102
ALFRED J . COULOMBRE
not collagen fibrils are responsible for water uptake (see van Walbeek,
1960). A hydrated cornea regains its transparency if it eliminates the
excess water.
Historically there have been two principal hypotheses concerning the
basis of corneal transparency. The older concept ascribes transparency
to the uniformity of refractive index as between the collagen fibrils on
the one hand and the extrafibrillar medium surrounding them on the
other. The requirements of this hypothesis could be met, if, after dehydration of the developing stroma, mucopolysaccharides achieved a
high enough local concentration to render the extrafibrillar substance
equal to collagen in refractive index. Maurice (1960) summarizes
objections to this hypothesis.
The alternative hypothesis has been put forward more recently by
Maurice (1957, 1960, 1962). I t suggests that a series of long rods will
interfere destructively with a light beam in every direction but the
direction of its propagation if the following conditions are met : that the
rods have a diameter which is small relative to the wavelengths of light
which are traversing the system ; that they be of uniform diameter ; and
that they be spaced in a regular parallel array such that the distance
between units is small relative to the wavelengths of light traversing the
system. I t is remarkable in what manner the developing corneal stroma
approximates these conditions. The collagen fibrils (elongated rods) have
a diameter (300 Â) which is small relative to wavelengths of visible light.
The fibrils, unlike most collagen fibrils elsewhere are relatively uniform
in diameter. During early development, while the cornea still has a
relatively high water content and is opaque, collagen fibrils are widely
separated in the stroma. I t is only during the course of dehydration of
the extrafibrillar compartment that they come close together. When
they do so they tend to line up not only equidistant from each other,
but also in such a manner that cross bands of the fibrils are in register
across any given fiber. Perhaps the regularity of this lattice will ultimately
be accounted for on the basis of stoichiometric and chemically specific interactions as among the collagen protein, noncollagenous protein and the
mucopolysaccharides.
The hypothesis of Maurice seems to fit the situation that obtains
within the individual fibers or lamellae. I t must, however, be borne in
mind, as Kikkawa (1959) has pointed out, that the corneal stroma is a
double matrix, one within the fibers (the fibrillar lattice) and the other
at the fiber level (the fiber matrix). At the level of the fiber matrix an
approach to uniform refractive index may be important. I t is possible
that both of these mechanisms may ultimately be found to contribute
to transparency. The work of Trokel (1962) on the transparency of the
crystalline lens brings into clear focus some of the physical considéra-
ALFRED J . COULOMBRE
not collagen fibrils are responsible for water uptake (see van Walbeek,
1960). A hydrated cornea regains its transparency if it eliminates the
excess water.
Historically there have been two principal hypotheses concerning the
basis of corneal transparency. The older concept ascribes transparency
to the uniformity of refractive index as between the collagen fibrils on
the one hand and the extrafibrillar medium surrounding them on the
other. The requirements of this hypothesis could be met, if, after dehydration of the developing stroma, mucopolysaccharides achieved a
high enough local concentration to render the extrafibrillar substance
equal to collagen in refractive index. Maurice (1960) summarizes
objections to this hypothesis.
The alternative hypothesis has been put forward more recently by
Maurice (1957, 1960, 1962). I t suggests that a series of long rods will
interfere destructively with a light beam in every direction but the
direction of its propagation if the following conditions are met : that the
rods have a diameter which is small relative to the wavelengths of light
which are traversing the system ; that they be of uniform diameter ; and
that they be spaced in a regular parallel array such that the distance
between units is small relative to the wavelengths of light traversing the
system. I t is remarkable in what manner the developing corneal stroma
approximates these conditions. The collagen fibrils (elongated rods) have
a diameter (300 Â) which is small relative to wavelengths of visible light.
The fibrils, unlike most collagen fibrils elsewhere are relatively uniform
in diameter. During early development, while the cornea still has a
relatively high water content and is opaque, collagen fibrils are widely
separated in the stroma. I t is only during the course of dehydration of
the extrafibrillar compartment that they come close together. When
they do so they tend to line up not only equidistant from each other,
but also in such a manner that cross bands of the fibrils are in register
across any given fiber. Perhaps the regularity of this lattice will ultimately
be accounted for on the basis of stoichiometric and chemically specific interactions as among the collagen protein, noncollagenous protein and the
mucopolysaccharides.
The hypothesis of Maurice seems to fit the situation that obtains
within the individual fibers or lamellae. I t must, however, be borne in
mind, as Kikkawa (1959) has pointed out, that the corneal stroma is a
double matrix, one within the fibers (the fibrillar lattice) and the other
at the fiber level (the fiber matrix). At the level of the fiber matrix an
approach to uniform refractive index may be important. I t is possible
that both of these mechanisms may ultimately be found to contribute
to transparency. The work of Trokel (1962) on the transparency of the
crystalline lens brings into clear focus some of the physical considéra-