Perisomatic changes in the maturing hypoglossal nucleus after axon injury

Abstract

The perisomatic retrograde reactions to peripheral nerve injury during postnatal maturation were investigated by quantitative light and electron microscopy. The hypoglossal nerve was crushed, ligated or transected in 10 and 21 day postnatal (dpn) rats. Only crush injury was made in 7 dpn rats. Survival periods ranged from 3 to 40 days postoperative (dpo). Normal and sham operated animals of corresponding ages served as controls. A remarkable, transient response of neuronal somata in the young (7-10 dpn) rats, following all three types of axon injury, was the formation of excrescences which engulfed neuronal processes: boutons, dendrites, small neurites and a few myelinated axons in adjacent neuropil. The somal engulfment was rarely evident after nerve injury to 21 dpn rats. Bouton displacement from the somal surface, accompanied by glial incursion, followed each type of nerve injury but was less extensive and occurred later in the young rats. There seemed to be no association in the amount of boutons displaced from neuronal somata with the type of nerve injury for any of the three experimental age groups. However, the rate and intensity of the perineuronal glial reaction were related to the severity of the nerve injury in the older (21 dpn) but not in the younger (7-10 dpn) rats. Substantial loss of neurons occurred in the affected nucleus after each type of trauma to the young neurons. The degree of neuronal loss was related to the age and the volume of axoplasm disrupted. Only nerve transection in 21 dpn rats resulted in appreciable neuronal loss. The responses of axotomized neurons, their afferent axon terminals and the surrounding glia are quantitatively and qualitatively different in the hypoglossal nucleus of rats of increasing postnatal ages.

Degenerating Synaptic Boutons in Prion Disease

“…54 Similarly it has also been shown that injured hypoglossal motor neurons envelop synapses innervating the cell soma. 55 It is not clear from these studies what is the mechanism of recognition of the supernumerary synapses or the fate of the enveloped synapses. A recent study has proposed that during development supernumerary retinal ganglion cell synapses in the dorsal lateral geniculate nucleus are tagged by C1q and C3 synthesized by the retinal ganglion cells and subsequently removed by the microglia present in the vicinity.…”

Intrasomatic changes in the maturing hypoglossal nucleus after axon injury

“…A detailed account of the methods used in this study has been published previously (Borke, 1982). Two hundred and thirty-two male rats of the Osborn-Mendel strain were used for this work.…”

“…Marked amplification of the existing structural arrangement in neuronal perikarya suggests acceleration of the patterns of protein synthesis operating in the young neuron at the time of injury. However, the continued production of organized RER as well as the elaborations of somal excrescences after nerve injury (Borke, 1982) provide evidence that the synthetic processes were not directed exclusively toward axonal replacement and point out that a certain level of somal maturation may be necessary for the selective production and/or utilization of materials for axonal regrowth.…”

“…The development of organized RER can be triggered by the establishment of afferent (Eschner & Glees, 1963;Giordano & Cunningham, 1982) and efferent (Pilar & Landmesser, 1976) connections. The formation of afferent connections on hypoglossal neurons initially was not halted by axonal injury (Borke, 1982) indicating that the production of organized RER in these neurons may be influenced primarily by afferent connections and therefore not disrupted by peripheral nerve injury.…”

“…Nerve ligation and nerve transection of older neurons therefore caused a greater magnitude of cell body responses. Moreover nuclear involvement was directly related to those injuries that produced a substantial incidence of nerve cell loss (Borke, 1982).…”