Morphologic features of the aqueous humor drainage pathways in horses

The iridocorneal angle of the horse was anteriorly lined by a pectinate ligament, consisting of several layers of vertically oriented collagenous beams, that were interrupted by a network of extracellular spaces. The anterior chamber angle consisted mostly of a large uveal trabecular meshwork that was composed of widely-spaced, thick trabeculae. These trabeculae were attached in part posteriorly to extensions of the ciliary body musculature and anteriorly inserted into the prominent pectinate ligament. The corneoscleral trabecular meshwork was shaped hemiobovately and formed the external boundary of the iridocorneal angle. The extracellular spaces were smaller and more evenly spaced than those in the uveal trabecular meshwork. Uveal and corneoscleral trabecular meshwork beams were mostly composed of cores of collagen and elastin surrounded by several layers of basement membrane-like material and, in turn, were entirely enveloped by trabecular cells. An anterior narrow zone of broad corneoscleral trabeculae was composed of elastin-collagen cores surrounded by circular bundles of long-spacing collagen. The angular aqueous plexus was extensive, forming much of the external border of the outer corneoscleral trabecular meshwork. A dense zone was associated with the angular aqueous plexus, completely encompassing these aqueous outflow vessels. Posteriorly, the iridocorneal angle was confluent with a well-defined band of trabeculae that was situated between the ciliary body musculature and subjacent sclera and extended posteriorly to the anterior-most region of the choroid where these trabeculae tapered and ended. This region is referred to as the supraciliary trabecular meshwork and indicates a potentially extensive route for outflow of aqueous humor in the equine eye.