Thestroma of the cornea(orsubstantia propria) is a fibrous, tough, unyielding, perfectly transparent and the thickest layer of thecorneaof the eye. It is betweenBowman's layeranteriorly, andDescemet's membraneposteriorly.
| Stroma of cornea | |
|---|---|
 Vertical section of human cornea from near the margin. (Waldeyer.) Magnified.
| |
| Details | |
| Identifiers | |
| Latin | substantia propria corneae |
| MeSH | D003319 |
| TA98 | A15.2.02.020 |
| FMA | 58306 |
| Anatomical terminology | |
At its centre, a human corneal stroma is composed of about 200 flattenedlamellae(layers ofcollagenfibrils), superimposed one on another.[1]They are each about 1.5-2.5 μm in thickness. The anterior lamellae interweave more than posterior lamellae. The fibrils of each lamella are parallel with one another, but at different angles to those of adjacent lamellae. The lamellae are produced bykeratocytes(corneal connective tissue cells), which occupy about 10% of the substantia propria.
Apart from the cells, the major non-aqueous constituents of the stroma are collagen fibrils andproteoglycans.The collagen fibrils are made of a mixture oftype Iandtype Vcollagens. These molecules are tilted by about 15 degrees to the fibril axis, and because of this, the axial periodicity of the fibrils is reduced to 65 nm (in tendons, the periodicity is 67 nm). The diameter of the fibrils is remarkably uniform and varies from species to species. In humans, it is about 31 nm.[2]Proteoglycans are made of a small protein core to which one or moreglycosaminoglycan(GAG) chains are attached. The GAG chains are negatively charged. In corneas we can find two different types of proteoglycans:Chondroitin sulphate/dermatan sulphate(CD/DS) andkeratan sulphate(KS). In bovine corneas, the length of the CS/DS proteoglycans is about 70 nm, while the KS proteoglycans are about 40 nm long. Proteoglycan protein cores attach to the surface of the collagen fibrils with the GAG chains projecting outwards. The GAG chains are able to form antiparallel links with other GAG chains from adjacent fibrils, perhaps through the mediation of positively charged ions. In such a way, bridges are formed between adjacent collagen fibrils. These bridges are subject tothermal motionwhich prevents them from assuming a fully extended conformation. This results in forces that tend to move adjacent fibrils close to each other. At the same time the charges on the GAG chains attract ions and water molecules by theDonnan effect.The increased water volume between the fibrils results in forces that tend to push the fibrils apart. A balance between attractive and repulsive forces is reached for specific inter-fibrillar distances, which depends on the type of proteoglycans present.[3]Locally, the separations between adjacent collagen fibrils are very uniform.
Stromal transparency is mainly a consequence of the remarkable degree of order in the arrangement of the collagen fibrils in the lamellae and of fibril diameter uniformity. Light entering the cornea is scattered by each fibril. The arrangement and the diameter of the fibrils is such that scattered lightinterferesconstructively only in the forward direction, allowing the light through to theretina.[4]
The fibrils in the lamellae are directly continuous with those of thesclera,in which they are grouped together in fibre bundles. More collagen fibres run in a temporal-nasal direction than run in the superior-inferior direction.
During development of the embryo, the corneal stroma is derived from theneural crest(a source ofmesenchymein the head and neck)[5]which has been shown to containmesenchymal stem cells.[6]
Disorders of stroma
edit- Keratoconusis a condition caused by disorganised lamellae, leading to thinned and conical-shaped cornea
- Macular corneal dystrophy,associated with the loss ofkeratan sulfate
References
edit- ^Oyster, CW (1999). "8".The human eye: structure and function.Sinauer.OL8562710W.
- ^Meek KM; Quantock AJ (2001). "The Use of X-ray Scattering Techniques to Determine Corneal Ultrastructure".Progress in Retinal and Eye Research.20(1, pp. 9–137): 95–137.doi:10.1016/S1350-9462(00)00016-1.PMID11070369.
- ^Lewis PN; Pinali C; Young RD; Meek KM; Quantock AJ; Knupp C (2010)."Structural Interactions between Collagen and Proteoglycans Are Elucidated by Three-Dimensional Electron Tomography of Bovine Cornea".Structure.18(2): 239–245.doi:10.1016/j.str.2009.11.013.PMID20159468.
- ^Meek KM; Knupp C (2015)."Corneal structure and transparency".Progress in Retinal and Eye Research.49:1–16.doi:10.1016/j.preteyeres.2015.07.001.PMC4655862.PMID26145225.
- ^Hoar RM (Apr 1982)."Embryology of the eye".Environ. Health Perspect.44:31–34.doi:10.1289/ehp.824431.PMC1568953.PMID7084153.
- ^Branch MJ, Hashmani K, Dhillon P, Jones DR, Dua HS, Hopkinson A (Aug 3, 2012)."Mesenchymal stem cells in the human corneal limbal stroma".Invest Ophthalmol Vis Sci.53(9): 5109–16.doi:10.1167/iovs.11-8673.PMID22736610.
