Fibrous Tunic of the Eye

Histological Organization of the Sclera and Limbus

The outer opaque part of the tunic is the sclera. It is formed by 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 often interpreted as a single very broad 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 houses the histological structures responsible for the circulation and outflow of aqueous humor.

Trabecular Apparatus and Drainage System

The drainage system of the eye forms a chain of interconnected spaces designed to ensure adequate outflow of fluid from the anterior chamber into the venous system. A key element here is the trabecular meshwork (formerly known as the pectinate ligament), located in the iridocorneal angle between the sclera and the iris.

The primary structural unit of the apparatus is the trabecula (a flat strand), which exhibits a distinct layered structure from center to periphery:

1. A thick collagen fiber runs in the very center.
2. It is surrounded by a dense sleeve of elastic fibers.
3. The outer layer is the so-called "glassy membrane"—a layer of thin collagen fibers embedded in an amorphous ground substance.

Logic and Sequence of Aqueous Humor Outflow:

• First, the fluid enters the spaces of Fontana (trabecular spaces) — characteristic slits between the interwoven trabeculae.
• Then it is directed into the canal of Schlemm (scleral venous sinus) — a circular channel within the limbus.
• The final destination for outflow from the canal of Schlemm is the scleral venous plexus.

Notably, all these drainage spaces (the spaces of Fontana and the canal of Schlemm) are lined on the inside by simple squamous epithelium (endothelium). This endothelium is continuous with the endothelial lining of veins.

General Morphology and Layers of the Cornea

Unlike the sclera, the histological structure of the cornea is significantly more complex, featuring two additional epithelial layers and two limiting membranes. There are 5 distinct layers in total (from outside to inside):

1. Anterior epithelium. This is a stratified squamous non-keratinizing epithelium covering the external surface. Its basal layer consists of columnar cells acting as stem cells. This is followed by the wing (intermediate) cell layer made of polygonal cells tightly bound together by numerous desmosomes (providing mechanical strength). The outermost layer consists of flattened surface cells. The epithelium is rich in free nerve endings, providing high sensitivity and protective reflexes (blinking, lacrimation).
2. Anterior limiting membrane (Bowman's layer). Located immediately beneath the epithelium.
3. Proper substance of the cornea (stroma). Occupies the bulk of the tissue. It consists of dense regular connective tissue formed by strictly parallel thin collagen lamellae, interspersed with process-bearing fibroblasts, amorphous ground substance, and nerve fibers.
4. Posterior limiting membrane (Descemet's membrane). Lies beneath the endothelium.
5. Posterior epithelium. A simple squamous epithelium (often referred to in clinical practice as the endothelium) lining the inner surface of the cornea.

Note: Under light microscopy, both limiting membranes appear as thickened basement membranes. They consist of collagen fibrils and amorphous ground substance, giving them a characteristic glassy appearance.

Histophysiology: Factors of Corneal Transparency

Despite its multilayered structure, the cornea transmits light unimpeded. This optical transparency is ensured by a unique combination of tissue and biochemical features:

• Ideal organization. The collagen lamellae of the stroma are arranged strictly parallel to one another.
• Optical homogeneity. The amorphous ground substance of the stroma is rich in specific glycosaminoglycans — keratan sulfates. Their light refractive index closely matches that of the collagen fibers.
• Avasularity. Normally, the cornea is completely devoid of blood vessels. Its nutrition occurs exclusively via diffusion from the aqueous humor of the anterior chamber.
• Absence of myelin. Nerve fibers running through the stroma lack a myelin sheath, which would otherwise scatter light.
• Special hydration. The tissue has a lower water content (about 75–80%) compared to the opaque sclera.
• Absence of keratinized layers. The anterior stratified epithelium is non-keratinizing.