Paper - The subdivisions of the neural folds in man
|Embryology - 7 Dec 2019 Expand to Translate|
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The Subdivisions of the Neural Folds in Man
G. W. Bartelmez
Department of Anatomy, The University of Chicago, and the Laboratory of Embryology, Carnegie Institution of Washington
Veit and Esch have recently given us the most complete and detailed study of a vertebrate embryo during the period of somite formation that has ever appeared. All the labor and study expended upon it has been Well worth while, as human embryos of this period are very rare. The specimen is certainly normal and the preservation above reproach. The embryo has eight somites and belongs to the beginning of the third Week, a period which Prof. H. M. Evans and I have been studying for some years. Most of Veit and Esch’s findings fit well into the sequence of events as We have interpreted it from our series of embryos. There is, however, a radical disagreement in our interpretations of the nervous system, and in View of the great importance of the Veit embryo to human embryology, it would seem wise to call attention to the matter.
In his first paper based upon this embryo (’18), as well as in the complete description (’22) Veit has adopted a slight modification of the traditional interpretation of the nervous system in young human embryos. This seems to have originated with Kollmann (’89) in his description of the celebrated embryo ‘Bulle,’ which he had studied simply as a whole mount in balsam. The identification of the regions of the brain was, in the nature of the case, almost wholly subjective. The subsequent Writers who have ventured interpretations of the nervous system of embryos younger than ‘Bulle’ have followed Kollmann more or less closely. They have all made the forebrain relatively enormous and the hindbrain insignificant in size. None of these workers had a series of stages, such as is generally recognized as essential for the interpretation of the differentiations which can be seen in young embryos. It is not surprising that gross errors have crept into our text-books in spite of the correct interpretations of Giglio-Tos (’02; 15—somite embryo) and Low (’O8; 14-somite embryo).
We have now made a minute study of fourteen human embryos ranging from two to sixteen somites. In addition, the Davis (’20, ’23) embryo, of twenty somites, and the perfect 4-mm. embryo no. 836 of the Carnegie Collection have been available for study. Of the latter there is a complete series of models in the Carnegie Laboratory, prepared (under the direction of Professor Evans) by Mr. O. 0. Heard, who has made most of the other models I have used in this work. I am indebted to Doctor Davis for the use of his series of models. The identification of certain landmarks has made it possible to trace the history of the primary subdivisions of the brain to the 4-mm. stage, where there can be little doubt as to the three primary brain vesicles or the rhombomeres. Two of these landmarks are present from the youngest embryos on; others appear a little later, and together they afford a substantial body of evidence upon which our analysis is based. This evidence may be summarized as follows:
- A marked enlargement of the neural folds (or tube as the case may be) in the region of the otic plate can be recognized in every embryo. It may be termed the otic segment and can be shown to become the fourth rhombomere of the usual terminology. It could not escape notice even in a superficial study, and most writers have called it the second brain Vesicle, viz., Kollmann (’89), Dandy (’10), Bujard (’21, fig. 2), and the authors of various texts. This may also be the interpretation of Keibel and Elze (_’08, p. 18) for ‘Klb.’ Wallin (’13) and Veit and Esch (’2‘2) termed it the rostral end of the hindbrain. Giglio-Tos C02) and .Low (’08) have correctly analyzed the nervous system in their specimens, which were older than any described by the authors just referred to. The peculiarities by which the otic segment can be recognized are these: a) The form of the neural folds characteristic at this level (cf. p. 236). 1)) From the four—somite stage on, the acoustico-facial primordium is arising from its dorsal edge, and this division of the neural crest is distinctive in form and behavior (cf. p. 242). c) The adjacent ectoderm is thickened as the otic plate which can be readily recognized in the two- somite embryo (Ingalls, ’20), although it does not begin to invaginate until the eleven-somite period. d) The relation of the otic segment to such structures as the pharyngeal pouches, the heart, the aortic rami, etc., shift but slowly during the few hours of development we are studying, and these relations serve to confirm our identification.
- The second landmark is the midbrain which constitutes the knee of the cranial flexure. The latter ca11 be recognized as the first abrupt bending of the neural axis which we encounter as We pass back from the rostral end. It is obvious at least as early as the two-somite stage, but the midbrain thus located does not always have well-defined boundaries, unfortunately. Other differentiations appear, however, which help to delimit it. The optic primordium is such a one, since its caudal end serves to locate the di-mesencephalic boundary.
- The behavior of the neural crest of the trigeminal segment is distinctive so that it can be used to identify this part of the hindbrain in the older members of the series.
- The first somite is unmistakable because of its form and the behavior of its cells, and during the period involved there is but little shifting with reference to the nervous system.
The following figures are from midsagittal projection reconstructions which, with the exception of the first, were made at amagnification of 200 diameters. In the original charts every section was plotted and all were controlled by the study of themodels. The figures are from accurate copies by Mr. J . F. Didusch of my original charts.
- Keibel and Elze (’08, p. 18) have called attention to a cranial flexure in the five- to six-somite embryo ‘Klb’ (Normentafel no. 3). It is not clear from the text whether they refer to the flexure as we have identified it or whether they mean the more caudal one. The latter is a more abrupt bend which appears in the Keibel model, but not in the figures of Kroemer which were reproduced by Pfannenstiel (’03). It is located at the rostral extremity of the otic segment which is well marked in this embryo.
Cite this page: Hill, M.A. (2019, December 7) Embryology Paper - The subdivisions of the neural folds in man. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_The_subdivisions_of_the_neural_folds_in_man
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