The morphology of the equine iridocorneal angle: a light and scanning electron microscopic study.

The research paper examines the unique characteristics of the iridocorneal angle – the part where the cornea and iris meet – in horses. The findings reveal that the structure of this angle, particularly its pectinate ligament and ciliary cleft, helps to explain why horses rarely develop glaucoma.

Morphological Characteristics of the Iridocorneal Angle

The study investigates the physical features of 20 horse eyes, detailing the morphology of the iridocorneal angle. The key points discussed include:

• The iridocorneal angle of the horse is characterized by a prominent pectinate ligament – a band of fibrous tissue – and a sizable ciliary cleft – a slit-like opening among the eye’s ciliary bodies.
• The pectinate ligament is made up of long and broad pigmented trabeculae (connective tissue) that form a dense and flat network that encircles the eye.
• The ciliary cleft, when viewed on cross-sections, presents as a wide, triangular space filled with a trabecular meshwork. This meshwork is divided into two parts: a larger, inner part, and a smaller, outer part.
• The inner part forms a three-dimensional network of large pigmented trabeculae with wide spaces in between, while the outer part fills the posterior angle of the ciliary cleft. The outer part forms a compact annular (ring-shaped) network of small, non-pigmented, circular trabeculae that enclose small spaces containing glycosaminoglycans – long unbranched polysaccharides found in the body.

Other Find Structures of the Equine Eye

The study also discusses other fine structures found in the equine eye:

• Immediately outside the outer trabecular network is the angular aqueous plexus, a circular, discontinuous network of small slit-like vessels. This plexus in horses was found to be rudimentary or underdeveloped.
• The intrascleral venous plexus is also weakly developed in the horse eye under study. In contrast, the internal collector channels and the episcleral venous plexus are notably visible.
• The large ciliary cleft is supported by robust trabeculae of both the pectinate ligament and the inner part of the trabecular meshwork. This makes the collapse of the ciliary cleft highly unlikely.

Implication in Glaucoma Prevalence

The researchers conclude that the unique morphological features of the equine iridocorneal angle could be the reason why horses infrequently develop glaucoma. This is particularly attributed to the ciliary cleft’s sizeable and well-supported structure preventing its collapse—a common precursor to glaucoma.