Paper - The relation of the myotomes to the ventrolateral musculature and to the anterior limbs in amblystoma (1910)
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Lewis WH. The relation of the myotomes to the ventrolateral musculature and to the anterior limbs in amblystoma. (1910) Anat. Rec. 4(5): 183-.
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The Relation of the Myotomes to the Ventrolateral Musculature and to the Anterior Limbs in Amblystoma
Warren H. Lewis
From the Anatomical Laboratory, Johns Hopkins University
With Eight Figures
The Normal Development
The formation of the myotomes begins at an early stage before the tail bud makes its appearance. At a stage, shown in fig. 1,^ in which there is a small tail bud present, there are fifteen myotomes and the sixteenth is partly formed. At this stage the anterior myotomes have just separated off from the lateral mesoderm. The first myotome is irregular in shape and lies m close relation with the gill mass. The third, fourth and fifth myotomes are still attached to the pronephros. The ventral processes of the first three myotomes are just beginning. Fig. 2, shows a stage, about two days later. There are no signs of the arm bud at this time, either in the dissected specimens or in the cross sections. From now on, there is a rapid growth of the ventral processes of the myotomes. The ventral processes of the anterior three myotomes pass in front of the pronephros, while the fourth lies behind. These ventral processes avoid the region of the pronephros and futiu'e arm bud and gradually grow over the lateral surface of the embryo, the first one faster than the second, the second faster than the third, and so on, so that the first and second may be well advanced before the seventh and eighth appear.
At a somewhat later stage than that shown in fig. 2, we find that ventral to the pronephros the ventral processes of the third and the fourth myotomes unite to make this lateral sneet continuous.
^ The figures are all from dissected specimens, which were fixed in corrosiveacetic solution.
Fig. 1. Operating stage, my I, first myotome ; pro, pronephros; sp, spinal cord; g, gill mass.
Fig. 2. Somewhat later stage showing ventral processes of anterior myotomes.
The ventral processes of the first, second, and third become completely separated from the myotomes, partly by the pronephros, and form the stemo-hyoid portion of the ventral musculature. From the first segment of the sterno-hyoid the genio-hyoid arises. The pronephros also gradually separates a portion of the ventral process of the fourth segment from its dorsal part. The segmentation of the ventro-lateral musculatiu'e can be observed even after the musculature is split into different layers. From the first and second myotomes a lateral chordal mass early splits ofif and is separated from the dorsal portion of the myotome by the vagus ganglion and the otic capsule. Fig. 3, shows normal relations of an embryo twenty days older than the one shown in fig. 1. The arm has been dissected away leaving only the myotomic musculature. The arm bud does not appear until sometime after the operation stage (fig. 1) and lies in close relation to the pronephros in the region of the second, third, fourth and fifth myotomes, mainly the third and fourth.
Fig. 3. Normal larva twenty days after operation stage, leg has been dissected away, my I, first myotome; oc, otic vesicle; gehy, genio-hyoid; sthy, stylohyoid; Ic, lateral chordal muscle; pro, pronephros.
The cells of the arm bud apparently arise from the somatopleure in the region of the pronephros and not from the myotomes, they gradually form a protuberance on the surface of the embryo. The lateral myotomic muscle sheet gradually spreads out beneath the arm mass, that is medial to it but superficial to the pronephros.
In the following series of experiments with the exception of the second, I have attempted to remove various myotomes at the stage shown in fig. 1, that is, at the time just after the myotomes have separated off from the lateral mesoderm. The following results are based not only upon the study of dissected specimens but upon serial sections as well.
In the first series attempts to remove the first myotome only were not entirely successful, although in a number of experiments it was found that twenty days after the operation the dorsal part of the first myotome was almost completely absent and in such cases the anterior segment of the lateral chordal mass was very small. In all of these experiments however, the anterior end of the ventral muscle mass, namely the sterno-hyoid was present, though smaller than normal in one case. The failure to extirpate completely the first myotome was probably on account of the difficulty of cutting out the ventral portion, which lies in such close relation with the gill mass, and which gives rise to the anterior end of the ventral musculature.
In the second series of experiments, an attempt was made to remove the ventral halves of the first three myotomes at a stage slightly older than that shown in fig. 1. In one partially successful experiment killed twenty days after the operation the dorsal part of these three myotomes was intact. The lateral chordal part of the first myotome was entirely wanting and that of the second myotome very much smaller than normal; the first segment of the ventral muscle mass, that is of the sterno-hyoid, was entirely wanting, and also the genio-hyoid which arises from it. The second ventral segment was very small and thin, while the third was apparently normal. In the remaining experiments of this series the dorsal and lateral chordal portions of the myotomes are apparently uninjured while the sterno-hyoid and geniohyoid muscles are smaller than normal especially at the anterior end where they are almost completely wanting in some of the experiments. It is evidently difficult to remove the ventral portion of the myotome entirely at this stage.
Fio. 4. Larva twenty days after removal of anterior three myotomes (first and third only partly removed). The lateral chordal and ventral derivatives of these myotomes are wanting.
In the third series of experiments an attempt was made to take out completely the first three myotomes with the ectoderm over them. In an embryo killed twenty days after the operation, the conditions were found as shown in figure 4. The dorsal part of the first myotome is small, the first segment of lateral chordal mass and the sterno-hyoid are wanting as well as the genio-hyoid.
The second myotome as well as its derivatives is entirely absent. The third myotome is small and the third segment of the sternohyoid is absent. It is very evident that one need not extirpate the entire myotome in order to prevent the development of the ventro-laterai derivatives. In another experiment, the embryo was killed ten days after the operation, the dorsal part of the first myotome was found to be very small, while that of the second and third were wanting. The lateral chordal as well as the ventrolateral musculature derived from these segments was found to be absent as in fig. 4. The fourth ventral segment was, however much more elongated than the one shown in fig. 4. The other experiments of this series show various degrees of extirpation of these myotomes, usually with the corresponding absence of their derivatives, namely the lateral chordal mass and the stemo-hyoid and genio-hyoid muscles, or in some cases where these derivative muscles are not entirely wanting, they are found to be smaller than normal.
Fig. 6. Lateral view of larva shown in fig. 5. The ventral muscular derivative of the fourth myotome wanting.
In the fourth series of experiments, I attempted to take out the fourth myotome only. In one very successful experiment, apparently this entire myotome was extirpated; as a result, we find nineteen days after the operation that the myotome and its muscle derivatives are entirely wanting, see fig. 5 and 6. In another experiment where the myotome was apparently entirely extirpated we find that the ventro-lateral muscle forms a continuous sheet. This is due to the elongation of the preceding and succeeding segments of the ventro-lateral musculature to fill in the gap.
In the fifth series of experiments, I attempted to remove the second, third, fourth and fifth myotomes. These myotomes were apparently completely extirpated in an embryo killed twentysix days after the operation. Figs. 7 and 8 from a dissection show the following results. The second, third, fourth and fifth myotomes are completely wanting, as are also the muscular derivatives of these myotomes. We find that the sixth myotome (both dorsal and ventral portions) has elongated, the dorsal portion to nearly the length of three myotomes, and the ventral portion so as to partly fill in the gap between it (figs. 7 and 8) and the first segment of sterno-hyoid muscle. In fact, the ventral edge of the ventral muscle has extended so as to meet the elongated first segment. In another experiment, the third and fourth myotomes were apparently completely extirpated and the second and fifth partially. The ventral lateral muscle shows complete absence of the third and fourth segments and imperfect development of the second and fifth, while the sixth segment is very much elongated. The gap, however, which might be expected from the complete absence of the third and fourth segments is nearly filled in by the elongation of the ventral portions of the remaining segments. Still other experiments show much the same results, the condition of the ventral musculature depending upon the degree of extirpation. This elongation of segments to fill in the gap caused by the extirpation of certain segments takes place in nearly all of the experiments and probably would be complete if the animals were allowed to live for a greater length of time after the experiment. The condition found by Miss Byrnes^ in her experiments on Amblystoma in which, after destruction of the ventral halves of the myotomes in the region of the posterior limb the ventral musculature was present, is evidently to be explained through elongation of the remaining myotomes or their ventral processes. It is very unlikely that this regeneration takes place from the muscle of the opposite side as there is a wide gap between the two sides in the mid- ventral line.
Figs. 7 and 8. Larva killed twenty-six days after complete removal of the second , third, fourth and fifth myotomes. Dorsal and lateral views. The muscular derivatives of these myotomes wanting.
The Relation of these Experiments to the Anterior Limb
It was found in most of these experiments that extirpation of myotomes, either the first, second, and third, the fourth, or the second, third, fourth and fifth, made no difference in the development of the musculatiu'e of the leg, even complete absence of these myotomes was not accompanied by defects in the musculature of the limb. In a few experiments, however, the anterior limbs were absent or defective or the development was retarded, probably due to extirpation along with the myotomes of all or some of the cells destined to form the limb rudiment. These experiments show then very conclusively that the musculature of the limb is not derived from the myotomes. The experiments were primarily directed toward this problem and the extirpation of the myotomes was done immediately after and in some cases even before separation of the myotomes in the limb region from the lateral mesoderm and before there was any chance for myotome processes to have entered the place where the limb was later to arise. These results agree with those of Byrnes on the relation of the limb muscles to the myotomes.
- Byrnes Journ. of Morph. 1898. Vol. 14.
We have seen from fig. 2, that in the normal development the myotome processes avoid the region of the pronephros and of the limb, and that it has been impossible to trace in a study of the normal development either myotome buds or cells into the limb bud.
Both the experimental evidence and the study of the normal development support the idea that the musculature of the anterior limbs of Amblystoma develops in situ and is in no way derived from the myotomes or their ventral processes.
Accepted by the Wistar Institute of Anatomy and Biology March 20, 1910. Printed June 6, 1910.
Cite this page: Hill, M.A. (2021, April 10) Embryology Paper - The relation of the myotomes to the ventrolateral musculature and to the anterior limbs in amblystoma (1910). Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_The_relation_of_the_myotomes_to_the_ventrolateral_musculature_and_to_the_anterior_limbs_in_amblystoma_(1910)
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