Hyaluronan production regulation from porcine hyalocyte cell line by cytokines

Vitreous substitutes and tamponades - A review of types, applications, and future directions

Vitreous substitutes and tamponades occupy the vitreous cavity following vitrectomy in the management of various conditions such as retinal detachment, macular hole, and diabetic retinopathy. Such replacements can be for the short term (gases such as sulfur hexafluoride (SF6) and perfluoropropane (C3F8) or long term (such as silicone oils). Certain substitutes such as perfluorocarbon liquids are used only transiently during surgery as "a third hand" or rarely till a few days post surgery. Hydrogels and hyaluronan derivatives are among the newer vitreous substitutes that are showing promise for the future, albeit still under investigation. still being investigated for use as vitreous substitutes. These materials have properties similar to the natural vitreous and may offer advantages such as improved biocompatibility and biodegradability. Although vitreous substitutes are valuable tools in treating vitreoretinal conditions, they carry risks and potential complications such as cataract formation, glaucoma, and inflammation. The current communication extensively reviews the available literature on vitreous tamponades. It details the composition and properties of various vitreous substitutes and tamponades available for the clinician, highlighting the techniques of usage, indications, and limitations.

Advances in Polysaccharide- and Synthetic Polymer-Based Vitreous Substitutes

The vitreous humour is a gel-like structure that composes the majority of each eye. It functions to provide passage of light, be a viscoelastic dampener, and hold the retina in place. Vitreous liquefaction causes retinal detachment and retinal tears requiring pars plana vitrectomy for vitreous substitution. An ideal vitreous substitute should display similar mechanical, chemical, and rheological properties to the natural vitreous. Currently used vitreous substitutes such as silicone oil, perfluorocarbon liquids, and gases cannot be used long-term due to adverse effects such as poor retention time, cytotoxicity, and cataract formation. Long-term, experimental vitreous substitutes composed of natural, modified and synthetic polymers are currently being studied. This review discusses current long- and short-term vitreous substitutes and the disadvantages of these that have highlighted the need for an ideal vitreous substitute. The review subsequently focuses specifically on currently used polysaccharide- and synthetic polymer-based vitreous substitutes, which may be modified or functionalised, or employed as the derivative, and discusses experimental vitreous substitutes in these classes. The advantages and challenges associated with the use of polymeric substitutes are discussed. Innovative approaches to vitreous substitution, namely a novel foldable capsular vitreous body, are presented, as well as future perspectives related to the advancement of this field.

Identifying Hyperreflective Foci in Diabetic Retinopathy via VEGF-Induced Local Self-Renewal of CX3CR1+ Vitreous Resident Macrophages

Intraretinal hyperreflective foci (HRF) are significant biomarkers for diabetic macular edema. However, HRF at the vitreoretinal interface (VRI) have not been examined in diabetic retinopathy (DR). A prospective observational clinical study with 162 consecutive eyes using OCT imaging showed significantly increased HRF at the VRI during DR progression (P < 0.01), which was reversed by anti-vascular endothelial growth factor (VEGF) therapy. F4/80+ macrophages increased significantly at the VRI in Kimba (vegfa+/+) or Akimba (Akita × Kimba) mice (both P < 0.01), but not in diabetic Akita (Ins2+/-) mice, indicating macrophage activation was modulated by elevated VEGF rather than the diabetic milieu. Macrophage depletion significantly reduced HRF at the VRI (P < 0.01). Furthermore, BrdU administration in Ccr2rfp/+Cx3cr1gfp/+vegfa+/- mice identified a significant contribution of M2-like tissue-resident macrophages (TRMs) at the VRI. Ki-67+ and CD11b+ cells were observed in preretinal tissues of DR patients, while exposure of vitreal macrophages to vitreous derived from PDR patients induced a significant proliferation response in vitro (P < 0.01). Taken together, the evidence suggests that VEGF drives a local proliferation of vitreous resident macrophages (VRMs) at the VRI during DR. This phenomenon helps to explain the derivation and disease-relevance of the HRF lesions observed through OCT imaging in patients.

Hyalocytes in proliferative vitreo-retinal diseases

Introduction

Hyalocytes are sentinel macrophages residing within the posterior vitreous cortex anterior to the retinal inner limiting membrane (ILM). Following anomalous PVD and vitreoschisis, hyalocytes contribute to paucicellular (vitreo-macular traction syndrome, macular holes) and hypercellular (macular pucker, proliferative vitreo-retinopathy, proliferative diabetic vitreo-retinopathy) diseases.

Areas covered

Studies of human tissues employing dark-field, phase, and electron microscopy; immunohistochemistry; and in vivo imaging of human hyalocytes.

Expert opinion

Hyalocytes are important in early pathophysiology, stimulating cell migration and proliferation, as well as subsequent membrane contraction and vitreo-retinal traction. Targeting hyalocytes early could mitigate advanced disease. Ultimately, eliminating the role of vitreous and hyalocytes may prevent proliferative vitreo-retinal diseases entirely.

Age-related increase of let-7 family microRNA in rat retina and vitreous

Vitreous alterations occur from early stages and continue through the normal aging, with gradual lamellae formation and the appearance of liquefied spaces, which eventually leads to complications, such as retinal tear, retinal detachment, and intravitreal hemorrhage. The aim of the present study was to investigate the expression of let-7 miRNA family in the vitreous and retina in newborn (1-3- day-old), young adult (2-month-old), and aging (12-month-old) rats, as well as their role as regulators of vitreous components. MicroRNAs are small, non-coding RNAs that post-transcriptionally regulate gene expression. Our results showed detection of all investigated let-7 isoforms (let-7a, let-7b, let-7c, let-7d, let-7e, let-7f and let-7i) in the retina and vitreous. Although most let-7 members were significantly upregulated in the vitreous during development, only let-7b, let-7c, and let-7e followed this same expression pattern in the retina. Let-7b and -7c increased in aging vitreous as well, and were expressed in vitro by Müller glial cells and their extracellular vesicles. Moreover, let-7 targeted hyaluronan synthase 2 (Has2) mRNA, a synthesizing enzyme of hyaluronan. These observations indicate that let-7 function is important during retina and vitreous development, and that isoforms of let-7 increased with aging, potentially modulating hyaluronan content.

[Hyalocytes of the vitreous body and their role in ophthalmic pathology]

Hyalocytes of the vitreous body are variety of tissue macrophages that exercise various functions - from ensuring the synthesis of extracellular matrix components and modulating the immune response in the vitreous body to participating in different stages of inflammatory processes. Some aspects of the biology of hyalocytes remain poorly understood and controversial. However, the vitreous cells are starting to be considered a potential point of application in the treatment of diseases of the vitreous body and the retina.

Differences in the molecular structure of the blood-brain barrier in the cerebral cortex and white matter: an in silico, in vitro, and ex vivo study

The blood-brain barrier (BBB) is the main interface controlling molecular and cellular traffic between the central nervous system (CNS) and the periphery. It consists of cerebral endothelial cells (CECs) interconnected by continuous tight junctions, and closely associated pericytes and astrocytes. Different parts of the CNS have diverse functions and structures and may be subject of different pathologies, in which the BBB is actively involved. It is largely unknown, however, what are the cellular and molecular differences of the BBB in different regions of the brain. Using in silico, in vitro, and ex vivo techniques we compared the expression of BBB-associated genes and proteins (i.e., markers of CECs, brain pericytes, and astrocytes) in the cortical grey matter and white matter. In silico human database analysis (obtained from recalculated data of the Allen Brain Atlas), qPCR, Western blot, and immunofluorescence studies on porcine and mouse brain tissue indicated an increased expression of glial fibrillary acidic protein in astrocytes in the white matter compared with the grey matter. We have also found increased expression of genes of the junctional complex of CECs (occludin, claudin-5, and α-catenin) in the white matter compared with the cerebral cortex. Accordingly, occludin, claudin-5, and α-catenin proteins showed increased expression in CECs of the white matter compared with endothelial cells of the cortical grey matter. In parallel, barrier properties of white matter CECs were superior as well. These differences might be important in the pathogenesis of diseases differently affecting distinct regions of the brain.

Correlative Microscopy of Lamellar Hole-Associated Epiretinal Proliferation

Purpose. To describe morphology of lamellar hole-associated epiretinal proliferation (LHEP) removed from eyes with lamellar macular holes (LMH). Methods. Based on optical coherence tomography data, 10 specimens of LHEP were removed from 10 eyes with LMH during standard vitrectomy. Specimens were prepared for correlative light and electron microscopy (CLEM) using an immunonanogold particle of 1.4 nm diameter that was combined with a fluorescein moiety, both having been attached to a single antibody fragment. As primary antibodies, we used antiglial fibrillary acidic protein (GFAP), anti-CD45, anti-CD64, anti-α-smooth muscle actin (α-SMA), and anticollagen type I and type II. Results. In LHEP, GFAP-positive cells possess ultrastructural characteristics of fibroblasts and hyalocytes. They represent the major cell types and were densely packed in cell agglomerations on vitreous collagen strands. Epiretinal cells of LHEP rarely demonstrated contractive properties as α-SMA-positive myofibroblasts were an infrequent finding. Conclusion. CLEM indicates that epiretinal cells in LHEP might originate from the vitreous and that remodelling processes of vitreous collagen may play an important role in pathogenesis of eyes with LMH.

Retinal Changes Induced by Epiretinal Tangential Forces

Two kinds of forces are active in vitreoretinal traction diseases: tangential and anterior-posterior forces. However, tangential forces are less characterized and classified in literature compared to the anterior-posterior ones. Tangential epiretinal forces are mainly due to anomalous posterior vitreous detachment (PVD), vitreoschisis, vitreopapillary adhesion (VPA), and epiretinal membranes (ERMs). Anomalous PVD plays a key role in the formation of the tangential vectorial forces on the retinal surface as consequence of gel liquefaction (synchysis) without sufficient and fast vitreous dehiscence at the vitreoretinal interface. The anomalous and persistent adherence of the posterior hyaloid to the retina can lead to vitreomacular/vitreopapillary adhesion or to a formation of avascular fibrocellular tissue (ERM) resulting from the proliferation and transdifferentiation of hyalocytes resident in the cortical vitreous remnants after vitreoschisis. The right interpretation of the forces involved in the epiretinal tangential tractions helps in a better definition of diagnosis, progression, prognosis, and surgical outcomes of vitreomacular interfaces.

Vitreous substitutes: the present and the future

Vitreoretinal surgery has advanced in numerous directions during recent years. The removal of the vitreous body is one of the main characteristics of this surgical procedure. Several molecules have been tested in the past to fill the vitreous cavity and to mimic its functions. We here review the currently available vitreous substitutes, focusing on their molecular properties and functions, together with their adverse effects. Afterwards we describe the characteristics of the ideal vitreous substitute. The challenges facing every ophthalmology researcher are to reach a long-term intraocular permanence of vitreous substitute with total inertness of the molecule injected and the control of inflammatory reactions. We report new polymers with gelification characteristics and smart hydrogels representing the future of vitreoretinal surgery. Finally, we describe the current studies on vitreous regeneration and cell cultures to create new intraocular gels with optimal biocompatibility and rheological properties.

Inflammatory mechanisms of idiopathic epiretinal membrane formation

The pathogenesis of idiopathic epiretinal membranes (iERMs), a common pathology found in retina clinics, still eludes researchers to date. Ultrastructural studies of iERMs in the past have failed to identify the cells of origin due to the striking morphologic changes of cells involved via transdifferentiation. Thus, immunohistochemical techniques that stain for the cytostructural components of cells have confirmed the importance of glial cells and hyalocytes in iERM formation. The cellular constituents of iERMs are thought to consist of glial cells, fibroblasts, hyalocytes, etc. that, in concert with cytokines and growth factors present in the vitreous, lead to iERM formation. Recently, research has focused on the role of the posterior hyaloid in iERM formation and contraction, particularly the process of anomalous PVD as it relates to iERM formation. Recent advances in proteomics techniques have also elucidated the growth factors and cytokines involved in iERM formation, most notably nerve growth factor, glial cell line-derived growth factor, and transforming growth factor β1.

Cloning and characterization of human vitreous tissue-derived cells

PURPOSE

Previously, we established a porcine vitreous tissue-derived hyalocyte cell line (PH5) and investigated the regulation of hyaluronan synthesis in these cells by cytokines. The objective of the current study was to establish human vitreous tissue-derived cells and to compare their characteristics with those of PH5 cells.

METHODS

Human vitreous specimens from two patients were cultured in the presence of 10% foetal bovine serum and immortalized by infection with human papilloma virus 16 genes E6 and E7. We used reverse transcription polymerase chain reaction (RT-PCR) to analyse and compare the expression profiles for several genes in the human vitreous tissue-derived cells and PH5 cells. To investigate the regulation of hyaluronan production in response to cytokine stimulation, the expression of hyaluronan synthase isoforms was examined using RT-PCR, and hyaluronan production was measured using enzyme-linked immunosorbent assay (ELISA).

RESULTS

Two types of cells, HV64 and HV65, were derived from human vitreous tissue. The HV64 and HV65 cell-doubling times were 58 r and 76 hr, respectively. The cells expressed messenger RNA (mRNAs) encoding collagen type I α1 (COL1A1), collagen type II α1 (COL2A1), CD11b, CD14, CD68, CD204 and CD206 but did not express mRNA for glial fibrillary acidic protein (GFAP). Cytokine stimulation did not induce the expression of hyaluronan synthase mRNA or the production of hyaluronan. In contrast, mRNAs for GFAP and hyaluronan synthase-2 were expressed in the porcine PH5 cells, and treatment with transforming growth factor-β1 and/or platelet-derived growth factor-BB induced the production of hyaluronan in PH5 cells.

CONCLUSION

The new human vitreous tissue-derived cells have macrophage-like characteristics and are different from our previously developed porcine hyalocyte cells. These human vitreous tissue-derived cells might be useful for studies of human intraocular diseases.

Vitreous substitutes: a comprehensive review

Vitreoretinal disorders constitute a significant portion of treatable ocular disease. Advances in vitreoretinal surgery have included the development and characterization of suitable substitutes for the vitreous. Air, balanced salt solutions, perfluorocarbons, expansile gases, and silicone oil serve integral roles in modern vitreoretinal surgery. Vitreous substitutes vary widely in their properties, serve different clinical functions, and present different shortcomings. Permanent vitreous replacement has been attempted with collagen, hyaluronic acid, hydroxypropylmethylcellulose, and natural hydrogel polymers. None, however, have proven to be clinically viable. A long-term vitreous substitute remains to be found, and recent research suggests promise in the area of synthetic polymers. Here we review the currently available vitreous substitutes, as well those in the experimental phase. We classify these compounds based on their functionality, composition, and properties. We also discuss the clinical use, advantages, and shortcomings of the various substitutes. In addition we define the ideal vitreous substitute and highlight the need for a permanent substitute with long-term viability and compatibility. Finally, we attempt to define the future role of biomaterials research and the various functions they may serve in the area of vitreous substitutes.

Hyalocytes: essential cells of the vitreous cavity in vitreoretinal pathophysiology?

PURPOSE

To review the present understanding of hyalocytes.

METHODS

A review of recent studies that investigated the roles of hyalocytes in the pathophysiology of the vitreous cavity.

RESULTS

Studies on immunocytochemistry and chimeric mice with green fluorescent protein transgenic mice show that hyalocytes belong to the monocyte/macrophage lineage and derive from bone marrow. The effects of hyalocytes on the vitreous cavity environment can be divided into three categories: synthesis of extracellular matrix, regulation of the vitreous cavity immunology, and modulation of inflammation. In noninflamed eyes, vitreous cavity is an immune-privileged site that is maintained by a system called vitreous cavity-associated immune deviation, in which hyalocytes play the role of antigen-presenting cells. However, cultured hyalocytes proliferate in response to inflammatory molecules and secrete vascular endothelial growth factor and urokinase-type plasminogen activator. A collagen gel embedded with hyalocytes contracts over time, which is enhanced by transforming growth factor-β but is inhibited by Rho kinase inhibitor. These results suggest that hyalocytes can be an exacerbating factor in inflamed eyes. Clinically, hyalocytes are frequently found in the surgically removed specimens of epiretinal membrane or proliferative vitreoretinopathy.

CONCLUSION

Elucidating the properties of hyalocytes is important to understand the biology of vitreous cavity and to develop novel treatments for vitreoretinal diseases.

Interactions between vitreous-derived cells and vascular endothelial cells in vitreoretinal diseases

PURPOSE

This study aimed to investigate the roles played by vitreous-derived cells in the pathogenesis of vitreoretinal vascular diseases.

METHODS

The vitreous was removed from porcine eyes and small pieces were cultured from which vitreous-derived cells were isolated. Polymerase chain reaction and ELISA were performed to determine the expression of vascular endothelial growth factor (VEGF) and interleukin 6 (IL-6) at the mRNA and protein levels, respectively. The viability of human retinal endothelial cells (HRECs) exposed to vitreous-derived cells was assessed by MTT assay.

RESULTS

Expression of the mRNA and protein of VEGF and IL-6 was increased by exposing the porcine vitreous-derived cells (PVDCs) to interleukin-1alpha (IL-1alpha), interleukin-1beta (IL-1beta) and tumour necrosis factor alpha (TNFalpha), but not to VEGF or IL-6. The percentage of living human vascular endothelial cells was increased by including VEGF and IL-6 in the culture media. The viability of HRECs was affected by co-culturing them with PVDCs that had been exposed to IL-1alpha, IL-1beta, IL-6, TNFalpha and VEGF.

CONCLUSIONS

Porcine vitreous-derived cells are stimulated by IL-1alpha, IL-1beta and TNFalpha, and produce VEGF and IL-6, which then enhance the proliferation of vascular endothelial cells. This network, including the cytokines and different types of cells, may contribute to the pathogenesis of proliferative vitreoretinal diseases.

Vitreoschisis in macular diseases

OBJECTIVES

Vitreoschisis is a possible pathogenic mechanism in macular diseases. Thus, the vitreoretinal interface was evaluated in monkey eyes and patients with various macular diseases in search of vitreoschisis. It is hypothesised that vitreoschisis is present in macular holes (MH) and macular pucker (MP), but not in other maculopathies.

METHODS

Histopathology was studied in 14 monkey eyes and a vitrectomy specimen of a patient with macular pucker. Optical coherence tomography/scanning laser ophthalmoscopy (OCT/SLO) was performed in 239 eyes: 45 MH, 45 MP, 51 dry age-related macular degeneration (AMD), 53 non-proliferative diabetic retinopathy (NPDR) and 45 controls.

RESULTS

Immunohistochemistry demonstrated lamellae in the posterior vitreous cortex of 12/14 (86%) monkey eyes. With OCT/SLO, vitreoschisis was detected in 24/45 (53%) MH and 19/45 (42%) MP eyes, but in only 7/53 (13%) NPDR, 3/51 (6%) AMD and 3/45 (7%) control eyes (p<0.001 for all comparisons). Rejoining of the inner and outer walls of the split posterior vitreous cortex was visible in 16/45 (36%) MH eyes and 15/45 (33%) MP eyes. Histopathology of the MP specimen confirmed a split with rejoining in the posterior vitreous cortex.

CONCLUSIONS

Vitreoschisis was detected in half of eyes with MH and MP, but much less frequently in controls, AMD and NPDR patients. These findings suggest that anomalous PVD with vitreoschisis may be pathogenic in MH and MP.

The in vitro response of human retinal endothelial cells to cytokines and other chemically active agents is altered by coculture with vitreous-derived hyalocytes

BACKGROUND

Ocular angiogenesis is regulated by polypeptides including cytokines, which are known to affect vascular endothelial cells. We have reported that hyalocytes interact with vascular endothelial cells, and some cytokines affect these interactions.

AIMS

To determine the effect of various chemically active agents on the viability of endothelial cells alone and cocultured with hyalocytes.

METHODS

The viability of human retinal endothelial cells (HRECs) was determined after exposure to IL-1alpha, IL-1beta, IL-6, TNFalpha and VEGF using the MTT assay. These results were compared to the viability when the HRECs were cocultured with porcine hyalocytes that had been exposed to different types of cytokines. The effects of bevacizumab, fenofibrate and dexamethasone on the viability of HRECs in coculture with hyalocytes were also assessed.

RESULTS

Ten micrograms/millilitre of bevacizumab decreased the percentage of living HRECs stimulated by VEGF without hyalocytes, but with the hyalocytes, 100 microg/ml of bevacizumab was required to decrease the percentage of viable HRECs stimulated by VEGF. Fenofibrate, at 5 microg/ml, decreased the viability of HRECs stimulated by IL-1beta and VEGF without hyalocytes but could not decrease the viability of HRECs cocultured with hyalocytes. Dexamethasone, at 50 microg/ml, decreased the viability of HRECs stimulated by IL-1alpha, IL-1beta, IL-6 and VEGF without hyalocytes but could not decrease the viability of HRECs cocultured.

CONCLUSIONS

Coculturing HRECs with vitreous-derived hyalocytes depressed the effects of cytokines, bevacizumab, fenofibrate and dexamethasone. This suggests that the vitreal hyalocytes may play a role in pathogenic endothelial cell proliferation in vivo. Future studies to better understand this pathobiology should utilize coculture systems of HRECs and vitreal hyalocytes.

Cloning and characterization of cell strains derived from human corneal stroma and sclera

PURPOSE

To establish human corneal stroma- and sclera-derived cells as models for studying diseases of the anterior segment of the eye.

METHODS

Using a recombinant retrovirus system, we transfected human papilloma virus 16 E6 and E7 (HPV16 E6/E7) into human corneal stroma- and sclera-derived cells. The primary cells and established cell strains were characterized by assessing the mRNA expression of collagen, matrix metalloproteinase, and tissue inhibitor of metalloproteinase by reverse transcription-polymerase chain reaction. We also examined the effects of inflammatory cytokines on hyaluronan synthase expression and hyaluronan products.

RESULTS

Both a corneal stroma-derived cell strain, Cs3, and a sclera-derived cell strain, Sc1, were obtained, and both cell strains could be passaged up to 25 times. The mRNA expression pattern observed in the primary cells was identical to that observed in the cell strains. Hyaluronan synthase 1 and 2 mRNAs were increased by transforming growth factor beta and platelet-derived growth factor BB. Significant differences were observed between the hyaluronan products with and without cytokine treatment.

CONCLUSION

Cell strains derived from corneal stroma and sclera fibroblast cells can be established using HPV16 E6/E7 immortalized genes of the same origin. The phenotypic cell characteristics did not change after transfection, immortalization, or successive passages in culture.

Dermal hyaluronan is rapidly reduced by topical treatment with glucocorticoids

Skin atrophy is part of the normal ageing process, but is accelerated by topical glucocorticoid (GC) treatments that are widely used in dermatology. Hyaluronan (HA) is one of the most abundant components of the cutaneous extracellular matrix and is involved in tissue homeostasis, hydration, and repair processes, but little is known about the effects of GCs on HA synthesis and stability. Here we examined the regulation of HA metabolism in human skin during GC therapy. Expression of the HA synthesizing enzymes hyaluronan synthase (HAS)-2 and HAS-3 and the HA degrading enzymes HYAL-1, HYAL-2, and HYAL-3 in response to GC treatment was evaluated. HAS-2 expression was markedly suppressed by dexamethasone treatment of cultured fibroblasts and HaCaT keratinocyte cells, and in human skin biopsies taken from volunteers treated with dexamethasone ointment. Consistently, the HA content of cell culture supernatants and in human skin was reduced after dexamethasone treatment. Hyaluronidase expression and activity, on the other hand, was not altered by dexamethasone treatment. These data show that the levels of skin HA rapidly decrease after short-term GC treatment due to a reduction in HA synthesis, while HA degradation is not changed. This may reflect an initiation of skin atrophy in response to topically applied GCs.

Morphological characterization of pecteneal hyalocytes in the developing quail retina

The periphery of the vitreous body contains a population of cells termed hyalocytes. Despite the existence for more than one century of publications devoted to the pecten oculi, a convoluted coil of blood vessels that seems to be the primary source of nutrients for the avian avascular retina, little information can be found concerning the pecteneal hyalocytes. These cells are situated on the inner limiting membrane in close relationship with the convolute blood vessels. To characterize the origin and macrophagic activity of pecteneal hyalocytes, we have analysed two different stages of quail eye development using histochemistry and immunohistochemistry. Pecteneal hyalocytes express the QH1 epitope and cKit, confirming that these cells belong to the haematopoietic system. They also express vimentin, an intermediate filament protein present in cells of mesenchymal origin and very important for differentiation of fully active macrophages. However, similarly as described in porcine hyalocytes, pecteneal hyalocytes express the glial fibrillary acidic protein, a recognized neuroglial marker. Pecteneal hyalocytes did not express other neuroglial markers, such as glutamine synthetase or S100. Acidic phosphatase was activated and Lep100 was found in secondary lysosomes, confirming phagocytic activity of pecteneal hyalocytes during ocular development. Pecteneal hyalocytes strongly react with RCA-I, WFA, WGA, PNA, SNA, LEA and SBA lectins, whereas other avian macrophages from thymus and the bursa of Fabricius did not bind PNA, SNA and LEA lectins. Interestingly, WGA lectin reacts with all kinds of avian macrophages, including pecteneal hyalocytes, probably reflecting the specific binding of WGA to components of the phagocytic and endocytic pathways. In conclusion, pecteneal hyalocytes are a special subtype of blood-borne macrophages that express markers not specifically associated with the haematopoietic system.