Communication compartments in the ectoderm of embryos of Patella vulgata

Patterns of gap junctional communication in the ectoderm of embryos of Patella vulgata have been studied by intracellular injection of the fluorescent dye Lucifer Yellow, and by analysis of its subsequent spread to adjacent cells (dye-coupling). We found that dye-coupling became progressively restricted to different domains of the ectoderm, forming communication compartments. These communication compartments are characterized by their high coupling abilities within the compartment, and reduction of coupling across their boundaries. During development, the pretrochal (anterior) ectoderm becomes subdivided into two communication compartments, the apical organ and the anlage of the head ectoderm. The posttrochal (posterior) ectoderm becomes subdivided into different communication compartments in two successive phases. Firstly, in the 15-h embryo the dorsal and ventral domains of the ectoderm form separate communication compartments. A dorso-ventral communication boundary restricts the passage of dye between the two domains. Secondly, in the 24-h embryo dye-coupling becomes further compartmentalized in both the dorsal and ventral domains. These compartments correspond to the anlagen of different ectodermal structures. In order to study whether any level of coupling persists between the ectodermal compartments we injected currents through a microelectrode inserted into one cell of one compartment and monitored its spread by means of a second microelectrode inserted into one cell of another compartment (electrical coupling). Despite the absence of dye-coupling, electrical coupling between the ectodermal dye-coupling compartments was detected, which suggests that some level of communication is maintained between compartments. Our results demonstrate that within the ectoderm layer of Patella vulgata the transfer of dyes becomes progressively restricted to communication compartments and, concomitantly with the specification of the different ectodermal anlagen, these compartments become subdivided into smaller communication compartments.