The body wall epithelium of starfish gastrulae change the barrier property of the septate junction against large molecules in response to hypertonic environment caused by small molecules such as glycine, arabinose, urea, and NaCl. Ultrastructural obesrveations reveal that the septal portion of the junction either becomes diffuse or disappears altogether while the opposing junctional membranes remain unaltered. Under such conditions, Molecules as large as IgG and IgM can penetrate the body wall without causing morphological abnormalities to the embryo. We have devised a method to detect the paracellular permeation by applying fluorescein‐labeled IgG into the stimulation medium and monitoring the fluorescence which penetrated into the blastocoel. Micropreciptiates of LaCl3 were found, in thin sections of embryos treated with glycine, to lie along the intercelluar spaces, showing, although indirectly, that macromolecules flow through this pathway instead of a transcellular one. The possible role of the septal plates in the barrier function of the septate junction is discussed. © 1995 Wiley‐Liss, Inc.
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