Detection of mucus glycoconjugates in human conjunctiva by using the lectin colloidal gold technique in TEM. I. A quantitative study in normal subjects

Meibomian Gland Dysfunction and Contact Lens Discomfort

Meibomian glands are located in the eyelids and secrete meibum, which gives rise to the lipid layer of the tear film. Changes to these glands can lead to the development of meibomian gland dysfunction (MGD), which is associated with various ocular symptoms such as fatigue, dryness, burning sensation, and heavy sensation. The diagnosis of MGD thus relies on evaluation of ocular symptoms, meibum condition, and lid margin abnormalities. The recent development of noninvasive meibography and tear interferometry has provided important insight into meibomian gland structure and function, respectively. Wearers of contact lenses complain of ocular symptoms that are thought to be attributable to a variety of causes, such as a diminished aqueous or mucin layer of the tear film, changes in tear protein concentration, and altered meibomian gland structure or function. Many studies have examined the relation between contact lens wear and meibomian gland changes. Such studies have found that lens wear is associated with adverse changes in meibomian gland morphology and in the condition of the lid margin and meibum, suggesting that contact lenses negatively affect meibomian glands. Meibomian gland dysfunction-like changes in meibomian glands induced by contact lens wear may thus be responsible for at least some of the ocular symptoms in lens wearers.

Evaluation of ocular surface glycocalyx using lectin-conjugated fluorescein

Purpose:

A glycocalyx plays important roles in the ocular surface epithelium, but there is no direct simple method to evaluate ocular surface glycocalyx. We tested a wheat germ agglutinin conjugate of fluorescein (F-WGA) as a marker to demonstrate ocular surface glycocalyx in vivo.

Methods:

After a 5% F-WGA solution was applied to the eyes of eight healthy volunteers, fluorescent intensities of the central cornea and bulbar conjunctiva were measured by fluorophotometry. A 10% fluorescein-conjugated dextran solution served as the control. Changes in fluorescent intensities of the ocular surface following a challenge with 5% N-acetyl cysteine, a conventional mucolytic agent, were tested in the second experiment. Saline instillation served as a control.

Results:

The ocular surface was diffusely stained by F-WGA. Breakup of the precorneal tear film was not apparent, possibly because F-WGA was bound to the glycocalyx of the ocular surface epithelium. F-WGA fluorescent intensities were high in the superior, nasal, and inferior regions of the bulbar conjunctiva and low in the temporal conjunctiva and cornea. No such regional differences were observed with fluorescein-conjugated dextran. F-WGA fluorescent intensities decreased significantly with N-acetyl cysteine instillation, whereas they were not affected by saline instillation.

Conclusion:

The fluorophotometric method may be used to evaluate the glycocalyx quantitatively in the ocular surface in vivo.

Tissue engineering: current and future approaches to ocular surface reconstruction

Although cells have been cultured outside the body for many years, research has only recently begun to develop complex three-dimensional tissue constructs that will, ideally, mature into fully functional tissues and organs. Tissue engineering is an emerging field in the area of biotechnology that combines the principles and methods of life sciences with those of engineering for the purpose of regenerating, repairing, or replacing diseased tissues. In this review, we describe the recent advances and current development of tissue engineering approaches as related to the ocular surface system, which comprises the three main integrated tissue units: conjunctiva, cornea and lacrimal glands.

Collagen gel-embedding culture of conjunctival epithelial cells

BACKGROUND

Collagen has effects on cell morphology, differentiation characteristics and function. Using collagen gel culture, several studies about cell differentiation were reported. In this study, the differentiation of rabbit conjunctival epithelial cells in a collagen gel-embedding culture system was investigated by electron microscope and lectin labeling.

METHODS

Rabbit bulbar conjunctival epithelial cells were cultured in type I collagen gel. After 1 and 2 weeks of culture, some of these cells were stained with PAS and seven kinds of lectins, and others were examined by transmission electron microscopy.

RESULTS

The conjunctival epithelial cells cultured within collagen gel formed stratified cell layers and globules with cavities. The inner layer cells facing the cavities showed PAS and lectin staining patterns similar to those of conjunctival goblet cells in vivo, whereas the staining patterns of the outer layer cells on the collagen matrices resembled the patterns of non-goblet epithelial cells. Microvilli on the surface of the innermost cells, basement membranes beneath the outermost cells, tight junctions, adherent junctions, interdigitating folds and desmosomes between cells were identified on electron microscopic examination.

CONCLUSION

These results indicate that cell junction structures of the conjunctival epithelial cells are well developed in collagen gel-embedding culture systems, and that the inner layer cells have carbohydrates similar to those of conjunctival goblet cells. Culture of conjunctival epithelial cells within collagen gel is a useful model for examining differentiation of these cells.

Galactose- binding sites in the acinar cells of the human accessory lacrimal gland

This investigation for the first time has collected evidence of a specific glycoconjugate contribution of the acinar cells from accessory lacrimal glands to human tears. Amongst group III lectin binding glycoconjugates, monosaccharides seem to be more prominent than disaccharides. alpha (and less obvious beta) Galactose sugar moieties appear to be specifically important. A need for further differentiating investigations is outlined.

Ultrastructural localization of lectin-binding sites on the surface of the guinea pig conjunctival epithelium

The cell-surface binding sites of two lectins, concanavalin A (Con A) and wheat-germ agglutinin (WGA) in the guinea pig conjunctiva were investigated at the ultrastructural level by means of pre-embedding staining of the tissue with lectin-colloidal gold complexes. Wheat-germ agglutinin, which recognizes N-acetylglucosamine and sialic acid residues, showed prominent and fairly uniform binding to the microvilli. The binding was markedly increased in the vicinity of the goblet cells, indicating that the same carbohydrate ligands were also present in the mucus. In contrast, no binding of concanavalin A, which recognizes mannose and N-glucose, was observed. The results suggest the presence of sialic acid and galactose as the constituent carbohydrates of glycoconjugates in the surface membrane of conjunctival epithelial cells as well as in the mucus produced by the goblet cells.

Mucus alteration and eye dryness. A possible relationship

From a very early phase we studied 15 patients suffering from a dry-eye condition ant associated to systemic diseases. Conjunctival biopsies were studied in Transmission (TEM) and Scanning (SEM) electron microscopy. Moreover, the lectin-gold cytochemistry at ultrastructural level was applied to investigate the distribution of some glycosidic receptors produced by both the goblet cells and the vesicles belonging to the Second Mucus System (SMS). No evidence of epithelial stratification and only a decrease in the goblet cell population was observed. The SMS vesicles and the superficial cell microvilli did not appear greatly reduced in number. A difference in the mucus composition in terms of content of glycosidic residues was detected in dry-eye patients compared to the normal subjects. The role of the mucus produced by both the goblet cells and the SMS vesicles in debated. A possible correlation between the alteration of the mucus content and the failure of the tear film stability is proposed. On the basis of these data, a new therapeutic approach for eye dryness is suggested.

An improved processing method for electron microscopy investigation of conjunctival biopsies

A new schedule in the processing of conjunctival biopsies for ultrastructural investigations is proposed. The specimens are first examined in Scanning Electron Microscopy (SEM), briefly rehydrated and secondly processed for observation in Transmission Electron Microscopy (TEM). The rehydration counterbalances the shrinkage due to the previous processing steps applied for SEM examination and allows a good TEM morphology of the recovered specimens to be achieved. The correlative SEM/TEM study on the same conjunctival biopsy provides complete informations on both the surface and the inner structure of the tissue. This appears of value in the investigation of conjunctival biopsy from eyes affected by tear film quantitative and/or qualitative alterations.

Detection of mucus glycoconjugates in human conjunctiva by using the lectin-colloidal gold technique in TEM. II. A quantitative study in dry-eye patients

The mucus glycoconjugates produced by conjunctival goblet cells in dry-eye patients were studied by a specific cytochemical reaction in Transmission Electron Microscopy (TEM). Four lectins, proteins of vegetal origin which specifically bind glycosidic residues, (WGA, PNA, SBA and ConA) were used conjugated with colloidal gold as ultrastructural marker. We performed a quantitative analysis by counting the colloidal gold particles present on mucus granules. The results were compared with normal conditions. We found a decrease in sialic acid, N-acetyl-glucosamine, N-acetyl-galactosamine and galactose-N-acetyl-galactosamine and an increase in mannose. The different content of glycoconjugates in goblet cells may reflect in the change of physical and functional properties of mucus. We think these data may be useful in the search for a therapeutic mucomimetic drug.