Neuronal changes in the forebrain of mice following penetration by regenerating olfactory axons

Abstract

Following total, unilateral bulbectomy in neonatal mice, the olfactory sensory axons regrow from a reconstituted population of sensory neurons, cross the lamina cribrosa, and invade the spared forebrain that has leaned forward toward the anteroventral wall of the cranial cavity. The sensory axons invade several regions of the spared forebrain, at times penetrating deeply into the brain parenchyma. These axons terminate in characteristic globose structures resembling the glomeruli of the olfactory bulb. However, they can be distinguished from the latter by the absence of periglomerular cells. These ectopic glomerular structures are formed by the commingling of the olfactory axon terminals and the dendrites of brain neurons that lie in their proximity. Previously we have established that synaptic contacts occur between the sensory axon terminals and the dendrites of the brain neurons. Our present study describes large neurons, resembling mitral cells, that expand their dendrites into the intracerebral glomeruli. These neurons are recognized by virtue of their relatively large diameter, their selective stainability with silver methods, and the unorthodox arrangement of their dendrites in comparison with the neurons of the region. Their appearance is contingent upon the presence of ectopic glomeruli. The possibility is discussed that the large argyrophilic neurons may be derived from developing neuronal elements of the brain.

Olfaction in Olfactory Bulbectomized Rats

“…Evidence from other studies suggests that these anomalous inputs induce some reorganization within the forebrain. Thus, sensory axon terminals form synaptic contacts on nearby multipolar neurons that extend their dendrites into these glomerular-like structures (Graziadei et al, 1978;Graziadei and Monti Graziadei, 1986), and at least some of these clusters have been observed to be associated with periglomerular-like tyrosine hydroxylase (TH)-positive neurons (Guthrie and Leon, 1989). Our tissue sections, processed for WGA-HRP and lightly counterstained with thionin, were inadequate to reveal such changes.…”

FGF signaling through FGFR1 is required for olfactory bulb morphogenesis

“…Previous studies in mice have shown that olfactory epithelial axons can induce OB-like neurons in the anterior telencephalon of bulbectomized neonates (Graziadei and Monti-Graziadei, 1986). It is possible that during later development, the inductive effect of sensory axons, along with the arrival of migrating neurons from the ventral telencephalon, account for the late forming bulb-like evagination in the Fgfr1-deficient telencephalon.…”

Differentiation of mitral cell dendrites in the developing main olfactory bulbs of normal and naris-occluded rats

“…Following early bulbectomy, olfactory sensory axons regenerate and invade the remaining forebrain to form glomerular structures, and dendrites of forebrain neurons entering the ectopic glomeruli develop tufted structures similar to the glomerular tufts in the olfactory bulb (Graziadei and Monti Graziadei, 1986a). It is likely that the primary afferents promote the differentiation of entire primary dendrites as well as the formation of glomerular tufts (Treloar et al, 1999).…”