Paper - The segmental flexures of the notochord (1907)
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The Segmental Flexures of the Notochord
By Charles S. Minot.
Harvard Medical School. With 6 figures.
The following note records observations on several species of mammals, in all of which they liave demonstrated the existence of segmental flexures of the notocliord in early embryonic stages. The general character of these flexures is the same in all cases." The notochord is confined to the median plane and makes a bend in the ventral direction corresponding to the center of each segment, and makes a series of corresponding bends in the dorsal direction, tlie apex of each dorsal bend being in the intervertebral disk. The general character of these flexures and of their relations to th(> vertebral and intervertebral structures is well shown in Fig. 2. Since the flexures have been found in all the species of mammals examined, it seems to me probable that they occur in all mammals at corresponding stages. The species examined are :
- All the figures are from specimens in the Harvard Embryological Collection, to the catalogue of which the numbers refer. The figures are all oriented alike, the cephalad end up, the dorsal side to the left, and are uniformly magnified, 35 diameters. Except in the pig, as detailed below.
The exact observations made may be briefly stated as follows :
Pig — At 5.5 mm
(sagittal series 916) the metameric curves of the notochord are just beginning to show clearly, for in each segment the notochord is bowed ventralwards, making a slight but distinct curve. The distance of the outer somewhat irregular surface of the spinal cord from the notochord is less than the diameter of the latter. The floor of the spinal cord is parallel to the notochord and makes a series of small segmental bends, yet in the frontal series of these stages (N'o. 917) I see no evidence of neuromere formation in the lateral walls of the spinal cord, although the metamerism in the median ventral wall is perfectly distinct.
Pig — At 6.0 mm
The conditions are very much the same, but the notochord is larger, its sheath more distinct and its curves more evident. In the median line there are still no blood vessels near the wall of the medullary tube, although they are present on both sides. There are as yet no clear evidences of intervertebral differentiation. We have, therefore, the noteworthy fact to record that ihe metamerism of the notochord and of the adjacent floor of the spinal cord both precede in their development ihe formation of the vertebral anlages.
Pig — At 7.0 mm
In an embryo of 7.0 mm. (Series 11) the segmental curves are again a little more distinct than in the earlier stages; the notochord is thickened and its distance from the spinal cord has increased ; capillaries have appeared in the median line under the spinal cord; the mesench^Tna above the notochord is still of a very loose texture, but below the notochord it is slightly condensed, and on careful examination it can. be recognized that this condensation is greater underneath the summit of each ventral bend of the notochord. The notochord itself, which in earlier stages was still somewhat flattened, has now become nearly circular in cross section.
In slightly older embryos (7.5 mm., No. 756, and 7.8 mm., No. 429,) conditions are nearly the same as in the 7.0 mm. specimen, but in both of these series the flexures of tlie notochord are very distinct and regular, as shown in Fig. 1. They make a series of rounded waves; the dorsal sum III it of cacli wave corresponds to an area of lighter mesenchyma; the ventral depressions are each lodged in condensed mesenchyma, and each patch of condensed mesenchyma is continued a little distance below the notochord by bands of smaller condensed tissue which unite with the mesenchyma specialized around tlie two adjacent depressions or ventrad curves of the notochord.
Fig. 1. Pig, 7.8 mm. Sagittal series 429, median section reconstructed from the actual sections, S9-102. Kch.. notochord; Sp. c. cavity of the spinal cord, bounded by thin median walls; Ao., main dorsal aorta.
At U.I) mm, (series 53) the notochordal waves are more pronounced, the distance Ijctween the uotochord and the spinal cord has increased and the areas of condensed mesenchvma are more definitely indicated and at earlier stages. The areas of condensation are easily recognized as the anlages of the intervertehral ligaments.
From the preceding descriptions it appears that in the pig up to this stage, the ventrad bends of the notochord are intersegmental, whereas, as stated in the opening paragraph, their characteristic position is segmental. It is thus especially interesting to note that at 12 mm. the change in the relation of the flexures to the segments is initiated. This is shown by the fact that the apices of the flexures are moving in each segment towards the head (series 7). Again, in an embryo of 14.2 mm. (series 1130) I find that the apex of each dorsal flexure has moved headwards and lies near the cephalad border of each vertebral body. Similarly the apex of each ventrad flexure has moved towards the cephalad border of the intersegmental anlage (ligament). In consequence of this change, the form of the curves has become more like that observed in the cats (compare Fig. 5). In this embryo and in another of exactly 14.0 mm. (series 66) the flexures are well developed, but the segmental curves of the floor of the medulla, which, we found in earlier stages (compare Fig. 1), have disappeared. The intervertebral anlages are very distinct, the cells composing them are much crowded and have smaller nuclei than the cells of the ventral anlage or of the neighboring mesenchvma. The formation of fibers in the anlage has apparently begun, and the differentiation of the vertebral perichondrium is easily recognized.
At 17.0 mm. (series 50) the medullary floor shows no metameric curves. The notochord is remote from the spinal cord; between the two the median blood-vessel is everywhere distinct. The vertebral bodies are clear, light; the intervertebral discs are broad, dark; but not cartilaginous. The relations of the flexures are now changed clearly, in that they have shifted, each dorsad apex being now vertebral in position, each ventrad apex intervertebral, see sections 23T-239. In the upper cervical region (section 22T), however, the relation is different and seems to be like that in the sheep. Unfortunately the sections are not true to the sagittal plane and the disposition of the cervical notochord is tlicri'for.' not readily followed.
At 20 mm. (series 60) the notochord shows no distintt fit-xures in the upper cervical region, but its bends are still to be traced in the dorsal and lumbar regions.
Fig. 2. Sheep, 14.6 ram. Sagittal series 1109. Reconstruction from sections 190-193. Fio. 3. Sheep, 20. 1 mm. Sagittal series 1112, Section 235.
The flexures are more distinct, that is to say of greater amplitude, in the sheep than in any other species I have examined. We have at present in our collection three sagittal series; the youngest stage is that of 13.7 ram. (series 1107). In this the notochordal waves are more marked than in pigs. The vertebral bodies are well but not sliarply defined, the intervertebral disks are wide, and owing to the crowding of their cells, they appear darkly stained. The notochord is of uniform diameter.
The next series (1109) is of an eml)ryo of 14.6 mm. and the condition here found is represented in Fig. 2. The series of flexures, as depicted, is very obvious. It will be noticed that from the fifth vertebra on, the segmental flexures are quite regular, there being a bend towards the ventral side in each vertebral body. In the first vertebral body we find the first dorsal flexure; in the second, third, and fourth vertebral bodies the "same is true, but in the fourth the apex of the dorsal flexure lies more cephalad. In the fifth body this change is still more marked, and in the sixth we find that the ventrad flexure has become vertebral. This is the relation which we find throughout the rest of the vertebral column. In the cervical region, then, the flexures are not strictly segmental ; we have for the six main vertebral bodies six and one-half waves. Whether this irregularity is the result of the peculiar development of the atlas and epistropheus or not, I am unable to say, but it is a natural supposition to connect the two phenomena.
In a sheep of 26.1 mm. (series 1112) I find the notochord beginning to degenerate, but its course, as shown in Fig. 3, is still traceable with certainty. The segmental flexures are conspicuous. In each vertebra the notochord makes a long ventrad bend, and where these bends unite in the intervertebral ligaments, there is t}T)ically a sharp apex formed, as is clearly shown in the figure. The differentiation of the vertebral cartilage has commenced. In no other animal which I have examined have I found the flexures so conspicuously developed as in this sheep.
Of the cow only a single sagittal series is at my disposal (Xo. 1126). The length of the embryo is 17.0 mm. The specimen has been hardened in formalin and its preservation was not very good. The notochord had shrunken within its sheath and I think it probable that in mounting the sections the two halves of the vertebral column have become a little more widely separated, so that the notochordal space has a greater diameter than is perhaps natural. Fig. 4 shows, however, that the notochordal flexures are present and characteristic; the ventral bend being vertebral, the dorsal intervertebral. It is noteworthy that in this specimen the fibrous differentiation of the intervertebral ligament is already quite distinct, although in the much larger sheep embryo (Fig. 3) it is hardly begun.
At 6.2 mm. (series 39T) the flexures may be observed in both the cervical and dorsal regions, though not yet very perfectly developed. They already differ in form from the wave-like flexures seen in ungulates. The dorsad apices are intervertebral and tend to lead off from the side towards the head into steep descents and towards the tail with
Fig. 4. vertebra
Fig. 5. 153-155.
Cow, 17.0 mm. Sagittal series 1126, Section 150. The uppermost in the figure is the sixth vertebral body. Cat, 10.7 mm. Sagittal series 475. Reconstruction from Sections
more gradual slopes; the resultant forms are shown in Fig. 5. The difference noted will be evident in comparing this figure with Fig. "2.
At 10.7 mm. (series 475) the vertebral bodies and intervertebral disks are well defined. (See Fig. 5). The notochordal flexures are well marked in the dorsal region, but are much less well marked in the lumbar region. This is the reverse of the difference l)etwcen the two regions in the rabbit.
At 12.0 mm. (series 400) the ventrad curves in the vertebra are 1)0Wshaped, wliile the intervertebral summits of tbe curves are somewhat pointed, as in the sheep. This is the condition to be seen in the dorsal and lower cervical vertebrae. In the lumbar region the flexures are indistinct, perhaps owing to the less advanced stage of vertebral development.
At 15.0 mm. (series 437) a slight trace of the flexures is still visible in the lumbar region, but they are a little more distinct in the cervical region: the long limb of the vertebral flexure tends to make a slight dorsad convexity.
Finally, in an embryo of 24.0 mm. (series 467) I find no trace of flexures; the vertebral notochord has become a mere thread, and the intervertebral notochordal expansion is large.
The single series at my disposal is an embryo of 12.5 mm. (Xo. 179). It shows perfectly clearly the slight segmental flexures of the notochord as a series of shallow waves. The specimen was hardened in picro-sulphuric acid, and there is no shrinkage of the notochord within its sheath, such as I have seen in many other specimens, even those hardened in Zenker's Fluid.
At 12.5' days (series 150) the intervertebral condensations are beginning; the notochordal flexures are unquestionably present, ])ut are very slight; their dorsad summits are intervertebral, their ventrad summits vertebral.
At 14 days (series 156) the flexures are more distinct though still much slighter than those which we observed in the pig.
At 15 days (series 159) the notochord in the dorsal region is nearly straight and shows slight vertebral enlargements, but from the level of the liver down towards the tail the segmental flexures are still distinct — a series of gentle waves.
As pointed out in connection with the description of the conditions in the cat, the two species show reversed relations; the flexures being most distinct in the lower half of the vertebral column in the rabbit, in the upper half in the cat.
At 16 days (series 162) the intervertebral expansions of the notochord are very marked and the degeneration of the notochord in the vertebrie has led to resorption; as at 15 days the notochord is nearly straight in the dorsal region, but in the lumbar and pelvic regions the slight flexures are readily recognized.
In man at 10.2 mm. (series 736) the notochordal flexures are slight but unmistakable. (Compare section 139). The vertebral anlages are distinct, and so also are Froriep's so-called vertebral bows, but neither are yet sharply defined. The flexures that are present are so inconspicuous they would perhaps hardly be noticed were not attention directed to them by previous acquaintance with the much more clearly defined flexures in other species. The no1x)chord is somewhat larger in diameter than in most mammals. At 22.0 mm. (series 851) I have succeeded in detecting no flexures whatever. This embryo is at the beginning of the chondrostyle stage.
At 8.0 mm. (series 718) we notice at once that the notochord is much larger than in the placental mammals and that it is histologically more like that of reptiles and birds. I have found no notochordal flexures; they seem to be wholly lacking. On the other hand, at 11.0 mm. (series 925) the embryo being in the chondrostyle stage, the flexures are indicated because the intervertebral thickenings of the notochord have appeared and each of these thickenings extends farther dorsad than ventrad. The same condition recurs in an embryo of 12.0 mm. (series G16) that shows still more clearly. (This condition is represented in Fig. 6.) If we imagine a line traced which would represent the center of the notochord, it would rise towards the dorsal side in each intervertebral anlage and fall towards the dorsal side 12.0 mm. Sagittal ill t'acli vertebral region, thus making it evident that series No. 616, \\^q course of the flexures in this marsupial is the Section 190. -,1,1 1 ^ -it same as in the other mammals we have considered.
The detailed observations above briefly recorded, togetlier with the accompanying flgures, seem to me to justify the statements made in the opening paragraph of this note.
As to the morphological significance of the flexures and as to the mechanism of their production, my observations have as yet furnished me no clue.
Harvard Medical School, December, 1906.
Cite this page: Hill, M.A. (2020, August 15) Embryology Paper - The segmental flexures of the notochord (1907). Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_The_segmental_flexures_of_the_notochord_(1907)
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