Abstract
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] glyhucosamine and H(2)35SO(4)35SO(4) 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 the 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 l4. 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
Highlights
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 cornea1 transparency, which occurs on Day 14 of development in the chick
The proportion of radioactivity incorporated per unit of time in total glycosaminoglycans relative to that in glycopeptides increased as development proceeded
Maximal incorporation rates into total glycosaminoglycans on a dry weight basis occurred on Day 12 of development for both isotopes
Summary
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 cornea transparency, which occurs on Day 14 of development in the chick. 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 cornea development may play an important role in cornea morphogenesis and the achievement of cornea transparency. Nearly two-thirds of the glycosaminoglycan is keratan sulfate I, with most of the remainder being chondroitin 4-sulfate and much smaller amounts of other glycosaminoglycans [9, 10] These polysaccharides may play a role in the maintenance of the physical characteristics of the cornea stroma [11]. Their work suggests that the biosynthesis of this glycosaminoglycan begins late in cornea development, nearly coincident with the increase in cornea transparency, onset of dehydration, and apperance of metachromasia (occurring on Day 14 of development in the chick [21]). The aim of this study was to determine the time course of appearance and properties of glycosaminoglycans during the development of the chick cornea, placing particular emphasis on keratan sulfate
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