The elastin fiber system between and adjacent to collector channels in the human juxtacanalicular tissue.

The elastin fiber system (EFS) is important in many systems of the body, imparting support, resilience, and recoil to regions under biomechanical stress.1,2 Historically the EFS was classified into three components: elastin, elaunin, and oxytalan, using light microscopy stains and biochemical analysis. As technology evolved, electron microscopy defined elastin as having an amorphous central region surrounded by microfibrils, elaunin as a mixture of amorphous microfibril material, and oxytalan as mainly microfibrils.3 Complexity and variability of the EFS expanded with the identification of elastin-associated proteins: fibrillin-1 (FBN-1), fibrillin-2 (FBN-2), versican, latent TGFβ-binding proteins (LTBP 1–4), and microfibrillar-associated proteins (MFAP 1–4).4,5 In many regions of the body, FBN-1 and FBN-2 are laid down as a scaffold for elastin fibers.6 However, in periodontal membrane and the zonular fibers of the lens, FBN-1 exists alone, and elastin proteins are not present.7,8 Numerous pathologic conditions have alterations of the EFS system, including Marfan's syndrome, pseudoexfoliation syndrome, and emphysema.4,9–11 A major source of biomechanical stress in the anterior chamber of the eye is intraocular pressure. Head and eye movement and ocular pulse result in continuous pressure changes that require tissue deformation and recoil.12,13 The juxtacanalicular tissue (JCT) adjacent to Schlemm's canal (SC) is uniquely equipped to adjust for pressure changes within the conventional outflow pathway. The JCT, also known as the cribriform layer, is located between the outermost corneoscleral trabecular beam and SC endothelial cells.14,15 This region contains endothelial cells arranged in layers, commonly referred to as JCT cells. Within this region and located between the JCT cell layers is a complex system of elastin-like fibers known as the cribriform plexus.15 The cribriform plexus has a porelike or reticular appearance when viewed in a tangential orientation.16,17 The cribriform plexus extends from type A, B, and C tendons that originate from the ciliary muscle tips.17,18 Type A tendons anchor the longitudinal fibers of the ciliary muscle, type B tendons pass through the trabecular meshwork and anchor in the cornea, and type C tendons enter the fiber system at the outermost corneoscleral meshwork and make a 90° turn before joining the cribriform plexus. These elastin-like fibers have a small amount of elastin surrounded by a microfibrillar sheath with a periodic structure.14,17,19,20 Proteins identified in the sheath include FBN-1, MFAP-1, and type VI collagen.21–23 Extending from the sheath of the elastin-like fibers of the cribriform plexus are connecting fibrils that link to SC endothelial and JCT cells.20,24 Fluid flow through the trabecular meshwork is not uniform but appears segmental.25,26 Within segmental flow patterns are preferential fluid flow routes that occur in regions of the JCT underneath collector channels in human and animal eyes.25,27–29 In regions with elevated pressure, the JCT expands providing increased flow for aqueous humor. Areas of distension and loss of contact with underlying tissue have been observed in focal areas of SC and after laser treatment in monkey and human eyes.29–32 These “foamy” regions are characterized by endothelial cells that expand and extend into SC adjacent to collector channel orifices. It is unknown how or why preferential flow and “foamy regions” occur. One possible explanation is that differences in the distribution of extracellular matrix proteins adjacent to collector channel orifices may provide an environment that allows expansion of SC endothelium. The EFS provides both support and distensibility, which suggests that proteins within this group may have a functional role in formation of preferential flow regions. The aims of this study were to elucidate the composition of the EFS adjacent to collector channels, compare EFS underlying collector channels and the area between collector channels, and characterize changes to EFS under elevated pressure.