Biosynthesis of glycosaminoglycans by the separated tissues of the embryonic chick cornea

Corneal tissues (epithelium, endothelium, and stroma) were isolated from chick embryos at 14, 17, and 20 days of incubation and immediately labeled in vitro with d-[6-³H]glucosamine and H₂ ³⁵SO₄. Amount of label incorporated into each type of glycosaminoglycan or into glycopeptides was determined by specific degradative techniques, in conjunction with gel filtration chromatography. Results suggested that corneal epithelium synthesized little, if any, corneal keratan sulfates, but that corneal endothelium may have synthesized small amounts of corneal keratan sulfates. Nearly all corneal keratan sulfates were derived from the stroma. Corneal heparan sulfates appeared to be derived predominantly from corneal epithelium at later stages of development. Corneal endothelium contributed large proportions of the hyaluronic acids of the cornea. Only epithelium produced a large proportion of sulfated glycoproteins. In addition, epithelium synthesized a large proportion of a sulfated, high molecular weight polysaccharide which was resistant to treatments degrading known types of glycosaminoglycans. Each corneal tissue may not only affect corneal morphogenesis directly by contributing a unique spectrum of glycosylated proteins to the extracellular matrix, but also may regulate the extracellular matrix composition indirectly by modulating the biosynthetic activities of the other corneal tissues.