Layer A1 Research Articles

Heterochrony of postnatal accumulation of nonphosphorylated heavy-chain neurofilament by neurons of the cat dorsal lateral geniculate nucleus.

Accumulation of the heavy-chain neurofilaments reflects the maturation status of neuronal structures. The spatial distribution and postnatal developmental dynamic of neurons expressing nonphosphorylated heavy-chain neurofilaments (labeled by SMI-32 antibody) were analyzed in the dorsal lateral geniculate nucleus (LGNd) of the cat. Both interlaminar and intralaminar differences in the dynamic of SMI-32 staining were observed. The following results were obtained: (a) Ascending dorsoventral gradient in the density of SMI-32 immunopositive (SMI-32(+)) neurons (the greatest neuronal density in layer Cm, the minor in the top sublayer of layer A). This gradient was most prominent at the earliest stages of postnatal development (1st-2nd weeks) and slowly flattened up to adulthood; (b) Layer A1 exhibits increases in SMI-32-positive cells earlier than layer A; (c) The general transient increment in the number and density of SMI-32(+) neurons around 2-5 postnatal weeks. Since SMI-32 antibody is considered to be a putative marker for Y cells forming a motion processing stream, we suggest that peculiarities of SMI-32 staining at geniculate level could reflect the heterogeneity of Y cell subpopulations and the heterochrony of their postnatal maturation.

Early Postnatal Development of the Lamination in the Lateral Geniculate Nucleus A-Layers in Cats.

The early postnatal development of the A-layers of the dorsal lateral geniculate nucleus (LGNd) was investigated in kittens aged 0-34days by immunohistochemistry for the selective marker for neuronal differentiation (NeuN protein) and parvalbumin. We report two new facts about the LGNd development. First, there is a transient stratification of NeuN labelling in layer A, and to a lesser extent in layer A1, in kittens aged 0 and 4days. Second, a transient population of large cells that are located between the LGNd A-layers (interlaminar cells) showed high expression levels of both NeuN and parvalbumin. These neurons possessed both the morphological and immunohistochemical features, similar to cells in the neighbouring perigeniculate nucleus. Both NeuN-stratification and double-stained interlaminar cells gradually disappeared during the second postnatal week, and almost completely vanished by the opening of the critical period. We discuss a possible linkage between these observed transitory networks and the ON-/OFF- and X-/Y-cells development and propose that the data obtained reflect the functioning of the early environmentally independent geniculate networks.

Processing of broadband stimuli across A1 layers in young and aged rats

Presbycusis can be considered a slow age-related peripheral and central deterioration of auditory function which manifests itself as deficits in speech comprehension, especially in noisy environments. The present study examined neural correlates of a simple broadband noise stimulus in primary auditory cortex (A1) of young and aged Fischer–Brown Norway (FBN) rats. Age-related changes in unit responses to broadband noise bursts and spontaneous activity were simultaneously recorded across A1 layers using a single shank, 16-channel electrode. Noise bursts were presented contralateral to the left A1 at 80 dB SPL. Aged A1 units displayed increased spontaneous (29%), peak (24%), and steady state response rates (38%) than did young A1 units. This was true across all A1 layers, although age-related differences were significantly greater for layers I–III (43% vs 18%) than lower layers. There was a significant age-related difference in the depth and duration of post-onset suppression between young and aged upper layer A1 units. The present functional differences across layers were consistent with studies showing greatest losses of gamma-aminobutyric acid (GABA) markers in superficial layers of A1 and with anatomic studies showing highest levels of inhibitory neurons located in superficial cortical layers. The present findings were also consistent with aging studies suggesting loss of functional inhibition in other cortical sensory systems.

Afferent connections of fields 17 and 18 of the cat cerebral cortex formed by neurons of the dorsal lateral geniculate body.

Horseradish peroxidase (HRP) was applied microiontophoretically to nine individual columns in fields 17 and six columns in field 18 of the cat cerebral cortex. Most labeled cells in the dorsal lateral geniculate body were identified in layer A. The ratio of number of labeled cells in layer A to the number of labeled cells in layer A1 was assessed (the A/A1 ratio). Application of HRP to columns in field 17 yielded A/A1 ratios of 0.5 to 2.5; application to field 18 gave ratios ranging from 0.2 to 0.9. This suggests that the fields studied here contained columns which were afferent in relation to the ipsilateral eye and the contralateral eye.

Direct Modeling of X-Ray Diffraction Pattern from Skeletal Muscle in Rigor

Available high-resolution structures of F-actin, myosin subfragment 1 (S1), and their complex, actin-S1, were used to calculate a 2D x-ray diffraction pattern from skeletal muscle in rigor. Actin sites occupied by myosin heads were chosen using a “principle of minimal elastic distortion energy” so that the 3D actin labeling pattern in the A-band of a sarcomere was determined by a single parameter. Computer calculations demonstrate that the total off-meridional intensity of a layer line does not depend on disorder of the filament lattice. The intensity of the first actin layer A1 line is independent of tilting of the “lever arm” region of the myosin heads. Myosin-based modulation of actin labeling pattern leads not only to the appearance of the myosin and “beating” actin-myosin layer lines in rigor diffraction patterns, but also to changes in the intensities of some actin layer lines compared to random labeling. Results of the modeling were compared to experimental data obtained from small bundles of rabbit muscle fibers. A good fit of the data was obtained without recourse to global parameter search. The approach developed here provides a background for quantitative interpretation of the x-ray diffraction data from contracting muscle and understanding structural changes underlying muscle contraction.

Effect of corylus clusters on the physicochemical properties of soil

Soil sample plots were specified and the soil in layer A0, A1 and AB were collected in Maoershan-Forest Experiment Farm of Northeast Forestry University for study of the effect of corylus clusters on soil in 1999. The result shows that the pH value, contents of organic matter, total nitrogen, alkali-discomposed nitrogen and total phosphorus under the corylus clusters are higher than that under the non-corylus clusters, except the available phosphorus content. The number of soil granular aggregates or the water stable aggregates under corylus clusters is more than that under the non-corylus clusters. The corylus clusters play an important role in improving the physicochemical properties of the soil, which should be conserved and developed in the forestry production.

Functional morphology of the feedback pathway from area 17 of the cat visual cortex to the lateral geniculate nucleus

Two approaches were adopted to study the pattern of connectivity between the cat visual cortex and lateral geniculate nucleus. Fourteen individual cortico-geniculate axons were labeled and reconstructed after intracellular or extracellular injection of biocytin into regions of known receptive-field position and ocular dominance preference, and the distribution of boutons from multi-axon clusters was mapped in three dimensions and compared with the locations of strategically placed geniculate recordings made in the same tissue. The results show that the feedback has an accurate retinotopic component but that individual axons are both more extensive and more selective than described previously. Area 17 feedback axons terminate primarily in layers A and A1, but the distribution of terminal boutons is strongly biased (3:1 ratio) toward the layer that matches their eye preference. Thus, those driven by the contralateral eye preferentially target layer A, and those driven by the ipsilateral eye target layer A1. Each axon also innervates the perigeniculate nucleus (PGN), but the pattern is otherwise variable, suggesting that there are different axonal classes. The terminal fields of individual axons are much larger than described previously, with a maximum spread of 500-1500 microns. Nevertheless, the projection from a given location in area 17 has a center of maximum terminal density 400-500 microns across, which is in retinotopic correspondence with the aggregate receptive field of the cortical cells of origin. The surrounding zone of relatively sparse boutons, however, must permit corticofugal cells to influence visual processing well beyond the regions over which their own responses summate. It follows that any geniculate cell receives corticofugal input covering an equally extensive area of visual space.

ON and OFF activity gradients in the lateral geniculate nucleus of the cat: a combined 14C 2-deoxyglucose and D,L-2-amino-4-phosphonobutyric acid study.

Experiments on the lateral geniculate nucleus (LGN) of the cat based on 14C 2-deoxyglucose (2-DG) autoradiography and intraocular injections of 2-amino-4-phosphonobutyric acid (APB) provided evidence for gradients of metabolic activity in the ON and OFF pathways in layer A, but only very weakly, if at all, in layer A1. Alert and freely moving cats were exposed to square-wave gratings over a 45-min period after injection of the 2-DG. When one eye had been treated previously with APB, contralateral layer A showed a clear gradient of 2-DG label indicating that the remaining OFF pathway was most active ventrally in the layer and, by implication, that the ON pathway is normally most active dorsally. No gradient was apparent in layer A1 ipsilateral to the APB eye. Control experiments based on binocular injections of tetrodotoxin (TTX) demonstrated that no gradients were present in the baseline activity within the layers. Finally, monocular injections of TTX provided evidence for gradients of nondominant eye activity in layers A and A1 that were maximal near the interlaminar zone between layers A and A1 and declined in mirror-symmetric fashion toward the dorsal border of A and the ventral border of A1. Combined with earlier anatomical studies showing depth-dependent patterns of geniculo-cortical projection, these results indicate that in the cat, as in several other species, the visual input to striate cortex is partly organized around ON and OFF pathways. In addition, the results suggest that a systematic variation of binocular interaction, perhaps related to ocular dominance, exists through the depths of the geniculate layers.(ABSTRACT TRUNCATED AT 250 WORDS)

Synapses formed by identified retinogeniculate axons during the segregation of eye input

The synaptic organization of identified retinogeniculate axons was studied during the prenatal development of eye-specific layers in the LGN of the cat. During this period, retinogeniculate axons undergo stereotyped morphological changes. Retinogeniculate axons originating from one eye and passing through LGN territory destined to be solely innervated by the other eye (inappropriate territory) initially give rise to many side branches. As the eye-specific layers emerge, these axons elaborate extensive terminal arbors within territory appropriate to their eye of origin and concurrently retract their side branches from inappropriate territory (Sretavan and Shatz, 1986). These transient side branches may therefore represent a morphological substrate for the observed functional convergence of inputs from the two eyes onto common LGN neurons during prenatal development (Shatz and Kirkwood, 1984). This possibility was investigated by examining whether identified axons and their side branches form synapses in inappropriate territory. Three retinogeniculate axons from two fetuses aged embryonic day 53 (E53) and E57 were filled with HRP in an in vitro preparation, prior to being processed for electron microscopy (EM). The HRP-filled axons, originating from the contralateral eye, were first reconstructed at the light microscope level. The portion of axon passing through the center of ipsilaterally innervated layer A1 was then serially sectioned and reconstructed by EM. Two sets of 450 serial EM sections revealed that all three contralateral axons established synaptic contacts in ipsilateral territory. Many of these synapses were made by side branches and a few were even formed by the main axon trunks. Both side branches and trunks formed mainly en passant asymmetrical contacts that were associated with spherical synaptic vesicles and that were apposed to immature dendritic elements and dendritic shafts. For comparison, a portion of the same E53 axon within the future contralateral layer A was also serially sectioned and reconstructed for EM. Within this contralateral zone, the E53 axon formed synaptic contacts similar to those established in the ipsilateral region, except that in the appropriate zone they contained significantly more synaptic vesicles. These results demonstrate that axons from the contralateral eye can establish synapses in territory simultaneously innervated by the ipsilateral eye, both via side branches and by means of contacts along the main axon trunk. Thus, the development of eye-specific layers is accompanied by the formation and subsequent elimination of synapses that almost certainly represent a morphological substrate for the known transient functional convergence of inputs from the two eyes.

Effect of eye removal on N-acetylaspartylglutamate immunoreactivity in retinal targets of the cat

The endogenous brain dipeptide N-acetylaspartylglutamate (NAAG) has previously been demonstrated in the somata of retinal ganglion cells and the neurophil of retinal targets. In this paper we report that the NAAG immunoreactivity of the neuropil in the retinal targets is dependent on an intact optic pathway. Removal of one eye produced a marked decrease in the staining of the neurophil in layer A of the contralateral geniculate nucleus (LGN) and layer A1 of the ipsilateral LGN. There was also decreased staining in the superficial layers of the superior colliculus contralateral to the removal. These results suggest that NAAG is present in the terminals of retinal ganglion cells and is consistent with a role for NAAG in visual synaptic transmission.

Three-dimensional form and a simple, interrupted flat map of a single layer in the lateral geniculate nucleus of the cat

A simple, computer-based procedure is described for measuring the 3-dimensional (3D) coordinates and making two-dimensional (2D) flat maps of a curved surface representing the middle depth of layer A1 in the lateral geniculate nucleus (LGN) of the cat. The surface was defined from histological sections and flattened by tiling it with small triangular tiles which were then laid in a single plane to form several flat strips or lunes. Though the resultant maps are discontinuous (interrupted), they preserve total area to within a few percent, have constant scales throughout and closely approximate true shape within individual tiles. The results represent a step in assessing the potential contribution of form to visual function in the LGN. They provide graphic 3D and 2D images of the complex shape of a single layer and a quantitative framework for constructing models of geniculocortical processing that can incorporate the 3D form of the LGN as well as other anatomical and physiological data.

Response properties in the dorsal lateral geniculate nucleus of the adult cat after interruption of prenatal binocular interactions.

1. Single-cell recordings were made in the magnocellular layer of the dorsal lateral genicule nucleus (dLGN) of five adult cats in which prenatal binocular interactions were interrupted by monocular enucleation at known gestational ages. Three cats (early enucleates) had one eye removed on either embryonic day 44.48, or 49, before retinogeniculate inputs are segregated into uniocular layers. Two other (late enucleates) underwent this procedure on embryonic days 55 and 58, when segregation is well advanced. Responses were compared with those obtained from recordings in the A and A1 layers of the dLGN of seven normal adult cats. 2. Cells were classified as ON or OFF by the use of spots of light and as X or Y based on a test of linearity of spatial summation with the use of counterphased sinusoidal gratings. Receptive-field size and spatial resolution were also obtained. 3. The dLGN of prenatally enucleated cats contains a dorsal magnocellular layer and a ventral parvocellular layer. In early enucleates, only an occasional hint of a cell-sparse interlaminar zone was apparent, located between the magnocellular and parvocellular layers. In late enucleates, a prominent cell-sparse band was observed contralateral to the remaining eye, in a region that would most likely correspond to layer A1 in the normal dLGN. No such cell-sparse band was seen ipsilateral to the remaining eye in late enucleates. 4. Eighty-six X cells and 22 Y cells were studied in the enucleates. Both cell types were found at all depths of the magnocellular layer. All but a few neurons had concentric ON-center or OFF-center receptive fields that were normal in size. The topography of receptive fields also appeared normal. In addition, spatial resolution of X and Y cells was similar in experimental and control animals. 5. In early enucleates there was a higher percentage of X cells and a lower percentage of Y cells than normal. The change in X-to-Y ratio was shown to be because of both a gain in cells with X properties and a loss of cells with Y properties. The distribution of dLGN somal sizes in the early enucleates was comparable with controls, so the change in X-to-Y ratio most likely did not result from an electrode sampling bias. It was suggested that the X-to-Y ratio difference could stem from the abnormalities in retinogeniculate terminal arbors that have been shown to follow early eye removal.(ABSTRACT TRUNCATED AT 400 WORDS)

The Salazie Cirque ignimbrite (Piton des Neiges volcano, Réunion Island): Chronostratigraphy, description and significance of lithic fragments and eruptive mechanisms

An ignimbrite sequence in the Salazie Cirque is characterized by a pyroclastic-flow unit composed of basal breccias (layers A1 and A2), a welded tuff (layer B), and an upper ashy layer (layer C). This eruption began a new eruptive cycle in the Piton des Neiges volcanic complex, the second stage of a differentiation series characterized by the emission of undersaturated lava. The paleogeography of the volcano's eastern flank is reconstituted at about 150,000 y. B.P. from the distribution of ignimbrite deposits that partially fill two large erosion cirques, the Salazie protocirque and the Marsouins protocirque. The total volume of these deposits is only a few km3, and no evidence indicates a related caldera collapse. Basal breccias are composed of: (1) juvenile magmatic clasts yielding a mugearitic composition and normative nepheline; and (2) xenoliths representative of the different units in a 1500–2000-m-thick volcanic pile which include differentiated lavas (first stage of differentiated series) basaltic lava flows and plutonic rocks (“oceanite” series). The appearance of hydrothermal parageneses only in subsurface facies emphasizes the striking role of volcanotectonic structures in mineralized-fluid circulation. The common features of magma mixing (abnormal mineralogical associations, reverse zonings, disequilibrium features) in layer B are interpreted as resulting from the assimilation of highly differentiated foreign rocks by the mugearite magma during and after the course of the eruption. The Salazie Cirque ignimbrite is of the overflowing type (non-Plinian) characterized by high emplacement temperatures. A model is proposed for its formation: (1) rising of a mugearitic eruption column; (2) vesiculation of magma at shallow depth beneath the plug; (3) increase of magma pressure and eruption of a mixture of gas, magmatic blocks and xenoliths down slope; deposition of fines-depleted heterogenous lithic-rich flow (layer A1); (4) explosion (flashing) of the gas-rich upper part of the magma column and eruption of the main welded tuff unit with segregation of a basal non-fines-depleted lithic concentration zone during flow (layer A2) and elutriation of an ash-cloud fall (layer C); (5) effusive phase.

Corticotectal circuit in the cat: a functional analysis of the lateral geniculate nucleus layers of origin.

1. The dorsal lateral geniculate nucleus (LGN) of the cat is a major thalamic relay between the retina and several visual cortical areas. These cortical areas in turn project to the superior colliculus (SC). The aim of the present experiment was to determine which LGN layers provide a necessary input to the corticotectal circuit. 2. Individual layers of the LGN were reversibly inactivated by microinjection of cobalt chloride during recording of visual responses in the retinotopically corresponding part of the superior colliculus. 3. For cells driven through the contralateral eye, inactivation of layer A or the medial interlaminar nucleus (MIN) had little effect on visual responsiveness in the superior colliculus. In contrast, inactivation of layer C abolished visual responses at one-quarter of the SC recording sites, reduced responses at another quarter, and left half of the recording sites unaffected. 4. For cells driven through the ipsilateral eye, inactivation of layer C1 or the MIN had no effect. Inactivation of layer A1 uniformly reduced visual responses in the superior colliculus and usually abolished them entirely. 5. These results are compatible with previous work showing that cortical input to the SC originates from Y-cells. They indicate that two of the five Y-cell containing layers (A1 and C) provide major inputs to the corticotectal circuit. The results suggest that layer A1 is functionally allied to layer C as well as to layer A.

May 18, 1980, Mount St. Helens deposits in South Coldwater Creek, Washington

Beginning at 0832 Pacific Daylight Time (PDT), Mount St. Helens, Washington, produced a rockslide debris avalanche and a lateral explosion (blast) that generated a laterally directed pyroclastic surge, called here a blast surge. The resulting deposits are thickest in the east‐west trending valley of North Fork Toutle River immediately north of the volcano but are almost as thick in South Coldwater Creek, a small tributary of the Toutle River, in the lee of an east‐west trending ridge (Johnston Ridge) just north of the North Fork Toutle River. The avalanche surmounted Johnston Ridge (300 m relief) and flowed down the valley of South Coldwater Creek. Closely following the avalanche and in part contemporaneously with it, the blast surge left thin (up to ∼0.5 m) deposits on Johnston Ridge and thick (up to ∼ 15 m) deposits in South Coldwater Creek. These deposits have a lower layer (layer A1) which is friable, massive, poorer in fines (particles < 1/16 mm), and more poorly sorted than the upper layer (layer A2). Layer A2 is less friable because it contains more silt‐ and clay‐sized material. Beneath layer A1 is a zone containing a small percentage of debris from the blast surge mixed with products eroded from the underlying surface. This zone, designated as layer A0, is a poorly mixed layer containing schlieren of soil, shattered volcanic fragments, and considerable quantities of organic material. Layer A1 is strongly sorted and fines depleted and is interpreted to have been deposited from just behind the turbulent head of the blast surge; layer A2 is interpreted to have been deposited from the body of the blast surge which is believed to have been less turbulent than the head; layer A0 is interpreted to have formed by shearing at the base of the head of the blast surge in the region between the front and the back of the head of the blast surge. It is concluded that a strongly fines‐depleted basal layer of a pyroclastic flow or surge deposit underlain by a thin sheared zone is the signature of an extremely high‐velocity particulate density current.

Axon trajectories and pattern of terminal arborization during the prenatal development of the cat's retinogeniculate pathway.

In this study we have examined the trajectories taken by populations of ganglion cell axons and the spatial gradients of terminal arbor maturity within the lateral geniculate nucleus (LGN) during the prenatal development of the cat's visual system. To do so, an in vitro method of labeling optic tract axons from fetal brains between embryonic day 37 (E37) and postnatal day 2 (P2) with horseradish peroxidase (HRP) was used. At the earliest ages studied (E37-E53), optic axons leave the optic tract to run across the LGN toward their sites of termination in straight trajectories parallel to each other. At later ages (E57-P2), however, axons with abrupt changes in their course across the nucleus can be clearly identified. When the detailed terminal arbor morphology of the set of retinogeniculate axons filled with HRP at a given age was examined, two different spatial gradients of maturation could be detected. The terminal arbors of axons within LGN layer A are always more mature than those ending in layer A1, an observation consistent with previous findings that axons from the contralateral eye arrive within the LGN several days before those from the ipsilateral eye. Moreover, the terminal arbors of axons projecting to the medial portions of each layer are always more mature than their more lateral counterparts. These gradients are likely to be a direct reflection of the central-first, peripheral-last gradient associated with the neurogenesis of the retinal ganglion cells themselves. In the oldest animals studied (E58-P2), a remarkable periodic pattern of terminal arbor labeling was seen following a localized HRP injection into the optic tract. Within the labeled portions of the LGN, densely filled axon terminal arbors are separated by unlabeled gaps of similar width. This pattern of labeling could reflect local topographic disorder within the optic tract or could arise if axons of different classes of retinal ganglion cells run in separate portions of the optic tract. Taken together, all of these observations suggest that there may be a fair degree of topographic order in the retinogeniculate projection within the cat's LGN early on in development. However, when topographic errors are present, some can be corrected by minor readjustments in axonal trajectories.

ON and OFF layers in the lateral geniculate nucleus of the mink

The lateral geniculate nucleus (LGN), a major target of the optic nerve in mammals, is a laminated structure, although the pattern of lamination varies widely among different species. It has long been known that one function of the lamination is to segregate input from the two eyes. Many animals, however, have more than one layer innervated by each eye; the functional significance of these duplications has been only partially elucidated. In the mink, the principal layers innervated by the contralateral eye (layer A) and the ipsilateral eye (layer A1) are each split into two adjacent leaflets1. We report here that the anterior leaflet of each pair contains ON-centre neurones and the posterior leaflets OFF-centre neurones.

The effect of monocular paralysis on the lateral geniculate nucleus of the cat.

The proportions and some of the properties of X and Y cells in lateral geniculate nucleus (LGN) of adult cats were measured 14-16 days after monocular paralysis. The paralysis was produced by sectioning cranial nerves III, IV and VI. No difference from normal was observed in the proportions of X and Y cells either in the A layer driven by the paralyzed eye or in the A1 layer driven by the mobile eye. The distribution of latencies to chiasm stimulation and the average visual spatial resolution were within the normal range in both A and A1 layers. These experiments indicate that monocular paralysis in the adult cat does not affect the numbers of X and Y cells in the LGN. However, the averages of cell body size in layers A and A1 contralateral to the immobilized eye were roughly equal, whereas in the normal cat cells in layer A1 are larger than those in layer A.

Excitation of units in the lateral geniculate and contiguous nuclei of the cat by stretch of extrinsic ocular muscles.

In cats, anaesthetized with chloralose and paralysed, the responses of units in the right lateral thalamus were recorded while the extrinsic ocular muscles (EOM) of the right eye were stretched in the dark. Phasic responses were found in all layers of the dorsal lateral geniculate nucleus (LGNd) and in the perigeniculate nucleus (PGN). A given unit usually responded to stretch of more than one EOM and thus to more than one direction of rotation of the eye in the orbit. LGNd. Of a sample of 76 units in LGNd, 55 (72%) gave visula but no muscle responses and 21 (28%) responded to EOM stretch. In all, 40 units with EOM responses were examined and 25 of the 27 tested (93%) also had visual responses. Of the 40 units, 32 could be allocated to layers, thus; layer A, 8 (25%); layer A1, 20 (63%); layer B, 3 (9%); central interlaminar nucleus, 1 (3%). It is interesting that most of the EOM responses were found in layer A1 which recieves the excitatory visual input from the eye whose EOM were stretched. Muscle responsive units occurred with ON- and OFF-centre visual responses of sustained and transient types. PGN. In PGN, 21 units gave EOM responses and most of them were also excited by visual input. The conclusion is that the LGNd and PGN receive an extraretinal proprioceptive signal which should be present during at least large saccadic eye movements. The anatomical pathways which may be involved and the significance of the signal are discussed briefly.

Abnormal retino‐geniculate and geniculo‐cortical pathways in several genetically distinct color phases of the mink (Mustela vison)

Several genetically distinct color phases of mink, which all show an abnormal reduction of pigment in the retinal pigment epithelium and which also show abnormalities of the retinofugal pathways, have been studied. Autoradiographic methods have been used to demonstrate the retino-geniculate pathways, and retrograde degeneration or the retrograde transport of horseradish peroxidase has been used for the geniculo-cortical pathways. The retino-geniculate abnormality is mild in some of the color phases and extremely severe in others, but within any one color phase the variability is relatively low. Although the severity of the abnormality varies between color phases, a rather specific pattern of abnormal geniculate innervation is recognizable for mink in general and this is distinct from that found in Siamese cats. In the abnormal mink the size of geniculate lamina A1 is reduced and there is an abnormal crossed input going to the intermediate sectors of this reduced layer. Layer C1 also receives an abnormal crossed input, but this is more variable than that going to A1 and there appears to be little correspondence, retinotopically, between the normal inputs to layers A1 and C1. In some of the abnormal mink there are interruptions within the cytoarchitectionically definable layer A1, and opposite these gaps reduplications of layer A are commonly seen, as though there is an intrinsic geniculate mechanism for generating the characteristic multilaminar geniculate structure. However, there are also numerous examples of fusions between layers receiving afferents from the same eye, and these demonstrate that the development of geniculate lamination must also be under the influence of the retinal inputs. The geniculo-cortical pathway shows a normal topography in most of the mink. Abnormal geniculo-cortical projections, comparable to the "Boston" pattern of Siamese cats are extremely rare, and their occurrence could not be correlated with the severity of the retino-geniculate abnormality or with the laminar pattern in the lateral geniculate nucleus. We suggest that the development of one or the other pattern of geniculo-cortical projection may depend upon the relative timing of the two mechanisms that produce the geniculate lamination.