Transitions in extracellular macromolecules during avian ocular development.

The macromolecular components of the extracellular matrix of avian ocular tissues undergo complex transitions during the course of development. Two of these tissues in particular, the cornea and vitreous body, have been studied in reference to their glycosaminoglycan and collagen components. During early stages of development, the corneal stroma is an acellular structure composed of orthogonally arranged fibrils containing types I and II collagens with associated chondroitin sulfate-proteoglycan. These are produced by the corneal epithelium. Type IV collagen and proteoglycan are also present in the epithelial basement membrane. The endothelium produces hyaluronate and possibly type IV collagen. Subsequently, the stroma becomes highly hydrated, swells and is invaded by mesenchymal cells which initially produce large amounts of hyaluronate. These cells then differentiate to corneal fibroblasts which synthesize mainly type I collagen and chondroitin sulfate and keratan sulfate-proteoglycans, the major components of the mature corneal matrix. At this time hyaluronidase activity increases in the cornea and the hyaluronate is removed; the tissue loses water, shrinks, and becomes transparent. Two major extracellular components of the avian vitreous body during the course of its development are chondroitin sulfate and type II collagen. Early in development these components are synthesized and secreted into the vitreous by the neural retina whereas subsequently they are derived from cells within the vitreous body itself. Possible structural and morphogenetic roles of these extracellular macromolecules relate to the stabilization of tissue phenotype and cellular migration.