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

This study has determined the ontogenetic schedule of the arrival of the ax ons from the hypothalamus and the diagonal band in the median eminence in r ats by using the fluorescent lipophilic carbocyanine dye, 1,1'-dioctadecyl- 3,3,3',3'-tetramethylindocarbocyanine perchlorate (Dil) as a retrograde tra cer. 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 da ys, and on the 2nd postnatal day. This was followed by fluorescent staining of the neuronal cell bodies in the hypothalamus. According to our data, th e axons of rare hypothalamic neurons first reached the primordium of the me dian eminence on the 14th embryonic day. For two subsequent days, the numbe r of neurons projecting the axons to the median eminence appeared to increa se considerably. They were widely distributed through the hypothalamus and in the ventromedial region of the more rostral forebrain. Till the 20th emb ryonic day, the majority of the fluorescent neurons were concentrated mainl y in the paraventricular nucleus (dorsal and medial parts) and the arcuate nucleus, and to a. lesser extent in the medial preoptic nucleus, the suprao ptic nucleus, the diagonal band, and the retrochiasmatic nucleus. In neonat es, Dil-labelled neurons appeared additionally in the accessory dorsolatera l nucleus, medial preoptic area lateral to the diagonal band, anterior hypo thalamic area, and in the anterior periventricular nucleus. Thus, the axons of differentiating neurons arrive in the median eminence from the 14th emb ryonic day till the neonatal period, providing the pathway for the neurohor mone transfer to the hypophysial portal circulation.