Intercellular bridges in the embryo of the Atlantic squid, Loligo pealei. I. Cytoplasmic continuity and tissue differentiation.
In the post-gastrulation embryo of the Atlantic squid, Loligo pealei, the cells of the developing blastoderm are joined to each other by intercellular bridges which may provide a means of cytoplasmic communication between the cells. This paper describes an electron microscope survey of bridges in the developing blastoderm just prior to, and during, the onset of differentiation. The bridges are similar to those described in the gonadal tissue of many animal species and appear to result from incomplete cytokinesis followed by the disappearance of the spindle remnant. The bridges persist and chains of cells result which are generally branched and coiled. In the undifferentiated blastoderm the chains of cells show no apparent orientation to each other. However, in the apical blastoderm undergoing differentiation, chains of bridged cells appear to coincide closely with the developing mantle and shell gland primordia. The configuration a chain of cells assumes depends upon the degree of branching (i.e. the number of cells having three bridges) and the degree of coiling of the chain. Whereas coiling is probably affected by the crowding of neighboring cells, both branching and coiling appear to be functions of spindle orientation relative to previous bridges. During mitosis the bridges appear to become occluded by systems of transverse membranous cisternae, and mitotic nuclei are thus isolated. However, the bridges apparently re-open during G1, and during periods of protein synthesis the cells within a group share a common cytoplasm. It is suggested that gene products are shared and protein synthesis of the entire bridged group may be synchronized. As the sharing of control molecules may also be facilitated, these essentially syncytial groups may respond uniformly to inducers from the yolk syncytium, or other tissues and differentiation may be synchronized within the group.