Overview
/ Basic structure of the eyeball
The eyeball consists of three principal layers.
- The outer layer, or sclera, consists of dense fibrous connective
tissue.
- The sclera is "white" of the eye.
- The sclera is continuous with the transparent substantia propria of the cornea.
- The exposed front surface of the eye, including the cornea, is also
covered by a thin, non-keratinized stratified squamous epithelium.
- The next layer, or choroid, consists of heavily pigmented loose
connective tissue, including many melanocytes.
- The choroid is normally not visible behind the "white" of the sclera.
-
The
choroid is continuous with the iris; together the
choroid and iris are called the uvea. - A hole in this layer, the pupil, allows light to pass through.
- The inner layer, or retina, includes two
portions.
- The neural retina is the photoreceptive, imaging-processing tissue.
- The pigmented epithelium lies behind
the neural retina; it also extends forward to line the hidden side of
the iris.
- The lens is a specialized epithelial structure, suspended
behind the pupil.
- The anterior chamber, a space filled with aqueous humor, lies between the iris and the cornea.
- The posterior chamber lies behind the iris.
Cornea
The cornea consists of a thin surface epithelium (non-keratinized stratified squamous) overlying a layer of dense fibrous connective tissue, called substantia propria.
Although the corneal tissues are made of the same tissue elements as other body parts (i.e., epithelial cells, collagen, fibroblasts, etc.), the cornea is quite unlike most tissues in that it is perfectly transparent.
Compare and contrast the tissue layers of the cornea with those of other body surfaces:
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The features which distinguish corneal tissues from those of other body surfaces are all related to its transparency.
The
epithelium of the cornea is continuous with the epithelium of the conjunctive,
both that of the eyeball itself and that of the inside of the eyelid, which
in turn is continuous with the epidermis of skin on the exposed surface of
the eyelid.
Corneal epithelium is very thin (only a few cells thick).
Notably (i.e., unlike most other stratified squamous epithelial), corneal epithelium lies flat against the underlying substantia propria. The absence of connective tissue papillae (compare with skin, where the basal surface of the epidermis is indented by many dermal papillae).
The basement membranes between corneal epithelium and substantia propria is exceptionally thick and is called Bowman's membrane.
Like most dense fibrous connective tissue, the substantia propria of the cornea is mostly collagen and ground substance, with fibroblasts as the most common cell type. Quite unlike most other dense, fibrous connective tissue, the corneal connective tissue is perfectly transparent.
Collagen of the cornea is organized into extremely regular layers. All the collagen fibers in one layer arranged in parallel, and alternating layers run in different directions.
Corneal connective tissue has no blood vessels. (You don't need a microscope to confirm this; just look in a mirror.)
Even though cells of the cornea are not very active metabolically, they still need oxygen and nutrients. As long as the cornea is in direct contact with air, oxygen can be absorbed directly. Nutrients can diffuse into cornea from aqueous humor.
Cells of corneal connective tissue are limited to fibroblasts. There is no immune-system component, hence the relative ease with which corneal tissue can be transplanted without need for careful tissue typing.
At the inner surface of the cornea, a thick basal lamina (Decemet's membrane) separates the substantia propria from a cellular layer, resembling a simple low cuboidal epithelium, called corneal endothelium. (This is a unique use of the term endothelium; it is not related to vascular endothelium.) It is believed that this corneal endothelium is essential for maintaining corneal transparency, by regulating the composition of ground substance in the substantial propria.
Corneal epithelium contains free nerve endings. Since pain seems to be the only sensory modality that functions for corneal tissue, biologists long ago decided that free nerve endings elsewhere may also represent pain fibers.
Corneal transparency.
The tissue elements of the cornea are specialized for transparency. The effectiveness of this design can be readily seen by looking in the mirror and comparing cornea with sclera (the white of the eye). However, apart from the cornea's obvious absence of blood vessels, its tissue composition appears almost identical to that of the sclera. Yet unlike the sclera, the cornea is marvellously transparent.
The transparency of the cornea is based primarily on the regularity of its tissue components, which minimizes the number of surfaces where light can be refracted or reflected.
At the limbus (the edge of the cornea, where the cornea meets the sclera), it is apparent that the epithelium of the sclera is almost identical to that of the cornea, except for a slightly less regular basal layer. Similarly, collagen of the corneal substantia propria is not noticably different from collagen of the sclera, except for being slightly more uniform in arrangement. But these small differences are significant; the cornea is transparent while the sclera is opaque. The effect is rather like the difference between packed snow and crystalline ice -- both have the same composition (frozen water), but one is opaque (by scattering the light off the surfaces of many tiny snow crystals) while the other is crystal clear.
Although most cells and fibers are colorless, the surfaces of these elements can scatter light when irregularly arranged. Similarly, light scattered from colorless and transparent ice crystals produce the familiar whiteness of winter snow. Such scattering (together with absorption of light by pigments such as melanin and hemoglobin) prevents light from passing freely through most tissues.
Features of the cornea which minimize the scattering of light include the following:
- Both outer and inner surfaces of the corneal epithelium are smooth.
- The collagen fibers of the substantia propria are arranged into uniform layers with parallel fibers within each layer (unlike the more irregular texture of collagen in the sclera and in the dermis of the skin).
Collagen fibers are formed extracellularly, self-assembling from tropocollagen molecules (secreted as procollagen by fibroblasts). The regulatory machinery responsible for the regular arrangement of collagen fibers in the cornea remains unknown.
- The water content of the ground substance is carefully regulated, to maintain uniform spacing among collagen fibers.
The low cuboidal cells which form the cornea's innermost layer, the so-called corneal endothelium, actively pump ions and water from the corneal ground substance into the aqueous humor, to prevent excess water from disturbing the regularity of the collagen layers and causing opacity.
Iris
Functionally, the iris is a rather simple opaque ring surrounding and controlling the diameter of its central aperture -- the pupil.
However, the iris does have some peculiar histological features.
- The iris is the only internal organ which is clearly visible from outside
the body. The visible surface of the iris consists of loose connective
tissue and includes blood vessels.
- The color of the iris ("eye color") results both from scattering of light by its trabecular meshwork of collagen fibers and from absorption of light by melanin granules in scattered melanocytes.
- Variation in eye color result from individual differences in the distribution
and density of melanocytes and trabecular meshwork.
- The posterior (hidden) surface of the iris is an intensely pigmented extension
of the embryological optic cup (the same tissue which forms the retina
and the pigmented epithelium).
This tissue continues as the ciliary processes
around the perimeter of the iris.
- A ring of smooth muscle surrounding the pupil comprises the pupillary
sphincter.
- The pupillary dilator is not ordinary smooth muscle but rather contractile extensions of myoepithelial cells of the deep layer of the two-layered pigmented epithelial tissue that comprises the posterior surface of the iris.
Lens
Histologically, the lens is bizarre. It is an isolated island of epithelial tissue with an anterior layer that is simple cuboidal and a posterior layer consisting of extravagantly elongated cells, called lens fibers, that are packed with lens protein.
This
pattern is most readily understood by considering the embryology of the eye.
The lens forms as a vesicle that pinches in from surface ectoderm. 
In
basic plan, it is therefore a "bubble" of epithelial tissue.
The cells on the posterior of this bubble then grow incredibly long until
they extend across almost the entire thickness of the lens, save only a thin
layer of cuboidal epithelium that remains on the lens' anterior surface.
At the edge of the lens, one can see where these two different epithelial cell shapes meet one another.
The
shape of the lens (and hence its focal length) is determined by tension exerted
through the suspensory fibers, controlled by smooth
muscle of the ciliary body.
Ciliary body and suspensory fibers (zonules)
Deep
to the limbus (i.e., the cite where the cornea meets
the sclera), the choroid layer
is thickened into the ciliary body.
- The ciliary body is a ring of smooth muscle fibers arranged concentrically
around the opening in which the lens is suspended.
- The lens is suspended from the ciliary body by thin fibers of collagen, called suspensory fibers or zonules.
Together,
the ciliary body and suspensory fibers control the shape of
the lens.
- When smooth muscle is in a relaxed state, the elasticity of the eyeball
(sclera) puts tension on the suspensory fibers, which in turn stretch the
lens radially and make it thinner. In this condition, the lens brings
distant objects into focus on the retina.
When the smooth muscle of the ciliary body contracts, the opening in which
the lens is suspended becomes (slightly) smaller and tension on the suspensory
fibers is relaxed. The lens is then allowed to adopt its "resting"
shape, which is more round. In this condition, the lens brings nearby
objects into focus on the retina.
- Note that a stretched lens, for distant focus, is maintained by
a relaxed muscle. Hence eyes at rest are focussed for distance
vision.
- Conversely, a contracted muscle is responsible for a relaxed lens, for near focus. Close work is tiring because the ciliary body must remain in contraction to maintain focus.
Ciliary
processes and aqueous humor
The surface of the ciliary body is covered by an extension of the embryonic optic cup ( the same tissue which forms the retina and the pigmented epithelium). Small projections of this tissue form the ciliary processes, which secrete the aqueous humor.
Aqueous humor flows from its site of formation in the posterior chamber (i.e., behind the iris) through the pupil into the anterior chamber. From there it drains into the canal of Schlemm and hence into venous drainage.
Clinical note: An imbalance between the formation and drainage of aqueous humor can create increased pressure. Increased fluid pressure inside the eyeball reduces blood flow into the eyeball, leading to glaucoma.
Canal
of Schlemm
The canal of Schlemm is a network of connective tissue spaces at the edge (limbus) of the cornea, through which aqueous humor can escape from the anterior chamber; from here the fluid seeps into venous drainage.
