Spatiotemporal patterns of secretomotor neuron generation in the parvicellular neuroendocrine system

Spatiotemporal patterns of parvicellular neurosecretory neuron generation (birthdates) were determined in the young adult male rat using a triple fluorescence labeling method. The six classic phenotypes were identified in histological sections with rabbit antisera to neurotransmitters (or related enzymes), nuclear bromodeoxyuridine was detected with a mouse monoclonal antibody, and an axonal projection to the median eminence was determined with the fluorescent retrograde tracer fast blue. The vast majority of triply labeled neurons are generated between embryonic days 12-14, during the time when magnocellular neurosecretory neurons are also generated. This pattern of neurogenesis is distinct from the well-known 'outside-in' pattern of hypothalamic neurogenesis, where the peak of lateral zone birthdates occurs on embryonic days 12 and 13, the peak of medial zone birthdates occurs on embryonic days 14 and 15, and the peak of periventricular zone birthdates occurs on embryonic days 16 and 17. Thus, neuroendocrine motoneurons may constitute 'pioneer neurons' for the various anatomically distinct regions of the periventricular zone. In addition, many intermixed neurons that express the same neurotransmitters as parvicellular neurosecretory neurons but do not send an axon to the median eminence, also appear to be generated between embryonic days 12 and 14. What these results imply about mechanisms underlying neuroendocrine motor zone differentiation is discussed.