Origin of the Sympathetic Trunks in the Chick Embryo

Abstract

In most current textbooks of neuroanatomy and embryology the sympathetic trunks are described as arising from the spinal ganglia. This conception is based on the older descriptions and upon the more recent experimental investigations of Mueller and Ingvar 1 or Van Campenhout. 2 On the other hand, the sympathetic trunks are described as arising from the neural tube by a few of the earlier workers and by the majority of later workers, particularly by Kuntz. 3

To gain further information on this subject 2 types of operations have been performed on chick embryos at 40-48 hours of incubation. First, the neural crests, with contiguous portion of the neural tube, were removed from the lower cervical and upper thoracic region for as many as 7 segments. The operation was performed under a binocular dissecting microscope by direct ablation with fine iridectomy scissors. The 25 embryos that survived this operation for a total incubation period of 7 days were then prepared for study by a modified Cajal technique, following which the nervous system in the region of operation was reconstructed graphically or in wax.

These 25 embryos may be divided into 2 groups, 13 of which (Group 1) have uninterrupted spinal cords extending throughout the region of operation and 12 of which (Group 2) have interrupted spinal cords, the gap ranging from one to 7 segments. The embryos of the first group show that the removal of neural crests and contiguous areas of the neural tube for as many as 7 segments does not prevent the formation of sympathetic trunks even though the spinal ganglia fail to develop in the area of operation. The embryos of the second group show that in cases where one to 3 segments of the spinal cord are missing (7 specimens) the sympathetic trunks will bridge the gap, but in cases where 2 to 7 segments of the cord are absent (5 specimens) the sympathetic trunks are interrupted.