90
ALFRED J . COULOMBRE
the cells continue to divide after they have invaded. The ability to divide
is apparently never lost and can be elicited in the mature cornea by
corneal wounding (Weimar, 1960). In the chick embryo the population
of stromal cells begins to stabilize at about 16 days of incubation as
reflected in deoxyribonucleic acid (DNA) measurements (Herrmann,
1958). As soon as stromal fibroblasts appear in the postepithelial layer
coarse collagen fibers begin to become apparent (Rones, 1932; Ghiani
and Bergamini, 1958a, b ; Neüschuler, 1931).
2. Synthetic Products
Collagen is deposited in the postepithelial layer by the fibroblasts
(Ladijenski, 1915; Laguesse, 1923a, b ; Levi, 1926; Neifach, 1952c). I n
this manner the stroma is formed. The electromicroscopic investigation
of Schwarz (1961) implicates the endoplasmic reticulum of these cells as
the site of synthesis of collagen precursors. The amorphous precollagen
which is present in the cisternae of the endoplasmic reticulum is
extruded from the cell surface into the surrounding space. Polymerization does not occur within the cell, but within the extracellular space
and at the cell surface. All of the fibrils which are thus formed have a
small diameter (250-300 Â in the rat, Jakus, 1954; and 300-350 Â in
man, Schwarz, 1953), have crossband macroperiods of about 640 Â
(van den Hooff, 1957), and are deposited in parallel arrays to form the
fibers and gross lamellae of the stroma. Schwarz notes that the fibrils
within these fibers, unlike those in other coUagenous deposits elsewhere
in the body, have essentially the same diameter throughout life.
Collagen, measured in terms of hydroxyproline, accumulates rapidly in
the chick embryo stroma until about the time of hatching ; then its rate
of deposition slows appreciably (Herrmann, 1958). Eventually the
fibroblasts of the stroma become inactive as evidenced by their cytology,
and are called stromal corpuscles. Thereafter they can be reactivated as
fibroblasts following wounding of the cornea (Weimar, 1959a, b, 1960;
Dunnington and Weimar, 1958). When this occurs the collagen fibrils
which are deposited in the scar have a wide distribution of diameters
similar to that in other collagen deposits of the body. The fact that the
stromal cells are incapable of making collagen fibrils of uniform diameter
in this situation suggests that they may not be responsible in the first
place for the uniformity of fibrillar diameter in the normal situation.
The postepithelial layer and conditions in the extracellular compartment may dictate this uniformity.
The oldest lamellae in the stroma probably lie toward the posterior
epithelium and the youngest are laid down sequentially beneath the
anterior epithelium (Meyer and O'Rahilly, 1959; Coulombre and Coulombre, 1961). Stromal cells invade the most posterior lamellae first and
ALFRED J . COULOMBRE
the cells continue to divide after they have invaded. The ability to divide
is apparently never lost and can be elicited in the mature cornea by
corneal wounding (Weimar, 1960). In the chick embryo the population
of stromal cells begins to stabilize at about 16 days of incubation as
reflected in deoxyribonucleic acid (DNA) measurements (Herrmann,
1958). As soon as stromal fibroblasts appear in the postepithelial layer
coarse collagen fibers begin to become apparent (Rones, 1932; Ghiani
and Bergamini, 1958a, b ; Neüschuler, 1931).
2. Synthetic Products
Collagen is deposited in the postepithelial layer by the fibroblasts
(Ladijenski, 1915; Laguesse, 1923a, b ; Levi, 1926; Neifach, 1952c). I n
this manner the stroma is formed. The electromicroscopic investigation
of Schwarz (1961) implicates the endoplasmic reticulum of these cells as
the site of synthesis of collagen precursors. The amorphous precollagen
which is present in the cisternae of the endoplasmic reticulum is
extruded from the cell surface into the surrounding space. Polymerization does not occur within the cell, but within the extracellular space
and at the cell surface. All of the fibrils which are thus formed have a
small diameter (250-300 Â in the rat, Jakus, 1954; and 300-350 Â in
man, Schwarz, 1953), have crossband macroperiods of about 640 Â
(van den Hooff, 1957), and are deposited in parallel arrays to form the
fibers and gross lamellae of the stroma. Schwarz notes that the fibrils
within these fibers, unlike those in other coUagenous deposits elsewhere
in the body, have essentially the same diameter throughout life.
Collagen, measured in terms of hydroxyproline, accumulates rapidly in
the chick embryo stroma until about the time of hatching ; then its rate
of deposition slows appreciably (Herrmann, 1958). Eventually the
fibroblasts of the stroma become inactive as evidenced by their cytology,
and are called stromal corpuscles. Thereafter they can be reactivated as
fibroblasts following wounding of the cornea (Weimar, 1959a, b, 1960;
Dunnington and Weimar, 1958). When this occurs the collagen fibrils
which are deposited in the scar have a wide distribution of diameters
similar to that in other collagen deposits of the body. The fact that the
stromal cells are incapable of making collagen fibrils of uniform diameter
in this situation suggests that they may not be responsible in the first
place for the uniformity of fibrillar diameter in the normal situation.
The postepithelial layer and conditions in the extracellular compartment may dictate this uniformity.
The oldest lamellae in the stroma probably lie toward the posterior
epithelium and the youngest are laid down sequentially beneath the
anterior epithelium (Meyer and O'Rahilly, 1959; Coulombre and Coulombre, 1961). Stromal cells invade the most posterior lamellae first and