Abstract
Mechanisms controlling the spatial configuration of the remarkably ordered collagen-rich extracellular matrix of the transparent cornea remain incompletely understood. We previously described the assembly of the emerging corneal matrix in the mid and late stages of embryogenesis and concluded that collagen fibril organisation was driven by cell-directed mechanisms. Here, the early stages of corneal morphogenesis were examined by serial block face scanning electron microscopy of embryonic chick corneas starting at embryonic day three (E3), followed by a Fourier transform analysis of three-dimensional datasets and theoretical considerations of factors that influence matrix formation. Eyes developing normally and eyes that had the lens surgically removed at E3 were studied. Uniformly thin collagen fibrils are deposited by surface ectoderm-derived corneal epithelium in the primary stroma of the developing chick cornea and form an acellular matrix with a striking micro-lamellar orthogonal arrangement. Fourier transform analysis supported this observation and indicated that adjacent micro-lamellae display a clockwise rotation of fibril orientation, depth-wise below the epithelium. We present a model which attempts to explain how, in the absence of cells in the primary stroma, collagen organisation might be influenced by cell-independent, intrinsic mechanisms, such as fibril axial charge derived from associated proteoglycans. On a supra-lamellar scale, fine cords of non-collagenous filamentous matrix were detected over large tissue volumes. These extend into the developing cornea from the epithelial basal lamina and appear to associate with the neural crest cells that migrate inwardly to form, first the corneal endothelium and then keratocytes which synthesise the mature, secondary corneal stroma. In a small number of experimental specimens, matrix cords were present even when periocular neural crest cell migration and corneal morphogenesis had been perturbed following removal of the lens at E3.
Highlights
The stroma of the cornea represents perhaps the most exquisite example in terms of its structural organisation of all connective tissues
The lens was removed from the right eye at E3 as described previously (Lwigale and Bronner-Fraser, 2007) after which development was allowed to continue until E4 or E6, with the left eye serving as a control
Primary stroma was first detected at E3 as a layer, around 15 μm thick, of loosely-distributed collagen fibrils located between surface ectoderm and the forming lens (Fig. 1A and D)
Summary
The stroma of the cornea represents perhaps the most exquisite example in terms of its structural organisation of all connective tissues. Within each lamella fibrils exhibit parallel alignment, with a gradual clockwise displacement in orientation in the distal two thirds of the stroma, progressing from superficial to deeper locations (Trelstad and Coulombre, 1971; Koudouna et al, 2018a). These highly ordered features are generally accepted to have functional significance in relation to the transparency and biomechanical properties of the cornea (Quantock and Young, 2008; Hassell and Birk, 2010; Chen et al, 2015).
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