The pattern of alkaline phosphatase activity in the developing mouse spinal cord.

The localization of alkaline phosphatase activity in the lumbosacral region of the developing spinal cord was studied in 9.5- to 17.5-day mouse embryos. The activity was uniformly distributed in the pseudostratified neuroepithelium of the 9.5-day cord. In the 11.5-day cord in which the lateral motor columns were being formed, the enzymatic activity was localized in the ventrolateral sector of the cord. The enzyme-positive ventricular cells tended to be located medially whereas radially oriented enzyme-positive processes extended into the marginal layer. The 13.5-day cord displayed a similar distribution pattern, but there were many more radial processes and the enzyme-positive cells had spread laterally. Close apposition between the processes and the ventricular cells was observed. By 15.5 and 17.5 days, when the intermediate layer was fully developed and the ventricular layer had regressed to a thin ependyma, the activity had become diffusely located in the ventral half of the cord. The enzyme-positive cells and processes became less conspicuous. The silver-stained processes in the cord were found to be organized in an entirely different pattern from that of the enzyme-positive processes, suggesting that the enzyme-positive processes were not neuronal processes. The enzymatic activity found in the developing spinal cord may be associated with the migration of neuroblasts along the radially aligned processes.