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| | #REDIRECT [[Paper - Observations on the neural crest of a ten-somite human embryo (1939)]] |
| 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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| [[Historic Embryology Papers]] | |
| {{Historic Disclaimer}}
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| ==Observations on The Neural Crest of a Ten-Somite Human Embryo==
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| By J. S. Baxter And J. D. Boyd
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| 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.
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| 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.<ref>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.</ref> 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.
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| 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.
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| 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).
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| 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.
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| 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.
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| It is, obviously, the crista neumlis rostralis of Bartelmez & Evans, or the ''craniale Kopfganglienleiste'' of Veit (1919). Although, caudally, there is a small
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| isolated portion of this neural crest material, we do not think that a distinct
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| subdivision into a “pars optica” and a “pars trigeminalis” is present. The
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| 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
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| Gasserian ganglion) and a crista neuralis prosencephali (from the dorsal edge of
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| 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
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| this portion of the neural crest. We incline to the opinion, however, which has
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| been expressed by Adelmann (1925), that the crista neuralis prosencephali
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| should be regarded with extreme caution. Adelmann was unable to find any
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| trace of such a portion of the neural crest in a large number of rat embryos of
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| the somite stage; he states (p. 52), “ absence of such a neural crest from at least
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| a major portion of the forebrain seems to be a general condition”. Schulte &
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| Tilney (1915) were also unable to find W neural crest cells arising from the
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| prosencephalon during the early development of the central nervous system of
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| the cat. Indeed, these investigators were so impressed by the absence of neural
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| crest cells in the region of the forebrain that they suggested the retention of
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| these cells in the wall of the neural tube to form an elementiadditional to the
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| alar and basal laminae. We are unable to find any evidence for such a retention
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| of alleged neural crest cells in the anterior portions of the neural plates, but we
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| have the impression that the cephalic portion of our crista neuralis rostralis is
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| not of prosencephalic origin but is an extension forwards of the main part of
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| the mass which is related primarily to the anterior part of the rhombencephalon
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| and the mesencephalon. It is, probably, that portion of the crest which will be
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| concerned with the development of the ophthalmic division of the trigeminal
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| nerve.
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| The cells constituting this right anterior neural crest mass do not show
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| continuity with the neural plate or the somatic ectoderm. In this character
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| (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
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| differ from the corresponding cells in the ten-somite embryo described by
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| Corner (1929), for he both described (p. 90), and figured, cells passing to this
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| mass from “the inner surface of the neural epithelium and the neuro-somatic
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| 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
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| question of a contribution from this source to the neural crest does not arise.
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| There are distinct differences between the neural crest cells and the
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| surrounding mesenchyme, but it is impossible to be definite in the identification
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| of the cells in the transition zone between the two groups. The bulk cochineal
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| stain is, as in Corner’s specimen,‘ not well adapted to show cytoplasmic
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| differences. We feel quite certain, however, that the neural crest material
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| extends towards, and merges with, the mesenchyme of the first branchial arch,
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| particularly with that portion of the arch which will later form the maxillary
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| process. The existence of this mingling of neural crest cells with the surrounding mesenchyme does not, of course, constitute evidence for the derivation of
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| mesenchyme from neural crest cells. On this point our material does not
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