ULTRASTRUCTURAL CHARACTERIZATION OF THE POSTNATAL-DEVELOPMENT OF THE THALAMIC VENTROBASAL AND RETICULAR NUCLEI IN THE RAT

Electron microscopy has been employed to analyze the normal maturation al sequence that characterizes the postnatal development of synaptic c ircuits in the ventrobasal (VB) and reticular (Rt) thalamic nuclei of rats at different ages (from birth to the end of the third postnatal w eek). Throughout the first postnatal week, similar signs of immaturity are observed in both nuclei, mainly consisting in scarcity of cytopla smic organelles, presence of wide extracellular spaces, and absence of myelinated fibers. Several synaptic terminals are however present fro m birth, thus indicating that some of the afferents have already reach ed and contacted their thalamic target during embryonic life. Most of the terminals are small and contain only a few round, clear vesicles, and therefore their cytological features do not allow the identificati on of their origin. In particular, in both nuclei, terminals with flat vesicles and symmetric specialization are only rarely observed, and i n VB the ascending terminals are not distinguishable from terminals of other sources as they are in adults. During the second postnatal week , progressive maturational changes in VB and Rt lead to neurons having well-developed cytoplasmic organelles and to an elaborate neuropil co ntaining myelinated fibers and synaptic terminals that are morphologic ally heterogeneous and resemble the adult ones. The permanence of grow th cone-like profiles and of numerous somatic and dendritic protrusion s, often contacted by synaptic terminals, indicates that a certain deg ree of reorganization is still taking place in both nuclei. By the end of the third postnatal week the synaptic organization of VB and Rt is indistinguishable from that observed in adults. This ultrastructural study shows that the appearance of the neuropil of VB and Rt and the m orphological complexity of the synaptic arrangements characteristic of the adult rat are not present in neonates, but are gradually acquired during the first three postnatal weeks, and that they result from pro gressive modifications in circuit organization involving both pre- and postsynaptic elements.