Schlemm's canal

Schlemm's canal
Anterior part of the human eye, with Canal of Schlemm at lower right.
The upper half of a sagittal section through the front of the eyeball. (Canal of Schlemm labeled at center left.)
Details
Identifiers
Latin	sinus venosus sclerae
TA	A12.3.06.109
FMA	51873
Anatomical terminology []

Schlemm's canal is a circular lymphatic-like vessel in the eye that collects aqueous humor from the anterior chamber and delivers it into the episcleral blood vessels via aqueous veins. It is named after Friedrich Schlemm (1795–1858), a German anatomist.

The canal is essentially an endothelium-lined tube, resembling that of a lymphatic vessel. On the inside of the canal, nearest to the aqueous humor, it is covered by the trabecular meshwork; this region makes the greatest contribution to outflow resistance of the aqueous humor.

Conventionally, the canal has been considered a blood vessel, but recent studies have revealed that the molecular identity of Schlemm's canal is strikingly reminiscent to the one of lymphatic vasculature.

While conventional wisdom has considered Schlemm's canal (also known as the scleral venous sinus) as vein, the canal shares several structural and functional features reminiscent of the lymphatic vasculature. Notably, it is never filled with blood in physiological settings as it does not receive arterial blood circulation.

Three recent independent pioneering studies by Aleksanteri Aspelund and Kari Alitalo from the University of Helsinki, Dae-Young Park and Gou Young Koh from KAIST and Krishnakumar Kizhatil and Simon W. M. John from the Howard Hughes Medical Institute, discovered that Schlemm's canal displays several features of lymphatic endothelium, including the expression of PROX1, VEGFR3, CCL21, FOXC2, but lacked the expression of LYVE1 and PDPN, indicating that Schlemm's canal is a lymphatic-like vessel.

Developmental studies revealed that Schlemm's canal develops via a unique mechanism involving the transdifferentiation of venous endothelial cells in the eye into lymphatic-like endothelial cells.