Ultrastructural and time-lapse studies of primary mesenchyme cell behavior in normal and sulfate-deprived sea urchin embryos

The mechanism of primary mesenchyme cell migration in the sea urchin, Lytechinus pictus, was studied in normal embryos and in sulfate-deprived embryos in which primary mesenchyme cells do not migrate. Based on scanning electron microscopy (SEM), cell processes were classified into six morphological types. Time-lapse cinematographic studies showed that two types of cell processes, a short finger-like process and a long process which accounted for 40 and 30% of the cell processes formed, respectively, in normal embryos, functioned as kinetic appendages during cell migration. Although the short finger-like process was formed to some extent in sulfate-deprived embryos, these processes were not able to attach to the ectodermal basal lamina, which is the migratory substratum. The long type of cell process was not observed at all in sulfate-deprived embryos. Transmission electron microscopy (TEM) demonstrated that cell processes in normal embryos were associated with 30 nm diameter granules in the basal lamina. Because these granules were absent in sulfate-deprived embryos, it is suggested that a specific component of the basal lamina substratum can be a limiting factor in cell migratory behavior.