Jm. Fitch et al., COLLAGEN FIBRIL ASSEMBLY IN THE DEVELOPING AVIAN PRIMARY CORNEAL STROMA, Investigative ophthalmology & visual science, 35(3), 1994, pp. 862-869
Purpose. The primary stroma of the developing avian cornea is a highly
organized extracellular matrix composed largely of striated collagen
fibrils synthesized by the epithelium. These fibrils are heterotypic s
tructures consisting of at least two different fibrillar collagen type
s (I and II) and probably a fibril-associated collagen (type IX). The
epithelial derivation and vectorial secretion of the components of thi
s matrix provide an advantageous system to study the steps in the asse
mbly of this developmentally regulated matrix, as well as in the assem
bly and maturation of heterotypic collagen fibrils in general. To asce
rtain when and where two of the collagens of the primary stroma (colla
gen types II and IX) are assembled into fibrils, monoclonal antibodies
were used that bind to the newly deposited, ''immature'' (non-cross-l
inked) forms of these molecules, but not to ones that have been assemb
led into fibrils and have become cross-linked. Methods. The patterns o
f immunoreactivity for newly deposited versus total collagens were com
pared by immunofluorescence histochemistry on sections of corneas from
developing chicken embryos. Pretreatments that affect collagen cross-
linking and enzymatic digestions with collagenase also were used. Resu
lts. In early corneas, immunoreactivity for the newly deposited forms
of both collagen types II and IX was confined to a narrow subepithelia
l zone, their epitopes being masked in the deeper layers of the stroma
. The masked immunoreactivity could be exposed in these layers by inhi
biting cross-link formation with beta-aminopropionitrile. At later dev
elopmental stages, after the stroma has swollen and become invaded by
mesenchymal cells, type IX collagen is no longer detectable either as
an ''immature'' or as a fibril-associated form. During most of this pe
riod, the distribution of ''immature'' type II collagen is noticeably
more restricted to the subepithelial zone than is total type II. Much
of the undetectable immunoreactivity for collagen type II could be ''u
nmasked'' in deeper stromal layers by brief digestions with bacterial
collagenase as well as by inhibition of cross-link formation. The exte
nt of such unmasking of type II, however, is more limited in older cor
neas, suggesting that some of the putatively masked epitopes at these
stages may in fact be proteolytically degraded. Conclusions. These res
ults conform with previous studies suggesting that the type II and typ
e IX collagens of the primary stroma are derived from the epithelium.
They also suggest that (1) the assembly and maturation of the heteroty
pic fibrils, including the addition of the fibril-associated collagen
type IX and covalent cross-link formation, occur shortly after synthes
is and secretion of the molecules; (2) most, if not ail, of the cornea
l type IX collagen becomes fibril-associated; and (3) during much of c
orneal development the N-telopeptide epitope in type II collagen is la
rgely retained in a sterically masked form, but in later stages, durin
g remodeling it may be removed by proteolytic degradation.