Function of the lens: The lens is one of the essential refractive media of the eye and focuses incident rays of light on the retina. It adds a variable element to the eye's total refractive power (10-20 diopters, depending on individual accommodation) to the fixed refractive power of the cornea (approximately 43 diopters).
Shape: The fully developed lens is a biconvex, transparent structure. The curvature of the posterior surface, which has a radius of 6 mm, is greater than that of the anterior surface, which has a radius of 10 mm.
Weight: The lens is approximately 4 mm thick, and its weight increases with age to five times its weight at birth. An adult lens weighs about 220 mg.
Position and suspension: The lens lies in the posterior chamber of the eye between the posterior surface of the iris and the vitreous body in a saucer-shaped depression of the vitreous body known as the hyaloid fossa. Together with the iris it forms an optical diaphragm that separates the anterior and posterior chambers of the eye. Radially arranged zonule fibers that insert into the lens around its equator connect the lens to the ciliary body. These fibers hold the lens in position (Fig. 7.1) and transfer the tensile force of the ciliary muscle (see Accommodation).
Embryology and growth: The lens is a purely epithelial structure without any nerves or blood vessels. It moves into its intraocular position in the first month of fetal development as surface ectoderm invaginates into the primitive optic vesicle, which consists of neuroectoderm. Apurely ectodermal structure, the lens differentiates during gestation into central geometric lens fibers, an anterior layer of epithelial cells, and an acellular hyaline capsule (Figs. 7.2a and b). The normal direction of growth of epithelial structures is centrifugal; fully developed epithelial cells migrate to the surface and are peeled off. However, the lens grows in the opposite direction. The youngest cells are always on the surface and the oldest cells in the center of the lens. The growth of primary lens fibers forms the embryonic nucleus. At the equator, the epithelial cells further differentiate into lens fiber cells (Fig. 7.2).
166 7 Lens Shape of the lens and its position in the eye.
— Embryology of the lens.
— Embryology of the lens.
These new secondary fibers displace the primary fibers toward the center of the lens. Formation of a fetal nucleus that encloses the embryonic nucleus is complete at birth. Fiber formation at the equator, which continues throughout life, produces the infantile nucleus during the first and second decades of life, and the adult nucleus during the third decade. Completely enclosed by the lens capsule, the lens never loses any cells so that its tissue is continuously compressed throughout life (Fig. 7.3a and b). The various density zones created as the lens develops are readily discernible as discontinuity zones (Fig. 7.4).
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