[Postnatal development of barreloid neuropils in the ventrobasal complex of mouse thalamus: a histochemical study for cytochrome oxidase].

Postnatal development of barreloid neuropils was studied in the ventrobasal complex of mouse thalamus. A single barreloid neuropil which topographically represented a corresponding facial vibrissa on the contralateral side was demonstrated as a solid reaction mass with the method for cytochrome oxidase. This architectural unit was called a vibrissa compartment. On postnatal Day 0, the nuclear area of the ventrobasal complex showed higher activity for the enzyme than in regions around, but no barreloids were seen. Barreloid neuropils became detectable on postnatal Day 3 in the dorsomedial subnucleus of the ventrobasal complex and the septal delimitation of barreloids was established on postnatal Day 6. The arrangement of barreloids showed a distinct orderlines similar to that of facial vibrissae on a horizontal section. Barreloid neuropils contained synaptic glomeruli ensheathed with glial processes, in which dendritic processes and their excrescences of nerve cells received large terminal knobs in well-organized manner. On postnatal Day 10, immature glomerular forms were found in barreloid neuropils. The synaptic glomerulus of mature type, however, was not formed till postnatal Day 13. The number of synaptic junctions was examined in barreloid neuropils from postnatal Day 3 to 21. The result that the growth rate of synaptic junctions was the highest in a short period from postnatal Day 10 to 13 suggested rapid maturation of barreloid neuropils in this period. On postnatal Day 21, several types of synaptic contacts became remarkable in matured glomeruli. One of them, a filamentous type with association of clear synaptic vesicles at presynaptic sites was indicated. And non-synaptic junctional complex accompaning tubular structures beneath dense materials at dendritic sites was remarked between terminal knobs and dendritic processes. From results the following conclusions are remarked that at the level of mouse thalamus, septal delimitation of vibrissa compartments appeared on postnatal Day 3 and was established until postnatal Day 6. On the other hand, barreloid neuropils containing glomerular synaptic complexes became a maturing state on postnatal Day 13, that is, about one week later after architectural compartmentalization.