Eyes: Tissue Function

Eyes are the organs of vision. Detection of light and its conversion into electrochemical impulses in neurons is their basic function. In higher organisms such as human beings, the eye is a complicated optical system that collects light from the surroundings, processes its intensity in the diaphragm, focuses it via lenses to form image, converts the formed image into electrical impulses, and finally transmits these impulses to the brain via neurons, which connect the optic nerve to the visual cortex and other areas of the brain.

[Page 401]Additional functions of the eye involve retinal photosensitive ganglion cells, which send signals along the retinohypothalamic tract to the suprachiasmatic nuclei to affect circadian cycle adjustment.

Regarding image formation, rays of light enter through a small hole known as the pupil and, with the help of the lens, they are focused on the retina; distance of the light rays from the image to the retina is also adjusted by the lens.

Movements of the eye are controlled by the ciliary body, which is a ring-shaped tissue having ciliary muscles within it. Focusing of the lens is also controlled by ciliary muscles. The amount of light entering the retina is controlled by the iris, which is a colored ring surrounding the pupil. In bright light it decreases in diameter, whereas its diameter increases in dim light.

The eye is a very delicate organ, so nature has provided a magnificent anatomical framework for its protection, including a hard white sheet known as sclera, which covers its outer surface. The cornea, the transparent anterior portion of the sclera, allows the light rays to enter the eye. Nourishment of the eye is provided by a vascular layer known as choroid. In addition, the lens, iris, and cornea are nourished by clear fluid called aqueous humor, formed by the ciliary body and filling the space between the lens and the cornea.

Conjunctiva of the eye is divided into two parts: conjunctival epithelium and stroma. The epithelium is about two to five cells thick; with chronic exposure it can become dry, whereas the stroma is highly vascularized tissue, separated from epithelium by a basement membrane. Follicular conjunctival reaction is absent in newborns, as the adenoid superficial layer doesn’t develop until about three months. It has mucin secretions comprising goblet cells, crypts of Henley, and Glands of Manz in association. In addition, it has accessory lacrimal glands located deep in the substantia propria.

The cornea consists of the outermost layer of epithelium, whose surface is covered with microplicae and microvilli, which facilitate the attachment of mucin. It has excellent regeneration ability, so it doesn’t scar. Epithelium stem cells are principally located at the superior and inferior limbus and are indispensible for the maintenance of healthy corneal epithelium; they also act as a junctional barrier, preventing conjunctiva tissue from growing into the cornea. The cornea is essential for sight; it is maintained and repaired by stem cells called limbal stem cells, found in an area at the edge of the eye. Blindness results from severe loss of corneal and the limbal stem cells, such as in corneal burns.

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