Paper - Observations on the Neural Crest of a Ten-Somite Human Embryo: Difference between revisions
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extends towards, and merges with, the mesenchyme of the first branchial arch, | extends towards, and merges with, the mesenchyme of the first branchial arch, | ||
particularly with that portion of the arch which will later form the maxillary | particularly with that portion of the arch which will later form the maxillary | ||
process. The existence of this mingling of neural crest cells with the surrounding mesenchyme does not, of course, constitute evidence for the derivation of | process. The existence of this mingling of neural crest cells with the surrounding mesenchyme does not, of course, constitute evidence for the derivation of mesenchyme from neural crest cells. On this point our material does not allow of any definite statement and, indeed, we doubt the value of any contributions to this problem which are based on description alone, particularly on the study of a single embryo or a very small series of embryos. | ||
mesenchyme from neural crest cells. On this point our material does not | |||
On the left side of the presumptive midbrain region there is a mass of | |||
neural crest cells which in position and relations is similar to the right crista | |||
neuralis rostralis (Pl. I, figs. 1, 2). It is, however, very much smaller than the | |||
corresponding mass of the right side. We correlate this difference in size with | |||
the relatively retarded development of the left side of the central nervous | |||
system in the head end of this embryo. As on the right side, there is no | |||
evidence of the migration of neural crest cells from the edge of the neural plate. | |||
The smaller size of this left crista neuralis rostralis probably accounts for its | |||
better delimitation and the absence of an appearance, on this side of the | |||
embryo, of an extension of the crest material into the branchial mesenchyme. | |||
Caudal to these two masses of neural crest cells there is a region of the | |||
developing nervous system which has no such cells related to it. A short | |||
distance behind the anterior limit of fusion of the neural folds there is found | |||
the next aggregation of neural crest cells (Pl. I, figs. 1, 2). This portion consists | |||
of a large mass of cells situated dorsal to the neural tube and intimately connected with it (see Pl. I, fig. 3 and PI. II, fig. 7). The cephalic part of this mass | |||
forms a continuum across the mid-line of the embryo with no trace of subdivision into right and left halves; but, when this mass of neural crest cells is | |||
traced caudally it separates into two, bilaterally symmetrical, horn-like processes which come to lie on either side of the dorsal portion of the neural tube | |||
(Pl. II, fig. 5). This group is the primordium acaustico-facialis (Bartelmez & | |||
Evans), or Veit’s caudale Kopfganglienleiste, and is that portion of the neural | |||
crest associated with the hyoid arch and the acoustico-facial nervous complex. | |||
It is situated, as the figures show, in close relation with the otic placodes, but | |||
there is no appearance which would suggest a contribution of placodal cells to | |||
the crest material. The neural crest cells can easily be differentiated from the | |||
cells of the adjacent neural tube, but most of the sections through this region | |||
show an appearance as if crest cells are being budded off from the line of | |||
apposition, in the median plane, of the two halves of the neural plate. The | |||
appearance, in fact, suggests that complete dorsal fusion of the neural folds | |||
has not yet taken place (see Pl. II, fig. 7), although they are completely separated from the overlying, continuous, somatic ectoderm by the relatively huge | |||
mass of neural crest cells. This acoustico-facial primordium is so large that it | |||
produces a definite protuberance upon the dorsal aspect of the embryo. | |||
The form taken by the acoustico-facial neural crest mass in this embryo is | |||
very different from that seen in Corner’s ten-somite human embryo (cp. | |||
Text-fig. 2B). In his specimen the anterior limit of fusion of the neural folds | |||
has not extended so far forward as in our embryo. It reaches only to the level | |||
of the acoustico-facial neural crest primordium which, here, is represented by | |||
bilateral and entirely separate cell aggregations; these, according to Corner’s | |||
description and microphotographs, seem to stream out of the angle of junction | |||
{{Footer}} | {{Footer}} | ||
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Baxter JS. and Boyd JD. Observations on The Neural Crest of a Ten-Somite Human Embryo (1939) J Anat. 73:318–326. PMID 17104759
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Observations on The Neural Crest of a Ten-Somite Human Embryo
By J. S. Baxter And J. D. Boyd
In the course of a study of a ten-somite human embryo in a good state of preservation we observed an unusual arrangement of those neural crest cells which form the acoustico-facial primordium. This led us to investigate the arrangement and distribution of the whole of the neural crest, and, as there are few detailed accounts of the neural crest in human somite embryos, we present our results as a separate contribution.
The embryo was obtained by laparotomy from a case of tubal gestation. The clinical history suggests that, if ovulation occurred about the mid-point of the menstrual cycle, the conceptional age of the embryo is approximately 28 days.[1] When received in the laboratory the intact chorionic vesicle had been fixed in formalin (of unknown strength) for several days. In its longest axis the vesicle measured 14 mm., in its shortest 12 mm. Having been dissected from the main part of the chorion, the embryo, with the connecting stalk and the adjacent chorionic wall, was stained in bulk with alum cochineal, dehydrated by the drop method, imbedded in paraffin and cut transversely at Sp. into a complete series of sections. The sections were then stained on the slide with orange G.
A number of reconstructions of this embryo have been made. Those of interest for the purpose of this description are a wax-plate model of the external form of the embryo, anda millboard-wax model (according to the method of Green, 1937), of the cranial portion of the nervous system and its associated neural crest primordia.
The sectioned embryo, and the reconstructions, show that the neural folds have united from a point somewhat cranial to the otic placodes to the level of the seventh somite. Here the closure of the neural tube is discontinuous, and the caudal extremity of the neural plate is a thickened, flattened area of columnar cells continuous laterally with the somatic ectoderm. The appearance of this caudal portion of the nervous system is very similar to that presented by other human embryos at a comparable stage of development. The cranial extremities of the neural folds, however, are not widely separated and thus contrast with the condition usually found at this stage. The anterior neuropore is, then, a narrow, deep cleft (Pl. I, figs. 1, 2); and, in our opinion, it is quite possible that this appearance may be ascribed to undue shrinkage of the embryo during fixation. There is a certain degree of asymmetry in the state of development of the two sides of the neural primordium, the right side of the central nervous system being more advanced than the left. This asymmetry has been described in a number of human somite embryos (cp. Bartelmez & Evans, 1926).
There are three regions in our embryo in which neural crest cells are found in relation with the developing nervous system (see Text-fig. 1). The most anterior of these regions is at the level where the cranial flexure is most pronounced, and here, on either side of the open neural folds, there is a discrete mass of neural crest cells. The right one is better developed and will be described first.
Text-fig. 1. Geometric projection of the right profile view of a reconstruction of the cephalic portion of the nervous system. The various primordia of the neural crest are shown. The hatched line indicates the margin of the left neural fold. The arrow shows the cranial limit of the first somite.
It is, obviously, the crista neumlis rostralis of Bartelmez & Evans, or the craniale Kopfganglienleiste of Veit (1919). Although, caudally, there is a small
isolated portion of this neural crest material, we do not think that a distinct
subdivision into a “pars optica” and a “pars trigeminalis” is present. The
neural crest cells do, however, extend forwards to the dorsal edge of the optic sulcus and, possibly, the aggregation of cells should be regarded as a compound of a crista neuralis rostmlis (associated with the trigeminal area and the
Gasserian ganglion) and a crista neuralis prosencephali (from the dorsal edge of
the optic primordium). If this interpretation is correct then the term primordium opto-cristale of Bartelmez & Evans might be used for the anterior part of
this portion of the neural crest. We incline to the opinion, however, which has
been expressed by Adelmann (1925), that the crista neuralis prosencephali
should be regarded with extreme caution. Adelmann was unable to find any
trace of such a portion of the neural crest in a large number of rat embryos of
the somite stage; he states (p. 52), “ absence of such a neural crest from at least
a major portion of the forebrain seems to be a general condition”. Schulte &
Tilney (1915) were also unable to find W neural crest cells arising from the
prosencephalon during the early development of the central nervous system of
the cat. Indeed, these investigators were so impressed by the absence of neural
crest cells in the region of the forebrain that they suggested the retention of
these cells in the wall of the neural tube to form an elementiadditional to the
alar and basal laminae. We are unable to find any evidence for such a retention
of alleged neural crest cells in the anterior portions of the neural plates, but we
have the impression that the cephalic portion of our crista neuralis rostralis is
not of prosencephalic origin but is an extension forwards of the main part of
the mass which is related primarily to the anterior part of the rhombencephalon
and the mesencephalon. It is, probably, that portion of the crest which will be
concerned with the development of the ophthalmic division of the trigeminal
nerve.
The cells constituting this right anterior neural crest mass do not show
continuity with the neural plate or the somatic ectoderm. In this character
(which may be due to the same shrinkage process which has caused the approximation of the two halves of the open neural plate in this region) they
differ from the corresponding cells in the ten-somite embryo described by
Corner (1929), for he both described (p. 90), and figured, cells passing to this
mass from “the inner surface of the neural epithelium and the neuro-somatic
junction” of the entire midbrain region and the rostral part of the rhombencephalon. We have been unable to find a trigeminal placode in our embryo so the
question of a contribution from this source to the neural crest does not arise.
There are distinct differences between the neural crest cells and the
surrounding mesenchyme, but it is impossible to be definite in the identification
of the cells in the transition zone between the two groups. The bulk cochineal
stain is, as in Corner’s specimen,‘ not well adapted to show cytoplasmic
differences. We feel quite certain, however, that the neural crest material
extends towards, and merges with, the mesenchyme of the first branchial arch,
particularly with that portion of the arch which will later form the maxillary
process. The existence of this mingling of neural crest cells with the surrounding mesenchyme does not, of course, constitute evidence for the derivation of mesenchyme from neural crest cells. On this point our material does not allow of any definite statement and, indeed, we doubt the value of any contributions to this problem which are based on description alone, particularly on the study of a single embryo or a very small series of embryos.
On the left side of the presumptive midbrain region there is a mass of
neural crest cells which in position and relations is similar to the right crista
neuralis rostralis (Pl. I, figs. 1, 2). It is, however, very much smaller than the
corresponding mass of the right side. We correlate this difference in size with
the relatively retarded development of the left side of the central nervous
system in the head end of this embryo. As on the right side, there is no
evidence of the migration of neural crest cells from the edge of the neural plate.
The smaller size of this left crista neuralis rostralis probably accounts for its
better delimitation and the absence of an appearance, on this side of the
embryo, of an extension of the crest material into the branchial mesenchyme.
Caudal to these two masses of neural crest cells there is a region of the
developing nervous system which has no such cells related to it. A short
distance behind the anterior limit of fusion of the neural folds there is found
the next aggregation of neural crest cells (Pl. I, figs. 1, 2). This portion consists
of a large mass of cells situated dorsal to the neural tube and intimately connected with it (see Pl. I, fig. 3 and PI. II, fig. 7). The cephalic part of this mass
forms a continuum across the mid-line of the embryo with no trace of subdivision into right and left halves; but, when this mass of neural crest cells is
traced caudally it separates into two, bilaterally symmetrical, horn-like processes which come to lie on either side of the dorsal portion of the neural tube
(Pl. II, fig. 5). This group is the primordium acaustico-facialis (Bartelmez &
Evans), or Veit’s caudale Kopfganglienleiste, and is that portion of the neural
crest associated with the hyoid arch and the acoustico-facial nervous complex.
It is situated, as the figures show, in close relation with the otic placodes, but
there is no appearance which would suggest a contribution of placodal cells to
the crest material. The neural crest cells can easily be differentiated from the
cells of the adjacent neural tube, but most of the sections through this region
show an appearance as if crest cells are being budded off from the line of
apposition, in the median plane, of the two halves of the neural plate. The
appearance, in fact, suggests that complete dorsal fusion of the neural folds
has not yet taken place (see Pl. II, fig. 7), although they are completely separated from the overlying, continuous, somatic ectoderm by the relatively huge
mass of neural crest cells. This acoustico-facial primordium is so large that it
produces a definite protuberance upon the dorsal aspect of the embryo.
The form taken by the acoustico-facial neural crest mass in this embryo is
very different from that seen in Corner’s ten-somite human embryo (cp.
Text-fig. 2B). In his specimen the anterior limit of fusion of the neural folds
has not extended so far forward as in our embryo. It reaches only to the level
of the acoustico-facial neural crest primordium which, here, is represented by
bilateral and entirely separate cell aggregations; these, according to Corner’s
description and microphotographs, seem to stream out of the angle of junction
Cite this page: Hill, M.A. (2024, April 18) Embryology Paper - Observations on the Neural Crest of a Ten-Somite Human Embryo. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_Observations_on_the_Neural_Crest_of_a_Ten-Somite_Human_Embryo
- © Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G
- ↑ We are indebted to Dr H. T. Laycock of Addenbrooke’s Hospital, Cambridge, for the opportunity of studying this specimen. Details of the clinical history of the patient from whom the embryo was obtained will be given in a subsequent contribution.
