STUDIES ON THE CORNEA I. The Fine Structure of the Rabbit Cornea and the Uptake and Transport of Colloidal Particles by the Cornea in Vivo

Physiological studies have demonstrated that ions, as well as large molecules such as hemoglobin or fluorescein, can diffuse across and within the cornea. Most of the substrates for corneal metabolism are obtained from aqueous humor filling the anterior chamber. In order to receive its nutrients and in order to maintain its normal conditions of hydration, the avascular cornea must transport relatively large amounts of solute and solvent across the cellular layers which cover this structure. I t has been suggested in the past that there may be a morphological basis for the transport of large amounts of solvents and solutes by cells by the mechanism of pinocytosis. The use of electron-opaque markers to study fluid movements at the electron microscope magnification level was described by Wissig (29). The present study describes the fine structure of the normal rabbit cornea and the pathways of transport of colloidal particles by the cornea in vivo. Rabbi t corneas were exposed in vivo to suspensions of saccharated iron oxide, thorium dioxide, or ferritin by injection of the material into the anterior chamber. In other experiments thorium dioxide or saccharated iron oxide was injected into the corneal stroma, producing a small bleb. Particles presented at the aqueous humor surface of the rabbit corneal endothelium are first attached to the cell surface and then pinocytosed. It appears that the particles are carried around the terminal bar by an intracellular pathway involving the pinocytosis of the particles and their subsequent transport in vesicles to the lateral cell margin basal to the terminal bar. Particles introduced at the basal surface of the endothelium (via blebs in the corneal stroma) are apparently carried through the endothelial cells in membrane-bounded vesicles without appearing in the intercellular space. There appears to be free diffusion of these particles through Descemet's membrane and the corneal stroma. The stromal cells take up large quantities of the particles when blebs are injected into the stroma. I N T R O D U C T I O N The earliest electron microscopic studies of the cornea (1-7) were primarily concerned with the characterization of the fibrous components of the corneal stroma and Descemet's membrane. Later work (5, 8I 1) dealt with the structure of the corneal epithelium in normal rat, mouse, and human tissue. Sheldon and Zetterqvist (12) described the corneal epithelium of the mouse with a vi tamin A deficiency. Only Speakman (13) has described the structure of the corneal endothelium in the rabbit, and attempted to correlate the normal morphology with the physiological activity of the cornea. Physiological studies by Maurice (14-16), 457 on F ebuary 2, 2013 jcb.rress.org D ow nladed fom Published March 1, 1962