The substantia propria (or stroma of cornea) is fibrous, tough, unyielding, and perfectly transparent.
It is composed of about 200 flattened lamellæ (plates of collagen fibrils), superimposed one on another. They are each about 1.5-2.5 micrometres in thickness. These fibrils run at different angles between the limbi. Fibres of the layers frequently interweave, the anterior lamellæ interweaving more than posterior lamellæ. The fibres of each lamella are parallel with one another, but at right angles to those of adjacent lamellæ. The lamellæ are produced by keratocytes (corneal corpuscles), which occupy about 10% of the substantia propria.
These lamellæ are made up of bundles of modified connective tissue, the fibres of which are directly continuous with those of the sclera. More collagen fibres run in a temporal-nasal direction than run in the superior-inferior direction.
In embryological development the corneal stroma is derived from the neural crest (a source of mesenchyme in the head and neck) and has been shown to contain mesenchymal stem cells.
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