Quantitative analysis of the striate cortex in the mutant microphthalmic rat

A quantitative analysis of the striate cortex of the mutant microphthalmic rat was conducted to determine whether or not transneuronal changes of the visual cortex were induced following the loss of eyes. The area of the striate cortex in the microphthalmic rat was approximately 60% of that in the normal rat. As for the thickness of each layer of the striate cortex, many layers of microphthalmia tended to be thin in comparison with the normal animal, except for layers I and III: the thickness of layers II, IV, V, and VI was about 74, 62, 82, and 82% of normal values, respectively. There was practically no difference between the number of neurons of each layer of the microphthalmic and the normal striate cortex per unit (10⁴ μm²), except for layer IV, in which the density had increased to 117% of the normal value. In many layers, the neurons of the microphthalmic striate cortex were smaller than normal and they had narrow neuroplasmic space. Our study demonstrated that the striate cortex of the microphthalmic rat underwent quantitative and morphometric transneuronal changes. Especially striking changes of the striate cortex were found in the inner granular layer with a reduction in thickness and a diminution of cell size.     

Intrinsic connections of layer III of striate cortex in squirrel monkey and bush baby: Correlations with patterns of cytochrome oxidase

This study used biocytin and horseradish peroxidase (HRP) to examine the intrinsic connections of the cytochrome oxidase (CO) rich blob and CO poor nonblob zones within layer III of striate cortex in two primate species, nocturnal prosimian bush babies (Galago crassicaudatus) and diurnal simian squirrel monkeys (Saimiri sciureus). Our main objective was to determine whether separate classes of lateral geniculate nucleus (LGN) cells projected to separate superficial layer zones or layers in either species. There were three significant findings. First, we confirm that layer III consists of three sublayers, IIIA, IIIB, and IIIC in both species. Layer IIIA receives input from layers IIIB, IIIC, and V, with little or no input from LGN recipient layers IV and VI. Layer IIIB receives its input from nearly every cortical layer. Layer IIIC, receives input principally from layers IVα [which receives its input from magnocellular (M) LGN cells] and from layers V and VI. Taken together with other findings on the extrinsic connections of these layers, our data suggest that IIIA and IIIC provide output to separate hierarchies of visual areas and IIIB acts as a set of interneurons. Second, we find that, as in macaque monkeys, cells in both IVβ and IVα of bush babies and squirrel monkeys projct to layer IIIB, converging within the blobs. These results suggest that information from all LGN cell classes [parvocellular (P), M, and the Koniocellular (K) or their equivalents] may be integrated within the blobs. Thus, blobs in all of these primates may perform a function that transcends visual niche differences. Third, our data show a species specific difference in the connections of the IIIB nonblobs; nonblobs receive indirect input via IVα from the LGN M pathway in bush babies but receive indirect input via IVβ from the LGN parvocellular (P) pathway in squirrel monkeys. These findings indicate that the role of nonblob zones within striate cortex differs from that of blob zones and takes into account visual niche differences. © 1993 Wiley-Liss, Inc.     

Identification of the trochlear motoneurons by retrograde transport of horseradish peroxidase

A common technique for demonstrating projections in the brain is to electrically stimulate one part of the brain and record mass or field potentials from another part. We showed in the visual system of the cat, where connections between retina, lateral geniculate nucleus, and superior colliculus are very well known, that the recording of field potentials is not at all sufficient to demonstrate connections. The most prominent potential after electrical stimulation of the optic tract is the field potential created by the Y-ganglion cell fibers of the optic nerve. We recorded this potential in the optic nerve head of the eye, in the lateral geniculate nucleus, and in the superior colliculus. To our surprise, we also could record this potential 7 mm in front of the retina, with the electrode in the vitreous, and 5 mm above the lateral geniculate nucleus and the superior colliculus, where there are no direct inputs from the optic tract. These results show quite clearly that field potentials can “stray” much farther than the underlying anatomical structure projects.     

Postnatal maturation of neurons in the cat's lateral geniculate nucleus

The maturation of dendrites in the cat's dorsal lateral geniculate nucleus was studied in Golgi Kopsch preparations of kittens from 3 days to 8 weeks postnatal. During the first postnatal week, more than a month after their birthdate, cells are immature and lack dendrites, bearing only multiple somatic processes or a few short thick extensions. Cells enter an active phase of dendritic extension during the second postnatal week. Growth cone-like structures and filopodia occur at the ends of dendrites and also at dendritic branch points. Assignment to general cell classes based on dendritic disposition is possible only after this period, and characteristic grapelike appendages are obvious after the third week. Mature cells in the lateral geniculate nucleus are not considered spiny, yet spines and hairs are ubiquitous on most cells once dendrites elongate and remain numerous on peripheral dendrites even after the soma and proximal dendrites become smooth, by 4–6 weeks. The decline of spine levels continues after this period. All cells go through a similar but nonsynchronous sequence of maturation. Large cells may mature first, but no correlation was noted between rate of maturation and laminar location or retinal representation. In the second and thirdpostnatal weeks, although the terminal arbors of retinal axonspre-synaptic to geniculate cells have already attained their final topography and laminar placement, the shape and synaptic relations of axon terminal swellings remain immature (Mason, 1982a,b) through the most active phase of dendritic outgrowth. After 3 weeks, both retinal axons and target geniculate cell dendrites finalize the shapes of characteristic appendages and synaptic relations in tandem. Potential interactions between immature axon terminal arbors and dendrite-bare geniculate cells during dendrite outgrowth and subsequent remodeling of structural details are discussed.     

X- and Y-cells identified in the cat lateral geniculate nucleus in vitro

Using an in vitro preparation of the cat dorsal lateral geniculate nucleus, we have studied the passive membrane properties and the electrotonic structure of single cells each identified as X or Y on the basis of their morphological features following intrasomatic injection of horseradish peroxidase. The input resistance of X-cells is higher and the membrane time constant longer than of Y-cells. The electronic length and the dendritic to somatic conductance ratio are similar for both classes of neurones.     

Reactions of lateral geniculate cells to chemical and electrical excitation of the visual cortex in rabbits

In anesthetized and paralyzed rabbits unitary discharges of lateral geniculate nucleus (LGN) were studied after cortical excitation by strychnine and following electrical stimulation of the visual cortex (VC). Results showed that local application of strychnine produced a general increase of the spontaneous and evoked activity of geniculate cells. By contrast, cortical depression with KCl led to a differential decrement of one of the evoked responses (on or off). Electrical cortical stimulation paired with on or off stimuli led to a differential increment of on or off responses. The results support the notion that, in rabbits, the corticogeniculate system is center-surround organized. A diagrammatic model is proposed to account for the relationship between the VC and the LGN in rabbits.     

Sublaminar organization within layer VI of the striate cortex in Galago.

In this study we examined the organization of projections from the striate cortex to the dorsal lateral geniculate (GL) and pulvinar (PUL) nuclei in the prosimian Galago by using retrograde transport methods. Injections of wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) into the PUL labeled two bands of cells in the striate cortex: the first consisted of large pyramidal cells in the upper half of layer V; the second consisted of small and medium-size pyramidal cells located in the deepest part of layer VI. The location of cells within layer VI coincided with a clear cytoarchitectonic sublayer, VIb, which contains fewer and paler staining cells than VIa. Injections of WGA-HRP involving all layers of the GL produced an uninterrupted band of pyramidal cells distributed throughout layer VI (a and b), including the region labeled after injections into the PUL. Thus as a first approximation, layer VI can be divided into an upper tier (VIa) that projects only to the GL and a lower tier (VIb) that projects to both the GL and PUL. Injections of WGA-HRP that were restricted to one or a few GL layers revealed a further refinement of the subdivisions within layer VI. Injections into the parvicellular and intercalated (or koniocellular) layers of the GL labeled neurons predominantly in the upper half of layer VIa, whereas injections restricted to the magnocellular layers labeled neurons in the lower half of layer VIa and in layer VIb. In order to determine whether individual neurons in layer VIb send axon collaterals to both the GL and PUL, we injected WGA-HRP into one nucleus and fluorescent rhodamine latex beads into the other. In three experiments, we found only one double-labeled cell. In sum, the results provide evidence that layer VI is divided into at least three sublayers: upper VIa, which projects to the intercalated and parvicellular GL layers; lower VIa, which projects to the magnocellular GL layers; and VIb, which sends separate projections to the magnocellular layers of the GL and to the PUL. The segregation observed is sufficiently discrete to propose the existence of multiple, descending pathways from layer VI of the striate cortex that complement those ascending from the GL and PUL.     

Laminar and cellular localization of cytochrome oxidase in the cat striate cortex

Cytochrome oxidase (C.O.) was histochemically localized in the cat striate cortex at the light and electron microscopic levels. The results indicate that the oxidative metabolic activity within the cat striate cortex may vary between (1) different laminae, (2) neurons and glia, (3) different neuron types, (4) dendrite and soma of the same cell, (5) different types of dendrites, (6) different segments of the same dendrite, and (7) different classes of symmetric and asymmetric axon terminals. Maximal laminar C.O. staining was localized within geniculoreceptive layer IV. Darkly reactive neurons include the large (presumed corticotectal) pyramids of layer V, and various classes of large and medium-sized presumed GABAergic nonpyramidal cells sparsely distributed throughout layers II-VI. The small and medium-sized pyramids of layers II, III, V, and VI, as well as many of the smaller presumed GABAergic neurons, were only lightly or moderately reactive. The darkly reactive neurons tended to be those that received convergent or proximally localized asymmetric axosomatic synapses, implying that they are strongly driven by excitatory synaptic input. The darkly reactive nonpyramids resembled those that form GAD+, symmetric axosomatic synapses with pyramidal cells. The dark reactivity of the symmetric synaptic terminals indicates that they mediate strong inhibition of neuronal discharge. The dark reactivity of a class of large asymmetric terminals in layer IV is likely to represent highly active geniculocortical terminals. The predominant distribution of elevated C.O. reactivity in dendrites is correlated with reported sites of (1) convergent excitatory synaptic input, (2) maximal field potentials, (3) highly active ion transport, and (4) Na+, K+-ATPase.     

Quantitative analysis of the lateral geniculate nucleus in the mutant microphthalmic rat

A quantitative analysis of the lateral geniculate nucleus was carried out in the mutant microphthalmic rat. In the dorsal lateral geniculate nucleus (LGNd) of the microphthalmic rat we found the total volume and neuronal population were reduced by 45 and 68% of normal values, respectively. The size of normal LGNd neurons was 8 to 20 μm and that of mutant LGNd cells from 6 to 16 μm. Neurons of the normal LGNd were medium-size and round or oval, and their cell bodies were filled with Nissl substance. Microphthalmic LGNd neurons, on the other hand, had narrow cytoplasmic spaces with few Nissl granules, and pale cell nuclei. In the microphthalmic rat, the lateral part of the ventral lateral geniculate nucleus (LGNvl) also showed a marked reduction in the total volume and neuronal population which were 42 and 76% of normal values, respectively. The size of normal LGNvl neurons was 8 to 20 μm and that of the microphthalmic neurons from 6 to 16 μm. These findings suggested that a marked reduction in the size of the LGNd and LGNvl in the mutant can be attributed to a decrease in neuronal population to a diminution of cell size.     

Distinct X- and Y-streams in the cat visual cortex revealed by bicuculline application

The projections from the central nucleus of the amygdala to the dorsal vagal complex were examined in the rat by means of anterograde and retrograde axonal transport of wheat germ agglutinin-horseradish peroxidase and anterograde degeneration. Light microscopic findings confirmed that the amygdala projects to the dorsal motor nucleus (DMV) and the nucleus of the solitary tract. Electron microscopic experiments demonstrated degenerating axosomatic and axodendritic terminals in the DMV following electrolytic lesions in the central nucleus of the amygdala.     

Differential effect of visual deprivation on cytochrome oxidase levels in major cell classes of the cat LGN

Cytochrome oxidase histochemistry was used to examine the effects of visual deprivation on the development of neurons in the lateral geniculate nucleus of the kitten. Early postnatal monocular suture results in a decrease in reactivity within the neuropil of visually deprived binocular laminae A, A1, magnocellular C, and medial interlaminar nucleus. Within these regions, monocular suture has a greater effect on the relative numbers of, and the growth of darkly reactive (normally large), presumed Y-cells than on other less reactive geniculate neuronal classes. The decreases in the reactivity of the neuropil may be attributed to the decreases in the number of mitochondria, the number of darkly reactive mitochondria, and/or the number of darkly reactive mitochondria localized within dendrites. Although all classes of dendrites appear to be adversely affected, the decrease in C.O. reactivity was most dramatic within the presumed proximal dendrites of class 1 Y-cells. These dendrites were identified by the type of synaptic contacts they formed with retinal terminals (Rapisardi and Miles, '84, J. Comp. Neurol. 223:515-534; Wilson et al., '84, Proc. R. Soc. Lond. [Biol.] 221:411-436). As with Y-cells, the effects of monocular suture on the large darkly reactive cells were not as dramatic at sites where binocular interactions were either absent or where they had been experimentally eliminated. Based on the present and previously reported findings from several laboratories, it is likely that the selective physiological and morphological effects of monocular suture on Y-cells are accompanied by metabolic deficits involving both dendrites and perikarya. These effects appear to be due more to binocular interactions than to visual deprivation per se.     

Laminar and columnar patterns of geniculocortical projections in the cat: Relationship to cytochrome oxidase

We examined the laminar and columnar arrangement of projections from different layers of the lateral geniculate nucleus (LGN) to the visual cortex in the cat. In light of recent reports that cytochrome oxidase blobs (which in primates receive specific geniculate inputs) are also found in the visual cortex of cats, the relationship between cytochrome oxidase staining and geniculate inputs in this species was studied. Injections of wheat germ agglutinin-conjugated horseradish peroxidase were made into the anterior “genu” of the LGN, where isoelevation contours of the geniculate layers are distorted due to the curvature of the nucleus. Consequently, anterograde labeling from the various LGN layers was topographically separated across the surface of the cortex, and labeling in a particular isoelevation representation of the cortex could be associated with a specific layer of the LGN. Labeling from the A layers, which contain X and Y cells, was coextensive with layers 4 and 6 in both area 17 and area 18, as previously reported. Labeling from the C layers, which contain Y and W cells, occupied a zone extending from the 4a/4b border to part way into layer 3 in area 17. The labeling extended throughout layer 4 in area 18. There was also labeling in layer 5a and layer 1 in both area 17 and area 18. Except in layer 1, labeling from the C layers was patchy. In the tangential plane, adjacent sections stained for cytochrome oxidase showed that the patches of labeling from the C laminae aligned with the cytochrome oxidase blobs. The cytochrome blobs were visible in layers 3 and 4a, but not in layer 4b in both areas 17 and 18. These results suggest that W cells project specifically to the layer 3 portion of the blobs, while Y cells, at least those of the C layers, project specifically to the layer 4a portion of the blobs in area 17. The heavy synaptic drive of the Y cells is probably the cause of the elevated metabolism, and thus, higher cytochrome oxidase activity, of the blobs. © 1996 Wiley-Liss, Inc.     

Histochemical localization of cytochrome oxidase activity in the visual system of the tree shrew:normal patterns and the effect of retinal impulse blockage

The tree shrew Tupaia belangeri has three functional pathways (ON-center, OFF-center, and W-like cells) that arise in the retina and proceed through separate LGN laminae to separate cortical targets. To determine whether these pathways have consistent differences in activity, cytochrome oxidase (C.O.) patterns were examined in the retina, LGN, and striate cortex. In six normal tree shrews the outer and inner plexiform layers of the retina were highly reactive for C.O. A pale, vascularized cleft zone separated the a (OFF) and b (ON) inner plexiform sublaminae, which seemed about equally reactive for C.O. In the LGN, laminae 1 and 2 (ON-center cells) and laminae 4 and 5 (mostly OFF-center cells) were highly reactive for C.O. LGN lamina 3 and 6 are part of an W-like afferent pathway. Lamina 3 was distinctly paler than laminae 1, 2, 4, and 5 while lamina 6 was intermediate. In the striate cortex, layer IV was the most reactive layer. Sublayer IVb (predominantly an OFF region) was consistently more reactive than sublayer IVa (predominantly ON). The middle portion, layer IVm, was paler than either IVa or IVb. This paler region includes, but extends above and below, the cell-sparse "cleft" region. Thus, considering all three levels of the retinogeniculostriate pathway, the ON and OFF systems were equally active until they reached the striate cortex, where the OFF system appeared to be more active than the ON. The W-cell laminae in the LGN exhibited the lowest level of activity. The contribution of ganglion cell activity to these patterns was assessed by intravitreal administration of tetrodotoxin (TTX) blockade either monocularly (three animals) or binocularly (two animals). In the TTX-treated retinae, the inner plexiform a and b sublaminae were paler for C.O., although visible, and were still separated by the pale cleft. The ganglion cell layer was very pale in comparison to the normal. In the LGN, monocular TTX blockade reduced the C.O. reactivity in the ON and OFF laminae that received input from the treated eye but had little effect on the W-like cell laminae. The ipsilaterally innervated ON and OFF laminae were more affected than were the contralaterally innervated laminae. Binocular TTX treatment resulted in a decrease of C.O. activity in the binocular segment of the ON and OFF LGN laminae. In the striate cortex, the most marked changes following TTX treatment occurred in layer IV.(ABSTRACT TRUNCATED AT 400 WORDS)     

Morphology and quantitative changes of transient NPY-ir neuronal populations during early postnatal development of the cat visual cortex

The early postnatal development of neuropeptide Y-containing neurons in the visual cortex of the cat was analyzed. Immunohistochemistry reveals several stages of morphological differentiation and degeneration. Completely undifferentiated neurons have very small somata with nuclei surrounded by a thin rim of cytoplasm and processes unclearly differentiated into dendrites and axons. Processes bear growth cones. Differentiating neurons show an increase in soma size and complexity of processes. Axons are recognizable. Fully differentiated neurons have well-defined axonal and dendritic patterns. Degenerating neurons are identified by thick, heavily beaded processes covered by hairy appendages and vacuolar inclusions in the somata. Cell death is expressed by shrunken somata and lysed, fragmented processes. According to their postnatal time course of differentiation and/or degeneration, NPY-immunoreactive neurons, which form several morphologically distinct cell types, are grouped into 3 neuronal populations. (1) Pseudopyramidal cells, bitufted "rectangular" cells with wide dendritic fields, unitufted cells, and small multipolar cells are located in the gray matter and have a rather primitive morphology resembling cell types found in lower vertebrate cortex and tectum. They constitute a first transient neuronal population, because all neurons are fully differentiated at birth and become largely eliminated by postnatal day (P) 12. (2) Axonal loop cells are mainly located in the white matter. Their most prominent feature is an often long hairpin loop formed by either the main axon itself or by a major collateral. The axonal branches pass through the cortex to connect the white matter and layer I. Axons do not form local plexusses and terminal elements in the gray matter. Neurons differentiate perinatally, form a first peak from P6 to P10, followed by a decrease in cell number and innervation density at P12, followed by a second peak from P15 to P20. After P20 the number of axonal loop cells steadily decreases, and they become eliminated by P48. (3) A third population consists of neurons with a higher degree of axonal ramification and a variety of axonal patterns. Early members are located mainly at the layer VI/white matter border, differentiate during the first postnatal week, and give rise to a diffuse innervation of the gray matter without forming specific terminal elements. Some of the early axonal patterns persist into adulthood, whereas others are not found in the adult brain.(ABSTRACT TRUNCATED AT 400 WORDS)     

Morphology of axons in the human lateral geniculate nucleus: A golgi study in prenatal and postnatal material

A study was made of rapid Golgi preparations from the lateral geniculate nucleus in humans aged from 28 weeks gestation to 70 years in order to identify axon terminals of afferent fibre systems. We describe three main axonal types using, as far as possible, nomenclature already adopted for other species. Type I axons were found only rarely. They are relatively straight with short, stalked side-branches and may represent cortico-geniculate fibres. Type II axons have complex, ball-like arborizations with large, irregular varicosities. They are common at all ages from gestation to maturity and are probably retinal in origin. Type IV axons (Type III was not used as no unequivocally intrinsic axons, for which the term has been used in the past, were identified) are branched, meandering and characterized by many, regular varicosities. Their origin is unclear, but may be related to non-specific brainstem sources. The basic morphology of Type II axons varies little between late gestation and adulthood, but Types I and IV seem to evolve during the perinatal period, perhaps from primitive forms that have similar morphological features. We conclude that the morphology of afferent axons to the human lateral geniculate nucleus is basically similar to that of lower mammalian species.     

Ultrastructural identification of synaptic terminals from the axon of type 3 interneurons in the cat lateral geniculate nucleus

Synaptic terminals from the axons of type 3 neurons in the A-laminae of the cat LGN impregnated with the Golgi gold-toning procedure were examined at light and electron microscopic levels. The axons were identified by their somatic origin, thin diameter, and, in one of these cells, by dense undercoating beneath the axolemma, which is a known characteristic of the axon initial segment. The axon of one of the analyzed cells was profusely branched and extended throughout most of lamina A within the dendritic domains of the cell, and both types of processes were oriented along projection lines in LGN. This suggests that the dendrites and axons of type 3 cells receive inputs and exert effects, of probably inhibitory nature, within restricted retinotopic regions of LGN. The vast majority of the axon terminals of these cells were distributed in series along axonal branches. In one of the type 3 cells, however, a dense cluster of terminals arising from a secondary axonal branch was observed. Ultrastructurally, the analyzed synaptic terminals of the type 3 cells contained flattened or pleomorphic synaptic vesicles, dark mitochondria, and established synapses that appeared to be of symmetrical type when the membranes were perpendicularly cut. On the basis of these characteristics these terminals are classified as F boutons, following Guillery's (Z. Zellforsch. 96:1-38, '69), nomenclature. The postsynaptic elements to the axon terminals were dendrites of small to medium size, which received "en passant" synaptic contacts in extraglomerular regions of the geniculate neuropil by the terminals distributed in series. The axon terminals located in clusters, however, made synapses with dendrites in glomerular regions of the neuropil, where they were not seen postsynaptic to retinal or other types of terminals. This is in contrast to the postsynaptic nature of F2 boutons in the same glomeruli, which have been identified as dendritic appendages of the GABA positive type 3 neurons in the cat LGN (Montero: J. Comp. Neurol. 254:228-245, '86). On the other hand, the axonal F terminals differ from F1 boutons in terms of synaptic relations and ultrastructure, since the latter have been shown to be presynaptic to F2s and somata and to contain crowded populations of flat synaptic vesicles which give them a characteristic dark appearance. Terminals equivalent to F1 boutons have been shown to originate from perigeniculate cells in the rat LGN. From these observations it is suggested that the geniculate GABAergic interneurons support two morphologically and functionally different type of inhibitory terminals synapsing the dendrites of relay cells.(ABSTRACT TRUNCATED AT 400 WORDS)     

Transient tectogeniculate projections in neonatal kittens: an autoradiographic study

By using anterograde transport autoradiography, the present experiments demonstrated that the pattern of tectogeniculate projections in young (birth-14 postnatal days) kittens is strikingly different from that present in adult cats. Rather than being confined to the ventral C laminae, the neonatal projection extended across all layers of the lateral geniculate nucleus. This projection, like that in the adult cat, originates from cells in superficial laminae and is visuotopically organized. Thus, labeling only a portion of the superior colliculus with tritiated leucine produced a topographically appropriate strip of labeling in the ipsilateral lateral geniculate nucleus that encompassed all laminae and was especially dense in all interlaminar zones. Transported label also invaded the medial interlaminar nucleus (MIN). The loss of tectogeniculate projections in the neonate from MIN and the dorsal laminae and interlaminar zones of the lateral geniculate nucleus does not appear to begin until 1-2 weeks postnatal. Once initiated, however, the process is nearly completed by 21 days postnatal. It is not yet known whether the loss of these "anomalous" projections is due to the pruning of axonal collaterals, cell death, or a combination of the two processes. However, by comparing these data with those from other laboratories, it does appear that the loss of tectogeniculate projections depends on the presence of the two eyes and may reflect the differential laminar distribution of W-, X-, and Y-cell types. The protracted postnatal anatomical maturation of tectogeniculate projections differs substantially from the earlier maturing patterns apparent in all other tectofugal pathways.     

Retinofugal projections following early lesions of the visual cortex in the ferret

Extensive lesions of the occipital cortex comprising the developing occipital visual areas and beyond in young ferrets (postnatal day 5) are followed by massive, but incomplete, degeneration of the lateral geniculate (LGN) and lateralis posterior (LP) nuclei of the thalamus, and minor volumetric reduction of the superior colliculus. Retinal projections (revealed by intraocular tracer injections), while reduced, remain confined to their territories of normal termination, both in the adult and throughout development. Comparisons with other mammalian species point to several common features in the developmental plasticity of retinofugal pathway.     

Development of the projections from the dorsal lateral geniculate nucleus to the lateral suprasylvian visual area of cortex in the cat.

In the study reported in the preceding paper, we used retrograde labeling methods to show that the enhanced projection from the thalamus to the posteromedial lateral suprasylvian (PMLS) visual area of cortex that is present in adult cats following neonatal visual cortex damage arises at least partly from surviving neurons in the dorsal lateral geniculate nucleus (LGN). In the C layers of the LGN, many more cells than normal are retrogradely labeled by horseradish peroxidase (HRP) injected into PMLS cortex ipsilateral to a visual cortex lesion. In addition, retrogradely labeled cells are found in the A layers, which normally have no projection to PMLS cortex in adult cats. The purpose of the present study was to investigate the mechanisms of this enhanced projection by examining the normal development of projections from the thalamus, especially the LGN, to PMLS cortex. Injections of HRP were made into PMLS cortex on the day of birth or at 1, 2, 4, or 8 weeks of age. Retrogradely labeled neurons were present in the lateral posterior nucleus, posterior nucleus of Rioch, pulvinar, and medial interlaminar nucleus, as well as in the LGN, at all ages studied. Within the LGN of the youngest kittens, a small number of retrogradely labeled cells was present in the interlaminar zones and among the cells in the A layers that border these zones. Such labeled cells were virtually absent by 8 weeks of age, and they are not found in normal adult cats. Sparse retrograde labeling of C-layer neurons also was present in newborn kittens.(ABSTRACT TRUNCATED AT 250 WORDS)