The ontogenesis of neurophysin-containing neurons in the mouse hypothalamus

Several aspects of the ontogeny of the neurophysin-containing neurons of the mouse hypothalamus were studied. The events investigated included: (1) the onset of appearance of neurophysin (NP) immunoreactivity in supraoptic (SON), paraventricular (PVN) and suprachiasmatic (SCN) neurons; (2) axonal outgrowth to the neural lobe and zona externa of the median eminence from SON and PVN, respectively; (3) the time course of dendritic outgrowth in selected SON neurons and (4) the time period for synaptic input to the SON. The first immunoreactive neurons were observed on embryonic day (E) 13 1 2 in the presumptive SON and in the region between the third ventricle and the SON. Neurophysin-positive axons and terminals from the SON were present in the posterior pituitary on E 14 1 2. Immunoreactive neurons were observed in the PVN on E 16 1 2 and their terminals in the zona externa of the median eminence were detected on E 17 1 2. Neurophysin immunoreactivity appeared in the SCN on E 18 1 2, just prior to birth. Dendritogenesis was studied in those SON neurons residing medial to the optic tract. Few dendritic processes could be observed in these neurons prior to postnatal day (PN) 5. On PN 10 many long dendritic processes oriented parallel to the pial surface were present. From day PN 15 onwards, the adult pattern, which consists of both parallel coursing dendrites and large finger-like dendritic branches oriented ventrally toward the pial surface, were observed. Synaptogenesis was studied from E 18 to PN 24 in tissue processed by standard electron microscopic procedures. Although a few synapses (as defined by the presence of membrane specializations) were present just before (E 18) and just after birth (PN 1), the majority of synaptic input was established between PN 5 and PN 15. These studies demonstrate that some characteristics of neuron differentiation (neurohormone synthesis and axonal outgrowth) are expressed early in fetal development while other aspects of neuronal maturity such as dendritic outgrowth and the establishment of synaptic input occur relatively late in post-natal life.