Diffusion Via Cell-to-cell Membrane Channels

Cell-to-cell (or gap junctional) membrane channels allow the direct movement of molecules from the cytoplasm of one cell to the cytoplasm of adjacent cells. While functioning to maintain homeostasis among cells in a tissue, these channels may also function in the transmission of signals involved in pattern formation in development. This thesis investigates the properties of cell-to-cell membrane channels in a developing tissue.;The rates of diffusion for the intercellular movement of fluorescent tracers, expressed as (D(,e)), have been measured among the developing epidermal cells of a larval beetle, Tenebrio molitor L. The spread of injected tracers in this monolayer was recorded with a low light level (SIT) television camera attached to a fluorescence microscope. Fluorescence intensities were digitized at successive times after the start of injection at various distances from the source cell by a microcomputer interfaced with a video analyzer. Using a mathematical model D(,e)\u27s were determined.;Intercellular diffusion is a function of cytoplasmic and junctional diffusion. Cytoplasmic diffusion coefficients (D(,cyt)) were determined by fluorescence photobleaching recovery (FPR) in which a small cytoplasmic region of fluorescent molecules is destroyed or \u27bleached\u27 and diffusion of fluorescent molecules back into this region results in recovery of fluorescence.;The rates of intercellular diffusion of molecules are also influenced by molecular size and charge. The diameter of the cell-to-cell membrane channels in Tenebrio epidermis is calculated to be between 24 - 34 (ANGSTROM). Using a series of nearly-identical fluorescent tracers which differ in their charge, increased rates of diffusion were found to be associated with increased negative charges on the fluorescent tracers.;The permeability of cell-to-cell membrane channels in Tenebrio can either be increased (using 20-hydroxyecdysone or EGTA) or decreased (using chlorpromazine). There is (i) a direct relationship between junctional conductance to inorganic ions and the junctional permeability to the fluorescent tracers, CF and LRB, and (ii) the ratio of D(,e)\u27s for the smaller CF to the larger LRB remains constant during the changes in junctional conductance. In this tissue, junctional channels thus open and close in an all-or-none rather than a graded manner, affecting the rates of movement of small and large channel-permeant molecules equally. (Abstract shortened with permission of author.