Glia-Neuronal and Glia-Vascular Interrelations in Blood-Brain Barrier Formation and Axon Regeneration in Vertebrates

Abstract

The capability to regenerate structures of the central nervous system (CNS) after injury differs among vertebrates. Certain regions of the CNS within the same species are distinguished by their regenerative potency following injury. The question: can each neuron grow after lesion, although hindered by environmental factors or, alternatively, do intrinsic factors also influence the regeneration capacity, are not conclusively answered as yet (Franklin and Blakemore, 1990; Fawcett, 1992). However, there is an immense body of experimental data on regeneration from which it can be concluded for the present that if central neurites are confronted with the milieu of a peripheral nerve they are supported to grow (Aguayo, 1985; Kromer and Cornbrooks, 1985; Politis and Spencer, 1986; Vidal-Sanz et al., 1987; Hausmann et al., 1989; Bunge and Hopkins, 1990; Montgomery and Robson, 1993). On the other hand, embryonic and perinatal CNS provide as well a milieu that is supportive for neuron survival and neuritic growth (Kalil and Reh, 1979, 1982; Sievers et al., 1989; Thanos et al., 1989; Schmalenbach and Müller, 1993; Wunderlich et al., 1994). But the molecular environments of perinatal CNS and adult peripheral nervous system (PNS) apparently are not identical. Moreover, the optic nerve of a fish does not need peripheral graft for successful regeneration. This seems to imply a striking difference of the molecular environment between fish and mammalian CNS. Indeed, several factors have been found recently which could explain the different growth capacity between the vertebrate groups (Bawnik et al., 1987; Bastmeyer et al., 1991; Vielmetter et al., 1991; Paschke et al., 1992; Schwab et al., 1993).