Distribution of the tecto-thalamic projection neurons in the hereditary microphthalmic rat

Projection neurons in the superficial layers of the superior colliculus in the rat: a topographic and quantitative morphometric analysis

The present study deals with qualitative and quantitative investigations in the superior colliculus of the rat. Tracer studies were correlated with Nissl staining to calculate the quantitative ratio between projection neurons and interneurons in the upper three layers of the superior colliculus. In order to reveal the projections from the superior colliculus, the first group of rats received injections of the tracer FluoroGold into the nucleus lateralis posterior thalami, the lateral geniculate body, the nucleus parabigeminalis, and the predorsal bundle. Commissural connections between the superior colliculus were traced in a second group of animals, which received Biocytin and FluoroGold injections in the upper layers of the right superior colliculus and small deposits of the carbocyanine tracer DiI in the deeper layers of the left superior colliculus. Additionally, double-labelling with FluoroGold tracing and the histochemical detection of NADPH-diaphorase activity was carried out to distinguish between projection neurons and interneurons. These experiments showed that 66% of the neurons within the superficial layers of the superior colliculus were represented by ascending projection neurons, whereas only 2-3% could be identified as descending neurons. Ascending neurons were scattered throughout the three laminae and descending neurons were localized in a cluster-like pattern. Approximately 2-3% of the neurons in the superficial layers were found to be commissural and interlayer neurons which were represented by an identical cell type, since both transcommissural and interlayer processes were originated from their somata. The somata of these commissural-interlayer neurons were all located in the mediorostral part of the superior colliculus and contained NADPH-diaphorase activity. The axon terminals of the interlayer-commissural neurons formed net-like structures which surrounded neuronal somata within the ipsilateral deep layers and within the contralateral upper layers of the superior colliculus, respectively.

Projections from the superior colliculus to the ventral lateral geniculate nucleus in the hereditarily microphthalmic rat

Projections from the superior colliculus (SC) to the ventral lateral geniculate nucleus (LGNv) were studied in hereditarily microphthalmic and normal rats by means of wheatgerm agglutinin conjugated with horseradish peroxidase (WGA-HRP). Unilateral injection of a tracer into the LGNv in normal rats revealed WGA-HRP positive neurons on both sides of the SC. In the ipsilateral SC, most of the labeled neurons were distributed in the upper part of the stratum opticum (SO) and the lower part of the stratum griseum superficiale (SGS). A few labeled neurons were also found in the same layers of the contralateral SC. After unilateral injections of the tracer into the LGNv of microphthalmic rats, labeled neurons appeared in similar layers of the SC on both sides. However, the number of labeled neurons in the ipsilateral SC decreased to 30% of normal, whereas on the contralateral side these neurons were apparently more numerous than those in normal rats. The soma size of the labeled SC neurons in microphthalmia was not significantly different from normal. These results indicate fundamentally that tecto-LGNv projecting neurons exist in microphthalmic rats despite the fact that they lack optic nerve afferents. Furthermore, the present results, taken together with our previous results, indicate that the diminution in the number of tecto-LGNd neurons was severest (3%), the tecto-LGNv neurons less severe (30%) and the tecto-LP neurons least severe (50% of that of normal).

Morphometric parameters of the superior colliculus of albino and pigmented rats

The superior colliculus (SC) or optic tectum of mammals consists of seven layers, numbered I-VII from superficial to deep, each of which has distinct connectivity patterns and electrophysiological response properties. The present study is devoted to a morphometrical analysis of neuronal diameters, densities, and numbers in different layers and regions of the SC of albino as well as pigmented rats in order to present a quantitative characterization of the collicular neuronal population involved in the different connectivities and functions of these compartments. The morphometric parameters were calculated from tracings of nuclei and cell bodies by means of Kontron-Videoplan equipment and a Micro PDP 11/23 computer. The mean soma diameter per superior colliculus appears to be 12.0 microns, the average neuronal density 70 cells per 0.001 mm3, and the total number of neurons about 600,000. The mean soma diameter gradually increases from superficial to deep layers (i.e., from 10.0 to 14.0 microns). Cellular density is highest in layer III, the retinal afferent layer (90 cells per 0.001 mm3), and decreases both in more superficial layers (to about 80 in layer I) and deeper layers (to about 44 in layer VII). About 25% of all collicular neurons are situated in layer II whereas layer I contains the lowest percentage of cells (4%). Rostrally within each collicular layer, cellular volumes are about 25% larger than caudally. On the other hand, neuronal densities are rostrally about 38% lower than caudally in all layers except for layers VI and VII. We conclude that collicular neurons, in contrast to collicular axons, are not arranged in distinct layers or clusters but basically establish a random network with only gradual transitions. In this respect, no statistically significant differences were observed between albino and pigmented rats.

Geniculotectal neurons of the lateral part of the ventral lateral geniculate nucleus in the hereditary microphthalmic rat: a retrograde WGA-HRP study

The distribution and morphological characteristics of the geniculotectal neurons from the lateral part of the ventral lateral geniculate nucleus in the hereditary microphthalmic rat were examined by the WGA-HRP method. Unilateral injection of the tracer into the microphthalmic superior colliculus showed that WGA-HRP-positive neurons were present in the ipsilateral lateral part of the ventral lateral geniculate nucleus. However, the number of labeled geniculotectal neurons of the microphthalmia was about 50% that of the normal rats. Furthermore, the size of labeled geniculotectal neurons of the microphthalmia was smaller and their dendrites were shorter than those of normal rats. These results demonstrated that, although geniculotectal neurons of the microphthalmic lateral part of the ventral lateral geniculate nucleus could differentiate just as in the normal rat, the size and number of microphthalmic geniculotectal neurons were remarkably smaller and fewer than those of normal rats. Moreover, the dendrites of microphthalmic geniculotectal neurons were less branched than usual.

Neurons of the dorsal lateral geniculate nucleus of the hereditary microphthalmic rat: a Golgi study

Neurons of the dorsal lateral geniculate nucleus (LGNd) in the microphthalmic rat were examined by the Golgi-Cox method. LGNd neurons in the microphthalmic rat were classified into the multipolar (I) and bipolar (II) types as in the normal rat. The multipolar type was further divided into two subclasses (Ia and Ib) on the basis of their dendritic patterns. The proximal portion of their primary dendrites was thinner than in normal LGNd neurons. The Ia cells had 6-7 primary dendrites extending radially, while the Ib cells had 3-4 primary dendrites spreading primarily parallel to the optic tract. Type II cells had two or three primary dendrites emerging from the cell bodies. In both types, primary dendrites were shorter in length or less branched than usual. These results suggested that LGNd neurons in the microphthalmic rat had smaller dendritic fields than those in the normal rat.