Paper - Factors Involved In The Formation Of The Filum Terminale
Embryology - 19 Apr 2024    Expand to Translate
|
|---|
| Google Translate - select your language from the list shown below (this will open a new external page) |
|
العربية | català | 中文 | 中國傳統的 | français | Deutsche | עִברִית | हिंदी | bahasa Indonesia | italiano | 日本語 | 한국어 | မြန်မာ | Pilipino | Polskie | português | ਪੰਜਾਬੀ ਦੇ | Română | русский | Español | Swahili | Svensk | ไทย | Türkçe | اردو | ייִדיש | Tiếng Việt These external translations are automated and may not be accurate. (More? About Translations) |
Streeter G.L. Factors Involved In The Formation Of The Filum Terminale 1919 Am J Anat. 22:1-11.
| Historic Disclaimer - information about historic embryology pages |
|---|
 |
Factors Involved In The Formation Of The Filum Terminale
George L. Streeter
From The Department Of Embryology, Carnegie Institution of Washington, Baltimore, Maryland.
Three Text Figures
In a study recently published by the writer1 on the development of the cartilaginous capsule of the ear in human embryos it was pointed out that the changes in size and form which the capsule undergoes during its development are accomplished not only by a progressive differentiation, but also in part by a retrogressive differentiation of its constituent tissues. The margins of the cartilaginous cavities are in a continual state of change; they exhibit an unstable equilibrium between two opposing tendencies: on one hand, toward the deposit of new cartilage, and on the other, toward the excavation of the old. The margins thereby are always advancing; or receding, and as a result of this there is provided a suitable suite of chambers for the contained membranous labyrinth in all stages of its development.
It is the feature of retrogressive differentiation or dedifferentiation that I wish particularly to recall here. The fact that certain areas of cartilaginous tissue'revert to an earlier embryonic type and are subsequently redifferentiated into a tissue of a Widely different histological character, is very clearly shown in the case of the otic capsule, and is a factor of great embryological significance. Such a process of retrogressive change, combined with redifferentiation of the same tissue, greatly in— creases the facilities for and the range of certain structural adjustments that occur in many regions in the development of the human embryo.
1 Streeter, G. L., 1917. The factors involved in the excavation of the cavities in the cartilaginous capsule of the ear in the human embryo. Amer. Jour. Anat.,
Another instance of dedifferentiation has recently been pointed
out by Kunitomo? This writer has published the results of a
careful study of the tail region in alarge number of human embryos,
representing the period of greatest development of the caudal
appendage, and also the later period of its gradual reduction.
He shows that in very young specimens the spinal cord reaches
the extreme tip of the tail and throughout its length is quite
uniform in structure. Somewhat later (11 to 15—mm. stage) it
can be divided at about the level of the thirty-second vertebra
into two parts—a cranial or main part, having a Wide central canal
and thick walls in which can be recognized well-developed mantle and marginal zones, and a caudal slender part, having a narrow canal with walls consisting only of an ependymal zone. Kunitomo shows that it is this caudal atrophic portion that eventually
forms the filum terminale. The main part lying cranial to the
thirty-second vertebra undergoes uninterrupted and progressive
differentiation, whereas the portion caudal to this undergoes regressive changes and, with the exception of the extreme tip,
finally becomes converted into a fibrous strand, the tip forming
the coccygeal medullary vestige. This, therefore, is another
instance in which an absorptive adjustment is brought about by
the reversion of the tissue to an earlier embryonic type with a
certain amount of subsequent redifferentiation.
Kunitomo further calls attention to the fact that in the formation of the filum terminale, in addition to the dedifferentiation of the caudal end of the medullary tube, there is also the mechanical disproportion between the growth of the medullary tube and that of the vertebral column. How much of one and how much of the other of these two factors is responsible for the further development of the filum terminale Was not determined by him. It has occurred to the writer that this question could be answered by the determination of‘the elongation of the nerve roots. ln the younger stages the spinal cord and the vertebral column lie alongside of each other in a metameric manner, corresponding in position segment for segment. Owing to their disproportion in growth, there occurs a relative displacement of their segment levels, so that, for instance, the thirtieth segment of the cord comes to lie opposite the twentieth segment of the vertebral column. The segment levels of the vertebral column are, of course, evident; in the spinal cord they are just as plainly marked byithe attachment of the nerve roots, for these become attached to the cord before the displacement begins, and thus permanently mark the various segmental levels. In the case of each segment of the spinal cord there are two fixed topographical points: the spinal ganglion, Which is held in the intervertebral foramen and registers the original position of the segment relative to the vertebral column, and the place at Which the dorsal root is attached to the cord and Which moves as the cord moves. By locating those points for the different stages one can determine the exact elongation of the nerve roots, and this in turn is the index of the relative displacement of the spinal cord as regards the vertebral column. Conversely, it Will be seen that the alteration not explained by mechanical displacement must be attributed to the retrogressive changes referred to above. The determination of the amount of displacement Was made by comparison of selected stages by means of profile reconstructions of the smaller specimens and actual dissection of the older ones. I Was assisted in this by Mr. James F. Didusch, of the Carnegie Embryological Laboratory, Who made careful dissections of these structures in several older fetuses, two of which Will be used for illustration. The results of this determination are given in the following note as a matter of interest to those Who have read the paper by Kunitomo, and also because it offers an opportunity to emphasize the significance of dedifferentiation of tissues in the processes of development in the human embryo.
2 Kunitomo, K., 1918. The development and reduction of the tail and of the caudal end of the spinal cord in the human embryo. Contributions to Embryology, vol. 8, Publication No.‘ 271, Carnegie Inst. of Wash. ‘
The part played by dedifferentiation in the caudal region of the spinal cord is more apparent in the younger stages of development, as pointed out by Kunitomo. The so-called ‘absorption’ of the tail is completed before the embryo reaches a length of 30 mm. It is also well known that the remodeling which takes
place in the gill region completes the obliteration of the gill bars
before the embryo is 20 mm. long. One well might expect these
processes of dedifferentiation and redifferentiation to be more
active in the earlier stages. They are not confined, however, to
this period, for in the case of the ear capsule they were found to
be very active throughout fetal life. In the case of the spinal
cord dedifferentiation is well demonstrated in the period represented by embryos between 11 and 30 mm. long. A" comparison
of these two stages can be made in figure 1. It will be noted in
the first place that the spinal ganglia show a regression varying
from arrest in development to complete disappearance. All but
two of the coccygeal ganglia have disappeared in the 30—mm.
specimen, and the remaining two are of about the same size as
the same two ganglia in the 11.5-mm. specimen}
As for the cord itself, the changes are equally marked. In the
younger stage (embryo 11.5 mm. long) the extreme caudal end
of the spinal cord, the part belonging to the non—vertebrated tail,
shows little differentiation, consisting only of indifferent cells
resembling embryonic ependyma. In the coccygeal region, how-
ever, the development is more advanced. Opposite the five
coccygeal ganglia the wall of the cord is differentiated into dis-
tinct ependymal, mantle, and marginal zones, with well—developed
rootlets entering into it from the first two ganglia. Sections
through it show nothing to indicate that this region is not going
on to complete its differentiation into the adult condition. When,
for comparison, one examines the very same region in the older
specimens (fig. 1, embryo 30 mm. long) it is found that its condition, relative to the remainder of the cord, has undergone a
marked change. While the precoccygeal cord has continued to
increase in the thickness of its walls and in the elaboration of the
mantle and marginal zones, the coccygeal region is less advanced
3 Throughout this paper the twenty-fifth to the twenty-ninth segments have
been uniformly regarded as sacral. The slight variation which is known to exist
in this respect is too small to be taken into account in our general conclusions,
and for convenience the regional terms, lumbar, sacral, and coccygeal, will be
used, upon the assumption that the specimen concerned has the usual regional
distribution of its segments.‘
Cite this page: Hill, M.A. (2024, April 19) Embryology Paper - Factors Involved In The Formation Of The Filum Terminale. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_Factors_Involved_In_The_Formation_Of_The_Filum_Terminale
- © Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G
