Morphologic and molecular changes during the post-natal development of the rabbit vitreous

Rabbits aged 1, 4, 10, 15, and 20 days, and 4 months were anesthetized and perfused with 4% formaldehyde. One eye of each rabbit was processed for paraffin embedding, while the other eye was embedded intact in methacrylate. Rabbits aged 1 and 15 days and 4 months were perfused with 2·5% glutaraldehyde, and the eyes were processed for Epon embedding. The paraffin sections were immunostained to allow detection of a high molecular weight cartilage matrix glycoprotein (CMGP), which is synthesized by the ciliary body and found in the vitreous in adult animals, using a specific mouse monoclonal antibody. CMGP was identified in the vitreous and in the inner layer of the ciliary epithelium only after the fifteenth day of life in amounts comparable to those detected in adult rabbits. Before this time immunostaining with the monoclonal antibody was seen only in the apical region of the inner ciliary epithelial cells. However, electron microscopic observations revealed that the cytoplasmic organelles responsible for the secretion of glycoproteins, i.e. the rough endoplasmic reticulum, Golgi apparatus, and vesicles, were present in the inner layer of ciliary epithelial cells as early as the first day of life. Anteroposterior sections of whole eyes embedded in methacrylate revealed a relatively dense meshwork of vitrous fibrils on the first day of life. The blood vessels were concentrated at the posterior region of the lens, and isolated cells were visible. The blood vessels were not seen after the age of 15 days, and the fiber meshwork and cells were inconspicuous by then. Electron microscopy demonstrated that the isolated cells had morphologic features similar to macrophages and were considered to be hyalocytes. The blood vessels were formed by a continuous layer of non-fenestrated endothelial cells and by a discontinuous layer of pericyte-like cells. No evidence was found for the participation of the hyalocytes in the synthesis of the components of the vitreous. It is likely that the amorphous components of the vitreous, which include hyaluronic acid and glycoproteins, e.g. CMGP, are produced at greater rates than type II collagen fibrils after birth in rabbits. Thus, the fibrils were a minor component by volume of the vitreous of adult rabbits in comparison to the amorphous portion of the vitreous.