Spatial organization of the retinal projection to the avian lentiform nucleus of the mesencephalon.

Utilizing the horseradish peroxidase retrograde tracing technique and the 2-deoxy-D-glucose metabolic mapping technique, we have demonstrated in chickens the distribution of retinal ganglion cells that project to the lentiform nucleus of the mesencephalon (LM) and the retinotopic organization of the projection in the LM. Retinal ganglion cells labeled after a nearly complete injection into the LM were found in the four quadrants, distributed in a wide horizontal belt lying along both sides of the retinal equator and stretching from the temporal to the nasal retina. The HRP-labeled cells, which appeared round or oval, ranged from 25 to 840 micron 2 in size with most in the smaller size range. Results of partial HRP injections into the LM and metabolic mapping patterns in the LM produced by stimulation of half the retina with horizontal visual motion suggest that there is an orderly mapping of the retina onto the LM. The inferior temporal quadrant projects to the rostrodorsal LM; the inferior nasal quadrant projects to the caudodorsal LM. The superior temporal quadrant projects to the middle and ventral LM, extending from the rostral to the caudal pole, whereas the superior nasal quadrant projects to a small zone in the caudal LM. The mapping of the retinal quadrants in the LM is remarkably similar to that reported in the optic tectum of birds. We suggest that a common embryological anlage with the optic tectum and the arrangement of retinal axons in the optic tract are important factors in establishing the retinotopic organization of the LM.

Light and electron microscopic study of an avian pretectal nucleus, the lentiform nucleus of the mesencephalon, magnocellular division.

Using several light microscopic methods we have identified the lentiform nucleus of the mesencephalon, magnocellular division, by its position in the pretectum, its cellular composition, and its complement of retinal afferents and have distinguished it from neighboring structures. At the light microscopic level large neurons (approximately 30 X 21 microns) and small neurons (approximately 13 X 9 microns), which are more numerous, are seen interspersed among myelinated axons. The large neurons are generally ovoid and contain an eccentrically located nucleus and large clumps of Nissl-stained material. In the electron microscope the most notable feature of these neurons is the presence of ribosome rosettes and many parallel arrays of rough endoplasmic reticulum (RER). On the basis of cytological and ultrastructural features, we conclude that only one class of large neuron is present. Although in the light microscope the small neurons appear to be similar, at the ultrastructural level three neuron types have been distinguished: (1) ovoid shape with cytoplasm densely packed with organelles especially RER, (2) round shape with very little cytoplasm with few organelles, and (3) triangular shape with a pale cytoplasmic matrix with some RER. Subsurface membrane configurations are often seen in the somata of all neuron types. In addition, axon terminals, some containing flat vesicles, and other less frequent ones containing round vesicles are seen terminating on the somata of all neuronal cell types. In the neuropil, three types of presynaptic profiles can be identified. Two of these profiles are axodendritic and the third is dendrodendritic. The type R profile, which is often as large as 4 micron 2, is the most numerous, contains large round synaptic vesicles, and is often seen synapsing on several dendritic profiles. The type F profile contains flat vesicles and a relatively dense cytoplasm, and is smaller in area than type R. The third profile, which contains small clusters of pleiomorphic vesicles and apparently is dendritic, forms synapses with dendritic profiles. We propose that the type R axon terminals originate mostly from the retina and the type F mostly from the visual telencephalon.

Functional postnatal changes in avian brain regions responsive to retinal slip: a 2-deoxy-D-glucose study.

The postnatal development of two avian brain areas, the nucleus of the basal optic root (nBOR) and the lentiform nucleus of the mesencephalon (LM), was studied using the 2-deoxy-D-glucose (2-DG) method. Previous studies have shown that these nuclei respond to whole field visual motion, which is used for optokinetic eye movements. In the present study vertical whole field visual motion presented to one eye resulted in the accumulation of 2-DG label in the contralateral nBOR in both hatchlings and 3-week or older chicks. In hatchlings the nBOR was diffusely labeled whether upward or downward motion was used as the test stimulus, whereas in older birds the label was localized within different subdivisions of the nBOR depending on the direction of vertical motion: upward motion resulted in concentration of 2-DG label in the dorsal nBOR, whereas downward motion resulted in label in the ventral nBOR. Horizontal whole field visual motion presented to one eye resulted in 2-DG label in the contralateral LM and in the lateral portion of the contralateral nBOR in animals of both ages. In hatchlings, visual motion in both temporal-to-nasal and nasal-to-temporal directions resulted in labeling of both subdivisions of the LM, LM magnocellularis (LMmc) and LM parvocellularis. In older birds, temporal-to-nasal motion resulted in labeling of only the LMmc, whereas nasal-to-temporal motion produced labeling in both subdivisions. These results strongly suggest that the nBOR and LM continue to develop their response properties postnatally and that different functional units become separated within particular subdivisions of the nuclei. Conceivably, the functional segregation within the nBOR is due to an intrinsic reorganization, whereas functional changes in the LM may be due to the postnatal development of a projection from the telencephalon.

Accessory optic system and pretectum of birds: comparisons with those of other vertebrates.

We compare the functional and anatomical organization in birds and other vertebrates of the accessory optic nuclei and of those pretectal nuclei implicated in optokinetic responses. In all vertebrate groups, the neurons in these nuclei respond most strongly to slow large-field visual motion in particular directions; the several nuclei differ in the direction of stimulus motion that evokes the best response. These nuclei are essential for optokinetic nystagmus (OKN) in all species examined; the pretectum is necessary for horizontal OKN and the accessory optic nuclei for OKN in other directions. At least in the accessory optic system of birds, the directional parcellation is not well-developed at hatching and requires visual experience to develop normally. There is evidence that the accessory optic system may play a role in transforming the visual motion signal from retinal coordinates into vestibular or oculomotor coordinates. In regard to anatomical connections, in all vertebrate groups studied, the accessory optic and pretectal nuclei project either directly or indirectly to the cerebellum; in addition, the accessory optic system and pretectum are extensively reciprocally connected. In some groups, but not in others, projections have been discovered from the accessory optic system and pretectum to the extraocular motor nuclei and from the accessory optic system to both the vestibular complex and the interstitial nucleus of Cajal.

Scanning and transmission electron microscopic study of the supraependymal macrophages in the lateral ventricles of the toad brain.

Transmission and scanning electron microscopy of the lateral ventricles of the toad brain revealed the presence of supraependymal cells that have the features of macrophages. Based solely on their surface morphology three different cell forms could be identified. The most frequently observed cells are flat and multipolar, and have a smooth or ruffled surface. The second type is spherical with a ruffled surface and occurs either singly, in which case it lacks processes, or in clusters from which processes radiate. The third type has surface blebs and numerous thin, smooth processes. However, when specimens that had been examined in the scanning electron microscope are viewed in the transmission electron microscope, all cells appear to belong to a single cell type. All cells viewed closely resemble macrophages in that they contain nuclei with clumped chromaffin, single cisternae of rough endoplasmic reticulum, numerous dense bodies, and many Golgi complexes. In addition, when horseradish peroxidase (HRP) was perfused into the ventricles, reaction product was found a short time thereafter within cytoplasmic vacuoles, and after a longer period within dense bodies. Because of their ultrastructural resemblance to macrophages and their capacity to ingest HRP, we suggest that these cells function as phagocytes and, as such, act to remove foreign materials from the cerbrospinal fluid.

Endocytic activity of subependymal microglial cells in the toad brain: a cytochemical study of peroxidase uptake.

A population of microglial cells that rapidly incorporate extracellular material introduced into the ventricular system has been identified just beneath the ependyma of all four cerebral ventricles in the toad (Bufo marinus). In untreated tissue these cells appear to be scattered, possess few processes and have an elongate shape with their long axes lying parallel to the ventricular surface. Their most distinctive ultrastructural features are nuclei containing clumps of chromatin, cytoplasmic dense bodies and single strands of granular endoplasmic reticulum. When horseradish peroxidase (HRP) is perfused through the ventricular system and the tissue processed using the DAB cytochemical method, the cells change shape and incorporate HRP into cytoplasmic structures. Even after very short perfusion periods (2-5 minutes) cells become rounded, the surface is ruffled and pseudopodia develop that contain characteristic flocculent material. Reaction product for HRP is contained in plain and coated vesicles, tubules, vacuoles and long structures composed of two closely apposed membranes. At these early times, relatively few multivesicular bodies and dense bodies contain reaction product, but when the cells are viewed at longer time periods after the ventricular perfusion of HRP an increasing proportion of the multivesicular bodies and dense bodies contain reaction product. By 320 minutes reaction product is found almost exclusively in these two organelles. In addition, many pseudopodia containing dense bodies with peroxidase activity are found in the neurophile; some, but not all, can be traced from the subependymal microglial cells. The cell bodies have resumed their flattened shape. When compared to the subependymal microglial cells, other brain cells--oligodendrocytes, astrocytes, ependymal cells and neurons--contain relatively little reaction product at short time intervals; only by 320 minutes are moderate amounts of HRP present. Because of the position of the microglial cells and their ingestive capacity, it is suggested that they function to protect the brain from foreign substances entering from the CSF.

Characterization of an unusual catecholamine-containing cell type in the toad hypothalamus. A correlated ultrastructural and fluorescence histochemical study.

A nucleus of catecholamine-containing cells bordering the preoptic recess of the toad hypothalamus has been studied by both fluorescence histochemical and electron microscopic methods. The perikarya of these cells form one to three rows immediately subjacent to the ependyma. They send brightly fluorescent apical processes between the ependymal cells to the ventricular surface, and also give rise to long basal processes, the proximal portions of which are also fluorescent. These cells contain two distinctive constitutents: juxtanuclear bundles of tightly packed filaments, the members of which are separated from one another by only approximately 100 A, and large numbers of dense-cored vesicles (400-2200 A in diameter), which appear to arise from an agranular tubular reticulum distinct from the Golgi apparatus. Axons containing either clear vesicles alone or clear and dense-cored vesicles form synapses on the subependymal cells, but no evidence has been found that the subependymal cells themselves form presynaptic contacts, or that axons originate from them. The cytological characteristics of these catecholamine-containing cells, plus the fact that they border directly on the cerebrospinal fluid, suggest that they may be more closely related to peripheral chromaffin cells than to the other cell types intrinsic to the central nervous system, and the name "encephalo-chromaffin cells" is therefore proposed for them. The possible functions of such cells in the central nervous system are discussed.