By establishing an avascular, highly elastic, region within the fetal area of high acuity (AHA), the developing primate eye has created a unique substrate on which the mechanical forces of intraocular pressure (IOP) and growth-induced retinal stretch (stretch) can act. We proposed (Springer & Hendrickson, 2004b) that these forces generate both the pit and high cone density found in the adult AHA. In this paper, we use quantitative measures to determine the temporal relationships between nasal and temporal retinal elongation, changes in pit depth, cone packing, and cone morphology over M. nemestrina retinal development. Retinal length increased rapidly to about 105 days postconception (dpc; Phase 1) and then elongation virtually ceased (Phase 2) until just after birth (180 dpc). Retinal elongation due to stretch resumed during Phase 3 until approximately 315 dpc (4-5 months), after which time the retina appeared mature (Phase 4). The pit appeared during the quiescent Phase 2, suggesting that IOP acts, in conjunction with molecular changes in the inner retina, on the highly elastic, avascular, AHA to generate a deep, narrow pit and causes inner retinal cellular displacements. Subsequently (Phase 3), the pit widened, became 50% shallower and central inner retinal lamina thinned slightly due to a small amount of retinal stretch occurring in the AHA. Centripetal movement of cones was minimal until just after birth when the pit reached 88% of its maximal depth. Accelerated cone packing during Phase 3 was temporally correlated with increased stretch.

Most primate retinas have an area dedicated for high visual acuity called the fovea centralis. Little is known about specific mechanisms that drive development of this complex central retinal specialization. The primate area of high acuity (AHA) is characterized by the presence of a pit that displaces the inner retinal layers. Virtual engineering models were analyzed with finite element analysis (FEA) to identify mechanical mechanisms potentially critical for pit formation. Our hypothesis is that the pit emerges within the AHA because it contains an avascular zone (AZ). The absence of blood vessels makes the tissue within the AZ more elastic and malleable than the surrounding vascularized retina. Models evaluated the contribution to pit formation of varying elasticity ratios between the AZ and surrounding retina, AZ shape, and width. The separate and interactive effects of two mechanical variables, intraocular pressure (IOP) and ocular growth-induced retinal stretch, on pit formation were also evaluated. Either stretch or IOP alone produced a pit when applied to a FEA model having a highly elastic AZ surrounded by a less elastic region. Pit depth and width increased when the elasticity ratio increased, but a pit could not be generated in models lacking differential elasticity. IOP alone produced a deeper pit than did stretch alone and the deepest pit resulted from the combined effects of IOP and stretch. These models predict that the pit in the AHA is formed because an absence of vasculature makes the inner retinal tissue of the AZ very deformable. Once a differential elasticity gradient is established, pit formation can be driven by either IOP or ocular growth-induced retinal stretch.

Mechanisms underlying the development of the primate area of high acuity (AHA) remain poorly understood. Finite-element models have identified retinal stretch and intraocular pressure (IOP) as possible mechanical forces that can form a pit (Springer & Hendrickson, 2004). A series of Macaca nemestrina monkey retinas between 68 days postconception (dpc) and adult were used to quantify growth and morphological changes. Retinal and pars plana length, optic disc diameter, disc-pit distance, and inner and outer retinal laminar thickness were measured over development to identify when and where IOP or stretch might operate. Horizontal optic disc diameter increased 500 mum between 115 dpc and 2 months after birth when it reached adult diameter. Disc growth mainly influences the immediate surrounding retina, presumably displacing retinal tissue centrifugally. Pars plana elongation also began at 115 dpc and continued steadily to 3-4 years postnatal, so its influence would be relatively constant over retinal development. Unexpectedly, horizontal retinal length showed nonlinear growth, divided into distinct phases. Retinal length increased rapidly until 115 dpc and then remained unchanged (quiescent phase) between 115-180 dpc. After birth, the retina grew rapidly for 3 months and then very slowly into adulthood. The onset of pit development overlapped the late fetal quiescent phase, suggesting that the major mechanical factor initiating pit formation is IOP, not retinal growth-induced stretch. Developmental changes in the thickness of retinal layers were different for inner and outer retina at many, but not all, of the ten eccentricities examined.

Purpose: Screening for detection of retinopathy of prematurity (ROP) currently is limited to indirect ophthalmoscopy, which requires considerable examiner skill and experience. We investigated whether conventional 10-MHz B-scan ultrasonography could document the clinical stages of ROP as accurately as indirect ophthalmoscopy.Methods: Thirty-four eyes of 18 neonates were examined by masked, independent observers with indirect ophthalmoscopy and digitally recorded 10-MHz B-scan ultrasonography. After pupil dilation and lid speculum placement, the retinologist recorded the stage of retinopathy with a retinal drawing. The ultrasonographer, without use of papillary mydriatics or lid speculum, determined the presence or absence of a ridge or tractional elements, if present on the ridge.Results: Ultrasound grade correlated with clinical grade (R =.79, P <.001). However, nine eyes were overdiagnosed by one stage, and one eye, in which a peripheral detachment was mistaken for an artifact, was underdiagnosed.Conclusions: Ten-megahertz ultrasonography offers the potential of imaging and detecting the clinical stages of ROP; the use of higher ultrasound frequencies, now becoming commercially available, is likely to enhance diagnostic accuracy. Care must be taken to distinguish between artifact and true anatomical structures in noncontact ultrasound examinations. Neonates with suspected ROP could be screened with B-scan ultrasonography by neonatal personnel without pupillary dilatation or lid speculum, thus eliminating potential morbidity, and clinically significant cases of ROP then could be referred to the retinologist.

In humans, an increasing density of foveal cone photoreceptors occurs slowly over several years after birth, and accounts for a region that subserves high visual acuity. Concurrently, inner retinal cells move centrifugally away from the foveal center. Such developmental rearrangements reflect complex cellular remodeling after the retinal neuronal cells have differentiated and have formed synapses. Explaining foveal morphogenesis is difficult, because differentiated neuronal cells seem incapable of moving actively. Presented here is a biomechanical explanation of how the above events occur. This hypothesis assumes that the cellular movements throughout the retinal layers occur passively as the eye grows and the retina is stretched. Retinal stretch was simulated using virtual engineering models that were analyzed with finite element analysis. A pit combined with retinal stretch causes the retinal layers to deform in a way that accounts for both the centrifugal and centripetal movement of various retinal cell types. Axially directed, tensile forces associated with stretching the retinal tissue surrounding the pit also accounts for the elongated morphology of foveal cone photoreceptors. These simulations suggest that a pit is required for both the centripetal displacement of cone cells toward the center of the fovea, and for the elongated foveal cone morphology. Since the primate fovea may have minimal impact on acuity, its primary role may be to initiate foveal morphogenesis in slowly developing eyes.

Tritiated thymidine ([3H]Tdr) was injected either intraocularly (i.o.) or intraperitoneally (i.p.). Autoradiography showed that more retinal cells were labeled by i.o. than by i.p. injection. Controls showed that the increased number of labeled retinal cells was not related to glioblast proliferation produced by trauma from the i.o. injection. After injection of identical doses, the peak concentration of [3H]Tdr available for incorporation into DNA was about 100 times greater after an i.o. than after an i.p. injection. The effective duration of the labeling pulse in goldfish was 9 times longer for an i.o. than an i.p. injection. This study shows that i.o. delivery of [3H]Tdr is preferable to i.p. injection for efficient delivery of label to proliferating retinal cells.

Experiments were designed to find the degree to which regenerated optic axons occupied their previous locations in the optic tracts. Following optic nerve crush and regeneration, either the dorsal, ventral, peripheral, temporal, or nasal part of the retina was ablated. The axons of the remaining retinal ganglion cells (RGCs) were labeled with cobalt. Density of the regenerated dorsal and ventral axons in the dorsal vs. ventral optic tracts was determined digitally. In addition, we determined the density of temporal and nasal axons in the temporal vs. nasal compartments of each optic tract and the density of central axons in the central vs. peripheral compartments of both optic tracts. Regenerated axons were not distributed randomly in the optic tracts. Instead, they were slightly but, significantly biased toward growing through the tract or compartment that they had occupied previously. Still, the pathway specificity exhibited by the regenerated axons was closer to random than it was to the pathway specificity seen in normal animals. Dorsal, ventral, and central RGC axons were significantly better localized to their correct tract or compartment than were temporal or nasal RGC axons. Also, over time, dorsal and ventral axons tended to disappear from incorrectly chosen optic tracts. The slight bias toward choosing the appropriate optic tract or optic tract compartment may be enough to account for the topographic specificity of the regenerated retinotectal projection. Near-randomness of the axonal positions in the tracts argues against the presence of any specific guidance cues in the optic tracts of adult animals. Axonal density was highest in the correct compartment and diminished progressively with increasing distance into the incorrect compartment. Such a gradient of axonal density suggests that regenerating axons "drift" away from their previous positions in the optic pathways.

Labeling severed axons with cobaltous-lysine ultimately leads to the degeneration of their distal segments. The present study was designed to determine whether microglia and radial glia have comparable roles in the elimination of degenerating axons. Another purpose was to determine whether the cobalt could escape from degenerating axons and enter intact neuronal cells. Optic axons were filled with cobaltous-lysine for 1 day and the retinal projections were examined from 1 to 106 days later. Optimal filling was obtained 1 day postlabeling. The number of filled axons in the optic tract was significantly reduced at the 2-day time point, indicating that many axons had disintegrated. Many axons contained large swellings that resembled cells. However, transneuronally labeled neuronal cells were never observed. Labeled, rounded microglia appeared among the degenerating axons at the 3-day time point, and the microglia changed shape at 5 days. They became elongated and manifested many processes. In addition, the microglia began to move toward, and entered, the ventricles and vasculature. Virtually all the labeled debris was removed between 17 and 28 days following the application of cobalt. The rapidity with which the axons were removed suggests that the cobalt accelerates the degenerative process either directly, or indirectly by accelerating the arrival of phagocytic cells. Radial glia appeared to play a smaller role in debris elimination. They took up labeled debris to a lesser degree than microglia and were briefly labeled. Interestingly, radial glia did not take up cobalt when it was injected intracranially and diffused through the brain. A previous claim of an axosomatic retinotectal projection to cells deep in the stratum periventriculare of a teleost fish is reinterpreted to represent cobalt within radial glia.

Cobaltous-lysine applied to the goldfish optic nerve backfilled retinal ganglion cells and their axons. Confined to the ventronasal and ventrotemporal retina was a small population of retinal ganglion cells whose axons traveled dorsally and parallel to the retinal margin. On reaching the boundary between dorsal and ventral retina, the axons arched, joined radially oriented bundles of axons, and traveled toward the optic disk. Control studies showed that the axons came from retinal ganglion cells rather than from retinopetal cells. The somatic area of retinal ganglion cells (RGCs) with circumferential axons was 30-50 microns, and was similar to that of average ganglion cells. The axons of these cells coursed between the optic fiber and ganglion cell layers or between the ganglion cell and inner plexiform layers. Many somata were displaced slightly toward the inner plexiform layer, but were not really displaced ganglion cells. The aberrant axonal trajectory may be related to the slightly displaced location of the cell. However, ganglion cells that are displaced to the edge of the inner nuclear layer usually have radially coursing axons. We digitized the coordinates of the bending points and the dorsoventral retinal boundary. On average, the bending points occurred within 100 microns of the dorsoventral retinal border. These findings suggest that some molecular, rather than mechanical, factor at the dorsoventral retinal boundary alters the course of the circumferential axons. Furthermore, because there are cells with circumferential axons throughout the ventral retina, the data imply that at least ventral RGC axons avoid mingling with the axons from dorsal RGCs.

Cell counts in hematoxylin-stained, flat-mounted retinae revealed that the inner nuclear (INL) and cone mosaic layers (CML) of the goldfish retina contained a high density of cells along the temporal boundary between dorsal and ventral retina. Our findings indicate that the goldfish contains a region situated in the temporal retina in which the density of a wide variety of cell types, that span several retinal layers, is elevated. This study indicates that the dorsotemporal retina, which is the first retinal region to develop, contains the highest density of cells.

The distribution of retinal ganglion cells (RGCs) in goldfish was determined by removing an eye and applying cobaltous-lysine to the optic nerve for 24 hr. This procedure allowed the cobalt label to be in continuous contact with the cut ends of the optic axons and thereby backfilled many RGCs. RGC density was determined across three different sizes of retinae by using fish with different eye sizes. Confirming earlier work, we found that RGC density diminished as retinal area increased. However, irrespective of the retinal size, the density of RGCs was elevated along the temporal boundary between the dorsal and the ventral retina. A conservative estimate indicated that the RGC density in the temporal retina was at least 1.8-2.5 times higher than the mean RGC density of the entire retina. Thus, the goldfish retina does not appear to have a homogeneous distribution of RGCs as was previously considered. Small and large retinae differed with respect to the percentage of cells in the RGC layer that was RGCs. In small retinae, even when the noncobalt-filled cells (glia and displaced amacrine cells) were added to the cobalt-filled RGCs, the density of all cell types was elevated in the temporal retina relative to the remainder of the retina. Furthermore, in small retinae, the percentage of cells in the RGC layer that was RGCs (75%) was constant across the radial and circumferential aspects of the retina. In marked contrast, in medium-large retinae, a homogeneous distribution of cells across the entire retina resulted when the noncobalt-filled cells were added to the cobalt-filled cells. However, the percentage of cells that was cobalt-filled RGCs was significantly greater in the temporal retina (50%) than in the remainder of the retina (35%). In large retinae, as in small retinae, the percentage of cells that was RGCs did not vary as a function of distance from the optic disc. These data suggest that, in the course of retinal maturation, cell density in the temporal retina is elevated initially and then declines subsequently to the level of the surrounding retina. Over time, more displaced to the level of the surrounding retina. Over time, more displaced amacrine cells may be added to the tissue surrounding the temporal retina. Alternatively, more RGCs outside the temporal retina may become displaced amacrine cells. Such events could account for the growth-associated, disproportionate decrease in the percentage of cells that is RGCs in the tissue surrounding the temporal retina.(ABSTRACT TRUNCATED AT 400 WORDS)

The rearrangements that retinal ganglion cell (RGC) axons undergo near the optic chiasm were determined by ablating either the nasal, temporal, dorsal, ventral, or peripheral retina. The axons of the remaining intact RGCs were then labelled with cobaltous lysine. RGC axons change their relationship with respect to the axes of the brain and with respect to one another. Toward the caudal end of the optic chiasm, the optic tract begins to rotate axially such that its rostral edge ultimately becomes located medially. Thereby, the column of ventronasal RGC axons shifts from a rostral to a medial position. In addition, columns of axons from other retinal sectors move with respect to one another. Ventrotemporal RGC axons, located initially at the caudal edge of the tract, move toward and come to be positioned laterally to, the column of ventronasal RGC axons. The column of dorsonasal RGC axons moves from the rostral to the lateral side of the column of dorsotemporal RGC axons. Concurrently, the axons within each column reorganize internally. Each chronological lamina of axons within a column twists such that the nasal and temporal axons within each column invert their positions with respect to the edges of the column. All of these reorganizations take place between the caudal end of the optic chiasm and the division of the main optic tract into the optic brachia. Furthermore, the rearrangements that occur do not involve any alterations in the positions of central and peripheral RGC axons with respect to the surface of the diencephalon. The results are discussed with respects to mechanisms that might influence the organization of the visual pathways.