PROBLEMS IN CORNEAL MORPHOGENESIS
91
begin to deposit coarser fibers of collagen to form the definitive lamellae.
During the development of many species the most anterior layers of the
stroma are devoid of cells. This zone has been referred to as Bowman's
membrane. While it persists to adult stages in primates and in some
submammalian forms, it disappears by adulthood in most species. Since
Bowman's membrane has all of the histochemical characteristics of the
extracellular compartment of the stroma with the exception that it does
not contain coarse fibers of collagen, it is reasonable to view it as a
region of stroma which is not yet fully matured. It may be that in those
species which still possess a Bowman's membrane in the adult stage the
period of active deposition of collagen by the stromal fibroblasts has
come to an end before the time at which the deposition of primitive
lamellae ceases beneath the anterior epithelium. On the other hand, in
those species which do not possess $ Bowman's membrane in the adult
stage, it is possible that the deposition of primitive lamellae anteriorly
comes to an end before the stromal cells cease to function as fibroblasts
and that there is, as a consequence, an opportunity for these cells to
populate all lamellar layers with coarse fibers of collagen.
Glycosaminoglycans (mucopolysaccharides) are also important constituents of the extracellular compartment of the stroma. A detailed
history of the investigation of these substances is given by Anseth
(1961a). They constitute about 5% of the dry mass of the stroma (Pirie
and van Heyningen, 1956). They are polymers of amino sugars (glucosamine and galactosamine) and uronic acid, and may or may not bear
sulfate side groups in ester linkage. Largely on the basis of work with
cattle corneae three different polysaccharides have been identified in
the corneal stroma: chondroitin-4-sulfate (A) (equimolar amounts of
i^-acetyl-galactosamine, uronic acid, and ester-sulfate) ; chondroitin
(equimolar concentrations of j^-acetyl-galactosamine and uronic acid,
with very little ester-sulfate); and keratan sulfate (keratosulfate)
(equimolar concentrations of iV-acetyl-glucosamine, galactose and estersulfate) (Meyer et al., 1953; Anseth, 1961a). This combination of glycosaminoglycans is specific for the corneal stroma, and one of them,
keratan sulfate, is not found in appreciable concentrations elsewhere in
the body. In the adult keratan sulfate accounts for about half of the
polysaccharide, while the two remaining polymers account for onequarter each.
The glycosaminoglycans are apparently synthesized by the stromal
fibroblasts (Schwarz, 1961). Jackson (1955) observed PAS + , toluidine
blue, and thionine metachromatic granules in the cytoplasm of cultured
fibroblasts.
The characteristic metachromasia of the stromal extracellular compartment with such dyes as toluidine blue is attributable to glyco-
91
begin to deposit coarser fibers of collagen to form the definitive lamellae.
During the development of many species the most anterior layers of the
stroma are devoid of cells. This zone has been referred to as Bowman's
membrane. While it persists to adult stages in primates and in some
submammalian forms, it disappears by adulthood in most species. Since
Bowman's membrane has all of the histochemical characteristics of the
extracellular compartment of the stroma with the exception that it does
not contain coarse fibers of collagen, it is reasonable to view it as a
region of stroma which is not yet fully matured. It may be that in those
species which still possess a Bowman's membrane in the adult stage the
period of active deposition of collagen by the stromal fibroblasts has
come to an end before the time at which the deposition of primitive
lamellae ceases beneath the anterior epithelium. On the other hand, in
those species which do not possess $ Bowman's membrane in the adult
stage, it is possible that the deposition of primitive lamellae anteriorly
comes to an end before the stromal cells cease to function as fibroblasts
and that there is, as a consequence, an opportunity for these cells to
populate all lamellar layers with coarse fibers of collagen.
Glycosaminoglycans (mucopolysaccharides) are also important constituents of the extracellular compartment of the stroma. A detailed
history of the investigation of these substances is given by Anseth
(1961a). They constitute about 5% of the dry mass of the stroma (Pirie
and van Heyningen, 1956). They are polymers of amino sugars (glucosamine and galactosamine) and uronic acid, and may or may not bear
sulfate side groups in ester linkage. Largely on the basis of work with
cattle corneae three different polysaccharides have been identified in
the corneal stroma: chondroitin-4-sulfate (A) (equimolar amounts of
i^-acetyl-galactosamine, uronic acid, and ester-sulfate) ; chondroitin
(equimolar concentrations of j^-acetyl-galactosamine and uronic acid,
with very little ester-sulfate); and keratan sulfate (keratosulfate)
(equimolar concentrations of iV-acetyl-glucosamine, galactose and estersulfate) (Meyer et al., 1953; Anseth, 1961a). This combination of glycosaminoglycans is specific for the corneal stroma, and one of them,
keratan sulfate, is not found in appreciable concentrations elsewhere in
the body. In the adult keratan sulfate accounts for about half of the
polysaccharide, while the two remaining polymers account for onequarter each.
The glycosaminoglycans are apparently synthesized by the stromal
fibroblasts (Schwarz, 1961). Jackson (1955) observed PAS + , toluidine
blue, and thionine metachromatic granules in the cytoplasm of cultured
fibroblasts.
The characteristic metachromasia of the stromal extracellular compartment with such dyes as toluidine blue is attributable to glyco-