Cytology of brain stem neurons projecting to the caudal neurosecretory complex: an HRP-electron microscopic study.

Brain stem projections to the neurons in the caudal neurosecretory complex (CNC) of Poecilia sphenops (molly) have been studied with HRP retrograde tracing. Using light microscopic procedures, HRP-labelled neurons were located in the reticular nucleus of the medulla (RMN) and in the vicinity of the nucleus of the medial longitudinal fascicle (NMLF). The present study was undertaken to examine the cytology of the brain stem neurons that project to the caudal neurosecretory complex using combined HRP-electron microscopic methods. Cells in the midbrain NMLF containing HRP-filled profiles were located bilaterally close to the midline and just beneath the ependyma. HRP reaction product was found additionally in the neurosecretory cells of the midbrain dorsal tegmental magnocellular nucleus (DTMN) located dorsal to the NMLF. Cells containing HRP-labelled profiles were also seen in the RMN and in neurons dorsal-lateral to the RMN. This latter group of neurons contained small dense core vesicles in addition to HRP labelled dense bodies.