Biosynthesis of glycosaminoglycans during corneal development

Glycosaminoglycan biosynthesis was studied in developing chick corneas, with particular attention paid to keratan sulfate I, the major glycosaminoglycan of this tissue. This polysaccharide is unique to the cornea and may be required for the development and maintenance of corneal transparency. Corneas from 5 to 20 day chick embryos were labeled in vitro with D [6 ³H]glucosamine and H₂³⁵SO₄, and the amount of label in each glycosaminoglycan was determined. The data indicate that, contrary to previous suggestions, keratan sulfate biosynthesis in the cornea begins at the time of fibroblast invasion of the primary stroma, at least 8 days prior to the onset of corneal transparency, which occurs on Day 14 of development in the chick. The rate of incorporation of radioactivity into keratan sulfates, on a dry weight basis, increases rapidly after Day 6 and levels off on Day 14. The proportion of ³H and ³⁵S in keratan sulfate reaches nearly maximal levels as early as Day 9. In contrast, the proportion of radioactivity in corneal heparan sulfates declines rapidly after Day 5. However, the rate of incorporation of radioactivity into heparan sulfates, on a dry weight basis, increases or remains the same during early development. On and after Day 14, keratan sulfates appear to become more highly sulfated. Moreover, the ratios of 4 sulfated to 6 sulfated chondroitin sulfates increase during development, reaching a maximum on Day 14. These changing patterns of glycosaminoglycan biosynthesis during corneal development may play an important role in corneal morphogenesis and the achievement of corneal transparency.