The stroma of the cornea (or substantia propria) is a fibrous, tough, unyielding, perfectly transparent and the thickest layer of the cornea of the eye. It is between
Bowman's membrane anteriorly, and
Descemet's membrane
Descemet's membrane ( or the Descemet membrane) is the basement membrane that lies between the corneal proper substance, also called stroma, and the endothelial layer of the cornea. It is composed of different kinds of collagen (Type IV and VIII) ...
posteriorly.
At its centre, human corneal stroma is composed of about 200 flattened ''lamellæ'' (layers of collagen fibrils), superimposed one on another. They are each about 1.5-2.5 μm in thickness. The anterior lamellæ interweave more than posterior lamellæ. The fibrils of each lamella are parallel with one another, but at different angles to those of adjacent lamellæ. The lamellæ are produced by
keratocytes (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 and
proteoglycans. The collagen fibrils are made of a mixture of
type I and
type V collagens. 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. Proteoglycans are made of a small protein core to which one or more
glycosaminoglycan
Glycosaminoglycans (GAGs) or mucopolysaccharides are long, linear polysaccharides consisting of repeating disaccharide units (i.e. two-sugar units). The repeating two-sugar unit consists of a uronic sugar and an amino sugar, except in the case ...
(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) and
keratan sulphate
Keratan sulfate (KS), also called keratosulfate, is any of several sulfated glycosaminoglycans (structural carbohydrates) that have been found especially in the cornea, cartilage, and bone. It is also synthesized in the central nervous system ...
(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 to
thermal motion
A thermal column (or thermal) is a rising mass of buoyant air, a convective current in the atmosphere, that transfers heat energy vertically. Thermals are created by the uneven heating of Earth's surface from solar radiation, and are an example ...
which 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 the
Donnan effect Donnan can refer to:
*Donnan (surname)
* Donnan, Iowa, a community in the United States
* Donnán of Eigg, Gaelic priest of the 7th century
* Donnan equilibrium
** Donnan potential
Donnan potential is the difference in the Galvani potentials whic ...
. 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. 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 ''lamellæ'' 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 light
interferes constructively only in the forward direction, allowing the light through to the
retina
The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then ...
.
The fibrils in the lamellae are directly continuous with those of the
sclera, 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 the
neural crest
Neural crest cells are a temporary group of cells unique to vertebrates that arise from the embryonic ectoderm germ layer, and in turn give rise to a diverse cell lineage—including melanocytes, craniofacial cartilage and bone, smooth muscle, per ...
(a source of
mesenchyme in the head and neck) which has been shown to contain
mesenchymal stem cells.
Disorders of stroma
*
Keratoconus is a condition caused by disorganised lamellæ, leading to thinned and conical-shaped cornea
*
Macular corneal dystrophy
Macular corneal dystrophy, also known as Fehr corneal dystrophy named for German ophthalmologist Oskar Fehr (1871-1959), is a rare pathological condition affecting the stroma of cornea. The first signs are usually noticed in the first decade of li ...
, associated with the loss of
keratan sulfate
References
External links
Diagram at pacificu.edu
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Human eye anatomy