Mammalian Eye Image

Mammalian Eye

FIGURE 3.12 Structure of the mammalian eye. The outer connective tissue capsule of the eye is the sclera, which grades into the cornea on the anterior side of the eye. The conjunctiva is a mucus membrane that covers the sclera. The iris (which includes its unseen part, the ora serrata) and lens form a partition between the anterior and posterior chambers of the eye. Muscles in the ciliary body control the configuration of the lens for focus. The anterior chamber is filled with an aqueous solution akin to blood plasma; the posterior chamber is filled with a gel-like vitreous body of hyaluronic acid and a complex protein called vitrein. The iris is a continuation of the pigmented (outer) layer of the retina. The inner layers of the retina are neural layers. The axons of the optic nerve originate in the ganglion layer of the neural retina and converge at the back of the eye, where they exit on their way to the brain. Reproduced with permission, courtesy of Dr. K. W. Condon, http://anatomy. iupui.edu/courses/histo_D502/D502f04/lecture.f04/Eyef04/1.jpg.

FIGURE 3.12 Structure of the mammalian eye. The outer connective tissue capsule of the eye is the sclera, which grades into the cornea on the anterior side of the eye. The conjunctiva is a mucus membrane that covers the sclera. The iris (which includes its unseen part, the ora serrata) and lens form a partition between the anterior and posterior chambers of the eye. Muscles in the ciliary body control the configuration of the lens for focus. The anterior chamber is filled with an aqueous solution akin to blood plasma; the posterior chamber is filled with a gel-like vitreous body of hyaluronic acid and a complex protein called vitrein. The iris is a continuation of the pigmented (outer) layer of the retina. The inner layers of the retina are neural layers. The axons of the optic nerve originate in the ganglion layer of the neural retina and converge at the back of the eye, where they exit on their way to the brain. Reproduced with permission, courtesy of Dr. K. W. Condon, http://anatomy. iupui.edu/courses/histo_D502/D502f04/lecture.f04/Eyef04/1.jpg.

is the retina, which is composed of two layers, a thinner pigmented retina, and a thicker neural retina of several cell layers that transduces light photons into electrical signals that pass over the optic nerve to the brain. Toward the anterior part of the eye, the vascular layer thickens to form a muscular ring around the eye called the ciliary body. The vascular layer extends further anteriorly to form the iris with its papillary opening. The iris contains smooth muscle that controls the diameter of the pupil. Lining the iris posteriorly are two layers of pigmented epithelial cells (PECS) that extend from the pigmented and neural layers of the retina. The lens is a transparent, flexible oblate body suspended just behind the PEC layers in the pupillary opening by ligaments attached to processes of the ciliary body. The suspensory ligaments hold the lens under tension; contraction of smooth muscle in the ciliary body eases this tension, allowing the lens to focus light.

The structure of the adult lens is shown in FIGURE 3.13. The lens consists of three components, anucleate transparent lens fibers characterized by their synthesis of a, P, and y crystallin proteins, an anterior lens epithelium, and a covering capsule that is a thick basement membrane laid down during eye development by lens epithelial cells (LECs). During development, the LECs of the posterior half of the epithelium are the first to differentiate into lens fibers. The LECs of the anterior

Germinal zone

Forming lens fiber rvM

Lens capsule (basement membrane)

Differentiated lens fiber

■ Anterior lens epithelium

FIGURE 3.13 Diagram of a slice of lens. The anterior lens epithelium is the source of new lens fibers at the equator in the germinal zone. The epithelial cells of the germinal zone elongate and differentiate into lens fibers that express a, P and y crystallins. The lens is surrounded by a basement membrane, the lens capsule, which is secreted by the epithelium early in development. Reproduced with permission, courtesy of Dr. K. W. Condon, http://anatomy.iupui.edu/courses/ histo_D502/D502f04/lecture.f04/Eyef04/1.jpg.

half of the epithelium persist into adult life. Their proliferation is responsible for the growth of the lens. LECs proliferate throughout the anterior epithelium of the growing lens, but differentiate into new lens fibers only at the equator of the eye.

The embryos of several vertebrate species, including fish, frogs, birds, and rats, as well as the tadpoles of the anuran, Xenopus laevis, and the adult newt, can regenerate the lens (Reyer, 1977; Stroeva and Mitashov, 1983; Mitashov, 1996). Lens regeneration is an epimorphic process involving formation of a blastema of dediffer-entiated cells. The source of the lens blastema in Xenopus tadpoles is the inner corneal epithelium (Freeman, 1963; Carinato et al., 2000). The newt lens blastema is derived by the dedifferentiation of pig-mented epithelial cells (PECs) of the dorsal iris (Reyer, 1977; Stroeva and Mitashov, 1983; Mitashov, 1996; Eguchi, 1998; Del-Rio Tsonis et al., 2003; Del-Rio Tsonis and Eguchi, 2004). The capacity for lens regeneration by the newt iris declines from dorsal to ventral. Pieces of ventral iris fail to form lens under experimental conditions in which the dorsal iris readily regenerates lens (Reyer, 1977). The cells of the ventral iris, however, have not lost the capacity for lens regeneration, because they can form lens when dissociated and cultured in vitro (Eguchi, 1998).

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