Transection of the spinal cord in developing Xenopus laevis.

INTRODUCTION T H E literature on regeneration in the central nervous system of vertebrates has been reviewed exhaustively by Windle (1955, 1956). Adult fish and urodeles reestablish physiological and anatomical continuity of the spinal cord after it has been completely transected while adult anurans (Piatt & Piatt, 1958) and mammals on the whole do not. In all groups of vertebrates regeneration is more successful in the period of early embryonic development, and becomes less so as development proceeds. Experiments designed to investigate the factors responsible for this change demand an animal in which the difference in the regenerative capacity of embryonic and adult form is marked, and all stages of development are easily accessible for operative procedures. These criteria are satisfied by Anura. For this reason regeneration in the anuran central nervous system merits further investigation. After spinal cord transection in urodele larvae, Piatt (1955) found that the Mauthner axons did not regenerate although other axons around them did. This suggests that the regeneration of the axons was controlled by factors which were independent of the local changes occurring consequent to the trauma. The fundamental nature of this hypothesis justifies more experiments to test it. Larvae of Xenopus laevis are especially suitable for such experiments as in the ventral white columns of their spinal cord are large descending axons of primary motor neurones as well as the Mauthner axons. The large axons of the primary motor neurones are almost as big as the Mauthner axons (Hughes, 1959). Also, Piatt (see Windle, 1955) has pointed out that observations on the behaviour of the Mauthner axons after spinal cord transection in Anura are lacking. Hooker (1915, 1925) showed that physiological and anatomical regeneration followed transection of the spinal cord in frog larvae at all stages of development. Lorente de No (1921) found that regeneration occurred after cutting partly across the spinal cord of frog larvae 20-35 mm. long. Recently Piatt & Piatt (1958) showed that regeneration did not take place after transection of the spinal 1 Author's Address: Department of Anatomy, University of Cambridge, Cambridge, U.K. [J. Embryol. exp. Morph. Vol. 10, Part 2, pp. 115-26, June 1962]