LESSON 30 EYE

• The eye is composed of several compartments, including the anterior and posterior compartments. The anterior compartment is located between the cornea and the vitreous body, contains the aqueous humor, and is divided into two chambers: the anterior and posterior chambers. The posterior compartment is filled by the vitreous body.

• The lens is a biconvex disk located behind the iris, connected to the ciliary body through zonular fibers. It is made up of three parts: the capsule, subcapsular epithelium, and fibers of the lens.

• The capsule is a homogeneous and refractile layer made up of type IV collagen fibers, glycoproteins, and proteoglycans. It lines the subcapsular epithelium of the lens and forms a basement membrane-like structure.

• The subcapsular epithelium of the lens is the lens's growth zone, composed of cuboidal cells arranged as a layer below the anterior capsule.

• The fibers of the lens are highly differentiated cells organized as a dense cell palisade, joined together by fissure junctions. They have undergone hyalinization and have lost all organoids.

• The eye's outer tunic consists of the fibrous tunic (sclera and cornea). The sclera is a fibrous membrane mainly made up of collagen fibers, providing the eyeball with strong resistance. The cornea is the anterior continuation of the sclera and is transparent and avascular.

• The uvea is the eye's vascular tunic, made up of the choroid, ciliary body, iris, and iridocorneal angle. The choroid is a layer rich in blood vessels and contains pigmented cells, preventing light reflection. The ciliary body is an anterior thickening of the choroid, with ciliary processes producing aqueous humor, and the iris is its most rostral expansion, with a pupil for light entry.

• The retina is the eye's neuroepithelial and photoreceptor membrane, composed of a set of specialized neurons and considered part of the central nervous system. It includes the retina proper, macula lutea, blind or ciliary retina, and blind spot, with 10 layers: pigment epithelium, layer of rods and cones, outer limiting membrane, and more. The pigment epithelium absorbs excess light, transports nutrients, and interacts with the choroid.

• The epithelium of the retina performs phagocytosis of degenerated parts of photosensitive neurons.

• Two types of photosensitive neurons exist: rods and cones.

• Rods have a large, euchromatin-rich nucleus located in the outer nuclear layer, and an elongated inner segment with numerous mitochondria and a basal body for the specialized cilium.

• Outer segment of rods contains rhodopsin and is phagocyted by the pigment epithelium.

• Cones have a similar morphology to rods but differ in their outer segment, which may have open disks with a relation to the external space.

• Bipolar cells come in different types: rod bipolar neurons, dwarf bipolar cells, and flat bipolar cells, each with different morphology and functions.

• Horizontal cells have a large, polygonal perikaryon, long thin cytoplasmic processes that contact bipolar and cone axonal terminations.

• Amacrine cells lack axons, have small oval somas, and long dendrites interrelating with axonal endings, mainly of bipolar neurons.

• Retina has distinct areas: macula lutea (fovea), ciliary or blind retina, and the blind spot.

• Macula lutea (fovea) has maximum acuity and contains mostly cones.

• Blind retina is the thinnest portion of the retina and has no retinal cells, only nerve fibers forming the optic nerve.

• Ganglion cells have spherical and large somas, a central euchromatic nucleus, and numerous organoids in the cytoplasm.

• Their axons form the optic nerve fibers, and the ganglion cell layer is covered by the inner limiting membrane.

Test your knowledge about the complex structure and functions of the human eye. This quiz covers the constituents, nerve membrane, ocular architecture, and refractive medium of the eye.