Falk Schroedl

The normal human choroid is endowed with a significant number of lymphatic vessel endothelial hyaluronate receptor 1 (LYVE-1)-positive macrophages.

PURPOSE Lymphatic vessel endothelial hyaluronic acid receptor (LYVE-1) is a newly discovered lymphatic endothelium-specific marker that is also expressed by a subpopulation of macrophages. To date, there is no report on its expression in the posterior human uvea. The purpose of this study was to investigate the expression of LYVE-1 in normal human choroids. METHODS Eyes of body/cornea donors (55-89 years of age; 4-9 hours postmortem) were obtained. Choroids were dissected and prepared for cryosections followed by immunohistochemistry with anti-human LYVE-1 antiserum and immunogold labeling. In addition, anti-human antibodies against macrophage markers (CD68, MHC class II) and lymphatic (podoplanin) and blood vascular endothelium (CD31, vWF) were used. For documentation, light-, fluorescence-, confocal laser scanning-, and electron-microscopy were used. RESULTS The normal human choroidal stroma contained 274 +/- 86 LYVE-1 positive cells/mm(2). The cells displayed irregular shapes with a relatively uniform diameter of 32 mum. Costaining with CD68 and negativity for CD31, podoplanin, and melan-A/HMB45, as well as electron microscopic features, suggest these LYVE-1(+) cells to be macrophages. Besides that, no classic LYVE-1(+)/podoplanin(+) lymphatic vessels were detected within the normal adult human choroid. CONCLUSIONS The normal adult human choroid does not contain typical lymph vessels, but is endowed with a significant number of LYVE-1 positive macrophages. These cells may be involved in choroidal hyaluronic acid metabolism or contribute to temporary formation of lymphatic channels under inflammatory conditions.

Brain and Retinal Pericytes: Origin, Function and Role

Pericytes are specialized mural cells located at the abluminal surface of capillary blood vessels, embedded within the basement membrane. In the vascular network these multifunctional cells fulfil diverse functions, which are indispensable for proper homoeostasis. They serve as microvascular stabilizers, are potential regulators of microvascular blood flow and have a central role in angiogenesis, as they for example regulate endothelial cell proliferation. Furthermore, pericytes, as part of the neurovascular unit, are a major component of the blood-retina/brain barrier. CNS pericytes are a heterogenic cell population derived from mesodermal and neuro-ectodermal germ layers acting as modulators of stromal and niche environmental properties. In addition, they display multipotent differentiation potential making them an intriguing target for regenerative therapies. Pericyte-deficiencies can be cause or consequence of many kinds of diseases. In diabetes, for instance, pericyte-loss is a severe pathological process in diabetic retinopathy (DR) with detrimental consequences for eye sight in millions of patients. In this review, we provide an overview of our current understanding of CNS pericyte origin and function, with a special focus on the retina in the healthy and diseased. Finally, we highlight the role of pericytes in de- and regenerative processes.

Neural Crest Origin of Retinal and Choroidal Pericytes

PURPOSE The origin of pericytes (PCs) has been controversially discussed and at least three different sources of PCs are proposed: a neural crest, mesodermal, or bone marrow origin. In the present study we investigated a potential neural crest origin of ocular PCs in a transgenic Rosa26-YFP-Sox10-Cre neural crest-specific reporter mouse model at different developmental stages. METHODS The Rosa26-YFP-Sox10-Cre mouse model expresses the yellow fluorescent protein (YFP) reporter in cells with an active Sox10 promoter and was here used for cell fate studies of Sox10-positive neural crest derived progeny cells. Detection of the YFP signal in combination with double and triple immunohistochemistry of chondroitin sulfate proteoglycan (NG2), platelet derived growth factor receptor β (PDGFRβ), α smooth muscle actin (αSMA), oligodendrocyte transcription factor 2 (Olig2), and lectin was performed and analyzed by confocal microscopy. RESULTS Sox10-YFP-positive cells and profiles were detected in the inner nuclear layer, the ganglionic cell layer, and the axons of the nerve fiber layer in postnatal retinas. An additional population has been identified in the retina, optic nerve, and choroid that displays strong perivascular localization. These cells were colocalized with the PC-specific markers NG2 and PDGFRβ in embryonic (E14.5) as well as postnatal (P4, P12, 6-week-old) vasculature. Beside PCs, vascular smooth muscle cells (vSMCs) were also labeled by the Sox10-YFP reporter protein in all ocular tissues investigated. CONCLUSIONS Since YFP-positive PCs and vSMCs are colocalized with NG2 and PDGFRβ, we propose that capillary PCs and vSMCs in the retina and the optic nerve, both parts of the central nervous system, as well as in the choroid, a tissue of mesodermal origin, derive from the neural crest.

Absence of blood and lymphatic vessels in the developing human cornea.

Purpose: The normal human cornea is devoid of both blood and lymphatic vessels and actively maintains this avascularity (corneal angiogenic privilege). Whether and when corneal angiogenic privilege is achieved during development is unknown. Methods: This study analyzed whether the cornea is primarily devoid of both blood and lymphatic vessels during intrauterine development or whether secondary regression of pre-existing vessels occurs before delivery. Indirect double immunohistochemistry was performed on 4-μm serial pupil-optic disc sections of paraffin-embedded human eyes stillborn at gestational ages of 17 to 41 weeks with antibodies against von Willebrand factor (vWF; factor VIII-associated antigen) as a panendothelial marker and with antibodies against lymphatic vessel endothelial hyaluronate receptor 1 (LYVE1) as a marker specific for lymphatic vascular endothelium. Results: Human corneas were devoid of both vWF+++/LYVE-1− blood vessels and vWF+/LYVE-1+++ lymphatic vessels at all time-points analyzed. In contrast, there were numerous blood and lymphatic vessels detectable in the adjacent conjunctiva. Conclusion: The normal human cornea is primarily avascular and devoid of both blood and lymphatic vessels. Corneal angiogenic privilege is already achieved very early during fetal intrauterine development. This suggests early and strong expression of both antiangiogenic and antilymphangiogenic factors in the human cornea during development.

The effect of vasopressin on choroidal blood flow, intraocular pressure, and orbital venous pressure in rabbits.

PURPOSE To investigate the effects of arginine-vasopressin (AVP) on intraocular pressure (IOP), orbital venous pressure (OVP), and choroidal blood flow (ChorBF) regulation in anesthetized rabbits. METHODS Mean arterial pressure (MAP), IOP, and OVP were measured by direct cannulation of the central ear artery, the vitreous, and the orbital venous sinus, respectively. Laser Doppler flowmetry was used to record ChorBF. To change the perfusion pressure (PP), MAP was manipulated mechanically with occluders around the aorta and vena cava. In the first group of animals (n = 11) the dose-response relationship was measured. In the second group of animals (n = 8) pressure-flow relationships were determined at baseline and in response to intravenous application of a low (0.08 ng/kg/min) and a high (1.33 ng/kg/min) infusion rate of AVP. RESULTS AVP caused a dose-dependent increase of MAP and choroidal vascular resistance (ChorR), whereas IOP, OVP, ChorBF, and heart rate (HR) were decreased. In contrast to the high infusion rate, the low infusion rate of AVP had no effect on baseline ChorBF. However, the pressure-flow relationship was shifted downward significantly by both infusion rates at PP below baseline. CONCLUSIONS AVP reduces IOP and OVP significantly and is a potent vasoconstrictor in the choroidal vascular bed. In the choroid, the effect of AVP is not only dose-dependent, but also PP-dependent, which is indicated by the reduced perfusion relative to control with low-dosed AVP at low PP.

Sufficient Evidence for Lymphatics in the Developing and Adult Human Choroid

We read with interest the recent article by Koina et al.1 suggesting evidence for the presence of lymphatic vessels in the developing and adult human choroid. However, this study does not meet the recently published consensus criteria on the immunohistochemical detection of ocular lymphatic vessels,2 and therefore, in our opinion, requires critical revision. First, appropriate positive and unequivocal negative controls are not presented in the study of Koina et al. In particular, when describing novel anatomical structures for the first time, and in order to change an existing dogma, a detailed documentation of blood and lymphatic vessel detection in the control tissue is mandatory. The provided supplementary data do not fulfill these criteria. Second, the immunohistochemical marker panel used is critical. Endomucin does not represent an established lymphatic marker,3,4 but is rather expressed by “endothelial cells along the whole vascular tree including lymphatic vessels.”5 Thus, an unequivocal discrimination between blood and lymphatic vessels is impossible with this marker. A further discrepancy is the use of the transcription factor prospero-related homebox gene-1 (Prox-1) as an extranuclear lymphatic endothelial precursor marker. Although reports of the extranuclear presence of PROX-1 in cell types other than lymphatic endothelium exist,6–8 PROX-1 clearly shows a nuclear expression in lymphatic endothelia in human,9 as well as mouse10 and avian,11 embryos, retaining its nuclear localization into adulthood.12–14 On the other hand, it is not clear why lymphatic endothelial surface markers, such as podoplanin, lymphatic vascular endothelial-specific hyaluronic acid receptor-1 (LYVE-1), and the vascular endothelial marker CD34 display nuclear expression in this study. Additionally, the only lymphatic endothelial cell marker used in whole mounts is VEGFR-3, which is also expressed in fenestrated blood vessels, and, as such, also in the choriocapillaris.15,16 Morphologically, the supposed lymphatic VEGFR-3–positive vessels are indistinguishable from the honeycomb-like lobular pattern of the choriocapillaris.17 Furthermore, the study of Koina et al. includes a blatant inconsistency in the use and documentation of immunohistochemical markers between fetal and adult eyes. Although one has to acknowledge that certain lymphatic markers might be expressed during embryogenesis, this pattern easily changes during maturation.18 Therefore, such an approach would require extensive comparison of the same markers in different ages, thus representing an extensive survey in its own right. However, this is not the case in the study of Koina et al. Third, the ultrastructural study would be greatly strengthened by immunoelectron microscopy. Indeed, anchoring filaments with a diameter of 40 to 100 A—becoming readily identifiable only at magnifications of 40,000× to 50,000×—are present in lymphatics,19 but could be easily present in the choroid as well without any association to lymphatic vessels,20–22 particularly in aged eyes with typical alterations of the extracellular matrix. For this purpose, as well as for ruling out Weibel-Palade bodies, serial ultrathin sectioning with appropriate labeling would be necessary. Despite possible postmortem tissue alterations, numerous previous studies successfully applied different detection systems for ultrastructural investigations using ocular human donor tissue.23–29 A limited use of immunomarkers for these investigations, as claimed, seems therefore not justified. In regard to the above-mentioned criticisms, the evidence presented in the study of Koina et al. does not justify the hypothesized paradigm shift that functional lymphatic vessels are present in the human choroid. Rather, the findings of Koina et al. confirm previous reports of net-like structures with a “pseudo-vessel” appearance in the human choroid endowed with lymphatic vascular precursor cells (represented as LYVE-1+ macrophages).25 Those “atypical” lymphatic-like cells (i.e., endothelial cells with divergent or uncommon immunohistochemical phenotypes) may also exist in other parts of the eye. For example, the endothelial cells of Schlemms canal display many, but not all, features of terminally differentiated lymphatic endothelial cells, including responsiveness to VEGF-C–induced lymphangiogenesis.30 In closing, we acknowledge that the work of Koina et al. is a further contribution to our understanding of the choroid, but although the existence of lymphatics in the human choroid cannot be ruled out per se, because of the aforementioned points and the sheer volume of evidence to date, we maintain that the inner human eye and in particular the choroid should still be considered an immune-privileged site devoid of lymphatic vessels. Further unequivocal evidence of “typical lymphatic vessels” in the human choroid is still missing.

Expression of Lymphatic Markers in the Adult Rat Spinal Cord

Under physiological conditions, lymphatic vessels are thought to be absent from the central nervous system (CNS), although they are widely distributed within the rest of the body. Recent work in the eye, i.e., another organ regarded as alymphatic, revealed numerous cells expressing lymphatic markers. As the latter can be involved in the response to pathological conditions, we addressed the presence of cells expressing lymphatic markers within the spinal cord by immunohistochemistry. Spinal cord of young adult Fisher rats was scrutinized for the co-expression of the lymphatic markers PROX1 and LYVE-1 with the cell type markers Iba1, CD68, PGP9.5, OLIG2. Rat skin served as positive control for the lymphatic markers. PROX1-immunoreactivity was detected in many nuclei throughout the spinal cord white and gray matter. These nuclei showed no association with LYVE-1. Expression of LYVE-1 could only be detected in cells at the spinal cord surface and in cells closely associated with blood vessels. These cells were found to co-express Iba1, a macrophage and microglia marker. Further, double labeling experiments using CD68, another marker found in microglia and macrophages, also displayed co-localization in the Iba1+ cells located at the spinal cord surface and those apposed to blood vessels. On the other hand, PROX1-expressing cells found in the parenchyma were lacking Iba1 or PGP9.5, but a significant fraction of those cells showed co-expression of the oligodendrocyte lineage marker OLIG2. Intriguingly, following spinal cord injury, LYVE-1-expressing cells assembled and reorganized into putative pre-vessel structures. As expected, the rat skin used as positive controls revealed classical lymphatic vessels, displaying PROX1+ nuclei surrounded by LYVE-1-immunoreactivity. Classical lymphatics were not detected in adult rat spinal cord. Nevertheless, numerous cells expressing either LYVE-1 or PROX1 were identified. Based on their localization and overlapping expression with Iba1, the LYVE-1+ cell population likely represents a macrophage subpopulation, while a significant fraction of PROX1+ cells belong to the oligodendrocytic lineage based on their distribution and the expression of OLIG2. The response of these LYVE-1+ and PROX1+ cell subpopulations to pathological conditions, especially in spinal cord inflammatory conditions, needs to be further elucidated.

The Effect of Vasopressin on Ciliary Blood Flow and Aqueous Flow

PURPOSE Previous experiments have shown that arginine-vasopressin (AVP) reduces intraocular pressure (IOP) dose-dependently. The present study investigated the relationships between IOP, ciliary blood flow (CilBF), and aqueous flow (AqF) responses to AVP in anesthetized rabbits. METHODS CilBF was measured by laser Doppler flowmetry and AqF by fluorophotometry. Mean arterial pressure (MAP) and IOP were monitored continuously and simultaneously. Perfusion pressure (PP) was varied mechanically. Four experimental protocols were performed: the dose-response (n = 11) and the pressure-flow relationship (n = 8) for CilBF and the effects on CilBF, and AqF at low (0.08 ng/kg/min; n = 14) and high AVP infusion rates (1.33 ng/kg/min; n = 12). RESULTS AVP decreased CilBF and IOP dose-dependently. At the low AVP infusion rate, AqF was reduced by 21.48% ± 2.52% without changing CilBF significantly. The high AVP infusion rate caused a 24.49% ± 3.53% decrease of AqF and a significant reduction in CilBF (35.60% ± 3.58%). IOP was reduced by 9.56% ± 2.35% at low and by 41.02% ± 3.19% at high AVP infusion rates. Based on the Goldmann equation, the decrease of AqF at the low AVP infusion rate accounted for 77.1% of the IOP reduction, whereas at the high AVP infusion rate, decreased AqF accounted for 28.4% of the IOP decline. CONCLUSIONS The results indicate that AVP can modulate IOP by different dose-dependent physiological mechanisms. The shifts of the CilBF-AqF relationship suggest that the reduction of AqF by the low AVP infusion rate is mainly provoked by inhibiting secretory processes in the ciliary epithelium. In contrast, at the high AVP infusion rate, the AqF reduction is caused by either reduced CilBF or more likely by a combined effect of reduced CilBF and secretory inhibition.

Aqueous humor ferritin in hereditary hyperferritinemia cataract syndrome.

Purpose Hereditary hyperferritinemia cataract syndrome (HHCS) is a rare autosomal dominant hereditary disease, characterized by hyperferritinemia but with absence of body iron excess and early onset of bilateral cataracts. Although 5- to 20-fold increased serum ferritin concentrations have been reported in HHCS patients, data of ferritin levels in aqueous humor have not been obtained. We therefore aimed to investigate the ferritin levels in aqueous humor and serum and further present histological and ultrastructural data of the lens. Methods During cataract extraction and intraocular lens implantation, aqueous humor and lens aspirate of a 37-year-old HHCS patient were obtained from both eyes. Ferritin levels in serum and aqueous humor were quantitatively analyzed via immunoassays in the HHCS patient and healthy control subjects (n = 6). Lens aspirate in HHCS was analyzed histologically and at the ultrastructural level. Further, genetic mutation screening by polymerase chain reaction and DNA sequencing in blood was performed. Results Serum ferritin levels in the control group were 142.2 ± 38.7 &mgr;g/L, whereas in the HHCS patient, this parameter was excessively increased (1086 &mgr;g/L). Analysis of ferritin in aqueous humor revealed 6.4 ± 3.8 &mgr;g/L in normal control subjects and 146.3 &mgr;g/L (OD) and 160.4 &mgr;g/L (OS) in the HHCS patient. DNA analysis detected a C>A mutation on position +18, a T>G mutation on position +22, a T>C mutation on position +24, and a T>G polymorphism on position +26 in the iron-responsive element of the light-chain ferritin (L-ferritin) gene. Conclusions In the HHCS patient, a 23-fold (OD) to 25-fold (OS) increased aqueous humor ferritin level was detected. Therefore, the formation of bilateral cataract in HHCS is most likely a result of elevated aqueous humor ferritin. In addition, a novel mutation in this rare disease in the iron-responsive element of L-ferritin gene is reported.

Lymphatic and vascular markers in an optic nerve crush model in rat

Abstract Only few tissues lack lymphatic supply, such as the CNS or the inner eye. However, if the scleral border is compromised due to trauma or tumor, lymphatics are detected in the eye. Since the situation in the optic nerve (ON), part of the CNS, is not clear, the aim of this study is to screen for the presence of lymphatic markers in the healthy and lesioned ON. Brown Norway rats received an unilateral optic nerve crush (ONC) with defined force, leaving the dura intact. Lesioned ONs and unlesioned contralateral controls were analyzed 7 days (n = 5) and 14 days (n = 5) after ONC, with the following markers: PDGFRb (pericyte), Iba1 (microglia), CD68 (macrophages), RECA (endothelial cell), GFAP (astrocyte) as well as LYVE‐1 and podoplanin (PDPN; lymphatic markers). Rat skin sections served as positive controls and confocal microscopy in single optical section mode was used for documentation. In healthy ONs, PDGFRb is detected in vessel‐like structures, which are associated to RECA positive structures. Some of these PDGFRb+/RECA+ structures are closely associated with LYVE‐1+ cells. Homogenous PDPN‐immunoreactivity (IR) was detected in healthy ON without vascular appearance, showing no co‐localization with LYVE‐1 or PDGFRb but co‐localization with GFAP. However, in rat skin controls PDPN‐IR was co‐localized with LYVE‐1 and further with RECA in vessel‐like structures. In lesioned ONs, numerous PDGFRb+ cells were detected with network‐like appearance in the lesion core. The majority of these PDGFRb+ cells were not associated with RECA‐IR, but were immunopositive for Iba1 and CD68. Further, single LYVE‐1+ cells were detected here. These LYVE‐1+ cells were Iba1‐positive but PDPN‐negative. PDPN‐IR was also clearly absent within the lesion site, while LYVE‐1+ and PDPN+ structures were both unaltered outside the lesion. In the lesioned area, PDGFRb+/Iba1+/CD68+ network‐like cells without vascular association might represent a subtype of microglia/macrophages, potentially involved in repair and phagocytosis. PDPN was detected in non‐lymphatic structures in the healthy ON, co‐localizing with GFAP but lacking LYVE‐1, therefore most likely representing astrocytes. Both, PDPN and GFAP positive structures are absent in the lesion core. At both time points investigated, no lymphatic structures can be identified in the lesioned ON. However, single markers used to identify lymphatics, detected non‐lymphatic structures, highlighting the importance of using a panel of markers to properly identify lymphatic structures. HighlightsPDPN expression was detected in GFAP positive astrocytes in the rat ON.LYVE‐1 is expressed in cells with microglial/macrophagic activity.Absence of lymphatic structures within the rat ON under physiological conditions.No formation of lymphatic structures within the rat ON following ONC trauma.