Schlemm's canal is a circular lymphatic-like vessel in the eye. It collects

aqueous humor The aqueous humour is a transparent water-like fluid similar to plasma, but containing low protein concentrations. It is secreted from the ciliary body, a structure supporting the lens of the eyeball. It fills both the anterior and the posterior ...

from the

anterior chamber The anterior chamber ( AC) is the aqueous humor-filled space inside the eye between the iris and the cornea's innermost surface, the endothelium. Hyphema, anterior uveitis and glaucoma are three main pathologies in this area. In hyphema, blood f ...

and delivers it into the episcleral blood vessels. Canaloplasty may be used to widen it.

Structure

Schlemm's canal is an endothelium-lined tube, resembling that of a lymphatic vessel. On the inside of the canal, nearest to the aqueous humor, it is covered and held open by the

trabecular meshwork The trabecular meshwork is an area of tissue in the eye located around the base of the cornea, near the ciliary body, and is responsible for draining the aqueous humor from the eye via the anterior chamber (the chamber on the front of the e ...

. This creates outflow resistance against the

Development

While Schlemm's canal has generally been considered as a vein or a scleral venous sinus, the canal is similar to the

lymphatic vasculature The lymphatic vessels (or lymph vessels or lymphatics) are thin-walled vessels (tubes), structured like blood vessels, that carry lymph. As part of the lymphatic system, lymph vessels are complementary to the cardiovascular system. Lymph vessel ...

. It is never filled with blood in physiological settings as it does not receive arterial blood circulation. Schlemm's canal displays several features of

lymphatic endothelium The lymphatic endothelium is a specialised form of epithelium, distinct from but similar to vascular endothelium. A lymph capillary endothelial cell is distinct from other endothelial cells in that collagen fibers are directly attached to its plasm ...

, including the expression of

PROX1 Prospero homeobox protein 1 is a protein that in humans is encoded by the ''PROX1'' gene. The Prox1 gene is critical for the development of multiple tissues. Prox1 activity is necessary and sufficient to specify a lymphatic endothelial cell fate i ...

VEGFR3 Fms-related tyrosine kinase 4, also known as FLT4, is a protein which in humans is encoded by the ''FLT4'' gene. This gene encodes a tyrosine kinase receptor for vascular endothelial growth factors C and D. The protein is thought to be involve ...

CCL21 Chemokine (C-C motif) ligand 21 (CCL21) is a small cytokine belonging to the CC chemokine family. This chemokine is also known as 6Ckine (because it has six conserved cysteine residues instead of the four cysteines typical to chemokines), exodus- ...

FOXC2 Forkhead box protein C2 (FOXC2) also known as forkhead-related protein FKHL14 (FKHL14), transcription factor FKH-14, or mesenchyme fork head protein 1 (MFH1) is a protein that in humans is encoded by the ''FOXC2'' gene. FOXC2 is a member of the ...

, but lacked the expression of

LYVE1 Lymphatic vessel endothelial hyaluronan receptor 1 (LYVE1), also known as extracellular link domain containing 1 (XLKD1) is a Link domain-containing hyaladherin, a protein capable of binding to hyaluronic acid (HA), homologous to CD44, the main ...

and

PDPN Podoplanin is a protein that in humans is encoded by the ''PDPN'' gene. Structure and function Podoplanin is a mucin-type protein with a mass of 36- to 43-kDa. It is relatively well conserved between species, with homologues in humans, mice, rat ...

. It develops via a unique mechanism involving the transdifferentiation of venous endothelial cells in the eye into lymphatic-like endothelial cells. This developmental morphogenesis of the canal is sensitive to the inhibition of lymphangiogenic growth factors. In adults, the administration of the lymphangiogenic growth factor VEGFC enlarged the Schlemm's canal, which was associated with a reduction in

intraocular pressure Intraocular pressure (IOP) is the fluid pressure inside the eye. Tonometry is the method eye care professionals use to determine this. IOP is an important aspect in the evaluation of patients at risk of glaucoma. Most tonometers are calibrated to ...

. In the combined absence of

angiopoietin 1 Angiopoietin 1 is a type of angiopoietin and is encoded by the gene ANGPT1. Angiopoietins are proteins with important roles in vascular development and angiogenesis. All angiopoietins bind with similar affinity to an endothelial cell-specific ty ...

and angiopoietin 2, Schlemm's canal and episcleral lymphatic vasculature completely failed to develop.

Function

Schlemm's canal collects

from the

. It delivers it into the episcleral blood vessels via aqueous veins.

Clinical significance

Canaloplasty

Canaloplasty is a procedure to restore the eye’s natural drainage system to provide sustained reduction of

. Micro

catheter In medicine, a catheter (/ˈkæθətər/) is a thin tube made from medical grade materials serving a broad range of functions. Catheters are medical devices that can be inserted in the body to treat diseases or perform a surgical procedure. Cath ...

s are used in a simple and minimally invasive procedure. A surgeon will create a tiny incision to gain access to Schlemm's canal. A microcatheter circumnavigates Schlemm's canal around the iris, enlarging the main drainage channel and its smaller collector channels through the injection of a sterile, gel-like material called

viscoelastic In materials science and continuum mechanics, viscoelasticity is the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation. Viscous materials, like water, resist shear flow and strain linearly wi ...

. The catheter is then removed and a suture is placed within the canal and tightened. By opening Schlemm's canal, the pressure inside the eye is relieved''.''

Angiopoietin deficiency

Schlemm's canal and episcleral lymphatic vasculature completely failed to develop in the combined absence of

and angiopoietin 2. This causes

buphthalmos Buphthalmos (plural: buphthalmoses) is enlargement of the eyeball and is most commonly seen in infants and young children. It is sometimes referred to as buphthalmia (plural buphthalmias). It usually appears in the newborn period or the first 3 mo ...

and

glaucoma Glaucoma is a group of eye diseases that result in damage to the optic nerve (or retina) and cause vision loss. The most common type is open-angle (wide angle, chronic simple) glaucoma, in which the drainage angle for fluid within the eye rem ...

Glaucoma

Glaucoma may be related to drainage of aqueous humor from the eye. This is mostly related to resistance on the internal surface of Schlemm's canal.

History

Schlemm's canal is named after

Friedrich Schlemm Friedrich Schlemm (11 December 1795 – 27 May 1858) was a German anatomist who was professor at the University of Berlin. He was born in Salzgitter. As his family could not afford higher education, he was apprenticed to a barber-surgeon in Brauns ...

(1795–1858), a

German German(s) may refer to: * Germany (of or related to) **Germania (historical use) * Germans, citizens of Germany, people of German ancestry, or native speakers of the German language ** For citizens of Germany, see also German nationality law **Ger ...

anatomist Anatomy () is the branch of biology concerned with the study of the structure of organisms and their parts. Anatomy is a branch of natural science that deals with the structural organization of living things. It is an old science, having its ...

Additional images

Gray870.png, Enlarged general view of the iridial angle. (Labeled with older label of 'sinus venosus scleræ' at center top.)

References

External links

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DiagramDiagram
{{Authority control Human eye anatomy