Fetal and early postnatal cyto- and synaptogenesis in the suprachiasmatic nucleus of the rat hypothalamus.

The fetal and early postnatal morphogenesis of the rat suprachiasmatic nucleus (SCH) was investigated by light and electron microscopy. Around the 15th fetal day many mitoses are present in the primordium of the SCH, and the 3rd cerebral ventricle is continuous with the eye cup. Between the 16th and 18th fetal days, tightly packed, uniform, small neurons, containing mainly polyribosomes, become differentiated. On the 17th fetal day, synapse-like formations (presynapses), while on the 18th day, synapses make their appearance on the dendrites. The number and degree of maturity of the synapses increase gradually. On the 20th fetal day, the neuropil of the SCH is characterized by a lattice-like structure; the larger dendrites cross each other, the spaces are filled with small bundles of delicate processes, supposedly axons. The fetal and early postnatal morphogenesis of the SCH seems to show a close resemblance to that of the arcuate nucleus investigated earlier by us.