Biomedical Engineering Reference
In-Depth Information
lens. However, its curvature is fixed, unlike the lens that can be adjusted to focus
when required. Together the cornea and the sclera are a roughly spherical, fibrous
envelope whose important optical properties—curvature and axial length, are
maintained by pressure from the intraocular fluids. Although both the cornea and
the sclera are alike in consisting mostly of collagen, they are structurally very
different. The transparency of the cornea sets it apart from the opaque sclera and
maintenance of its integrity and transparency is essential for the proper functioning
of the whole eye.
The cornea consists of 5 different layers (Fig.
3.1
a); an outermost stratified,
squamous epithelial layer, limited posteriorly by the Bowman's layer—an acel-
lular layer 8-12 lm thick lying next to the basal lamina of the epithelium. Next is
the corneal stroma, which accounts for around 90 % of corneal thickness and
comprises aligned collagen fibrils interspersed with keratocytes. It is this precise
arrangement of collagen fibrils in the cornea that are responsible for corneal
transparency and what sets it apart from the disorganised sclera. The stroma is
separated from the anterior most endothelial layer by the Descemet's membrane.
Descemet's is considered the basement membrane of the endothelium consisting
of several different collagen types that are secreted by the endothelial cells. In
young eyes, the membrane is 3-4 lm thick but this can increase to 10-15 lm with
age. The endothelial layer is a single layer of metabolically active cells that act as
pumps to maintain the normal stromal water content, which in turn maintains the
transparency of the cornea. Corneal endothelial cells are postmitotic and divide
rarely, if at all, in the adult human cornea. Instead, wounding of the layer causes
cells to enlarge to fill the gaps left by lost cells. If cell numbers fall below a certain
threshold then corneal edema can occur, leading to disruption of the collagen fibril
spacing, creating light scatter and ultimately, impaired vision.
3.2 The Corneal Epithelium
The ocular surface epithelium is a multilayered, renewable barrier consisting of 6-8
cell layers with a thickness of about 50 lm (Fig.
3.1
b). The epithelial layer is
stratified and cells differ in morphology depending on their location within the
epithelium. The basal cells are a single layer of cells that rest on the basement
membrane, adjacent to the Bowman's layer, are columnar in shape and are the
newest mitotically active cells. Lying above this layer are the middle zone wing
cells, so called because of their lateral wing-like side projections. This layer can be
1-3 cells thick containing cells with convex anterior surfaces, concave posterior
surfaces and oval or round nuclei. This layer is then topped by several layers of
flattened, squamous cells with horizontal nuclei. These cells form many tight
junctions to act as a barrier to fluid loss and their superficial surfaces present
numerous microvilli and microplicae, which support and stabilise the tear film on
the ocular surface. The anterior most of this layer are the cells that are sloughed off
first
as
virtually
all
the
cells
in
the
epithelium
are
born,
transformed
into
