Studies on the Cornea

Abstract

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. It 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. Rabbit 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.

An Electron Microscopic Study of the Passage of Colloidal Particles From the Blood Vessels of the Ciliary Processes and Choroid Plexus of the Rabbit

“…Similarly the wall of the capillaries in the cerebral cortex, although continuous, becomes thinner as the animal progresses to the adult stage (19). In recent studies (20,21) marker particles were found to traverse the corneal endothelium (Descemet's mesothelium) by an intercellular pathway. In this area, the particles appear to be streaming out of the intercellular space where they fan out into Descemet's membrane.…”

Studies on the Cornea

“…5) particles are never found in the terminal bar. Studies of normal rabbit cornea (1), and all observations thus far made on experimentally treated rabbit corneas, indicate that a terminal bar is found between all endothelial cells and is continuous in three dimensions. T h a t is to say, the terminal bar forms a continuous ring about any given endothelial cell, just below its apical surface.…”

“…Electron-opaque colloidal particles can be added to the fluid bathing either the epithelial or the endothelial surface of the cornea. The present study describes the pathways of transport of colloidal particles by the living cornea maintained in vitro and compares these results with those found in the cornea in vivo (1). A comparison of transport phenomena in mammalian (rabbit) corneas and amphibian (frog) corneas will be presented in another communication (3).…”

“…Dehydra-= tion in graded ethanol solutions, orientation of the tissue, and embedding in methacrylate were carried -o out as in previous work (1).…”

“…The literature on corneal fine structure and physiological studies of corneal permeability has been reviewed in a previous communication from this laboratory (1), in which the fine structure of the rabbit cornea and the uptake and transport of colloidal particles by the rabbit cornea in vivo were described.…”