Distribution of nitric oxide synthase-immunoreactive neurons in fetal rat brains at embryonic day 15 and day 19.
Although nitric oxide (NO) is hypothesized to play a role in the modelling of neuronal connections in the developing brain, no precise histological information has been reported on nitric oxide synthase immunoreactive (NOS-IR) structures in the embryonic brain. Thus, we examined the distribution of NOS-IR structures in the rat brain at embryonic day 15 (E15) and day 19 (E19), as a stage of early embryonic neurogenesis and active synaptogenesis, respectively. In the brain at E15, a few NOS-IR cell bodies were observed in the diencephalon near the 3rd ventricle and in the pons. By E19, the number of NOS-IR cells was greatly increased. Many NOS-IR cells were distributed throughout the forebrain and the medulla oblongata. NOS-IR cells of the neostriatum were located in its ventrolateral two-thirds. NOS-IR cells in the hypothalamus were distributed densely in the medial part while sparsely in the latero-ventral part, suggesting medio-lateroventral directions of NOS development. In the lower brainstem, on the other hand, NOS-IR cells were distributed in a similar pattern to that reported in adult rat, indicating earlier development of synaptic refinement in this area. The number of NOS-IR cells in the cerebral cortex and the cerebellum was very few.