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adult cornea (Leuenberger 1978). The PLL is synthesized and secreted by
the corneal endothelium (see below) so that the most anterior portion is the
oldest and the least uniform in texture. This is the foetal part of the PLL and
it is known as the banded layer, whereas the post- natal component of the
PLL is uniform in texture and is also the portion that thickens with age.
The corneal endothelium
A single layer of cells forms the corneal endothelium across the most
posterior (innermost) aspect the cornea (Leuenberger 1978, Doughty 1989a,
Edelhauser 2000). The distinct organization of this cell layer (cell mosaic)
is now a well-known clinical feature of the cornea, with images being
readily obtainable with a specular microscope (Doughty 1998, Edelhauser
2000). The corneal endothelium can also be viewed by confocal microscopy
(Jonuscheit et al. 2011). With either method, the endothelial cells can form
a rather uniform or regular mosaic (i.e. all cells seem to have similar sizes)
or be non-uniform in which the cells exhibit a range of sizes; the latter
appearance is widely referred to as polymegethism. It can be shown that
a relatively large proportion of the endothelial cells in a regular mosaic
are in fact 6-sided, but that there are notable numbers of other polygons,
especially 5- and 7-sided cells (Doughty 1998). While it might be considered
that the normal adult cornea has a slight defi ciency of 6-sided cells (i.e. all
cells should be 'hexagonal'), detailed studies of the corneal endothelia of
children also clearly indicate that there are quite a large number of non-6-
sided cells (Müller et al. 2000). In younger adults (c. 20 years of age), the
average area of the endothelial cells, regardless of shape (polygonality),
is around 330 µm 2 ; this translates to a calculated endothelial cell density
(ECD) of close to 3000 cells/mm 2 .
After birth, it is generally considered that these cells do not normally
replicate in the human or primate cornea (Edelhauser 2000). Since human
corneal endothelial cells have apparently lost their ability to replicate in situ ,
any cells lost by stress, trauma, or damage will not be replaced (Edelhauser
2000). Therefore, there is a natural age-related decline in the number of
corneal endothelial cells. At birth, the ECD is likely to be around 5500 cells/
mm 2 (Doughty et al. 2000, Müller and Doughty 2002). In the early post-natal
years, there is a dramatic decline in cell density by around 1000 to 2000
cells to reach values of close to 4000 cells/mm 2 by childhood (Doughty et
al. 2000, Müller and Doughty 2002) ( Fig. 17 ). This initial apparent reduction
is associated with the dramatic growth (enlargement) of the cornea that
occurs during this period (Müller and Doughty 2002) ( Fig. 11A ). Thereafter,
although somewhat debatable, there is a predictable further decline in cell
density as a result of cell loss so that ECD values that are rather less than
3000 cells/mm 2 and even closer to 2000 cells/mm 2 may well be observed
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