PROBLEMS IN CORNEAL MORPHOGENESIS
103
tions that must be recognized in the search for the physical basis of
transparency either in predominantly cellular structures (lens, corneal
epithelium) or in structures with a large extracellular compartment
(stroma).
VIII. Growth and Shaping of the Cornea
In many vertebrates the cornea has a greater curvature (smaller
radius of curvature) than the remainder of the eye wall. Cucchia (1926)
has followed the development of corneal curvature in man. In land
vertebrates the anterior surface of the cornea is the locus of highest
refraction in the eye since it separates media of quite different refractive
index. For this reason the mechanisms which control the shaping of the
cornea during development are of some importance. Mechanical forces
appear to play an important role (Neifach, 1952a; Coulombre, 1956,
1957; Coulombre and Coulombre, 1958c). When the vitreous substance
is drained from the developing eye of the chick embryo through an
indwelling glass tube, the vitreous body does not increase in size. Under
these conditions the eye wall is not kept under tension and the cornea
fails to grow in diameter. Mechanical forces also help to give the cornea
a greater curvature from the rest of the eye. The eye of the chick
embryo expands as a sphere early in development when its wall is soft.
On the 8th day a hyaline cartilage begins to differentiate in the sciera
just outside the corneal limbus. Thereafter the limbic region expands
less rapidly than the remainder of the eye wall in response to intraocular
pressure. As a consequence the corneal curvature develops anteriorly,
and a scierai curvature of larger radius develops posteriorly. This event
does not occur until after birth in the rabbit (Smelser and Ozanics,
1960) where thé deposition of collagen at the limbus may play a
mechanical role similar to that played by cartilage in submammalian
forms.
The cornea continues to increase progressively in diameter not only
throughout development but for a considerable period following birth
or hatching. Norrby (1958) has shown a continual increase in corneal
diameter even after 200 days post partum in the Norwegian rat. Since
the fibrils of corneal collagen are in the plane of the cornea and follow
relatively straight courses from one side of the limbus to the other, and
since collagen has a relatively low elastic limit (about 3% of its length),
this expansion is probably not effected by a simple stretching of the
cornea, but must reflect continued synthesis of collagen and other constituents at the corneal margin. Since the fibrillar lattice and the fiber
matrix show no discontinuities at any stage in development this process
of deposition of new matrix must be an extremely orderly process which
accurately extends the old matrix on which it builds. The fact that, at
D2
A.M.4
103
tions that must be recognized in the search for the physical basis of
transparency either in predominantly cellular structures (lens, corneal
epithelium) or in structures with a large extracellular compartment
(stroma).
VIII. Growth and Shaping of the Cornea
In many vertebrates the cornea has a greater curvature (smaller
radius of curvature) than the remainder of the eye wall. Cucchia (1926)
has followed the development of corneal curvature in man. In land
vertebrates the anterior surface of the cornea is the locus of highest
refraction in the eye since it separates media of quite different refractive
index. For this reason the mechanisms which control the shaping of the
cornea during development are of some importance. Mechanical forces
appear to play an important role (Neifach, 1952a; Coulombre, 1956,
1957; Coulombre and Coulombre, 1958c). When the vitreous substance
is drained from the developing eye of the chick embryo through an
indwelling glass tube, the vitreous body does not increase in size. Under
these conditions the eye wall is not kept under tension and the cornea
fails to grow in diameter. Mechanical forces also help to give the cornea
a greater curvature from the rest of the eye. The eye of the chick
embryo expands as a sphere early in development when its wall is soft.
On the 8th day a hyaline cartilage begins to differentiate in the sciera
just outside the corneal limbus. Thereafter the limbic region expands
less rapidly than the remainder of the eye wall in response to intraocular
pressure. As a consequence the corneal curvature develops anteriorly,
and a scierai curvature of larger radius develops posteriorly. This event
does not occur until after birth in the rabbit (Smelser and Ozanics,
1960) where thé deposition of collagen at the limbus may play a
mechanical role similar to that played by cartilage in submammalian
forms.
The cornea continues to increase progressively in diameter not only
throughout development but for a considerable period following birth
or hatching. Norrby (1958) has shown a continual increase in corneal
diameter even after 200 days post partum in the Norwegian rat. Since
the fibrils of corneal collagen are in the plane of the cornea and follow
relatively straight courses from one side of the limbus to the other, and
since collagen has a relatively low elastic limit (about 3% of its length),
this expansion is probably not effected by a simple stretching of the
cornea, but must reflect continued synthesis of collagen and other constituents at the corneal margin. Since the fibrillar lattice and the fiber
matrix show no discontinuities at any stage in development this process
of deposition of new matrix must be an extremely orderly process which
accurately extends the old matrix on which it builds. The fact that, at
D2
A.M.4