Intercellular permeability in the earlyAsterias embryo

It has been previously observed that the initiation of electrical coupling between cells of the starfish embyo (Asterias forbesi) is delayed until the early blastula stage. It can be demonstrated that the flow of small ions (e.g., K⁺) carrying the current between cells at this stage occurs through intercellular low-resistance pathways. The present experiments suggest that these low-resistance pathways are not permeable to somewhat larger ions than those most probably carrying current between cells. The fluorescent dyes fluorescein, molecular weight 330, and Procion Yellow M4RS, molecular weight approximately 500, have been introduced iontophoretically into cells at various stages of development. In general, subsequent to iontophoretic injection of these molecules, the cells maintain electrical coupling and exhibit normal cleavage but do not allow transfer of the fluorescent dyes to adjacent cells. This selective permeability of the intercellular pathways may play a significant role in the differentiation of this embryo and, furthermore, it represents a clear difference between similar low-resistance pathways of adult tissues, several of which are freely permeable to fluorescein, Procion, and several other molecules.