PROBLEMS IN CORNEAL MORPHOGENESIS
89
would be a most important undertaking in connection with morphogenetic
studies of the vertebrate cornea. If the geometry of the stromal matrix is
laid down in miniature underneath the anterior epithelium early in the
development of the cornea, it would invite attention to the possibility
that epithelial structures elsewhere in the embryo might dictate the
initial internal structuring of the more deeply situated connective
tissues. If these observations are confirmed we may have an explanation
for the inability of stromal fibroblasts to deposit collagen in an orderly
array during the healing of corneal wounds in the adult. For it may be
that the matrix is laid down during a restricted period in early development layer by layer underneath the primitive corneal epithelium and
that it cannot subsequently be replaced.
B. Stromal Fibroblasts
1. Origin
Following an interval which varies with the species the postepithelial
layer becomes populated with fibroblasts. There is virtually universal
agreement that these cells migrate in from the mesenchymal pool that
has accumulated in the limbic region (reviewed by Redslob, 1935). This
event seems always to be preceded by the migration of mesenchyme
along the posterior face of the postepithelial layer (directional membrane) to form the posterior epithelium of the cornea. There is, however,
no indication that the presence of the posterior epithelium is a necessary
condition for the migration of the stromal cells. Since an aggregation of
mesenchyme cells is present in the limbus in some species for a considerable period after the postepithelial layer has begun to form, it is possible
that their failure to migrate indicates either that they have not matured
to the point where they are capable of doing so, or that the postepithelial
layer has not differentiated sufficiently to stimulate and support such
migration, or both. When these cells do enter the postepithelial layer,
they invade the posterior lamellae first (Meyer and O'Rahilly, 1959). As
they enter the presumptive stroma the cells quickly become differentiated cytologically from those in the mesenchymal pool from which
they arose (Rones, 1932). That the differentiation of these cells is
dictated by the primitive stroma is indicated by the experiment of
Attardi (1954) who explanted pieces of chick embryonic stroma which
had already become populated with fibroblasts. Under these conditions
those cells which migrate away from the stromal framework dedifferentiate cytologically and those which remain in contact with the stroma
retain the cytological characteristics of stromal fibroblasts. The increase
in the cellular population of the stroma is not dictated solely by the
process of migration for, as Rones (1932) and Iasvoin (1939) point out,
89
would be a most important undertaking in connection with morphogenetic
studies of the vertebrate cornea. If the geometry of the stromal matrix is
laid down in miniature underneath the anterior epithelium early in the
development of the cornea, it would invite attention to the possibility
that epithelial structures elsewhere in the embryo might dictate the
initial internal structuring of the more deeply situated connective
tissues. If these observations are confirmed we may have an explanation
for the inability of stromal fibroblasts to deposit collagen in an orderly
array during the healing of corneal wounds in the adult. For it may be
that the matrix is laid down during a restricted period in early development layer by layer underneath the primitive corneal epithelium and
that it cannot subsequently be replaced.
B. Stromal Fibroblasts
1. Origin
Following an interval which varies with the species the postepithelial
layer becomes populated with fibroblasts. There is virtually universal
agreement that these cells migrate in from the mesenchymal pool that
has accumulated in the limbic region (reviewed by Redslob, 1935). This
event seems always to be preceded by the migration of mesenchyme
along the posterior face of the postepithelial layer (directional membrane) to form the posterior epithelium of the cornea. There is, however,
no indication that the presence of the posterior epithelium is a necessary
condition for the migration of the stromal cells. Since an aggregation of
mesenchyme cells is present in the limbus in some species for a considerable period after the postepithelial layer has begun to form, it is possible
that their failure to migrate indicates either that they have not matured
to the point where they are capable of doing so, or that the postepithelial
layer has not differentiated sufficiently to stimulate and support such
migration, or both. When these cells do enter the postepithelial layer,
they invade the posterior lamellae first (Meyer and O'Rahilly, 1959). As
they enter the presumptive stroma the cells quickly become differentiated cytologically from those in the mesenchymal pool from which
they arose (Rones, 1932). That the differentiation of these cells is
dictated by the primitive stroma is indicated by the experiment of
Attardi (1954) who explanted pieces of chick embryonic stroma which
had already become populated with fibroblasts. Under these conditions
those cells which migrate away from the stromal framework dedifferentiate cytologically and those which remain in contact with the stroma
retain the cytological characteristics of stromal fibroblasts. The increase
in the cellular population of the stroma is not dictated solely by the
process of migration for, as Rones (1932) and Iasvoin (1939) point out,