Canal of Schlemm

Location

The scleral venous sinus is found within a canal lying in the corneoscleral junction. This canal is bounded anterolaterally by the internal scleral sulcus, posteriorly by the scleral spur, and posteromedially by the trabecular meshwork.

To have a complete picture of how this canal looks like, let’s recall what these bounding structures are:

• The internal scleral sulcus is a shallow groove on the posterior surface of the corneoscleral junction, which extends along the entire circumference of the junction.
• The scleral spur is a projection of the posterior lip of the internal scleral sulcus. It is triangular in cross-section, with its base attached to the lip of the internal scleral sulcus, and its tip oriented anteriorly and inwards. The ciliary muscle attaches to the posterior surface of the spur and plays a role in aqueous humor outflow.
• The trabecular meshwork is a connective tissue network that extends between the tip of the scleral spur and the Schwalbe’s line (external margin of the corneal endothelium). It contains trabecular cells (trabeculocytes) separated by intratrabecular free spaces.

The canal of Schlemm, lying within a canal bounded by these three structures, is lined by endothelial cells.

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Aqueous humor pathway

When the ciliary muscle contracts, it stretches the trabecular meshwork, expanding the open space between the trabeculocytes. The aqueous humor from the anterior chamber of the eye pours through these free spaces and reaches the inner surface of the canal of Schlemm.

It is important to note that there is no open communication between the canal of Schlemm and the trabecular meshwork. Instead, the endothelial cells of the canal of Schlemm collect the aqueous humor by a specialized form of transcellular transport. More specifically, each endothelial cell contains a giant vacuole that opens both on the extraluminal and intraluminal surfaces of the cells, allowing the passage of aqueous humor from the trabecular meshwork into the lumen of Schlemm’s canal.

The Schlemm’s canal is drained by 25-30 collector channels. These convey the aqueous humor to the deep scleral plexus, which then empties into the intrascleral plexus. The latter drains into the episcleral plexus, which finally opens into the anterior ciliary veins. The anterior ciliary veins drain into the ophthalmic veins.

A small amount of the collector channels (~10%) bypass the deep scleral plexus and open into the conjunctival veins. These channels are known as the aqueous veins that drain directly into the ophthalmic veins.