The paraventricular organs (PVO) of the pigeon and domestic chicken contain at least three types of serotonin-immunoreactive (serotonin-ir) CSF-contacting neurons. Type 1 neurons were predominant. They had two bipolar extending processes. The somata were mostly found in the pars hypendymalis. Type 2 neurons were characterized by thin and long apical processes. Their perikarya were found in the pars distalis of the PVO or the more lateral area of this organ. Type 3 neurons were considerably smaller and had round somata. They were mostly bipolar with thin and short dendritic processes and thin basal processes. A small number of this type was conspicuous along the cranial peripheral region of the PVO. In addition to the PVO area, aggregations of small, bipolar serotonin-ir CSF-contacting neurons were shown in the most caudal wall of the third ventricle of both species, distributed medially or paramedially. Immunoelectron microscopy revealed many dense granules in apical ventricular processes and perikarya. Synaptic connections were frequently observed on basal processes.
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