Projections from the hypothalamus to the posterior lobe in rats during ontogenesis: 1,1′-dioctadecyl-3,3,3′,3′-tetramethylindocarbocyanine perchlorate tracing study

The objective of this study was to determine the schedule of the arrival of the axons from the hypothalamus to the posterior lobe of the pituitary (PL) in rats during ontogenesis by using the fluorescent lipophilic carbocyanine dye 1,1′‐dioctadecyl‐3,3,3′,3′‐tetramethylindocarbocyanine perchlorate (DiI) as a retrograde tracer. After preliminary fixation of the brain, DiI crystals were implanted in the PL on embryonic day 15 (E15), E16, E17, and E19 as well as on postnatal day 2 (P2) and P9. This was followed by a DiI retrograde diffusion along the plasma membrane and subsequent staining of hypothalamic neuronal cell bodies. The supraoptic nucleus (SO) contained an accumulation of fluorescent cells that extended toward the diamond‐like swelling of the third ventricle as early as E15. These data suggest that the magnocellular neurons of the SO send their axons to the PL at the very beginning of differentiation, perhaps even before reaching their final position. The initial axons of the neurons of the paraventricular nucleus proper (PV) appeared to reach the PL significantly later, at E17. In addition to the SO and the PV, accessory magnocellular nuclei contributed to the innervation of the PL in perinatal rats. The neurons of the retrochiasmatic accessory nucleus first sent their axons to the PL on E16–E17. Axons that originated from other accessory hypothalamic nuclei reached the PL after birth, suggesting a delay in their involvement in the regulation of visceral functions compared with other magnocellular nuclei. Thus, the axons of magnocellular neurons reach the PL unexpectedly early in embryogenesis, raising the possibility of the functional significance of vasopressin and oxytocin as fetal neurohormones. J. Comp. Neurol. 422:327–337, 2000. © 2000 Wiley‐Liss, Inc.