ABSTRACT
In the amphibian embryo, beginning in the late neurula and continuing through midtailbud stages, the developing medulla exerts an inductive influence on the prospective ear, effecting its determination. Fine structural analysis of the region of closest apposition between the two tissues in the axolotl (Ambystoma mexicanum) reveals that during this period, there is a significant increase in the surface area of the apposed cells through the projection of long finger-like processes that traverse the medulla–ear interspace and the appearance of many focal contacts between the two cell types. These contacts are small, varying in diameter from 10–30 nm and they exhibit the septilaminar appearance and 2–4 nm intercellular cleft characteristic of gap junctions. Once the ear has been determined, both the cell processes and the focal junctions between apposed cells virtually disappear.
We suggest that the projection of processes from the surfaces of the apposed cells enhances the opportunity for cell interaction through the formation of very small gap junctions and that the junctions could provide the structural substrate for direct communication between medulla and ear during their inductive interaction.
