An experimental study of spinal nerve segmentation in Amblystoma with reference to the plurisegmental contribution to the brachial plexus

Abstract

THIRTY-EIGHT FIGlJRES INTRODUCTIONThat the mesodermic somites exert a morphogcnetic role upon the development and segmentation of the spinal ganglia is a fact that has been supported by a number of experimental studies (Lehmann, '27 ; Detwiler, '32, '33 a, '33 b). Working up011 tlic embryos of Plenrodeles shortly after the closing of the neural folds, Lehmann excised the somites as well as grafted a region of the spinal cord lateral to thc host's somites. From his findings Lehmann drew essentially the following coriclusioris : 1) that in the complete absence of the somites of a definite i*egion, the developmcnt of the respective spinal ganglia is entirely suppressed (p. 102) ; 2) "that the segmental arrangement of the spinal ganglia o~w r s only wlieii therc is a degree of normal arrangement of the mesodermal structiires, and that in almost all cases disturbances in the mesoderm are accompanied by an abnormal develo1)-meiit of tlie ganglion cells"; 3) that the normal location of the sensory and motor roots is subservient to a normal arrangement of the mesoderm, and that disturbances in the latter bring about alteration in the location of the out-growing fiber bundles; 4) that it is the presence of thc mesial surface of the somites mhich brings about differentiation and segmen- iation of the ganglia. The lateral surface of tlic somite lacks this formative quality, according to Lehmann, for when the cord was grafted lateral to the sornites, segmentation in the ganglia failed and little or no differentiation of cells occurred.Similar. experiments by tlie author upon the embryos of Amblystoma ('32) have yielded a number of cases in which discrete spinal ganglia developcd and differentiated in tlie complete absence of developing muscle and cartilage-a condition which, according to Lehmann, did not exist in Pleurodelcs. Moreover, a number of eases were obtained in which spinal ganglia developed along the lateral surface of the host 's somites when tlie spinal cord WRS grafted to this region. That tlie lateral surface of the somite inhibits ganglion cell differentiation, as coiicluded by Lehmann, could not be substantiated in my experiments.From the results so far at hand, it would liardly seem justifiable for us to conclude that no differentiation of spinal ganglion cells can take place in the absence of the mesial face of the somite or of the axial skeleton regardless of the morpliogenctic influence these tissues may exert under normal conditions of development. It would appear that the crest cells possess a certain amoiint of self-differentiating capacity in the same sense that Harrison ('10) foirnd the embryonic neuroblasts to develop in vitro. It may be that tissues other than developing myotome or sclerotome maS influence differentiation, for in several of my cases ganglia developed when adjacent 1 o glandular tissue (pronephros) and the notochord.The most conviiicing evideiice supporting the view that spinal ganglia can develop and diff ercntiate in the absence of muscle and cartilage was obtained from those case...

Vertebral Abnormalities Following Heat Shock in Xenopus Embryos

“…In chick (11) and mouse (1, 7 , 13), somite manipulations such as transplantation, excision and disturbance induce vertebral abnormalities. In amphibian Ambystoma, on the contrary, the excision of somites does not always lead to an abnormal vertebrae (5,9). This observation is not consistent with that of other species.…”

“…These results strongly suggest that the arrangement of somites plays an important role in determining the location of vertebral primodia in Xeno- pus embryos. Variance of previous observations in amphibians (5,9) may relate to the extirpated zone which may injure the vertebral rudiments (9) and/or tissues adjacent to the somites, i.e. the spinal cord and the notochord (9,12,18).…”

Observations on the migration and differentiation of neural crest cells in somite extirpated salamander larvae.

“…Experiments similar to the present study have already been done by LEHMANN (1927) and DETWILER (1927DETWILER ( , 1932DETWILER ( , 1934. They excised somites and investigated the participation of mesodermal tissues in the metamerism of spinal ganglia and the development of neurons in them.…”

“…Observations suggestive of the fragmentation of the cell cord corresponding to the somites or myotomes were never obtained. LEHMANN (1927) and DETWILER (1927, 1932, 1934 development of spinal ganglia and the differentiation of neurons in them in spite of the absence of muscles and axial cartilages and suggested a self-differentiation capability for neural crest cells. However, he did not mention the differentiation of satellite cells in the irregular-developed spinal ganglia.…”

Neuroanatomy of the brachial plexus of the dog

“…Lehmann (1927) concluded that the mesodermal somites are responsible for the development of spinal ganglia and the location of the sensory and motor roots. Detwiler (1934Detwiler ( , 1935 working with Amblystoma embryos obtained irregular development of spinal ganglia in the complete absence of somites. He concluded that the crest cells have a certain s elf-differentiating capacity and can develop independently of developing muscles and cartilage.…”