Monkey Optic Nerve Research Articles

A Workflow for Three-Dimensional Reconstruction and Quantification of the Monkey Optic Nerve Head Vascular Network.

A comprehensive characterization of the three-dimensional (3D) vascular network of the optic nerve head (ONH) is critical to understanding eye physiology and pathology. Current in vivo imaging technologies, however, do not have simultaneous high spatial resolution and imaging depth to resolve the small vessels deep within the ONH. We describe a workflow for the 3D reconstruction and quantitative morphological analysis of the ONH vasculature. The vessels of a normal monkey ONH were perfusion labeled. Serial cryosections of the ONH were imaged using fluorescence microscopy (FM) and instant polarized light microscopy (IPOL) to visualize the labeled vessels and label-free collagen, respectively. The IPOL images were registered and used to form a stack of FM images from which the vessels were segmented and skeletonized to reconstruct the 3D vascular network. The network consisted of 12,966 vessel segments, 7989 branching points, and 1100 terminal points at the boundaries. For each vessel segment, we measured its length, tortuosity, inclination (θ), and polar orientation (φ). The length followed a lognormal distribution, whereas the distribution of the tortuosity followed an exponential decay. The vessels were mainly oriented toward the coronal plane (θ = 90 deg). For orientation, there were nearly as many vessels aligned circumferentially (φ = 90 deg) and radially (φ = 0 deg). Our results demonstrate the workflow for 3D eye-specific reconstruction and quantification of the monkey ONH vascular network. This is a critical first step to analyze the blood flow and oxygenation within the ONH, which will help understand the role of vascular dysfunction in glaucoma.

Wilbrand's Knee: To Be or Not to Be a Knee?

Wilbrand's knee of the optic chiasm refers to crossing fibers from one optic nerve that stray for a short distance into the opposite optic nerve before joining the optic tract. This loop of aberrant axons, although small, has generated much controversy. In a previous study, labeling of the optic pathway in normal monkeys with a radioactive tracer revealed no Wilbrand's knee. Monocular enucleation induced a typical knee to form. These findings suggested that Wilbrand's knee is absent normally, but appears after atrophy of one optic nerve. This conclusion has been challenged by images showing Wilbrand's knee in the normal human chiasm using anisotropic light scattering. It has also been resisted by some clinicians who believe that Wilbrand's knee is necessary to explain the anterior chiasmal syndrome. Early in his distinguished career, William F. Hoyt examined the fiber organization of the monkey optic nerve and chiasm. He found no evidence for Wilbrand's knee and rejected its importance for the topical diagnosis of chiasmal lesions. His conclusion is supported by new data showing that anisotropic light scattering is not a reliable method for tracing axons. Hence, that method has given a misleading impression that Wilbrand's knee exists in normal subjects. Although Wilbrand's knee has fascinated neuro-ophthalmologists for more than a century, it is an inconsequential structure that develops only after a longstanding monocular optic neuropathy.

Cupping in the Monkey Optic Nerve Transection Model Consists of Prelaminar Tissue Thinning in the Absence of Posterior Laminar Deformation.

PurposeTo use optical coherence tomography (OCT) to test the hypothesis that optic nerve head (ONH) “cupping” in the monkey optic nerve transection (ONT) model does not include posterior laminar deformation.MethodsFive monkeys (aged 5.5–7.8 years) underwent ONH and retinal nerve fiber layer (RNFL) OCT imaging five times at baseline and biweekly following unilateral ONT until euthanization at ∼40% RNFL loss. Retinal nerve fiber layer thickness (RNFLT) and minimum rim width (MRW) were calculated from each pre- and post-ONT imaging session. The anterior lamina cribrosa surface (ALCS) was delineated within baseline and pre-euthanasia data sets. Significant ONT versus control eye pre-euthanasia change in prelaminar tissue thickness (PLTT), MRW, RNFLT, and ALCS depth (ALCSD) was determined using a linear mixed-effects model. Eye-specific change in each parameter exceeded the 95% confidence interval constructed from baseline measurements.ResultsAnimals were euthanized 49 to 51 days post ONT. Overall ONT eye change from baseline was significant for MRW (−26.2%, P = 0.0011), RNFLT (−43.8%, P < 0.0001), PLTT (−23.8%, P = 0.0013), and ALCSD (−20.8%, P = 0.033). All five ONT eyes demonstrated significant eye-specific decreases in MRW (−23.7% to −31.8%) and RNFLT (−39.6% to −49.7%). Four ONT eyes showed significant PLTT thinning (−23.0% to −28.2%). The ALCS was anteriorly displaced in three of the ONT eyes (−25.7% to −39.2%). No ONT eye demonstrated posterior laminar displacement.ConclusionsSeven weeks following surgical ONT in the monkey eye, ONH cupping involves prelaminar and rim tissue thinning without posterior deformation of the lamina cribrosa.

Chemoarchitecture of glial fibrillary acidic protein (GFAP) and glutamine synthetase in the optic nerve of the monkey (&lt;i&gt;Macaca fuscata&lt;/i&gt;): An immunohistochemical study

An immunohistochemical analysis of the chemoarchitecture of glial fibrillary acidic protein (GFAP) and glutamine synthetase (GS) was conducted in the monkey optic nerve. The optic nerve has been divided into 3 regions: the prelaminar, lamina cribrosa, and retrolaminar regions. However, it currently remains unclear whether the chemoarchitecture of GFAP and GS is homogeneously organized, especially in the retrolaminar region. Strong-to-moderate GFAP immunoreactivity was observed in all 3 regions. The retrolaminar region was further divided into anterior (RLa) and posterior (RLp) retrolaminar regions. More GFAP immunoreactive punctations were observed in the RLa region than in the RLp region. Regarding GS immunoreactivity, moderately GS immunoreactive glial cells were observed in the prelaminar and retrolaminar regions. In the retrolaminar region, there were more of these cells in the RLa region than in the RLp region. GS immunoreactivity was markedly weaker in the prelaminar and retrolaminar regions than in the retina. Thus, the chemoarchitecture of GFAP and GS was heterogeneously organized in the retrolaminar region, and the RLa region was the main GS distribution site in the retrolaminar region. Since GS is a key enzyme of glutamate metabolism, these results may provide clues as to how glutamate is metabolized in the primate optic nerve.

Effect of Acute Intraocular Pressure Elevation on the Monkey Optic Nerve Head as Detected by Spectral Domain Optical Coherence Tomography

To determine whether acutely elevated IOP alters optic nerve head (ONH) structural parameters characterized in vivo using spectral domain optical coherence tomography (SD-OCT). Five rhesus macaques were tested under isoflurane anesthesia. SD-OCT images of the ONH of both eyes were acquired 30 minutes after IOP was stabilized to 10 mm Hg and 60 minutes after stabilization to 45 mm Hg. The internal limiting membrane, Bruch's membrane/retinal pigment epithelium, neural canal opening (NCO), and anterior lamina cribrosa surface (ALCS) were delineated using custom software. Differences in SD-OCT structural parameters between the two IOP levels were assessed using generalized estimating equations. In six eyes of three animals, images were acquired after 10 minutes and 30 minutes of IOP stabilization to 10 mm Hg (control experiment). Acute IOP elevation resulted in a reduction in prelaminar tissue thickness (mean, -47 μm; SD, 25 μm; P = 0.002), rim volume (-0.05 mm(3), 0.02 mm(3); P = 0.002), rim width (-30 μm, 7 μm; P = 0.002), and in an increase in NCO depth (38 μm, 15 μm; P = 0.002). An increase in ALCS depth was significant relative to peripheral Bruch's membrane (48 μm, 24 μm; P = 0.002) but not relative to the NCO. No significant parameter changes were detected in the control eyes. Surface compliance changes in the normal monkey ONH primarily reflect prelaminar and peripapillary deformation. SD-OCT compliance testing will further our understanding of the effects of IOP on the ONH and help improve and validate numerical models of ONH biomechanics.

Deformation of the Early Glaucomatous Monkey Optic Nerve Head Connective Tissue after Acute IOP Elevation in 3-D Histomorphometric Reconstructions

To retest the hypothesis that monkey ONH connective tissues become hypercompliant in early experimental glaucoma (EEG), by using 3-D histomorphometric reconstructions, and to expand the characterization of EEG connective tissue deformation to nine EEG eyes. Trephinated ONH and peripapillary sclera from both eyes of nine monkeys that were perfusion fixed, with one normal eye at IOP 10 mm Hg and the other EEG eye at 10 (n=3), 30 (n=3), or 45 (n=3) mm Hg were serial sectioned, 3-D reconstructed, 3-D delineated, and quantified with 3-D reconstruction techniques developed in prior studies by the authors. Overall, and for each monkey, intereye differences (EEG eye minus normal eye) for each parameter were calculated and compared by ANOVA. Hypercompliance in the EEG 30 and 45 eyes was assessed by ANOVA, and deformations in all nine EEG eyes were separately compared by region without regard for fixation IOP. Hypercompliant deformation was not significant in the overall ANOVA, but was suggested in a subset of EEG 30/45 eyes. EEG eye deformations included posterior laminar deformation, neural canal expansion, lamina cribrosa thickening, and posterior (outward) bowing of the peripapillary sclera. Maximum posterior laminar deformation and scleral canal expansion co-localized to either the inferior nasal or superior temporal quadrants in the eyes with the least deformation and involved both quadrants in the eyes achieving the greatest deformation. The data suggest that, in monkey EEG, ONH connective tissue hypercompliance may occur only in a subset of eyes and that early ONH connective tissue deformation is maximized in the superior temporal and/or inferior nasal quadrants.

Deformation of the Normal Monkey Optic Nerve Head Connective Tissue after Acute IOP Elevation within 3-D Histomorphometric Reconstructions

To characterize optic nerve head (ONH) connective tissue deformation after acute (15 or 30 minutes) intraocular pressure (IOP) elevation in six adult normal monkeys using three-dimensional (3-D) histomorphometry. Trephined ONH and peripapillary sclera from both eyes of six monkeys, each perfusion fixed with one eye at IOP 10 mm Hg (IOP-10) and the other at IOP 30 or 45 mm Hg (IOP-30 or IOP-45, by anterior chamber manometer), were serially sectioned, 3-D reconstructed, 3-D delineated, and quantified according to standard parameters. For each monkey, intereye differences (high-IOP eye minus IOP-10) for each parameter were calculated and compared by ANOVA and EPIDmax both overall and regionally. EPIDmax deformations for each parameter were defined to be those statistically significant differences that exceeded the maximum physiologic intereye difference within six bilaterally normal monkeys in a previous report. Regional EPIDmax laminar thinning, posterior bowing of the peripapillary sclera, and thinning and expansion of the scleral canal were present in most high-IOP eyes and were colocalized in those demonstrating the most deformation. Laminar deformation was minimal, not only posteriorly but in some cases anteriorly in the high-IOP eyes. No increase in deformation was seen in the IOP-45 versus the IOP-30 eyes. ONH connective tissue alterations after acute IOP elevation involve regional thinning, stretching, and deformation of the lamina cribrosa and peripapillary sclera that are minimal to modest in magnitude. The time-dependent character of these alterations and their compressive, expansile, and shear effects on the axons, the astrocytes, and the laminar and posterior ciliary circulations remain to be determined.

Immunohistochemistry of the monkey retina with a monoclonal antibody against subunit V of cytochrome c oxidase.

A monoclonal antibody against subunit V of cytochrome c oxidase was newly prepared from purified bovine heart muscle enzyme. This antibody showed cross-reactivity against the bovine and the human cytochrome c oxidase on Western blot. Cytochrome c oxidase in paraffin embedded sections of monkey retinas and optic nerves was examined immunohistochemically using the monoclonal antibody. The inner segment of cone and rod photoreceptors, the inner nuclear layer and the inner plexiform layer showed marked reactivities. The optic nerve fiber axons also showed high reactivities, with more in the unmyelinated fibers peripheral to the lamina cribrosa. These immunohistochemical results using the antibody are mostly agreeable with previously reported histochemical localization of cytochrome c oxidase activity in the retina, but there were some noticeable differences in the plexiform layers.

Physiologic Intereye Differences in Monkey Optic Nerve Head Architecture and Their Relation to Changes in Early Experimental Glaucoma

To characterize physiologic intereye differences (PIDs) in optic nerve head (ONH) architecture in six normal rhesus monkeys and compare them to intereye differences in three previously reported cynomolgus monkeys with early experimental glaucoma (EEG). Trephinated ONH and peripapillary sclera from both eyes of six normal monkeys were serial sectioned, 3-D reconstructed, 3-D delineated, and parameterized. For each normal animal and each parameter, PID was calculated (both overall and regionally) by converting all left eye data to the right eye configuration and subtracting the right eye value from that of the left eye. Physiologic intereye percent difference (PIPD) was calculated as the PID divided by the measurement mean of the two eyes. For each EEG monkey, intereye (EEG minus normal) differences and percent differences for each parameter overall and regionally were compared to the PID and PIPD maximums. For all parameters the PID maximums were relatively small overall. Compared to overall PID maximums, overall intereye differences in EEG monkeys were greatest for laminar deformation and thickening, posterior scleral canal enlargement, cupping, and prelaminar neural tissue thickening. Compared with the regional PID maximums, the lamina cribrosa was posteriorly deformed centrally, inferiorly, inferonasally, and superiorly and was thickened centrally. The prelaminar neural tissues were thickened inferiorly, inferonasally, and superiorly. These data provide the first characterization of PID and PIPD maximums for ONH neural and connective tissue parameters in normal monkeys and serve to further clarify the location and character of early ONH change in experimental glaucoma. However, because of the species differences, the findings in EEG should be confirmed in EEG rhesus monkey eyes.

3-D Histomorphometry of the Normal and Early Glaucomatous Monkey Optic Nerve Head: Prelaminar Neural Tissues and Cupping

To introduce a three-dimensional (3-D) histomorphometric strategy for characterizing the connective tissue (laminar) and prelaminar neural tissue (prelaminar) components of optic nerve head (ONH) cupping in one bilaterally normal monkey and three monkeys with early experimental glaucoma (EG) in one eye. Trephined ONH and peripapillary sclera from both eyes of four monkeys were serially sectioned at either 3-mum thickness (three EG monkeys) or 1.5-microm thickness (the bilaterally normal monkey) with the embedded tissue block face stained and imaged after each cut. Digital section images were aligned and stacked to create a 3-D reconstruction of each ONH. Within 40 digital radial sagittal sections of each reconstruction, Bruch's membrane opening (BMO), the neural canal wall, and the anterior laminar surface were delineated by two delineators. The 80 BMO points were used to establish a BMO-zero reference plane. The parameters prelaminar tissue volume, post-BMO cup (the estimate of the clinical cup), and post-BMO total prelaminar volume (a global measure of ONH connective tissue deformation) were calculated overall and within 15 degrees radial regions. The parameter prelaminar tissue thickness was calculated at each delineated anterior laminar surface point. For each monkey, an intra-animal difference map was generated for each parameter. Overall volume and thickness data were compared between normal and EG eyes by analysis of variance (ANOVA). Regionally variable expansion of post-BMO cup volume and post-BMO total prelaminar volume were present in all three EG eyes and far exceeded the intra-animal, physiologic differences for these parameters in the bilaterally normal monkey. Prelaminar tissue thickness was increased in all three EG monkeys, with the greatest effects present within the peripheral regions of the canal. These data suggest that in young adult monkeys with more compliant connective tissues, clinical cupping in early glaucoma is primarily due to fixed deformation of the ONH connective tissues and occurs in the setting of prelaminar tissues that are thickened rather than thinned.

3-D Histomorphometry of the Normal and Early Glaucomatous Monkey Optic Nerve Head: Lamina Cribrosa and Peripapillary Scleral Position and Thickness

To delineate three-dimensionally the anterior and posterior surfaces of the lamina cribrosa, scleral flange, and peripapillary sclera, to determine the position and thickness of these structures within digital three-dimensional (3-D) reconstructions of the monkey optic nerve head (ONH). The trephinated ONH and peripapillary sclera from both eyes of three monkeys with early glaucoma (EG; one eye normal, one eye given laser-induced EG) were serially sectioned at 3-mum thickness, with the embedded tissue block's face stained and imaged after each cut. Images were aligned and stacked to create 3-D reconstructions, within which Bruch's membrane opening (BMO) and the anterior and posterior surfaces of the lamina cribrosa and peripapillary sclera were delineated in 40 serial radial (4.5 degrees interval) digital sagittal sections. For each eye, a BMO zero reference plane was fit to the 80 BMO points, which served as the reference from which all position measurements were made. Regional laminar, scleral flange, and peripapillary scleral position and thickness were compared between the normal and EG eyes of each monkey and between treatment groups by analysis of variance. Laminar thickness varied substantially within the normal eyes and was profoundly thicker within the three EG eyes. Laminar position was permanently posteriorly deformed in all three EG eyes, with substantial differences in the magnitude and extent of deformation among them. Scleral flange and peripapillary scleral thickness varied regionally within each normal ONH with the scleral flange and peripapillary sclera being thinnest nasally. Overall, the scleral flange and peripapillary sclera immediately surrounding the ONH were posteriorly displaced relative to the more peripheral sclera. Profound fixed posterior deformation and thickening of the lamina are accompanied by mild posterior deformation and thinning of the scleral flange and peripapillary sclera at the onset of confocal scanning laser tomography (CSLT)-detected ONH surface change in young adult monkey eyes with early experimental glaucoma.

Three-Dimensional Histomorphometry of the Normal and Early Glaucomatous Monkey Optic Nerve Head: Neural Canal and Subarachnoid Space Architecture

To delineate three dimensionally the neural canal landmarks-Bruch's membrane opening (BMO), anterior sclera canal opening (ASCO), anterior laminar insertion (ALI), posterior laminar insertion (PLI), and posterior scleral canal opening (PSCO)-and the anterior-most aspect of the subarachnoid space (ASAS), within digital three-dimensional (3-D) reconstructions of the monkey optic nerve head (ONH). The trephinated ONH and peripapillary sclera from both eyes of three early glaucoma (EG) monkeys (one eye normal, one eye with laser-induced EG) were serial sectioned at 3-microm thickness, with the embedded tissue block face stained and imaged after each cut. The images were aligned and stacked in a 3-D volume, within which the BMO, ASCO, ALI, PLI, PSCO, and ASAS were delineated in 40 digital, radial, and sagittal sections. An ellipse was fitted to the 80 BMO points to establish a BMO zero reference plane, on which all other points were projected. The distance from each projected point to the BMO centroid (offset) and BMO zero reference plane (depth) were calculated and compared regionally between normal and EG eyes, both overall and within each monkey, by analysis of variance. BMO was the clinically visible optic disc margin in all six eyes. The neural canal architecture was highly variable in the three normal eyes. Radial expansion of the neural canal was greatest posteriorly in the EG eyes. Axial elongation of the canal was less pronounced overall but was regionally present within all three EG eyes. ASAS was regionally radially expanded and anteriorly displaced within two of the three EG eyes. Profound deformation of the neural canal and ASAS architecture are present in young adult monkey eyes at the onset of ONH surface change in early experimental glaucoma.

Three-Dimensional Reconstruction of Normal and Early Glaucoma Monkey Optic Nerve Head Connective Tissues

To introduce high-resolution, digital three-dimensional (3-D) reconstruction of the connective tissues of the optic nerve head (ONH). Trephinated ONH and peripapillary sclera from both eyes of three monkeys with early glaucoma (EG; one eye normal, one eye given laser-induced EG) were embedded in paraffin and serial sectioned at 3-mum thickness from the vitreous surface through the orbital optic nerve, with the embedded tissue block face stained and imaged after each cut. Each image was aligned, and then the scleral canal wall, sclera, border tissue of Elschnig, Bruch's membrane, lamina cribrosa, optic nerve septa, pial sheath, and vasculature were delineated as unique objects. Delineated images were stacked, color mapped, and volume rendered and then serial sagittal and transverse digital sections of the resultant voxel geometries were viewed and measured. Substantial differences in the 3-D architecture of the peripapillary sclera, scleral canal wall, and lamina cribrosa were present among the three normal eyes. All three EG eyes displayed permanent posterior deformation of the central lamina cribrosa, as well as expansion of the anterior and posterior neural canal openings in comparison with their respective contralateral normal control eyes. Peripherally, whereas laminar deformation was greatest inferiorly or superiorly in all three EG eyes, statistically significant deformation was present in all four quadrants of all three eyes. High-resolution, digital 3-D reconstructions of the load-bearing connective tissues of the monkey ONH confirm that the ONH connective tissues are profoundly altered at the onset of detectable ONH surface change in experimental glaucoma.

Effects of age on the glial cells in the rhesus monkey optic nerve

The optic nerve is a circumscribed white matter tract consisting of myelinated nerve fibers and neuroglial cells. Previous work has shown that during normal aging in the rhesus monkey, many optic nerves lose some of their nerve fibers, and in all old optic nerves there are both myelin abnormalities and degenerating nerve fibers. The present study assesses how the neuroglial cell population of the optic nerve is affected by age. To address this question, optic nerves from young (4-10 years) and old (27-33 years) rhesus monkeys were examined by using both light and electron microscopy. It was found that with age the astrocytes, oligodendrocytes, and microglia all develop characteristic cytoplasmic inclusions. The astrocytes hypertrophy and fill space vacated by degenerated nerve fibers, and they often develop abundant glial filaments in their processes. Oligodendrocytes and microglial cells both become more numerous with age, and microglial cells often become engorged with phagocytosed debris. Some of the debris can be recognized as degenerating myelin, and in general, the greater the loss of nerve fibers, the more active the microglial cells become.

Distribution and Expression of Transforming Growth Factor- β and Platelet-derived Growth Factor in the Normal and Glaucomatous Monkey Optic Nerve Heads

Purpose: Remodeling of the extracellular matrix occurs in the lamina cribrosa in progressed glaucomatous optic nerve damage including disc cupping. We examined immunohistochemical changes in the transforming growth factor (TGF)-β and platelet-derived growth factor (PDGF) in the optic nerve head in an experimentally induced glaucoma model. Methods: We used 3 cynomolgus and 2 Japanese monkey eyes. Glaucoma was induced by repeated argon laser photocoagulation of the chamber angle. Eyes were enucleated after disc cupping had formed 3 to 5 months after photocoagulation. The optic nerve head was examined for expression of TGF-β1, -β2 and -β3 and PDGF-A and -B in frozen sections and by the biotin ExtraAvidin-alkali phosphatase method. Results: Normal monkey eyes showed TGF-β1, -β2 and -β3, and PDGF-A and -B in the optic nerve head including the nerve fibers, glial cells, and vascular cells. Glaucomatous eyes showed stronger expression of TGF-β1 and -β2 in the glial cells around the lamina cribrosa. The staining intensities for TGF-β3, PDGF-A and -B were the same as in normal eyes. Conclusions: Eyes with experimental glaucoma showed higher expression of TGF-β1 and -β2 around the lamina cribrosa. These findings may show upregulation of extracellular matrix production as related to remodeling of the lamina cribrosa in glaucoma.

Vascular and Glial Changes in the Retrolaminar Optic Nerve in Glaucomatous Monkey Eyes

Vascular and glial changes of the retrolaminar optic nerve were studied in monkey eyes with increased intraocular pressure (IOP) from 1 to 4 years and with different stages of optic nerve atrophy. In histological cross-sections of retrolaminar optic nerves of 11 rhesus and 6 cynomolgus monkeys the entire area, number of axons and vessels and area of pial septa were quantitated and three different kinds of nerve degeneration classified. Ultrathin sections of these different stages were performed and the number of open and occluded vessels was determined. In addition, in cynomolgus monkey optic nerves immunohistochemical staining for αB-crystallin, glial fibrillary acidic protein (GFAP) and vimentin was performed. Even in animals with the same duration of glaucoma and comparable mean IOP values the axon degeneration varied considerably. Independently of axon loss the number of capillaries in the rhesus monkeys remained constant, whereas there was a slight decrease in the cynomolgus monkeys. Some of the vessels, especially in the most severely damaged regions, were occluded. The density of glial cells increased whereas the total number remained nearly constant. In control sections all astrocytes stained for GFAP and αB-crystallin. In the glaucomatous optic nerves the density of αB-crystallin- and GFAP-positive cells was significantly increased. The vascular reaction in the retrolaminar glaucomatous optic nerves differs from that described in the prelaminar region. We assume that in the postlaminar region in areas with diminished nutritional needs vessels occlude and finally degenerate.

Effects of age on nerve fibers in the rhesus monkey optic nerve.

During normal aging there is a reduction in white matter volume in the cerebral hemispheres and structural abnormalities in myelin in some parts of the central nervous system, but whether nerve fibers are lost with age and whether the myelin changes are ubiquitous is not known. Studying the optic nerve, which is a circumscribed bundle of nerve fibers, offers an opportunity to gain further insight into the effects of normal aging on white matter. The present study examined the optic nerves from young (4-10 years) and old (27-33 years) rhesus monkeys using light and electron microscopy. These nerves had been perfused transcardially to obtain optimal preservation of the tissue. Varying degrees of degeneration were encountered in all the optic nerves from the old monkeys. The changes included myelin abnormalities, similar to those reported in other parts of the central nervous system; the presence of degenerating axons and their sheaths; changes in neuroglial cells; and thickening of the trabeculae of connective tissue in the nerve. The total number of nerve fibers was reduced from an average of 1.6 x 10(6) in the young optic nerves to as few as 4 x 10(5) in one old monkey, and with one exception in all of the old optic nerves the packing density of nerve fibers was less than in any of the young optic nerves. The degenerative changes were most marked in those optic nerves that contained the fewest nerve fibers.

Morphological and quantitative analysis of the fascicular pattern of monkey optic nerve.

Fascicular formation by processes of astrocytes was investigated immunohistochemically with an antibody against glial fibrillary acidic protein in the monkey optic nerve. Fascicular patterns were analyzed using an image analyzer. A steep decrease in total fascicular number, following "fusion" of small fascicles, was seen in the rostral part of the optic nerve; this became more gradual in the caudal part of the optic nerve. Just behind the eyeball, most of the fascicles were small (75.4% of all fascicles, n=958); these decreased sharply in number (16.7% of all fascicles) in the rostral part of the optic nerve. In contrast, the numbers of the middle-sized, large, and largest fascicles increased gradually in the rostral part of the optic nerve. The fascicular number in each fascicular group was constant at the caudal part of the optic nerve, except for the largest fascicles, which varied in number throughout the optic nerve. The change in fascicular size along the optic nerve was strikingly similar to that of the fiber dispersion. Distribution patterns of the small and middle-sized fascicles were different from that of the largest fascicles, which were preferentially distributed at the periphery of the optic nerve. The pattern of fascicular distribution according to size seemed to be related to the centroperipheral retinotopic fiber order.

Differences in dispersion of monkey optic nerve fibers according to their terminal layers in the lateral geniculate nucleus

Dispersion of retinogeniculate fibers in the monkey optic nerve was investigated in relation to their terminal layers in the lateral geniculate nucleus (LGN) using a retrograde axonal tracer (wheat germ agglutinin conjugated to horseradish peroxidase, WGA-HRP). Dispersion of fibers projecting to the parvocellular layer of the LGN (LGNp) is clearly less extensive compared with that of fibers projecting to the magnocellular layer of the LGN (LGNm) in the most distal part of the optic nerve. The occupation rate of retrogradely labeled fibers projecting from the periphery of the nasal retina to the LGNp increases only 1.2 times in the initial part of the nerve. In contrast, the occupation rate increases up to 1.6 times for labeled fibers projecting from a similar part of the nasal peripheral retina to the LGNm in the same part of the optic nerve. The morphological features of the labeled ganglion cells indicate that these fibers arise from Pβ and Pa ganglion cells, respectively.

Optic Nerve and Peripapillary Choroidal Microvasculature in the Primate

Using methylmethacrylate corrosion microvascular castings, the three-dimensional angioarchitecture of the monkey optic nerve and peripapillary choroid was examined with scanning electron microscopy. The centripetal blood supply to the optic nerve is from a peripheral arterial supply (branches of the short posterior ciliary arteries) toward the central venous drainage (the central retinal vein). Several choroidal branches of the short posterior ciliary arteries also form around the optic nerve a marginal arterial ring, which supplies blood to the peripapillary choroid. Large choroidal veins emerge ~300 (mum away from the margin of the optic nerve and drain this region. Comparison of the anterior optic nerve vasculature of the primate and nonprimate are discussed, and controversies regarding the choroidal involvement in the optic nerve blood supply are revisited.