Introduction
The primary purpose of the corneoscleral and uveal coats of the eye is to provide protection to the retina in addition to providing nourishment and enabling ocular movement. The retina is derived embryologically from the optic vesicle, an outpouching of the embryonic forebrain. The bilayered neuroepithelial structure of the mature retina reflects the apex-to-apex arrangement of the original optic cup. It also forms the wall of a cavity, the vitreous cavity, which is filled with glycosaminoglycans and collagen. The ocular cavity is homologous to a leptomeningeal cistern in that both vitreous and choroid are derived from mesenchyme that sandwiches the neuroepithelium on its path away from the brain. The ocular neuroepithelial cyst has two openings. Anteriorly lies the pupil, which is a full-thickness aperture, and posteriorly lies the optic nerve in which, similar to a coloboma, only derivatives of the inner retinal layers are found. Because the cell apices are oriented inwardly, the two layers of the optic cup and their derivatives are enveloped externally by basement membrane ( Fig. 6.1.1 ).
The relationship of the epithelial layers to each other is modified from anterior to posterior. Anterior to the ora serrata, the pigmented and nonpigmented epithelia of the iris and ciliary body are joined at their apices by a system of intercellular junctions ( Fig. 6.1.2 ), which is continuous with the external limiting layer of the neural retina and the apical junctional girdles of the retinal pigment epithelium (RPE; Fig. 6.1.3 ). At the ora serrata, the pigmented epithelium is continued as RPE; its basement membrane becomes Bruch’s membrane. The nonpigmented epithelium of the ciliary body and pars plana is continued posteriorly as the neural retina; its basement membrane becomes the internal limiting membrane. The union of the epithelial layers delimits the anterior cul-de-sac of the subretinal space at the ora serrata.
The apex-to-apex arrangement between the epithelia that clearly exists anterior to the ora is continued posteriorly by Müller cells that face and intermittently contact the RPE (see Fig. 6.1.1 ). Here, the contact is maintained not by apical junctions (even though an interreceptor matrix exists) but by the pressure of the vitreous and by suction forces of the RPE. Müllerian glia are the main structural cells of the neural retina and are found throughout the retina from the ora to the optic nerve head.
At the optic nerve head, the internal limiting membrane continues as the basement membrane of Elschnig, supported by the glial meniscus of Kuhnt ( Fig. 6.1.4 ). The (glial) external limiting membrane joins the apices of the RPE to form the posterior cul-de-sac of the subretinal space at the optic nerve, which is delimited by a glial border tissue, the intermediary border tissue of Kuhnt. This border tissue continues posteriorly at the choroidal level as the border tissue of Elschnig; both tissues separate the outer retina and choroid from the axons of the inner retina. The axons, in turn, fixate the posterior retina to the scleral lamina cribrosa and its glial system. The retina, therefore, is fixed to the choroid directly by the apical junctional system at the ora serrata (anterior cul-de-sac of the subretinal space) and indirectly, via the ciliary body and choroid, to its attachments at the scleral spur and sclera. At the nerve head, all neuroepithelial and choroidal layers are fixed by both the junctional tissues and the exiting axons. The corneoscleral coat protects, moves, and holds the retina in the appropriate position and allows the object of regard to be focused on the center of the retina.
Center of the Macula: Umbo
The fovea represents an excavation in the retinal center and consists of a margin, a declivity, and a bottom ( Fig. 6.1.5 ). The bottom corresponds to the foveola, the center of which is called the umbo. The umbo represents the precise center of the macula, the area of retina that results in the highest visual acuity. Usually, it is referred to as the center of the fovea or macula. Although both terms are commonly used clinically, neither is a precise anatomical designation.
The predominant photoreceptor of the foveola and umbo is the cone. The foveal “nuclear cake” results from the centripetal migration of the photoreceptors and the centrifugal lateral displacement of the bipolars and ganglion cells during foveal maturation, which occurs 3 months before and 3 months after term. Although their individual diameters are narrowed because of extreme crowding, central cones maintain their volume through elongation, up to a length of 70 µm. The central packing of cones takes place in an area of 1500 µm diameter. The greatest concentration of cones is found in the umbo, an area of 150–200 µm diameter, referred to as the central bouquet of cones. Estimates of central cone density are 113 000 and 230 000 cones/mm 2 in baboons and cynomolgus monkeys, respectively. For the central bouquet, the density of cones may be as high as 385 000 cones/mm 2 . The inner cone segments (myoids) are connected laterally by a junctional system, the external limiting membrane. Their inner fibers (axons) travel radially and peripherally as fibers of Henle in the outer plexiform layer ( Fig. 6.1.6 ). As a result of their high concentration and crowding, the central cones have their nuclei arranged in multiple layers in a circular shape, which resembles a cake ( gateau nucleaire ).
