Fibrous tunic of the eyeball

Histological organization of the sclera and limbus

The outer opaque part of the tunic is the sclera (white tunic). It is formed by strong, dense regular fibrous connective tissue. The bulk of the sclera consists of numerous lamellae of collagen fibers oriented predominantly parallel to the surface of the eyeball. In histology, each such lamella is considered a single, very wide collagen fiber. The spaces between the collagen lamellae are filled with cellular and elastic components: flattened fibroblasts and isolated elastic fibers reside here.

The zone of gradual transition from the opaque sclera to the optically transparent cornea is called the limbus. This is a strategically important region because it is precisely within the thickness of the limbus that the histological structures responsible for the circulation and outflow of aqueous humor are localized.

Trabecular apparatus and drainage system

The drainage system of the eye forms a chain of communicating cavities whose function is to ensure adequate fluid outflow from the anterior chamber into the venous bed. The key element here is the trabecular meshwork (in older terminology, the pectinate ligament), located in the angle between the sclera and the iris.

The main structural unit of the apparatus is the trabecula (a flat cord), which has a distinct layered structure from the center to the periphery:

1. Right in the center runs a thick collagen fiber.
2. It is surrounded by a dense sheath of elastic fibers.
3. Externally lies the so-called "glassy sheath"—a layer of fine collagen fibers embedded in an amorphous substance.

Logic and sequence of aqueous humor outflow:

• First, the fluid enters the spaces of Fontana—characteristic clefts between the interlacing trabeculae.
• Then it is directed into the canal of Schlemm (scleral venous sinus)—ring-shaped cavities within the thickness of the limbus.
• The final destination of outflow from the canal of Schlemm is the scleral venous plexus.

It is important to note that all these drainage cavities (spaces of Fontana and Schlemm's canal) are lined from the inside by a simple squamous epithelium (endothelium). This epithelium is continuous with the endothelial lining of the veins.

General morphology and layers of the cornea

Unlike the sclera, the histological structure of the cornea is significantly more complex—it features two additional epithelial layers and two limiting membranes. In total, 5 layers are distinguished (from outside to inside):

1. Anterior epithelium. This is a stratified squamous non-keratinized epithelium covering the outer surface. Its basal layer consists of prismatic cells that act as a cambium. It is followed by the spinous (intermediate) layer of polygonal cells firmly connected to each other by a huge number of desmosomes (which gives the epithelium mechanical strength). The outermost layer is the layer of flattened cells. The epithelium is rich in free nerve endings, which provides high sensitivity and protective reflexes (blinking, lacrimation).
2. Anterior limiting membrane (Bowman's layer). Located directly beneath the epithelium.
3. Proper substance of the cornea (stroma). Occupies the bulk of the tissue. It is a dense regular connective tissue consisting of strictly parallel thin collagen lamellae, between which reside process-bearing fibroblasts, an amorphous substance, and nerve fibers.
4. Posterior limiting membrane (Descemet's membrane). Lies beneath the endothelium.
5. Posterior epithelium. A simple squamous epithelium (frequently referred to as endothelium in clinical practice) lining the inner surface of the cornea.

Note: Both limiting membranes appear under the light microscope as thickened basement membranes. They consist of collagen fibrils and an amorphous substance, giving them a characteristic glassy appearance.

Histophysiology: factors of corneal transparency

Despite its multi-layered structure, the cornea is able to transmit light unhindered. This optical transparency is ensured by a unique combination of tissue and biochemical features:

• Ideal ordering. The collagen lamellae of the stroma are arranged strictly parallel to each other.
• Optical homogeneity. The amorphous substance of the stroma is rich in specific glycosaminoglycans—keratan sulfates. Their light refraction index exactly matches that of the collagen fibers.
• Avascularity. Normally, the cornea is completely devoid of blood vessels. Its nutrition is carried out exclusively by diffusion from the fluid of the anterior chamber of the eye.
• Absence of myelin. Nerve fibers passing through the stroma lack a myelin sheath, which could otherwise scatter light.
• Special hydration. The tissue has a lower water content (about 75–80%) compared to the opaque sclera.
• Absence of keratinized masses. The anterior stratified epithelium is non-keratinized.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Cellular composition of the differons of the anterior stratified corneal epithelium.
• The role of desmosomes in spinous layer cells in ensuring tissue strength.
• Biochemical features of keratan sulfates and their effect on the optical density of the stroma.
• Histological structure of the simple squamous epithelium of Schlemm's canal.
• Mechanisms of transition of the epithelium of Fontana's spaces into venous endothelium.

Other topics in "Sensory organs: organs of vision and olfaction"

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