Development of non-pyramidal neurons in the rat sensorymotor cortex during the fetal and early postnatal periods.

The development of non-pyramidal neurons was studied in the rat sensorymotor cortex during the fetal and early postnatal periods with Golgi's method. Additional data of the early stages were obtained using autoradiography and electron microscopy. Horizontal neurons in the marginal layer were found from the 15th postconceptional (pc.) day (cellular birthday: 13th-15th pc. day). Horizontal neurons in the inner margin of the early cortical plate were found from the 16th pc. day (cellular birthday: 14th-16th pc. day). Coarse cell profiles were seen with Golgi's method, but, under electron microscope, increased numbers of organelles were observed in these cells, as compared to those of the cortical plate. From the 17th day of gestational age to the moment of birth, a small number of non-pyramidal neurons were stained with Golgi's method, including horizontal, stellate, and neurogliform neurons, at different levels of the cortical plate. The greatest development of the non-pyramidal system was observed during the second postnatal week, while the greatest increase of dendritic branches and synaptic spines was observed during the third and fourth weeks. This pattern is similar to that observed in the pyramidal system, but an ascending gradient was not observed.