Biomedical Engineering Reference
In-Depth Information
Fig. 3.1 The characteristics of human cornea and limbus. a The human cornea in histological
cross-section stained with haematoxylin and eosin showing all five layers. b The human corneal
epithelium showing stratification into three different layers. c The CFE assay with examples of
holoclones (H) and meroclones (M). d Scanning electron microscope image of a decellularised
human limbus showing the limbal crypts (image taken from the area indicated in the inset).
e High magnification scanning electron microscope image of a single limbal crypt. f Confocal
image of cells in the limbal crypts and closely associated keratocytes (white arrows) stained with
propidium iodide (red) to label nuclei and fluorescein isothiocyanate—phalloidin (green) to label
cell cytoskeleton. g Clinical signs of limbal epithelial stem cell deficiency resulting in
neovascularization, epithelial surface breakdown and corneal opacity. h Fluorescein staining
showing a large central epithelial defect of the cornea following unilateral alkali injury. i After
receiving a cultured limbal epithelial autograft with cells harvested from his healthy eye, 3 years
post-op the patient has a clear cornea with a healthy epithelium and limited neovascularization
differentiated epithelial cells and then shed within about 10 days in a healthy
cornea. This concept of cell loss and replacement of the corneal epithelium was first
described by Thoft in his X, Y, Z hypothesis [
1
]. He suggested that with X repre-
senting the proliferation of the basal cells, Y the proliferation and centripetal
migration of the cells from the corneal periphery, an area known as the limbus, and
Z the loss of cells from the surface, for healthy corneal equilibrium, X ? Y must
equal Z. This theory has later been substantiated by additional evidence that the
limbus is the source of cells that replace the lost squamous epithelial cells.
