty acids are composed of C14:0, C15:0, and C16:0 and mass spectroscopy combined with chemical techniques placed the hydroxyl group at the 2-carbon. 2-(O-acyl)Hydroxy fatty acid esters are not found in the pink portion of the rabbit harderian gland nor have they been reported to occur in harderian glands or other species.