Differentiation of chemically defined neuronal populations in the transplanted olfactory bulb without olfactory receptor innervation.

Olfactory bulbs (OBs) from embryonic day 15 and 17 and postnatal day 1 mice were transplanted into the lateral ventricle of juvenile host mice without bulbectomy, and fine structural and chemical features of neurons and glia in the OB transplants were investigated immunocytochemically and electron microscopically. In the OB transplants there were neither clearly defined glomeruli nor layers, nor olfactory marker protein immunoreactive elements. However, chemically defined neuronal populations resembling those in the normal OBs such as those immunoreactive for gamma-aminobutyric acid (GABA), tyrosine hydroxylase and Ca(2+)-binding proteins (calbindin-D28K, calretinin, parvalbumin) were observed. Electron microscopically, dendrodendritic and somatodendritic reciprocal synapses, that is, synapses characteristic of the OB, were occasionally observed in the OB transplants. These results indicated that at least some embryonic or newborn mouse OB neurons and/or precursor cells could exhibit chemical properties and form typical synaptic contacts observed in normal OB, even when they received no inputs from olfactory receptor cells.