Axonal projections from the hypothalamus to the median eminence in rats during ontogenesis: DiI tracing study

This study has determined the ontogenetic schedule of the arrival of the axons from the hypothalamus and the diagonal band in the median eminence in rats by using the fluorescent lipophilic carbocyanine dye, 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) as a retrograde tracer. After fixation of the brain, the crystals of the dye were implanted in the median eminence on the 13th, 14th, 15th, 16th, 17th, 20th embryonic days, and on the 2nd postnatal day. This was followed by fluorescent staining of the neuronal cell bodies in the hypothalamus. According to our data, the axons of rare hypothalamic neurons first reached the primordium of the median eminence on the 14th embryonic day. For two subsequent days, the number of neurons projecting the axons to the median eminence appeared to increase considerably. They were widely distributed through the hypothalamus and in the ventromedial region of the more rostral forebrain. Till the 20th embryonic day, the majority of the fluorescent neurons were concentrated mainly in the paraventricular nucleus (dorsal and medial parts) and the arcuate nucleus, and to a lesser extent in the medial preoptic nucleus, the supraoptic nucleus, the diagonal band, and the retrochiasmatic nucleus. In neonates, DiI-labelled neurons appeared additionally in the accessory dorsolateral nucleus, medial preoptic area lateral to the diagonal band, anterior hypothalamic area, and in the anterior periventricular nucleus. Thus, the axons of differentiating neurons arrive in the median eminence from the 14th embryonic day till the neonatal period, providing the pathway for the neurohormone transfer to the hypophysial portal circulation.