Fig. 1.

Simplified model of the principal lamellar orientations in the human cornea. (a) and (c) are based on X-ray data; (b) and (d) are based on second harmonic generated (SHG) microscopy. (a) and (b) are en face views from the front of the cornea. The X-ray data (a) show the model (black lines) superimposed on the distribution of preferentially aligned collagen lamellae and indicates preferred lamellar orientations in the inferior-superior and nasal-temporal directions, predominantly in the posterior stroma. Many of these seem to become part of a circum-corneal annulus at the limbal region. In addition, anchoring lamellae with larger collagen fibrils are thought to enter the stroma in alignment with the extraocular musculature. The lines in the model show the predominant orientation of lamellae, not the actual course of individual lamellae. SHG microscopy from the posterior stroma (b) reveals an interwoven lattice of lamellae, such that adjacent lamellae are not orthogonal; thus no preferred orientation of collagen is obvious in the central cornea using this technique. (c) and (d) are views through the thickness of the stroma. In (c) the X-ray data are shown on the right. The corresponding model on the left suggests that lamellae are more interwoven anteriorly, particularly in the central 4 mm zone. Outside this zone, interweaving is more prominent in deeper stromal layers also. At the limbus (indicated by broken red line), the circumcorneal annulus appears in cross-section (represented by black circles) and anchoring lamellae appear to enter the stroma from the deeper layers and gradually move towards the surface. SHG microscopy (d) also demonstrates the highly interwoven anterior lamellae, which are often referred to as fibres using this imaging modality. A 3-D reconstruction shows bow spring fibres (blue), anchoring fibres inserting from the limbus (green), and the highly intertwined anterior fibre meshwork (teal) near Bowman's layer (gold). (d) is reproduced from Winkler et al. (2011) with permission of the copyright holder.