Ontogenesis of the central serotonin neuron system of the rat--an immunohistochemical study

The peroxidase-antiperoxidase technique was used to study the ontogenesis of serotonin-containing neurons in fetal, neonatal and adult rats. Serotonin-containing neurons were first detected in a rat embryo of crown-rump length (CRL) 5 mm, bilaterally distributed in the ventral metencephalon near the isthmus rhombencephali. Serotonin fibers were first detected in CRL 6 mm embryos. There was a rapid increase in serotonin-immunoreactive neurons of embryos of 7-8 mm CRL. Serotonin neurons appeared initially as independent, bilateral groups of neurons, but as early as CRL 16 mm, the adult mid-line distribution was present, while the neurons that occurred as single groups in the adult already existed as single groups or at least were only partially connected across the mid-line. A continuous distribution of serotonin neurons was observed in the mesencephalic and pontine reticular formation from CRL 13 mm to the adult stage. It is suggested that the rapid increase in serotonin neurons at 7 and 8 mm CRL is associated with the completion of mitosis which occurs at this time.