Evidence that at least some of the motor nerve cells that die during development have first made peripheral connections

Abstract

The paper sets out to determine whether the spinal motoneurons that die during the normal development of Xenopus tadpoles have been peripherally connected before death. The numbers of cells in the control ventral horns were compared with the numbers remaining after early (stage 50 or 52/53) amputation of the limb bud. Up to stage 52, there was no difference between the two sides, but thereafter the numbers on the amputated side fell away until at stage 57 none were left. Thus, as neurons mature, they become dependent for survival on contacting the limb. During this time, there was no difference between amputees and controls in the numbers of mitoses in that part of the ventricular layer that gives rise to ventral horn cells, indicating no change in proliferation. Thus, the numbers of ventral horn cells remaining after early amputation is a measure of the numbers of cells in the normal animal that are still independent of the limb (Phase I cells) and hence by subtraction, the other cells (post-Phase I cells) are those that only survive by virtue of having contacted the limn. A graph of numbers of post-Phase I cells shows a peak at stage 54 and thereafter declines as metamorphosis approaches. This is interpreted to indicate that a number of cells that survive at stage 54 by reason of peripheral contacts or connections in the limb subsequently themselves die, that is, that al least some of the cells that die during normal development have previously made contacts within the limb.