Histology
The collagen of the sclera is continuous with the cornea. From outer to innermost, the four layers of the sclera are:
- episclera
- stroma
- lamina fusca
- endothelium
The sclera is opaque due to the irregularity of the collagen fibers, as opposed to the near-uniform thickness and parallel arrangement of the corneal collagen. Moreover, the cornea bears more mucopolysaccharide (a carbohydrate that has among its repeating units a nitrogenous sugar, hexosamine) to embed the fibrils.
The cornea, unlike the sclera, has five layers. The middle, thickest layer is also called the stroma. The sclera, like the cornea, contains a basal endothelium, above which there is the lamina fusca, containing a high count of pigment cells.
Sometimes, very small gray-blue spots can appear on the sclera, a harmless condition called scleral melanocytosis.
Read more about this topic: Sclera