Novel discovery of LYVE-1 expression in the hyaloid vascular system

PURPOSE. The hyaloid vascular system (HVS) is a transient network nourishing developing eyes and has been widely used as a natural model to study blood vessel regression. Failure of its regression in humans leads to several blinding diseases. Lymphatic vessel endothelial hyaluronic acid receptor (LYVE-1) is a recently defined lymphatic marker that is also expressed by a subpopulation of macrophages. To date, there is no report on its expression in the HVS. This study was conducted to investigate whether LYVE-1 is expressed in the HVS and how it is associated with the vascular structure and macrophage phenotype. METHODS. Normal C57BL/6 mouse eyeballs were sampled from embryonic day (E) 10.5 to postnatal (P) and adult stages for immunofluorescent microscopic studies with antibodies against LYVE-1, CD31 (panendothelial cell marker), and F4/80 (macrophage marker). Additionally, Angiopoietin-2 (Ang-2) knockout mice with abnormally persistent HVS were examined. RESULTS. The LYVE-1 expression was detected on normal HVS between E12.5 and P14. The LYVE-1 ⁺ cells were F4/80 ⁺ but CD31 ^-, indicating a macrophage lineage. Additionally, LYVE-1 ⁺ cells bud on CD31 ⁺vessels and constitute an integral part of the network in both normal developing and Ang-2 knockout mice. CONCLUSIONS. This study provides the first evidence that the HVS contains a LYVE-1 ⁺ cellular component in both physiological and pathologic conditions. This novel finding not only provides a new concept in defining the embryogenesis and pathogenesis of the HVS, it also leads to a completely natural model in which to study the functions of the LYVE-1 pathway, an important topic for lymphatic research as well.