Extensive regeneration in vitro by early embryonic neurons on immature and adult CNS tissue

Abstract

The failure of axon regeneration in the injured adult CNS has been ascribed to axon growth inhibitory molecules expressed by the resident glial cell populations, especially oligodendrocytes. Unlike their adult counterparts, however, early embryonic neurons are able to send lengthy axons through myelinated fiber tracts when transplanted into the adult brain. One explanation is that they have yet to express receptors for factors that inhibit the growth of older neurons. To test this possibility, we have used the cryoculture technique to study the regeneration of rat central and peripheral neurons, over a developmental period that encompasses the stages before, during, and after target contact, when cultured on either unmyelinated (neonatal) or myelinated (adult) optic nerve tissue sections. Early embryonic (days 14–15) retinal ganglion cells extended neurites on neonatal optic nerve, but few grew on adult optic nerve. In the case of early embryonic dorsal root ganglion neurons, however, neurite outgrowth on either neonatal optic nerve or on adult optic nerve was extensive. This response declined sharply with age. In contrast, neurite outgrowth by dorsal root ganglion neurons on laminin substrata remained relatively constant (> 80% extended neurites) over the same period. This suggests that (a) inhibition of neurite outgrowth within the optic nerve is mediated not only by oligodendrocytes, but also by molecules expressed prior to the onset of myelination; (b) neurons acquire receptors for these inhibitors only late in embryonic development; (c) differences exist between developing central and peripheral neurons in the response to myelin-associated axon-growth inhibitors.

Lens Injury Stimulates Axon Regeneration in the Mature Rat Optic Nerve

“…Thus, although lens puncture makes surviving RGCs competent to regenerate their axons, the scar prevents these axons from growing further. Myelin also normally represents a major impediment to axon growth in the CNS Schwab and Caroni, 1988;Shewan et al, 1995). However, as shown here and elsewhere, mature RGCs can extend lengthy axons into the myelinated optic nerve when appropriately stimulated.…”

Extensive reinnervation of the hippocampus by embryonic basal forebrain cholinergic neurons grafted into the septum of neonatal rats with selective cholinergic lesions

“…Thus, McKeon et al (1991) and Cadelli and Schwab (1991) have argued that the high regenerative potential seen in neonates may be linked to the low expression of growth-inhibiting molecules in the immature CNS tissue environment. Similarly, Cohen and collaborators (Cohen et al, 1989;Shewan et al, 1995) have proposed that the age-dependent decline in the capacity for axonal regeneration not only reflects a general downregulation of growth-promoting molecules and/or their receptors but may also involve the increased expression of, and/or the acquisition of receptors for, putative inhibitory factors along the growth trajectory. In the present study, it is interesting to note that the grafted septal neurons were taken from the donor fetuses at a time point (E14) when they normally start to grow axons towards the immature hippocampus (Koh and Loy, 1989;Semba, 1992) and that, in the host, at the time of grafting (PD8), the septohippocampal cholinergic projection is still in a phase of active growth and terminal expansion (Milner et al, 1983), suggesting that the target-directed axonal outgrowth patterns observed here are critically dependent on developmentally regulated expression of axonal guidance molecules in both the donor and the host tissue.…”

Neuronal Cyclic AMP Controls the Developmental Loss in Ability of Axons to Regenerate

“…Extensive regeneration occurred because the myelin antibodies neutralized myelin-specific inhibitors of regrowth, allowing regeneration to commence immediately after injury. On the other hand, myelin does not exert this inhibitory effect on embryonic neurons, either in culture (Shewan et al, 1995) or when transplanted in vivo (Li and Raisman, 1993). Identification of the molecular difference between embryonic and adult neurons that is responsible for this change in regenerative capacity has long been believed to be the key to encouraging adult axons to regrow after injury.…”