Occasional intracellular staining was observed in the epithelium of hair follicles in the eyelids (Figure 1A,C). western blot analysis was performed on four eyelids from 2 animals. == Results == Lubricin-positive meibomian cells were seen in cis-Urocanic acid the glands in all eight animals evaluated immunohistochemically. The percentage of lubricin-positive cells ranged from was 8%50% in the upper and 3%50% in the lower eyelid, with no significant difference between the upper and lower eyelid. Western blot analysis confirmed the presence of lubricin ranging from 10 to 40 ng in four eyelids from the other two rabbits. Occasional staining was seen in the epithelium of the hair follicles of the eyelid. No lubricin was evident on the ocular surface or in the lacrimal gland. == Conclusions == Lubricin is secreted by the meibomian gland. The results provide a basis for the hypothesis that lubricin plays a role in boundary lubrication and in preventing adhesions in the eye, as well as in contributing to other cis-Urocanic acid functions of the meibomian gland. Moreover, if lubricin functions to decrease the friction between the eyelid and ocular surface, this study provides a rationale to supplement the amount of lubricin in cases of compromised meibomian gland function and other conditions. == Introduction == The meibomian glands, also known as tarsal glands, are modified sebaceous glands located within the tarsal plates of both the upper and lower eyelid. The secretions of these holocrine glands, along with dead cell remnants, are released into the lumen by rupture of the plasma membrane. The secretion of the gland acini, meibum, enters into the ductule. Ductules join together to form the central duct which opens into the lid margin at the mucocutaneous junction of the eyelid. During blinking, contraction of the orbicularis oculi and muscle of Riolan results in meibum being released onto the posterior lid margin [1-3]. Neuronal and hormonal control of the gland has previously been demonstrated [4,5]. Meibum contributes to the lipid bilayer of the tear film situated at the air-tear film interface. This layer comprises non polar lipids (wax esters and steroid esters) and lesser amounts of polar lipids [6]. The inner layer of the bilayer, adjacent to the aqueous layer, comprises polar lipids and the outer layer, at the air-tear film interface, comprises non polar lipids [7]. The secretion of the meibomian gland functions to: maintain stability of the tear film; help prevent tear evaporation; prevent contamination of the tear film; and lower surface tension [8,9]. The ocular tear film is essential for lubrication between the inner surface of the eyelids and cornea, and it also provides an optically smooth surface [7]. The tear cis-Urocanic acid film has three layers: the innermost mucous layer secreted by the adjacent corneal epithelium; an intermediate aqueous layer produced by the lacrimal gland; an outermost lipid layer produced by the meibomian gland [10-12]. During blinking, frictional forces can be rationalized to be highest at the outermost lipid bilayer of the tear film, prompting the search for a boundary lubricating molecule synthesized by the meibomian gland. That the mucinous glycoprotein, lubricin, has been identified in recent years as the principal lubricating protein in the body focused attention on its synthesis by meibomian cells. Lubricin, which has been shown to play a vital role in the lubrication of joints [13,14], was first isolated from bovine synovial fluid by Swann, et al., in 1982 [15], and found to be synthesized by synovial cells [16]. cis-Urocanic acid Later work by Schumacher, et al. [17], described a proteoglycan, superficial zone protein (SZP), produced by the chondrocytes in superficial zone of bovine articular cartilage [17]. Subsequent studies showed that SZP was homologous to megakaryocyte stimulating factor [18] and to lubricin [19], and that these homologous glycoproteins were encoded by 12 exons of the proteoglycan 4 (Prg4) gene [20]. Immunohistochemical localization of lubricin has since demonstrated its presence in tendon [21], meniscus [22], ligament [23], muscle [23], skin [23], and intervertebral disc [24]. Lubricin has a molecular weight of around 2105Da (by sedimentation analysis), intrinsic viscosity of 92 ml/g, and a diffusion coefficient of 1 1.10107cm2/s (light-scattering measurements) [15]. The molecule is an alternately spliced 1,404 amino acid protein, having a NH2-terminal somatomedin B (SMB)-like EP domain and a COOH-terminal hemopexin (PEX)-like domain, joined together by a central mucin-like domain having heavy O-linked glycosylation with NeuAc (2,3)-Gal(1,3)- GalNAc [25,26]. Strong repulsive forces through hydration and steric forces generated by the high negative charge and hydrated sugars in the central domain contribute to lubricins lubricating property [25,27,28]. Studies have further shown the amphiphilic and strong absorbent nature of the molecule. On hydrophobic surfaces the molecule adopts a compact loop-like conformation to adsorb anywhere on its hydrophobic domain and on hydrophilic surfaces it adopts a tail like conformation to adsorb anywhere on the hydrophilic central domain [29]..