Localization and role of acetylcholinesterase in zebrafish development (somitogenesis, neurogenesis, vertebrate, diisopropyl fluorophosphate)

Acetylcholinesterase (AChE) is hypothesized to play a role in embryonic development. I have used histochemical techniques to localize AChE in the developing zebrafish, and an inhibitor of AChE to disrupt its function, in order to study its role in development. The nervous system of the embryonic zebrafish includes a distinctive population of primary neurons that follow a common developmental program consisting of an early withdrawal from the mitotic cycle, expression of AChE, and axonal outgrowth. During development AChE activity in the spinal cord appears in a rostro-caudal sequence. AChE appears to label a unique set of neurons since AChE-containing neurons increase in size, while other neurons do not. Outgrowth of identified motor axons does not occur in a strict rostro-caudal sequence as ascertained both by AChE and ZN-1 monoclonal antibody staining. This suggests that the appearance of AChE and axogenesis may be under separate developmental control. The first cells to stain for AChE in the hindbrain and spinal cord appear at regular segment-length intervals, suggesting a common segmental plan for the development of these two regions. Later in development, the segmental relationships are preserved, but the pattern of labeled cells is different in the two regions, suggesting regional differences in the control of neuromere development. Cells in the mesoderm express AChE prior to somitogenesis. As somites form, these cells elongate, forming the first muscle fibers. AChE activity also appears at the borders between the somites and at the neuromuscular junction. Diisopropylfluorophosphate (DFP), an inhibitor of AChE, was used to examine the role of AChE in development. Embryos soaked in DFP from 50% epiboly until 24 hours exhibited an inverse relationship between the number of somites formed and the concentration of DFP. Identified motor axons grew normally in the presence of DFP. These results suggest that (1) expression of the cholinergic phenotype and axon outgrowth may be controlled by different mechanisms, (2) the hindbrain and spinal cord are segmentally arranged, and (3) AChE appears to have a role in somitogenesis, but not axogenesis.