Cell proliferation during early hindbrain development in Alligator.

Cell proliferation, as determined by immunoreactivity to proliferating cell nuclear antigen (PCNA) was investigated during early hindbrain development in Alligator. At the earliest stage examined, stage 3, when five rhombomeres are present, PCNA immunoreactivity is more robust laterally towards the pial margin and interrhombomeric boundaries are clearly seen. Subsequently, PCNA immunoreactivity fills each respective rhombomere and increases in intensity but respects the boundaries between segments until stage 7. At this time, borders between rhombomeres become less distinct. At stage 8, when the full compliment of rhombomeres is present, two longitudinal columns of PCNA immunoreactivity are seen rostrally which coalesce into a single column caudally. Interrhombomeric borders are indistinct. Later, at stage 9/10, two longitudinal columns span the entire length of the hindbrain and interrhombomeric boundaries remain less clear. The more lateral column fades whereas the medial stripe persists through stage 12. Thereafter, immunoreactivity fades. These observations confirm the findings in other species that rhombomeres are centers of cell proliferation. This feature is most likely common to hindbrain development in all gnathostomes. It is hypothesized that these two longitudinal columns of cell proliferation might be important for future patterning of the hindbrain.