SEM shows that each zonule consists of bundles (arrows) of fibrils, which are most apparent next to the lens (L). (Original magnification, 78×.) (c) SEM shows termination of zonular fibrils, which unravel to form a dense meshwork over the capsule that greatly increases the surface area of attachment. A, zonular fiber. (Original magnification, 1600×.)
Vitreous
The vitreous humor is a transparent hydrogel that comprises a portion of the clear ocular media and accounts for up to two‐thirds of globe volume. Anteriorly, the vitreous provides support for the lens as it rests in a shallow concavity (i.e., the patella fossa), while posteriorly, the vitreous abuts the neurosensory retina. As a result, the vitreous functions to transmit light, to maintain the shape of the eye, and to help maintain the normal position of the lens and retina.
Embryologically, the vitreous is composed of three components: (i) primary vitreous (containing the hyaloid artery system); (ii) secondary (definitive, or adult) vitreous; and (iii) tertiary vitreous (lens zonules) (Figure 1.51). The primary, or primitive, vitreous develops first, as the hyaloid artery system courses through it to provide a blood supply to the avascular developing lens. The secondary vitreous then forms around the primary vitreous, leaving the primary vitreous at the central core of the vitreal compartment. The secondary vitreous becomes the definitive, or adult, vitreous. Within the adult vitreous exist several anatomical structures, potential spaces, and connection points between the vitreous and adjacent tissues. The core of the primary vitreous around which the adult vitreous develops is occupied by Cloquet's canal (i.e., the hyaloid canal), and the remnant of the anterior insertion of the hyaloid artery appears as a dense, white, small dot (i.e., Mittendorf's dot) with a variable “corkscrew” tail extending from the posterior pole of the lens.
Figure 1.51 Schematic illustrating the various components of and spaces within the vitreous. The secondary, or adult, vitreous is composed of the cortical and central (intermediate zone) components. Asterisk denotes not a true “membrane.”
Retina
The retina and optic nerve are derivatives of the forebrain; consequently, their morphology and physiology are similar to those of the brain. The nine layers of the neurosensory retina are connected to the brain by the optic nerve and the optic tracts (Figure 1.52). The rods and cones, the primary retinal photoreceptors, comprise a complex layer of specialized cells, which contain photopigments that convert light energy into a series of biochemical events. The RPE furnishes important metabolites to the photoreceptors; it also actively phagocytizes the outermost photoreceptor segments as they are shed during normal outer segment renewal. The retina has one of the highest rates of metabolism of any tissue in the body and receives almost all its nutrition from the retinal and choroidal capillaries.
The function of the retina is to turn light stimuli from the external environment into nervous impulses and transmit this information accurately to the brain, where it is then interpreted as vision. Once photoreceptors are stimulated by light, their release of a neurotransmitter is altered and this response is then received and modified by cells whose nuclei are in the inner nuclear layer (i.e., amacrine cells, bipolar cells, and horizontal cells). The modified message is then transferred to ganglion cells, whose axons form the nerve fiber layer and extend through the optic nerve to targets in brain (including the lateral geniculate nucleus and occipital cortex) (see Chapter 2). Recent studies indicate that a considerable amount of processing of visual impulses occurs within the retina. Classically, 10 layers are described in retinal histology. The neurosensory retina contains nine, and the supportive pigmented epithelium is the tenth layer. Remember that the retina develops from both inner (which invaginates) and outer optic cups. Hence, light and images must pass through the entire neurosensory retina to reach the photoreceptors. The 10 identifiable layers are considered, sclerad to vitread, in the following order: (i) RPE; (ii) photoreceptor layer (rod and cone layer); (iii) outer limiting membrane; (iv) outer nuclear layer; (v) outer plexiform layer; (vi) inner nuclear layer; (vii) inner plexiform layer; (viii) ganglion cell layer; (ix) nerve fiber layer; and (x) inner limiting membrane (Figure 1.53).
Figure 1.52 Relationship between different neuronal cells within the retina. The amacrine cell has a reciprocal inhibitory response onto the bipolar cell from which the information originated and acts to adjust the sensitivity of the ganglion cell synapse after receiving a signal. Horizontal cells interconnect laterally to integrate and regulate input from multiple photoreceptors.
Retinal Pigment Epithelium
The RPE is a monolayer of flat, polygonal cells that forms the outermost layer of the retina. It is the continuation of the outer pigmented epithelial layer of the ciliary body. The RPE is more adherent to the choroid than to the rest of the retinal tissue, and it serves an important role in nutrient transport from the choriocapillaris to the outer layers of the retina. Each cell sends cytoplasmic processes inward to surround the photoreceptor outer segments, which help to filter out excessive amounts of light and increase the photoreceptors' individual sensitivity. They also phagocytize the outer segments of photoreceptors as they are continuously shed. The RPE cells are usually densely pigmented, but there is variability in the intensity of pigmentation among individual animals.
Figure 1.53 The retina consists of nine discrete layers and a supportive pigmented epithelium that forms an outer, tenth layer, as demonstrated by light microscopy in the dog. G, ganglion cell; 1, RPE; 2, photoreceptor layer; 3, outer limiting membrane; 4, outer nuclear layer; 5, outer plexiform layer; 6, inner nuclear layer; 7, inner plexiform layer; 8, ganglion cell layer; 9, nerve fiber layer; 10, inner limiting membrane. The outer and inner limiting membranes are denoted by dashed lines.
Neurosensory Retina
The neurosensory retina varies in thickness, being thickest near the optic disc and tapering toward the ora ciliaris retinae. Ophthalmoscopically it is clear, and any disease usually results in increases in its transparency! The width of all layers decreases toward its periphery from the optic nerve head, but the nerve fiber layer contributes most to the variation in thickness. Most domestic animals have a central retina of approximately 200–240 μm and a peripheral retina of 100–190 μm. In animals with poorly vascularized or avascular retinas, retinal thickness rarely exceeds 140 μm, which is the proposed oxygen diffusion maximum for retinal tissue.
The retinal photoreceptors are the primary visual cells of the eye and are the first‐order neurons (Figure 1.54). Rods function in dim or reduced illumination, and cones function in bright light. The rods allow detection of shapes and motion, while the cones provide sharp visual acuity and color sensitivity. Primates and many avian and reptilian species possess cone‐rich regions completely free of rods; these are
