The architecture of the corneal stroma
- Nuffield Laboratory of Ophthalmology, University of Oxford, Walton Street, Oxford OX2 6AW, UK
In recent years the evolution of modern refractive surgery has focused attention on the architecture and biological properties of the cornea. In this issue of theBJO (p 437) Müller et al address the differential behaviour of the anterior and posterior stroma during corneal swelling and draw interesting conclusions about the factors maintaining corneal shape.
Transparency of the corneal stroma depends particularly on the degree of spatial order of its collagen fibrils which are narrow in diameter and closely packed in a regular array.1-8 The collagen fibrils themselves are weak scatterers, since their fibril diameter is less than the wavelength of light, and fibril refractive index is close to that of the ground substance. There is little variation in fibril diameter and separation between the anterior and posterior cornea.
The stromal fibrils are further organised into bundles, or lamellae, of which there are approximately 300 in the central cornea and 500 close to the limbus.9 The posterior lamellae course directly across the full width of the cornea without a break, having their origins in fibres which wind around the limbus at the corneoscleral junction10-12 or, according to Radner,9 have a pseudocircular organisation at the limbus, forming the ligamentum circulare corneae. On the basis ofx ray diffraction studies, about 49% of the stromal lamellae are preferentially aligned orthogonally, along the vertical and horizontal meridians, while about 66% lie within a 45° sector.1112 Fibrils within a lamella are in parallel array, except where branching of lamellae occurs. Branching in the horizontal plane occurs throughout the stroma, whereas anteroposterior branching is found only in the anterior third.13
The anterior and posterior stroma differ in specific ways. In general the posterior stroma is more ordered,14 more hydrated,15 more easily swollen, and has a lower …