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Essentials of Veterinary Ophthalmology. Kirk N. Gelatt
Читать онлайн.Название Essentials of Veterinary Ophthalmology
Год выпуска 0
isbn 9781119801351
Автор произведения Kirk N. Gelatt
Жанр Биология
Издательство John Wiley & Sons Limited
Zonular Attachment
The lens is circumferentially suspended from the ciliary body by fibers called zonules. Zonular attachment is achieved by a complex arrangement of fibers that insert onto the lens capsule in a zone encompassing the equator and a short distance both anterior and posterior to the equator (Figure 1.50a and b). Each zonular fiber is made of numerous small fibrils, which are visible under SEM as they attach to the lens capsule. The zonular fibers spread out near the equator and terminate into smaller bundles. Each of these bundles also fans out and forms a network that ramifies over the surface of the lens capsule, approximately 1.5–2.0 mm away from the lens equator.
Figure 1.50 Zonular attachments to the lens in a dog. (a) SEM shows that zonules (Z) extend from the ciliary body onto the equator of the lens (L) in a ringlike manner, covering each ciliary process. (Original magnification, 30×.) (b) 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.