Contribution to the Problem of the Origin and Development of the Sympathetic Nervous System.

Abstract

Three theories have been proposed regarding the origin of the sympathetic elements, the neural crest, the ventral half of the spinal cord, or the mesodermal tissue. The experimental method alone is able to give a satisfactory answer to this problem.

In embryos of Rana pipiens, at the stage of open or closing neural folds, the author has removed the neural crest from the hindbrain to the tip of the tail; the entire spinal neural crest and the dorsal half of the spinal cord were excised by that procedure. Many operated embryos live as long as 45 days after the operation. Their size was very much reduced if compared with the control animals and they showed marked deficiency in their sensitivity. The microscopic study of these embryos shows the absence of the spinal and sympathetic ganglia throughout the entire trunk region. The motor roots are present in all segments but are lacking sheath cells. The spinal cord shows normal motor neurones; the Rohon Beard elements are absent and the central canal is more or less dilated. The chromaffin part of the adrenals is undeveloped and the digestive tube does not show any nervous elements in its walls.

The removal of the hindbrain at the same embryonic stage does not interfere with the development of a vagal ganglion. Its visceral branch is, however, absent. The visceral portion of the vagus nerve seems to be exclusively derived from the medullar neural crest, the other components originating from the placodes. In these embryos the central and peripheral parts of the sympathetic system are well developed; the adrenals show their 2 constituting anlagen and the intestinal walls contain nervous elements.

The 2 experimental series strikingly confirm each other and they show that, in the frog, the neural crest represents the exclusive source of sympathetic elements, that the visceral branch of the vagus nerve is derived from the medullar neural crest and that the nervous elements in the intestinal walls are to be related to the sympathetic system.