Stimulation of axon growth from the spinal cord by a regenerating limb blastema in newts

The effects of limb blastemas of Pleurodeles waltl on axon growth from frag ments of spinal cord were studied in vitro. Cultured in a defined medium, s pinal cord fragments regenerated sparse, short axons. The culture of spinal fragments in the presence of blastemas greatly enhanced the length, number and survival of axons. Testing separately each of the two components of th e blastema showed that only the mesenchyme exerts a neurotropic effect on t he spinal fragments. Other tissues such as muscle or skin had a limited neu rotrophic effect. Additionally, the neurotrophic activity of blastemas seem s to be dependent of its proliferation status. Compared with blastemas of r egenerating limbs from young animals, irradiated blastemas (devoid of mitot ic activity) and blastemas of regenerating limbs from old animals or differ entiated blastemas (both characterized by a low mitotic activity), exhibite d a weaker neurotrophic influence. The blastema neurotrophic factor is not an attachment molecule but a soluble one and cannot be nerve growth factor (NGF) or fibroblast growth factor (FGF). It has a relatively low molecular weight (less than 15 kDa) and its protein nature was ascertained by its sen sitivity to heating and proteases. As the production of this mesenchyme-der ived neurotrophic factor depends upon mesenchymal cell proliferation of the blastema, we suggest that there is loop of positive regulation between spi nal nerves and blastema. Blastema tissues may stimulate nerve regeneration allowing the stimulation of proliferation of blastema cells by regenerating nerve fibers. Alternatively, blastema cells may produce a neurotrophic fac tor whose secretion might be dependent on cell proliferation. (C) 2000 Else vier Science B.V. All rights reserved.