Morphologic and Volumetric Studies of the Meibomian Glands in Elderly Human Eyelids

Alpha- and beta-melanocyte stimulating hormone positively impact lipogenesis of meibomian gland cells in vitro and ex vivo

PURPOSE

The meibomian glands produce a lipid-rich secretion that forms the superficial layer of the tear film, preventing excessive evaporation. Dysfunction of these glands (MGD) is the primary cause of dry eye disease (DED), a growing public health concern. Currently, there are limited pharmacological treatments for DED. However, α-/β-melanocyte-stimulating hormones (α-/β-MSH), ligands of the melanocortin receptors (MCR), are known to regulate lipogenesis and differentiation in sebaceous glands. This study investigated the influence of α-/β-MSH on exocrine secretion in human meibomian glands.

METHODS

Immunohistochemistry and RT-PCR for MCR expression were performed in human meibomian glands and an immortalized human meibomian gland epithelial cell line (ihMGECs). The effects of α-/β-MSH (agonists) and JNJ-10229570 (antagonist) in ihMGECs on lipid production and MCR response were analyzed using Oil-Red-O staining, transmission electron microscopy, qPCR, and a cAMP assay. Additionally, the effect of α-/β-MSH on an ex vivo organotypic slice culture (OSC) of human eyelids was investigated.

RESULTS

MCR expression was confirmed in human meibomian glands. Stimulation with α-/β-MSH increased cAMP levels and MCR expression. α-/β-MSH dose-dependently induced lipid production in ihMGECs and OSC, resulting in increased lipid droplet formation and upregulation of lipogenesis markers. Co-administration of JNJ-10229570 suppressed this effect.

CONCLUSION

Our data show for the first time that human meibomian glands express MCRs and that stimulation/inhibition of MCRs alters cAMP response, MCR expression, and lipogenesis markers, thereby affecting the genesis of meibum. Therefore, α-/β-MSH positively impacts meibum production and should be considered in the context of changes in glandular secretion in MGD and potential treatments.

Meibomian gland stem/progenitor cells: The hunt for gland renewal

Meibomian glands (MGs) secrete lipid (meibum) onto the ocular surface to form the outermost layer of the tear film. Proper meibum secretion is essential for stabilizing the tear film, reducing aqueous tear evaporation, and maintaining the homeostasis of the ocular surface. Atrophy of MG as occurs with aging, leads to reduction of meibum secretion, loss of ocular surface homeostasis and evaporative dry eye disease (EDED). Since MGs are holocrine glands, secretion of meibum requires continuous self-renewal of lipid-secreting acinar meibocytes by stem/progenitor cells, whose proliferative potential is dramatically reduced with age leading to MG atrophy and an age-related meibomian gland dysfunction (ARMGD). Understanding the cellular and molecular mechanisms regulating meibocyte stem/progenitor cell maintenance and renewal may provide novel approaches to regenerating MG and treating EDED. Towards that end, recent label retaining cell and lineage-tracing experiments as well as knock-out transgenic mouse studies have begun to identify the location and identities of meibocyte progenitor cells and potential growth and transcription factors that may regulate meibocyte renewal. In addition, recent reports have shown that ARMGD may be reversed by novel therapeutics in mice. Herein, we discuss our current understanding of meibocyte stem/progenitor cells and the hunt for gland renewal.

A New Organotypic 3D Slice Culture of Mouse Meibomian Glands Reveals Impact of Melanocortins

The meibomian glands (MGs) within the eyelids produce a lipid-rich secretion that forms the superficial layer of the tear film. Meibomian gland dysfunction (MGD) results in excessive evaporation of the tear film, which is the leading cause of dry eye disease (DED). To develop a research model similar to the physiological situation of MGs, we established a new 3D organotypic slice culture (OSC) of mouse MGs (mMGs) and investigated the effects of melanocortins on exocrine secretion. Tissue viability, lipid production and morphological changes were analyzed during a 21-day cultivation period. Subsequently, the effects on lipid production and gene expression were examined after stimulation with a melanocortin receptor (MCR) agonist, α-melanocyte-stimulating hormone (α-MSH), and/or an MCR antagonist, JNJ-10229570. The cultivation of mMGs OSCs was possible without impairment for at least seven days. Stimulation with the MCR agonists induced lipid production in a dose-dependent manner, whereas this effect was tapered with the simultaneous incubation of the MCR antagonist. The new 3D OSC model is a promising approach to study the (patho-) physiological properties of MG/MGD while reducing animal studies. Therefore, it may accelerate the search for new treatments for MGD/DED and lead to new insights, such as that melanocortins likely stimulate meibum production.

The morphogenesis of the rabbit meibomian gland in relation to sex hormones: Immunohistochemical and transmission electron microscopy studies

Rabbits have been proposed as a model for the human meibomian gland (MG), a large specific sebaceous gland in the eyelid that consists of secretory acini arranged laterally and related to the central duct via short ductules, with the central duct continuing as an excretory duct to open at the free margin of the lid. First detected at embryonic day 18 as an aggregation of mesenchymal cells in the tarsal plate, it completes its development approximately 2 weeks postnatal when the separation of the eyelids is completed. The Transmission electron microscopy supports the meibocytes’ gradient maturation to the meibum’s synthesis. While the differentiating cells, their cytoplasm, are well packed with lipid droplets, the basal cells are characterized by a high nuclear to cytoplasm ratio. The androgen and estrogen receptor proteins are expressed in the basal cell and the meibocytes, and increase in age increases in the expression of these proteins. Additionally, the cytokeratin (CK14) is expressed in the basal and differentiating cells of the acini and the ductal epithelium. Therefore, the duct cells of the MG are common in all stem cells. These data concluded that the MG plays a major role in maintaining the health of the ocular surface and preservation of visual acuity. Any abnormalities in the structure of the MG lead to its dysfunction and changes in lipid secretion.

Pathophysiology of Meibomian Glands - An Overview

Purpose: The meibomian glands are located in the tarsal plate of the upper and lower eyelid and are responsible for the production of a lipid-rich secretion, the meibum, which forms the outer component of the tear film. Meibomian gland dysfunction results in excessive evaporation of the tear film and is the leading cause of dry eye disease (DED). Despite the high prevalence of DED, the etiology of meibomian gland dysfunction is only basically understood. In addition, the molecular mechanisms of meibomian gland maturation and physiological function are currently the focus of research.Methods: A systematic literature search was performed using the main scientific databases, including all relevant published articles up to September 2020.Results: This article provides an overview of the current state of knowledge about meibomian gland stem cells, cell surface marker expression and PPARγ signaling, as well as the pathological causes of meibomian gland dysfunction.Conclusion: Androgen deficiency, hyperkeratinization, PPARγ signaling and inflammatory reactions including neutrophil extracellular traps (NETs) seem to be key factors within the pathological processes of the meibomian gland.

Reliable, Noncontact Imaging Tool for the Evaluation of Meibomian Gland Function: Sirius Meibography

OBJECTIVES

To investigate the correlation between the clinical grading of the severity of meibomian gland dysfunction (MGD) and meibomian gland area (MGA) loss detected using Sirius meibography in the upper and lower eyelids and to correlate these parameters with dry eye signs and symptoms.

METHODS

One hundred thirty eyes of 130 volunteer patients were enrolled. Meibomian gland dysfunction was defined as the presence of signs consistent with meibomian gland terminal duct obstruction and categorized between grades 1 and 4. Upper and lower MGA loss percentages were evaluated using Sirius meibography by two blinded examiners. Patients were categorized into two groups (positive and negative for MGD) based on their Ocular Surface Disease Index (OSDI) scores (≥15 and <14, respectively) and MGD grades (≥1 and 0, respectively).

RESULTS

The MGA loss percentage and ocular surface test results of the MGD group were greater than in the non-MGD group in both eyelids (P=0.001). Meibomian gland dysfunction grade and MGA loss for the lower eyelid was greater than that of the upper eyelids (P=0.002 and P<0.001, respectively). The OSDI score, tear film break-up time, and lissamine green staining were all significantly correlated with MGD grade and MGA loss in both eyelids (P<0.001). For the average measures of the MGA loss percentage in each scan, the intraclass correlation value was found as 0.994 (95% confidence interval [CI]: 0.992-0.995) for reader 1 and 0.988 (95% CI: 0.982-0.992) for reader 2.

CONCLUSIONS

Our findings suggest that Sirius meibography, a noncontact imaging device, is a reliable tool for evaluating meibomian gland structure in patients with MGD and non-MGD of various ages and regardless of sex.

Distribution and Characteristics of Meibomian Gland Dysfunction Subtypes: A Multicenter Study in South Korea

PURPOSE

We sought to evaluate the distribution and characteristics of meibomian gland dysfunction (MGD) and the treatment patterns for symptomatic MGD patients in South Korea.

METHODS

One hundred ninety-six right eyes of 196 MGD patients were enrolled. For each patient, meibum expressibility in the central eight glands in both the upper and lower eyelids was examined. Each upper and lower eyelid was separately classified into one of the following three subtypes: nonobvious obstructive (low-delivery without lid margin abnormality), obvious obstructive (low-delivery with lid margin abnormality), and hypersecretory (high-delivery with lid margin abnormality). All treatment plans were also recorded.

RESULTS

The mean number of expressible glands of the central eight glands in the upper eyelids (3.9 ± 2.6) was significantly higher than that in the lower eyelids (2.2 ± 2.4, p < 0.001). Obvious obstructive MGD was the most common subtype, followed by the hypersecretory and nonobvious obstructive subtypes in both the upper and lower eyelids. Of the 196 subjects, 38 (19.4%) had upper and lower eyelids that were assigned to different categories. Eyelid hygiene was the most prescribed treatment (74.5%), followed by lubricant eye drop usage (71.5%). Physicians tended to determine treatment plans based on the subtype of the upper eyelid rather than that of the lower eyelid.

CONCLUSIONS

The majority of subjects were classified as having the obvious obstructive subtype of MGD, and 19.4% had upper and lower eyelids that were different subtypes. Eyelid hygiene was the most prescribed treatment for MGD patients, and treatment patterns were mostly determined based on the subtype of the upper eyelids.

Intraductal meibomian gland probing: background, patient selection, procedure, and perspectives

Obstructive meibomian gland dysfunction is the most common cause of dry eye. Its conventional treatment has focused on using heat and pressure with anti-inflammatory and antimicrobial therapies but has often been a frequent, frustrating experience for patient and physician. New evidence from Meibomian gland intraductal probing suggests fixed intraductal strictures and obstruction correlating to periductal fibrosis first described in 1997. The use of intraductal probing has been reported, by this author and in at least ten independent peer-reviewed academic papers from around the world, to consistently lead to statistically significant improvement in signs and symptoms of gland dysfunction including cases refractory to other extensive treatments. This review will focus on the background of meibomian gland intraductal probing as well as patient selection, procedure, and perspectives.

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Meibomian gland features in a Norwegian cohort of patients with primary Sjögren´s syndrome

Purpose

To assess the tear film and meibomian gland (MG) features in a Norwegian cohort of patients with primary Sjögren´s syndrome (pSS) and in age- and gender-matched control subjects.

Methods

Thirty-four female patients with pSS (age 52.9±11.9 years) and 32 female control subjects (age 49.0±11.5 years) were recruited. After completion of Ocular Surface Disease Index (OSDI) questionnaire and McMonnies Dry Eye Questionaire, participants underwent measurements of tear osmolarity, tear break-up time (TBUT), ocular surface and corneal staining, Schirmer I test, corneal sensitivity, MG expressibility evaluations, and lid margin morphology examination using slitlamp microscopy. Non-contact infrared meibography images were assessed by computer-assisted analysis. The MG loss, calculated as (tarsal area-MG area)/tarsal area, was evaluated in both upper (UL) and lower lids (LL).

Results

Compared to the control group, pSS patients demonstrated higher MG loss in both UL (33.8±13.2% vs. 24.4±8.5%, p< 0.01) and LL (52.5±15.7% vs. 43.0±9.6%, p<0.05), as well as higher lid abnormality score (0.8±0.8 vs. 0.2±0.6, p< 0.01). Furthermore, pSS patients showed higher OSDI and McMonnies questionnaire scores, elevated osmolarity, shorter TBUT, shorter blink interval, less wetting in Schirmer I test, more ocular surface staining and more corneal staining. MG loss in UL correlated negatively with TBUT (r = -0.386, p = 0.029) in the pSS group, whereas MG loss in LL correlated negatively with TBUT (r = -0.380, p = 0.035) in the control group.

Conclusions

Significantly elevated dry eye symptoms and signs were found in the pSS group compared with the control group, which might be attributed to both decreased aqueous tear production and increased tear evaporation.

Functional lacrimal gland regeneration by transplantation of a bioengineered organ germ

The lacrimal gland has a multifaceted role in maintaining a homeostatic microenvironment for a healthy ocular surface via tear secretion. Dry-eye disease, which is caused by lacrimal gland dysfunction, is one of the most prevalent eye diseases that cause corneal epithelial damage and results in significant loss of vision and a reduction in the quality of life. Here we demonstrate orthotopic transplantation of bioengineered lacrimal gland germs into adult mice with an extra-orbital lacrimal gland defect, a mouse model that mimics the corneal epithelial damage caused by lacrimal gland dysfunction. The bioengineered lacrimal gland germs and harderian gland germs both develop in vivo and achieve sufficient physiological functionality, including tear production in response to nervous stimulation and ocular surface protection. This study demonstrates the potential for bioengineered organ replacement to functionally restore the lacrimal gland.

Lacrimal glands maintain a healthy corneal epithelium but are dysfunctional for example in dry-eye disease. Here, the authors transplant bioengineered lacrimal and harderian gland germs into mice, where they connect to the host duct and nervous system and restore lacrimal gland function.

Effect of retinoic acid on the nitrergic innervation of meibomian glands in rats

The purpose of this study was to investigate the effect of prenatal administration of retinoic acid (RA) on the development of nicotinamide adenine dinucleotide diaphorase (NADPH-d) positive structures in the rat Meibomian glands. One mg/kg of RA was applied to pregnant Wistar rats intraperitonaelly during the gestational period in each of the 12th-14th embryonic days (totally 3 mg/kg). Sections of the central upper eyelids were investigated in rat pups on the 14th postnatal day. They were processed histochemically for NADPH-d, to study the presence and distribution of nitric oxide synthase (NOS) positive nerve structures. NADPH-d staining of Meibomian glands was compared in two groups of rat pups. In the control group, eyelids of 14 day-old rats were studied with no experimental intervention. The second group consisted of rat pups which were prenatally administered the excess of RA. Histochemical analysis of control eyelids revealed numerous NADPH-d well-stained acini of Meibomian glands arranged tightly into groups. Intensively stained vessels and NADPH-d/NOS-positive nerve fibers bordered acini of Meibomian glands. These structures were present in the submucosal layer as well. The analysis of RA group showed less numerous, shrunken acini of Meibomian glands that were seen not only smaller in size, but also in density of their staining and the amount of nitrergic nerve fibers around acini were considerably lowered. In the submucosa differences were noticed compared to the control group, there were numerous NADPH-d stained vessels accompanied by NADPH-d/NOS-positive nerve fibers. The excess of RA during the prenatal period may influence on the development and morphology of NADPH-d positive structures of rat's Meibomian glands.

A solute gradient in the tear meniscus. I. A hypothesis to explain Marx's line

Marx's line is a line of mucosal staining behind the mucocutaneous junction. It can be demonstrated throughout life in all normal lids by staining with lissamine green and related dyes. Of all the body orifices, only the mucosae of the eye and mouth are directly exposed to the atmosphere. In this paper, we suggest that for the eye, this exposure leads to the formation of Marx's line. The tear meniscus thins progressively toward its apex, where it is pinned at the mucocutaneous junction of the lid. It also thins toward the black line, which segregates the meniscus from the tear film after the blink. We predict that, because of the geometry of the tear meniscus, evaporation generates a solute gradient across the meniscus profile in the anteroposterior plane, which peaks at the meniscus apices at the end of the interblink. One outcome would be to amplify the level of tear molarity at these sites so that they reach hyperosmolar proportions. Preliminary mathematical modeling suggests that dilution of this effect by advection and diffusion of solute away from the meniscus apex at the mucocutaneous junction will be restricted by spatial constraints, the presence of tear and surface mucins at this site, and limited fluid flow. We conclude that evaporative water loss from the tear meniscus may result in a physiological zone of hyperosmolar and related stresses to the occlusal conjunctiva, directly behind the mucocutaneous junction. We hypothesize that this stimulates a high epithelial cell turnover at this site, incomplete epithelial maturation, and a failure to express key molecules such as MUC 16 and galectin-3, which, with the tight junctions between surface epithelial cells, are necessary to seal the ocular surface and prevent penetration of dyes and other molecules into the epithelium. This is proposed as the basis for Marx's line. In Part II of this paper (also published in this issue of The Ocular Surface), we address additional pathophysiological consequences of this mechanism, affecting lid margins.

Volumetric reconstruction of the mouse meibomian gland using high-resolution nonlinear optical imaging

Recent studies suggest that mouse meibomian glands (MG) undergo age-related atrophy that mimics changes seen in age-related human MG dysfunction (MGD). To better understand the structural/functional changes that occur during aging, this study developed an imaging approach to generate quantifiable volumetric reconstructions of the mouse MG and measure total gland, cell, and lipid volume. Mouse eyelids were fixed in 4% paraformaldehyde, embedded in LR White resin and serially sectioned. Sections were then scanned using a 20× objective and a series of tiled images (1.35 × 1.35 × 0.5 mm) with a pixel size of 0.44 microm lateral and 2 microm axial were collected using a Zeiss 510 Meta LSM and a femtosecond laser to simultaneously detect second harmonic generated (SHG) and two-photon excited fluorescence (TPEF) signals from the tissue sections. The SHG signal from collagen was used to outline and generate an MG mask to create surface renderings of the total gland and extract relevant MG TPEF signals that were later separated into the cellular and lipid compartments. Using this technique, three-dimensional reconstructions of the mouse MG were obtained and the total, cell, and lipid volume of the MG measured. Volumetric reconstructions of mouse MG showed loss of acini in old mice that were not detected by routine histology. Furthermore, older mouse MG had reduced total gland volume that is primarily associated with loss of the lipid volume. These findings suggest that mice MG undergo "dropout" of acini, similar to that which occurs in human age-related MGD.

NADPH-diaphorase expression in the meibomian glands of rat palpebra in postnatal development

In the current study, we aimed at investigating the presence of nitric oxide synthase (NOS) positive nerve fibers in rat meibomian glands (MGs) at various stages of development. There is good evidence to suggest that nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) is a surrogate for neuronal nitric oxide synthase (NOS). Sections of the central, upper eyelids of Wistar rats were processed histochemically for NADPH-d to investigate the presence and distribution of NOS-positive nerve fibers at the following time points: day 1 and weeks 1, 2 and 3 post partum, and in adult controls. At day 1, MG acini were lightly stained and located at a distance from the mucosal border. Vessels were accompanied by intensely stained NADPH-d positive nerve fibers. At the week 1 time point, both the vessels and the NADPH-d positive fibers were still present, but less numerous. MGs were now closer to the mucosa, so that the submucosa was thinner. The acini were mostly pale but occasionally darker. At week 3, there were fewer blood vessels in both the sub-mucosa and within the septa. Darker acini were more common than lightly stained acini. NADPH-d positive dots were observed in the vicinity of the MGs. At the week 3 time point, MGs were adjacent to the mucosal border and stained more intensely than at earlier times; almost all acini were stained. The microscopic appearances were almost identical with those of adult palpebra. Submucosal and septal blood vessels and NADPH-d positive nerve fibers were less numerous. NADPH-d histochemical staining confirmed differences in the density of stained nerve fibers at different developmental stages. The greatest density of NADPH-d -positive nerve fibers occurred in 1-day-old rats whereas they were less numerous in adult rat eyelids. Nerves innervating MGs utilize nitric oxide (NO) as a neurotransmitter mostly in early developmental stages and this need thereafter decreases and stabilizes at 3 weeks postnatally.