PROBLEMS IN CORNEAL MORPHOGENESIS
101
in the stroma and the other is the appearance of a water-pumping
mechanism ('corneal pump') which begins to function at the epithelial
surfaces of the cornea during its dehydration. In the rabbit Smelser and
Ozanics (1960) described an increase in water-binding capacity with
developmental time rather than a loss. Additional study will be required
to retest a possible difference in this respect between the domestic fowl
and the rabbit.
Whatever the mechanism of the observed dehydration may be, the
studies of the chick embryo (Coulombre and Coulombre, 1959; and unpublished work) have clearly demonstrated that it not only has its onset
at the time the thyroid becomes active, but that exogenous L-thyroxin
will concomitantly accelerate the dehydration of the cornea and its
increase in transparency, and that thiouracil will retard the dehydration
of the cornea as well as the onset of clearing.
The time course of development of transparency in the cornea correlates with another well-established phenomenon. As noted above it
occurs during the time when an intense metachromasia appears in the
inner layers of stroma and spreads toward the anterior portions.
What constitutes the physical basis of the transparency of the cornea?
This question may have to be answered twice. This is so because both
the anterior epithelium and the stroma are capable of interfering
significantly with the transmission of a coherent beam of light through
the cornea (Coulombre, unpublished data). Thus it would seem essential
that during the course of development both of these portions of the
cornea be rendered transparent. Since they are of such different composition it is reasonable to assume that the physical basis of transparency is probably different in these two layers. It is, therefore, more
appropriate to speak of the physical bases, rather than the physical
basis of corneal transparency. We have yet to separately chart the developmental change in the transparency in the anterior epithelium of the cornea
and of the stroma. In the past attention has been focused almost exclusively on the stroma. Most of the stroma is extracellular, and an all
but exhaustive inventory of its constituents yields a relatively modest
list of substances (Pirie and van Heyningen, 1956). The stroma of the
adult vertebrate cornea is about 75% water by weight. Of its dry substance approximately 80% is collagen. Large aggregates of collagen
elsewhere in the body (including the adjacent sciera) reflect most of the
light that impinges on them and appear white. It seems paradoxical,
therefore, that the collagenous stroma is so highly transparent. Limited
information concerning the physical basis of corneal transparency can
be obtained by experimental manipulation of adult corneas. For example, it is known that when the water content of the stroma is increased
there is a progressive loss of transparency. Mucopolysaccharides and
101
in the stroma and the other is the appearance of a water-pumping
mechanism ('corneal pump') which begins to function at the epithelial
surfaces of the cornea during its dehydration. In the rabbit Smelser and
Ozanics (1960) described an increase in water-binding capacity with
developmental time rather than a loss. Additional study will be required
to retest a possible difference in this respect between the domestic fowl
and the rabbit.
Whatever the mechanism of the observed dehydration may be, the
studies of the chick embryo (Coulombre and Coulombre, 1959; and unpublished work) have clearly demonstrated that it not only has its onset
at the time the thyroid becomes active, but that exogenous L-thyroxin
will concomitantly accelerate the dehydration of the cornea and its
increase in transparency, and that thiouracil will retard the dehydration
of the cornea as well as the onset of clearing.
The time course of development of transparency in the cornea correlates with another well-established phenomenon. As noted above it
occurs during the time when an intense metachromasia appears in the
inner layers of stroma and spreads toward the anterior portions.
What constitutes the physical basis of the transparency of the cornea?
This question may have to be answered twice. This is so because both
the anterior epithelium and the stroma are capable of interfering
significantly with the transmission of a coherent beam of light through
the cornea (Coulombre, unpublished data). Thus it would seem essential
that during the course of development both of these portions of the
cornea be rendered transparent. Since they are of such different composition it is reasonable to assume that the physical basis of transparency is probably different in these two layers. It is, therefore, more
appropriate to speak of the physical bases, rather than the physical
basis of corneal transparency. We have yet to separately chart the developmental change in the transparency in the anterior epithelium of the cornea
and of the stroma. In the past attention has been focused almost exclusively on the stroma. Most of the stroma is extracellular, and an all
but exhaustive inventory of its constituents yields a relatively modest
list of substances (Pirie and van Heyningen, 1956). The stroma of the
adult vertebrate cornea is about 75% water by weight. Of its dry substance approximately 80% is collagen. Large aggregates of collagen
elsewhere in the body (including the adjacent sciera) reflect most of the
light that impinges on them and appear white. It seems paradoxical,
therefore, that the collagenous stroma is so highly transparent. Limited
information concerning the physical basis of corneal transparency can
be obtained by experimental manipulation of adult corneas. For example, it is known that when the water content of the stroma is increased
there is a progressive loss of transparency. Mucopolysaccharides and