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Editors Selection IGR 15-2

Anatomical Structures: Ocular Lymphatic Drainage: A novel pathway?

Comment by Alex Huang & Seung Hyen Lee on:

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This research represents an interesting contribution to understanding Schlemm's canal (SC) and its association with nearby subconjunctival lymphatics. PROX1 is a relatively specific lymphatic marker, and transgenic mice exist where lymphatics are natively fluorescent. SC is also fluorescent in these mice because SC has a partial lymphatic identity. The authors isolate eyes from these mice and apply optical clearing to make the sclera translucent. Next, using light sheet fluorescence microscopy, the authors demonstrate connections between the fluorescent SC and nearby smaller fluorescent vascular pathways. These pathways are determined to be lymphatics given concurrent PROX1 and CD31 immunofluorescence as opposed to collector channels which are PROX1 negative but CD31 positive. The authors term these connections 'lymphatic bridges'. The authors then introduce 70kD dextran tracers into the eye where the authors claim that the tracer enters the lymphatic-bridge pathways.

Just because a structure is fluorescent in this mouse does not mean it is guaranteed to be a lymphatic

Despite these interesting findings, methodological concerns exist. Firstly, this is a very small paper showing data from very few eyes. The authors claim to study 34 mouse eyes although, at most, data from only eight eyes are shown. There is no quantitative analysis to include data from all eyes. Thus, the remaining 26 eyes were completely and strangely omitted. Further, the PROX1 mouse demonstrates fluorescence in non-lymphatics. Our group has also published research using this mouse, and it is well-known that the phakic lens is fluorescent. Therefore, just because a structure is fluorescent in this mouse does not mean it is guaranteed to be a lymphatic. Lastly, there is a lack of a positive control in the perfusion studies. When the 70 kD tracers were placed into the anterior chamber, their size should allow flow into trabecular pathways. However, all fluorescent pathways designated by the authors were deemed to be lymphatic. Not observing trabecular aqueous humor outflow is strange.

A direct communication between the trabecular/conventional outflow pathways and subconjunctival lymphatics will be interesting to study in glaucoma pathophysiology as well as for developing new eye pressure lowering therapeutics

Ultimately, given methodological questions, considerable future validation and replication must be performed. Additionally, species-specific differences exist, and the presence of lymphatic bridges need to be shown in human eyes. If confirmed, a direct communication between the trabecular/conventional outflow pathways and subconjunctival lymphatics will be interesting to study in glaucoma pathophysiology as well as for developing new eye pressure lowering therapeutics.

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