Long-term survival of glial segments during nerve regeneration in the leech

Nerve injury that severs axons also disrupts ensheathing glial cells. Specifically, crushing or cutting the leech nerve cord separates the glial cell's nucleated portion from an anucleate segment, or `stump'. In experiments reported here, the fate of the stump and the response of the nucleated portion were determined by intracellular recording, by intracellular injection of Lucifer Yellow dye and horseradish peroxidase (HRP) as tracers, and by electron microscopy. The nucleated portion of the glial cell did not divide, degenerate, or grow appreciably. The severed glial stump remained isolated from the nucleated portion but maintained its resting potential and normal morphology for months. Stumps typically began to deteriorate after 3 months. Small macrophage-like cells, or 'microglia' increased in number after injury and ensheathed axons, thus partially replacing the atrophying glial stump. Some axons in the nerve cord degenerated; the remainder appeared morphologically and physiologically normal. Thus, both nucleated and anucleate glial segments persisted throughout the one to two months required for axons to regenerate functional connections. Glial cells in the leech are therefore available to guide physically the growing axons or to contribute in other ways to nerve regeneration.