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PubMed1990

Gadolinium-DTPA-enhanced magnetic resonance imaging in experimental optic neuritis.

Magnetic resonance imaging (MRI) with gadolinium-diethylenetriaminepentaacetic acid (Gd-DTPA) was used to investigate disruption of the blood-optic nerve barrier associated with acute autoimmune demyelination. Leakage of Gd-DTPA was seen in the optic nerves and optic chiasm of adult guinea pigs sensitized for acute experimental allergic encephalomyelitis, but not in normal unsensitized animals. This finding occurred as early as 5 to 8 days after antigenic sensitization with the myelin emulsion and before the onset of paralysis or ataxia. Pathologic examination at this early stage of experimental allergic encephalomyelitis showed an absence of demyelination in the optic nerves and optic chiasm, although scant perivascular foci of inflammatory cells were seen. Leakage of Gd-DTPA in the optic nerve before demyelination of this white matter tract illustrates that increased permeability of the blood-optic nerve barrier is an early, if not the initial, event in autoimmune demyelination, and the optic nerve is a common site of central nervous system involvement during the initial phase of acute experimental allergic encephalomyelitis. Findings in this animal model appear comparable with the results of MRI with Gd-DTPA in patients with optic neuritis, and they suggest that disruption of the blood-optic nerve barrier is a common denominator for both disorders of primary demyelination.

Animals↗
PubMed1996

The optic nerve.

Recent articles in the scientific literature have described major advances in our understanding of the anatomy and vascular relationships of the optic nerve (cranial nerve II) and of the diagnosis and treatment of a variety of disorders affecting this nerve, including congenital anomalies of the optic disc, dominant hereditary optic neuropathy, anterior and retrobulbar arteritic and nonarteritic ischemic optic neuropathy, optic neuritis, Cuban epidemic optic neuropathy, toxic and nutritional optic neuropathies, radiation-induced optic neuropathy, AIDS-related optic neuropathy, optic neuropathies caused by tumors, and papilledema.

Humans↗
PubMedDate not supplied

Ocular and systemic findings associated with optic disc colobomas.

PURPOSE: To determine the ocular and systemic anomalies associated with optic disc colobomas. PATIENTS AND METHODS: The records of patients with a diagnosis of isolated optic disc coloboma and chorioretinal coloboma with optic disc involvement were retrospectively reviewed. RESULTS: Fifteen patients were included in the study. Of the 30 eyes, the optic disc and choroid were involved in 18, an isolated disc coloboma was present in 5, a normal optic disc was present in 4, and the optic disc could not be identified because of extreme microphthalmia in 1. Of the optic discs outside the fundus colobomas, 1 had an abnormal shape and 1 had optic atrophy and hypoplasia. Eight patients had bilateral but asymmetric involvement of the optic nerve with the coloboma. Fourteen eyes of 9 patients were microphthalmic. Nine eyes of 6 patients had microcornea. One patient had a nonrhegmatogenous retinal detachment at the time of diagnosis. One eye had a retrobulbar cyst with microphthalmia. Eight (53%) of the patients had sensory strabismus. Two patients had unilateral cortical lens opacities. Associated systemic findings were present in 6 (40%) of the 15 patients. CONCLUSIONS: Optic disc colobomas have a wide variety of presentations. Poor visual acuity was observed mostly in eyes with macular involvement and microphthalmia.

Adolescent↗
PubMed2006

Optic nerve hyperintensity on T2-weighted images among patients with pituitary macroadenoma: correlation with visual impairment.

PURPOSE: Visual acuity (VA) disturbance other than field defect is important in evaluating patients with pituitary macroadenoma. The purpose of this study was to evaluate MR imaging appearances of optic nerves in patients with pituitary macroadenoma and to ascertain whether visual impairment was correlated with abnormality in optic nerve signal intensity. PATIENTS AND METHODS: Twenty-seven patients with pituitary macroadenoma were examined. Optic nerves were evaluated on T2-weighted images and correlations of signal intensity abnormality with VA disturbance, visual field disturbance, degree of optic chiasm compression, pathologic findings of surgical specimen, and disease duration were statistically analyzed. Correlations between recovery of VA after treatment and the above-mentioned factors were also determined. RESULTS: Coronal T2-weighted images demonstrated unilateral optic nerve hyperintensity lesions in 9 patients. Bilateral signal intensity abnormality of the optic nerve was seen in 5 patients. Signal intensity abnormality of the optic nerve was seen at the site of compression and in the ventral side of the tumor. These patients did not demonstrate signal intensity abnormality posterior to the tumor. Presence of such signal intensity abnormalities was correlated with the degree of optic chiasmal compression and with VA disturbance. Recovery of VA after treatment was correlated with disease duration. CONCLUSION: Hyperintensity of the optic nerves ventral to the pituitary macroadenoma was associated with VA impairment. Recovery of VA after treatment was correlated with disease duration. MR imaging of the optic nerves can provide valuable information for management of pituitary macroadenoma.

Adenoma↗
PubMedDate not supplied

Phased-array surface coil MR of the orbits and optic nerves.

PURPOSE: To devise a practical technique for high-resolution evaluation of the anterior optic apparatus using a phased-array surface coil system, and to evaluate this system in patients with suspected optic pathway abnormalities. METHODS: A four-element phased-array coil pair was placed on each side of the head, and signal-to-noise measurements were obtained using a head phantom. Comparison between the phased-array coil, the quadrature coil, and a single-turn 12.7-cm (5-in) surface coil was done. T1 spin-echo and T2 fast spin-echo sequences were obtained in the oblique axial and oblique sagittal planes, to approximate the long axis of the optic nerves and the nonoblique coronal plane. RESULTS: The phantom signal-to-noise measurements at simulated locations of the optic nerve head, optic canal, and optic chiasm revealed an improvement of at least 30% using the phased-array system. Of 24 imaged cases, 9 had trauma, 5 had suspected neoplasms, and 2 had optic neuritis. In 3 patients, an unexpected diagnosis of optic pathway contusion or infarction was made. The remaining 8 patients had various suspected visual pathway lesions. CONCLUSION: Phased-array surface coils allow rapid, thin-section imaging of the entire anterior optic pathway, with improved signal-to-noise ratio. This may improve evaluation of optic pathway lesions over conventional techniques.

Adolescent↗
PubMed1988

Histochemical localization of cytochrome oxidase in the retina and optic tectum of normal goldfish: a combined cytochrome oxidase-horseradish peroxidase study.

Cytochrome oxidase (C.O.) was histochemically localized in the normal retina and optic tectum of goldfish in order to examine the laminar and cellular oxidative metabolic organization of these structures. In the optic tectum, C.O. exhibited a distinct laminar, regional, and cellular distribution. The laminae with highest C.O. levels were those that receive optic input, suggesting a dominant role for visual activity in tectal function. This was demonstrated by colocalizing C.O. and HRP-filled optic fibers in the same section. However, the distribution of C.O. within the optic laminae was not uniform. Within the main optic layers, the SFGS, four metabolically distinct sublaminae were distinguished and designated from superficial to deep as sublaminae a, b, c, and d. The most intense reactivity was localized within SFGSa and SFGSd, followed by SFGSb, then SFGSc. In SFGSd, intense reactivity was found to occur specifically within a class of large diameter axons and terminals that were apparently optic since these were also labeled with HRP and cobaltous lysine applied to the optic nerve. Regional C.O. differences across the tectum were also noted. Low levels were found in neurons and optic terminals along the growing immature medial, lateral, and posterior edges of tectum, but were higher at the more mature anterior pole and central regions of tectum. This suggests that the oxidative metabolic activity is initially low in newly formed tectal neurons and optic axons, but gradually increases with neuronal growth and functional axon terminal maturation. Most C.O. staining was localized within neuropil, whereas the perikarya of most tectal neurons were only lightly reactive. Only a few neuron classes, mostly the relatively larger projection neurons, had darkly reactive perikarya. In the retina, intense C.O. reactivity was localized within the inner segments of photoreceptors, the inner and outer plexiform layers, and within certain classes of bipolar and ganglion cells. The large ganglion cells in particular were intensely reactive. Like the large diameter optic terminals in SFGSd, the large ganglion cells were preferentially filled with HRP, suggesting that they may project to tectum and are the source of the darkly reactive large diameter axons and terminals in sublamina SFGSd. We propose a new scheme to describe tectal lamination that integrates laminar differences in C.O. reactivity with classical histological work.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1988

Laminar and sublaminar ultracytochemical localization of cytochrome oxidase in the optic tectum of normal goldfish.

Distinct laminae and sublaminae in the goldfish optic tectum exhibit substantial differences in cytochrome oxidase (C.O.) reactivity. To determine whether these differences are associated with differential reactivity of different neuronal profiles, each tectal sublamina was examined at the ultrastructural level following C.O. treatment. The greatest abundance of darkly reactive mitochondria was found in the optically innervated layers within both pre- and postsynaptic profiles in correspondence with the most intense staining of these layers at the light microscopic level. Many reactive mitochondria were localized within terminals that were presumed to be optic on the basis of cytological criteria or were shown to be optic by filling optic fibers with HRP and processing so as to simultaneously demonstrate both mitochondrial C.O. reactivity and HRP labeling. These optic terminals tended to differ from each other in size and level of reactivity. The largest terminals were located within sublamina d of the stratum fibrosum et griseum superficials (SFGSd), and these were the most intensely reactive and contained the greatest number of darkly reactive mitochondria. Medium-sized terminals were found within sublaminae SFGSa, SFGSb, and a and c of the stratum album centrale (SACa,c). These were also darkly reactive but contained fewer mitochondria. Other medium-to-small optic terminals were found in stratum opticum a and b (60a,b), SFGSb, SFGSc, and stratum griseum centrale c (SGCc). These typically contained fewer mitochondria that also tended to be relatively less reactive, although darkly reactive mitochondria were also present. We suggest that the metalbolic differences within optic terminals of different size and sublaminar stratification arise from different ganglion cell classes and that the different optic layers of tectum are functionally substratified. As expected, darkly reactive mitochondria were most abundant in th intensely stained sublaminae, which included the optic lamina SFGS and nonoptic sublamina SGCa, and they were found not only within optic terminals but also within dendrites, presynaptic dendrites, and nonoptic terminals as well. Glial processes tended to contain less reactive mitochondria. The most prominent of the nonoptic terminals were the large-diameter P1 terminals, which contained pleomorphic vesicles and formed symmetric (presumed inhibitory) synapses. In stratum marginale most of the darkly reactive mitochondria were localized within dendrites. In the rest of the tectal layers most of the darkly reactive mitochondria were found in both presynaptic terminals and dendrites.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed2000

Cortical and subcortical influences on the nucleus of the optic tract of the opossum.

In the present work we propose a new phylogenetic hypothesis for the role played by cortical and subcortical afferents to the nucleus of the optical tract, the main visual relay station of the horizontal optokinetic reflex in mammals. The hypothesis is supported by anatomical and physiological data obtained in the South American opossum (Didelphis aurita) using the following experimental approaches: (i) single-unit recordings in the nucleus of the optic tract and simultaneous electrical stimulation of the contralateral nucleus of the optic tract; (ii) single-unit recordings in the nucleus of the optic tract and simultaneous electrical stimulation of the ipsilateral striate cortex; (iii) injection of cholera toxin subunit B into the striate cortex and subsequent immunohistochemical reaction to reveal the presence of the marker in the thalamus and mesencephalon; and (iv) single-unit recordings in the nucleus of the optic tract both before and after ablation of the ipsilateral visual cortex. The main results are: (i) there is a strong inhibitory reciprocal effect upon the nucleus of the optic tract following stimulation of its contralateral counterpart; (ii) electrophysiological and anatomical data imply that the visual cortex does not project directly to the nucleus of the optic tract. Rather, cortical terminals seem to target the nearby anterior and posterior pretectal nuclei and orthodromic latencies in the nucleus of the optic tract following stimulation of the visual cortex were twice as large as in the superior colicullus; and (iii) ablation of the entire visual cortex did not have any effect upon binocularity of cells in the nucleus of the optic tract. These results strengthen the model proposed here for the role of the interactions between the nuclei of the optic tract under optokinetic stimulation. The hypothesis in the present work is that the cortical influences upon the nucleus of the optical tract, in addition to the subcortical ones, appeared only recently in phylogenesis. In more primitive mammals, such as the opossum, subcortical interactions are thought to play a relatively important role. With the emergence of retinal specializations, such as the fovea, one might suppose that there followed the appearance of new ocular movements, such as the smooth pursuit and certain types of saccades, that came to join the pre-existent optokinetic reflex.

Animals↗
PubMed2003

Effect of the optic size of a single-piece acrylic intraocular lens on posterior capsule opacification.

PURPOSE: To investigate the effect of the optic size of an intraocular lens (IOL) with sharp optic edges on posterior capsule opacification (PCO). SETTING: Nishi Eye Hospital, Osaka, Japan. METHODS: In 5 rabbits, a single-piece 5.5 mm optic AcrySof IOL (Alcon Laboratories) was implanted in 1 eye and a specially fabricated single-piece 7.0 mm optic AcrySof IOL was implanted in the contralateral eye. Histopathological examinations were performed 3 weeks after surgery. RESULTS: On posterior views, less PCO was noted with the 5.5 mm optic in all rabbits except 1. Histopathological sections demonstrated adhesion of the anterior and posterior capsules between the haptic and optic and the formation of a sharp capsular bend at the posterior optic edge in 3 eyes with the 5.5 mm optic but in no eye with the 7.0 mm optic. Abundant PCO was noted when a sharp capsular bend had not formed. CONCLUSIONS: Capsular adhesion is a prerequisite of capsular-bend formation. The sharp optic edge alone does not provide a substantial barrier when a capsular bend is not formed. Bulky haptics such as those of the single-piece AcrySof and large optics may hamper capsular adhesion and bend formation.

Acrylic Resins↗
PubMed2000

Surgical prevention of posterior capsule opacification. Part 3: Intraocular lens optic barrier effect as a second line of defense.

PURPOSE: To emphasize an important aspect of preventing posterior capsule opacification (PCO), the barrier effect established by the optic of a posterior chamber intraocular lens (PC IOL), and present a new classification regarding capsular bag status after extra-capsular cataract extraction, including phacoemulsification. SETTING: Center for Research on Ocular Therapeutics and Biodevices, Storm Eye Institute, Medical University of South Carolina, Charleston, South Carolina, USA. METHODS: This analysis included 150 consecutive eyes obtained postmortem with United States-manufactured PC IOLs including (1) poly(methyl methacrylate), (2) silicone, and (3) hydrophobic acrylic designs that were accessioned in the Center from September 1995 to January 1, 1998. Gross photographs from behind (Miyake-Apple views) were taken and serial histologic sections prepared. RESULTS: Microscopic analysis of the 150 eyes showed that the morphologic appearance of the capsular bag could be grouped into 2 categories: (1) those with little or no evidence of retained cortical material and cells, and (2) those with retained cortical material and cells in which a Soemmering's ring formed. With the latter, when a distinct barricade to cellular migration created by the IOL optic was noted, 2 discrete configurations occurred, depending on the different geometries of the optic components. With a classic biconvex optic with a curved and tapered edge, in many instances some ingrowth of cells proceeded posteriorly around the edge of the IOL optic in the direction of the central axis. With a lens optic that had a squared, truncated, and relatively thick edge, there was often abrupt termination of cells at the peripheral edge of the optic. The posterior capsule subtending the entire optic zone was therefore relatively or totally cell free. CONCLUSIONS: The barrier effect of the IOL optic appears to be of critical importance in retarding ingrowth of cells, functioning as a second line of defense when cortical cleanup is incomplete. Analysis of PC IOLs obtained postmortem showed that a square, truncated optic edge seemed to provide the maximum impediment to cell growth behind the IOL optic.

Acrylic Resins↗
PubMed2001

Optical phase contrast measurement of ultrasonic fields.

This report describes an optical phase contrast imaging technique for the measurement of wide bandwidth ultrasound fields in water. In this method, a collimated optical wavefront (lambda(l) = 810 nm) impinges on a wide bandwidth ultrasound pulse. The method requires that refractive index perturbations induced by the ultrasound field be sufficiently small. Specifically, on exit from the acoustic field, the phase of the optical wavefront must be proportional to the ray sum of local density taken in the direction of propagation of the incident optical wave. A similar restriction is placed on the dimensions of the ultrasound pulse. Repeated measurement of this phase as the ultrasound field is rotated through 180 degrees about an axis normal to the direction of propagation of the incident optical wave generates the Radon transform of the ultrasonically induced refractive index perturbation. Standard tomographic reconstruction techniques are used to reconstruct the full three-dimensional refractive index perturbation. A simple two-lens imaging system and an optical signal processing element from phase contrast microscopy provide a method of directly measuring an affine function of the desired optical phase for small optical phase shifts. The piezo- and elasto-optic coefficients (the first partial derivatives of refractive index with respect to density and pressure) relate refractive index to density and pressure via a linear model. The optical measurement method described in this paper provides a direct, quantitative measurement of the piezo- and elasto-optic coefficients (from the density or pressure fields).

Journal Article↗
PubMedDate not supplied

Near-infrared imaging of fast intrinsic optical responses in visible light-activated amphibian retina.

High performance functional imaging is needed for dynamic measurements of neural processing in retina. Emerging techniques for visual prosthesis also require advanced methodology for reliable validation of electromagnetic stimulation of the retina. Imaging of fast intrinsic optical responses associated with neural activation promises a variety of technical advantages over traditional single and multichannel electrophysiological techniques for these purposes, but the application of fast optical signals for neural imaging has been limited by low signal-to-noise ratio and high background light intensity. However, by using an optimized near-infrared probe light and improved optical system, we improve the optical signals substantially, allowing single pass measurements with approximately micron resolution. We image fast intrinsic optical responses with different optical modalities, i.e., bright field, dark field, and cross-polarization, from isolated retina activated by visible light stimulation. At single cell resolution, bright-field imaging discloses the maxima of optical responses approximately 5% dI/I, where dI is the dynamic optical change and I is the baseline light intensity. Dark-field imaging techniques further enhance the sensitivity of optical measurements, and show the maxima of optical responses exceeding 10% dI/I. Cross-polarized imaging provides optical sensitivity similar to dark-field imaging, but different patterns of neural activation are observed.

Action Potentials↗
PubMed2001

Optical recording of the spatiotemporal propagation of neuronal excitation in the rat hippocampal CA2-CA1 pathway.

Changes in the membrane potential of neurons in the hippocampal CA2 and CA1 regions were recorded by optical recording techniques. After stimulation of the Schaffer collaterals at the hippocampal CA2 region, excitatory optical signals first occurred adjacent stimulus electrode and then flamed-up signals spread toward the hippocampal CA1 region. The optical signal was blocked by tetrodotoxin (TTX) (1 microM). Propagation of the optical signal was blocked in an artificial cerebrospinal fluid (ACSF) containing 0 mM Ca2+ and 6 mM Mg2+. 6,7-Dinitroquinoxaline-2,3 (1H,4H)-dione (DNQX) (20 microM) also blocked the optical signals that spread to the hippocampal CA1 region. The time course of the optical signal recorded at a unit area (49 pixels) on the propagation pathway was characterized by fast and slow components. TTX (1 microM) blocked both fast and slow components of the optical signal. The slow component of the optical signal was preferentially depressed by either removal of external Ca2+ or by bath-application of DNQX (20 microM). When bicuculline (15 microM) was applied to the bath-solution, the intensity and propagation area of the optical signal were increased. The results indicate that stimulation of the Schaffer collaterals in the hippocampal CA2 region produces the propagation of the optical signal to the hippocampal CA1 region, and that the optical signal involves the action potential and excitatory and inhibitory postsynaptic potentials.

Action Potentials↗
PubMed1997

Chronotopic fiber reordering and the distribution of cell adhesion and extracellular matrix molecules in the optic pathway of fetal ferrets.

We have examined the age-related reordering of optic axons as they pass through the chiasmatic region in fetal ferrets. Proportions of young and old optic axons were determined from electron micrographs taken sequentially through the prechiasmatic nerve, chiasm, and tract. This "chronotopic" reordering of axons was shown to emerge gradually, beginning rostral to the fusion of the two optic nerves, but continuing to develop caudal to the chiasmatic midline. Segregation of young from old optic axons was most pronounced within the optic tract. We then compared the emergence of this fiber reorganization to the distribution of cell adhesion and extracellular matrix molecules and to the glial architecture within the pathway. Using immunohistochemistry, the distributions of the cell adhesion molecules L1, NCAM, and TAG-1 and the extracellular matrix molecules laminin-1 and chondroitin sulfate proteoglycans (CSPGs) were determined. Among these, only the distribution of CSPGs was observed to change in a manner that complemented the segregation of young from old optic axons. CSPGs were densest in the deeper parts of the optic tract, coincident with radial glial fibers that turn to course within the region of the oldest optic axons. Both the glial architecture and the CSPG distribution form as a consequence of the invasion of the first optic axons, shown by the developmental sequence of each, and by the fact that these glial and molecular features fail to form in the absence of optic axons. The data suggest a model in which the gradient of CSPGs across the depth of the tract contributes to the formation of the chronotopic fiber reordering by providing a relatively unfavorable environment for subsequent axonal growth. The CSPGs may do so by interfering with adhesion molecules on optic axons that normally promote elongation.

Animals↗
PubMed2001

Optic perineuritis: clinical and radiographic features.

BACKGROUND: Optic perineuritis is an uncommon variety of orbital inflammatory disease that is distinct from demyelinating optic neuritis. OBJECTIVE: To describe the clinical and radiographic features of idiopathic optic perineuritis, with particular emphasis on those features that help to distinguish this condition from optic neuritis. METHODS: We reviewed the medical records of 14 patients with optic perineuritis who were seen in 2 neuro-ophthalmology clinics. RESULTS: Patients ranged in age from 24 to 60 years; 5 were older than 50 years. All patients had visual loss, eye pain, or both. The visual acuity was 20/20 or better in 8 of the 15 eyes. The results of visual field testing were normal in 2 eyes, and a paracentral scotoma or an arcuate defect was seen in 7. Magnetic resonance imaging scans demonstrated circumferential enhancement around the optic nerve, sometimes with intraorbital extension. Response to corticosteroids was dramatic; however, 4 patients had a relapse with lowering of the dose. CONCLUSIONS: In contrast to those with optic neuritis, patients with optic perineuritis are often older at onset and are more likely to show sparing of central vision. Magnetic resonance imaging scans demonstrate enhancement around, rather than within, the optic nerve. Response to corticosteroids is more dramatic than in patients with optic neuritis, and patients are more likely to experience recurrence after stopping treatment.

Adult↗
PubMed1981

The aberrant retino-retinal projection during optic nerve regeneration in the frog. II. Anterograde labeling with horseradish peroxidase.

Previous experiments have shown that a substantial number of regenerating optic axons in adult frogs (Rana pipiens) are misrouted into the opposite optic nerve and retina during early stages of regeneration. This projection is maximal at 5 and 6 weeks after optic nerve crush. To further characterize this anomalous projection, small quantities of horseradish peroxidase (HRP) were injected into the right eye or right optic nerve 5 or 6 weeks after right optic nerve crush. Twenty-four hours later the animals were killed and regenerating axons anterogradely filled with HRP were reacted with the tetramethyl-benzidine method or a diaminobenzidine-CoCl2 method. Serial reconstruction tracing the course of individual axons through the optic chiasm showed that few of the axons projecting into the opposite optic nerve were collaterals of axons projecting centrally. Instead, the majority of labeled axons misdirected into the opposite nerve or contributing to an expanded projection into the ipsilateral optic tract turned out of the chiasm without branching. Many of the labeled regenerating axons had unusual trajectories within the chiasm, making abrupt turns or changing their direction of growth. Most of the axons misrouted into the opposite nerve came from portions of the chiasm nearest to the nerve of other eye. In three of eight frogs with an intact optic nerve, a small number of HRP-labeled axons were found in the left nerve after right nerve injection, but there was no indication that these axons reached the left eye. The results from this investigation suggest that the most parsimonious explanation for the chiasmal misrouting of regenerating frog optic axons is that axons are mechanically deflected into inappropriate pathways.

Animals↗
PubMed1981

Axonal guidance during development of the optic nerve: the role of pigmented epithelia and other extrinsic factors.

It is well established that a congenital lack of ocular melanin (albinism) can lead to developmental abnormalities of the central visual pathways. However, it is yet unknown how the pigmentation per se acts to influence formation of the optic projection. In order to study the possible interaction between eye pigment and optic axons during development, we have examined, with the use of serial section techniques, a series of timed embryos at stages when the ocular pigment and outgrowing axons first become apparent. Our results have demonstrated that, in mice and rats, the upper wall of the distal half of the primitive eye stalk (a region which lies along the potential route to be taken by the earliest developing nerve fibers) is transiently pigmented prior to and during the migration of the pioneer optic axons. All outgrowing neurites avoid this stretch of melanotic tissue and instead grow preferentially through a system of extracellular tunnels in the ventral, pigment-free zones of the distal eye stalk. The stalk remains unpigmented from about its midpoint and continuing toward the brain. At the pigment/pigment-free interface many of the axons shift upward from their ventral positions, forming a marginal annulus. In the chick, on the contrary, pigmentation of the stalk does not occur and as the optic axons exit the globe they grow immediately in an annulus configuration. In Xenopus, the entire stalk becomes pigmented and the optic fibers congregate in one discrete bundle of fascicles along the length of the stalk's most ventral margin. These observations suggest that melanin-producing stalk cells may play a role in controlling the topographic patterning of optic fibers within the developing nerve by inhibiting the lateral spread of axonal growth cones into or within their territory. To test this hypothesis we have charted the distribution of optic fibers in the developing optic stalks of timed albino rat embryos. Indeed, as fibers leave the mutant eye, it was found that a small but consistent number of pioneering axons (day E15) become ectopic and immediately invade nonpigmented regions (those normally pigmented and axon-free) in the distal optic stalk. Thus, the usual topographic arrangement of the collection of pioneer optic fibers is altered in the albino.

Animals↗
PubMed1984

Optic axons ignore denervated foreign territory in goldfish tectum.

Ultrastructural morphometry was used to test firstly, whether regenerating optic axons in goldfish tectum will form connections in adjacent denervated foreign territory and secondly, whether intact optic axons will collaterally sprout to innervate this territory. The stratum fibrosum marginale of the tectum was partially denervated by removing the torus longitudinalis and cutting the tectal commissure. Optic axons do not normally synapse in this layer. Thirty days after partial denervation and optic tract section the numbers of normal synapses in a micron 2 column through the stratum fibrosum marginale reached a minimum and then started to increase so that by 159-173 days after the operation they had returned to control levels. Optic terminals, recognised by their degeneration after optic nerve section, did not contribute to these increasing numbers of synapses. Instead, optic axons preferentially reinnervated their normal tectal layers. Similarly, there was no evidence of collateral sprouting of intact optic axons into the partially denervated stratum fibrosum marginale and numbers of normal synapses in the layer also returned to control levels. These probably arose from collateral sprouting of terminals remaining after the partial denervation, raising the possibility that optic axons were prevented from synapsing there by the rapid occupation of vacant sites by other axons. However, delaying the partial denervation with respect to the tract section did not alter the result. These results support the idea of a specific affinity between optic axons and their postsynaptic targets.

Animals↗