Paper - The origin of the neural crest

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Conel JL. The origin of the neural crest. (1942) J Comp. Neurol. 76(2): 191-215.

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This historic 1942 paper investigates the origin of neural crest in a number of species. Our current understanding of neural crest origin from the edge of the neural plate is a significantly more accurate description.


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Historic Embryology - Neural Crest  
1879 Olfactory Organ | 1905 Cranial and Spinal Nerves | 1908 10 mm Peripheral | 1910 Mammal Sympathetic | 1920 Human Sympathetic | 1939 10 Somite Embryo | 1942 Origin | 1957 Adrenal
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The Origin of the Neural Crest

J. Leroy Conel

Boston University School of Medicine, Massachusetts

Four Plates (Nine-Teen figures)

Introduction

While studying transverse sections of embryos of Bdellostoma stouti the writer has observed that in the very early stages of development the neural crest is a hollow pouch whose wall consists of a single layer of cells. Each pouch is connected to the dorsal surface of the alar plate of the neural tube, and in some places the cavity of the pouch is continuous with the cavity of the neural tube. 0. Von Kupffer observed these pouches and stated that each was a hollow sac whose wall was a single layer of cells, so illustrating them in his figures 10, 11 and 21 (’00).


From the study of a fairly complete series of Bdellostoma embryos collected in 1930 with the assistance of grants from the National Research Council, the Bashford Dean Memorial (lommittec of the American Museum of Natural History, and the Committee on the Permanent Science Fund of the American Academy of Arts and Sciences (Conel, ’31), the writer ventures the opinion that these pouches explain the manner of origin of the neural crest and the dorsal root ganglia of the cranial and spinal nerves.


When traced through serial sections the walls of the pouches are observed to be directly connected to the dorsal surface of the neural tube and the cavity of each pouch is continuous with the cavity in the tube. In the region of the spinal cord enlargements in the pouches eventually become segmentally arranged; between the enlargements the neural crest is continuous but constricted. A slit—1ike cavity can be seen in the constricted parts of the neural crest, and the cavity in this part of the crest likewise opens directly into the cavity of the neural tube. In early stages of development the walls of the hollow pouch are formed by a single layer of cells, and the pouch might be called the ganglionic evagination or vesicle. In all stages the ganglionic vesicle is more advanced in development at the cephalic end than at the caudal end, that is, development proceeds in a cephalocaudal direction.


In the young stages, before the medullary folds have fused in the mid-dorsal line, the wall of the ganglionic vesicle is directly continuous with the ectoderm dorsally and with the epithelium of the medullary fold ventrally (fig. 1). In later stages the connection of the walls of the vesicle to the dorsal surface of the neural tube disappears, but the cavity still remains in the vesicle for a time.


An investigation of early embryos of Squalus acanthias revealed that in this animal also the neural crest arises as an evagination of the dorsal part of the alar plate and that the progressive stages in the development of the crest can be followed from its earliest appearance. Many embryos are required for this purpose, because favorable sections showing the continuity of the cavity of the evaginated pouch with the cavity of the neural tube are seen only infrequently. At least a portion of the cavity of the pouch, however, can be seen in almost every section. In the following descriptions the embryos marked H. E. C. are in the collection of embryos in the Harvard Medical School and those marked B. U. are in the collection of Boston University School of Medicine.


Observations

Squalus acanthias Embryo B. 17.19, mm. This embryo corresponds to Scammon’s (’11) normal plate series no. 11, figure 10, Taf. I. A very pearly stage in the differentiation of the neural crest is seen in an embryo of this length. The optic vesicle is the first part of the ganglionic evagination to appear; in this embryo ORIGIN or NEURAL CREST 193 the vesicle is evident as a slight lateral evagination of the wall of the medullary (neural) fold at the extreme anterior end of the alar plate. The medullary folds have not closed and the optic vesicle is merely a pit in the fold on each side. The evagination is interrupted between the optic part and the trigeminal part. The latter is merely an outward bulging of the lateral surface only of the wall of the medullary fold immediately ventral to its junction with the ectoderm, as shown in figure 2 by the ‘X’ mark. This outward bulging begins slightly posterior to the optic vesicle on each side and extends throughout the cephalic region of the medullary fold, graduallyi diminishing i.n size as it proceeds caudally. Posterior to the cephalic region the outward bulging of the lateral surface of the medullary fold is not apparent.


Embryo H. E. C. 978, 3.2 mm. This is the identical embryo used by Scammon for his normal plate series no. 11, figure 11, Taf. I. A slight pit has appeared on the medial surface of the out—bulging of the dorsal part of the medullary fold (fig. 3). The pit, or sulcus cristae, is present only at the anterior end of the medullary fold, and in the sections is filled with carmine granules from the stain used. The optic evagination has grown larger.


Embryo H. E. C. 980, 3.3 mm. Scammon used this same embryo for illustration of normal plate series no. 13, figure 13, Taf. I. The medullary tube is closed in the region of the optic evaginations. Immediately posterior to the region of the optic vesicles, however, the medullary folds have not quite met in the mid-line; they have fused in the mid-region of the spinal cord, but are still open throughout the caudal region of the cord. Beginning at a slight distance posterior .to the optic vesicle, the ganglionic evaginati-on is continuous to the anterior region of the spinal cord, diminishing gradually in size as it proceeds caudally. At the anterior end of the ganglionic vesicle the sulcus cristae, on the medial surface, has become a narrow crevice projecting into the evaginated cellmass. The latter consists of a single layer, of cells which is continuous laterally (dorsally) with the ectoderm and medially (ventrally) with the epithelium of the medullary tube (fig. 4). The crevice-like cavity cannot be identified in all sections and it is not of uniform width.


Embryo H. E. C. 463, 3.5 mm. This stage corresponds to Scammon’s normal plate series no. 14, figure 14., Tat‘. I. A graphic reconstruction of an embryo of this stage is illustrated in Seammon’s text figure 5 on page 47. The medullary tube is still open at the anterior and posterior ends, but is closed in the mid-region. At this stage of development the ganglionic evagination is a continuous mass which is limited to the cephalic region of the medullary tube. The wall of the ganglionic vesicle consists of a single layer of cells, and the cavity of the vesicle is continuous with the cavity of the medullary tube. The vesicle in the trigeininal region is illustrated by figure 5 and in the facial-acoustic region by figures 6 and 7. Posterior to the region of the facial-acoustic mass the ganglionic evagination has not differentiated and no crest cells can be identified with certainty. There is a slight thickening of the ectoderm in the region where the otocyst will appear later.


Embryo B. U. 32, 4.0 mm. This stage corresponds to Sca1nmon’s normal plate series 110. 16, figure 16, Taf. I. A graphic reconstruction of an embryo of this size is illustrated by Scammon’s text figure 7, on page 48. The medullary tube is now closed throughout its entire length. The neural crest is more advanced in development at its rostral end and gradually diminishes in size as it proceeds caudally, but cannot be identified beyond the anterior region of the spinal cord. In all places the ganglionic evagination is a hollow pouch whose cavity here and there opens into the cavity of the medullary tube. The former connections of the cavity of the ganglionic vesicle with the cavity of the tube are almost closed at therostral end of the evagination, but still remain open toward the caudal, younger part. figure 8 is a photomicrograph showing a transverse section through the thalamic part of the crest, and figure 9 illustrates a transverse section through the trigeminal portion of the ganglionic vesicle where a conORIGIN or NEURAL CREST 195 nection with the cavity of the medullary tube is suggested on the left side. The thalamic part of tl1e vesicle is in relation to the optic vesicle, while the maxillo-mandibular (trigeminal) part is in relation to the first and second (premandibular and mandibular) mesodermal somites. The trigeminal ganglion is indicated by a proliferation of cells in the ganglionic vesicle, and here the original cavity is now broken up into many small intercellular spaces. The vesicle extends ventrally to the dorsal surface of the mandibular somite and lies between the latter and the ectoderm. The cavity of the vesicle persists in this ventral extension.


The ganglionic Vesicle is interrupted in Embryo B. U. 32 between the trigeminal and facial—acoustic regions. The facial—acoustic part of the vesicle is in relation to the third (hyoid) mesodermal somite. In this region also the cavity of the medullaryitube is continuous with the cavity in the evaginated vesicle, as is apparent in figures 10 and 11. The black lines in figure 10 are pointing to the communication between the cavity of the neural tube and the cavity in the evaginated vesicle. The ectoderm lateral to the ganglionic vesicle is now thickened, indicating the anlage of the otic Vesicle.


The ganglionic vesicle is interrupted between the facial— acoustic and the glossopharyngeal areas. In this embryo the ganglionic evagination for the glossopharyngeal and vagus nerves does not descend ventrally as far as that illustrated by Scammon, but is represented as a slight thinning and upward bulging of the roof of the neural tube into which here and there slight crevices extend from the cavity of the tube. The bulging projects a little beyond the lateral margin of the tube on each side (fig. 12). The ganglionic vesicle for the glossopharyngeal and vagus nerves is one continuous mass which lies in the region of the fourth and succeeding mesodermal somites. Here the somites are not separated, but constitute a continuous mesodermal pouch.


The ganglionic evagination is differentiated at the extreme rostral end only of the spinal cord and is indicated as a slight upward bulging of the roof here and ‘there. No other evidence of a neural crest is apparent.


Embrg/0 B. U. 37, 4.29 mm. In this embryo the ganglionic evagination extends slightly more caudad than in the preceding stage, but is not present in the middle and posterior regions of the spinal cord.


In the thalmic region of the evagination the connection of the cavity of the vesicle with the cavity of the medullary tube has been entirely obliterated. The cavity still persists in the vesicle throughout its entire extent, but proliferation of the cells has broken up the cavity into many intercellular spaces (fig. 13). In the maxillo—Inandibular portion of the evagination the cells have proliferated by mitosis to form the ganglion, and here also the original connection of the evagination with the dorsal surface of the alar plate of the neural tube has been lost. In the ganglion more or less large spaces indicate the original cavity of the neural vesicle. The cavity still remains in its original condition in the ventral extension of the vesicle from the ganglion to the ventral surface of the mandibular somite. This ventral extension consists of two layers of cells between which the cavity can be seen here and there (fig. 14).


The ganglionic evagination is interrupted between the trigeminal and facial-acoustic regions. The roof of the medullary tube from the posterior end of the trigeminal region to the facial-acoustic region is becoming broader and thinner. The broadening is being accomplished by mitosis, while the thinning seems to be due to degeneration of cells. Many cells in various stages of atrophy can be seen in the roof of the tube, in the cavity of the tube near the roof, and between the roof and ectoderm. The ganglionic evagination in the facialacoustic region still remains connected to the dorsal part of the medullary tube, and the cavity of the vesicle is still continuous with the cavity of the medullary tube. Proliferation of cells in the evagination indicates the formation of the ganglion of the facial nerve, and the evagination has grown ventrally between the mesodermal somite (hyoid) and the ectoderm. The otic vesicle is indicated by thickening of the ectoderm in the region of the facial-acoustic ganglionic evagination. The internal surface of the ectoderm here presents a roughened and slightly vacuolated appearance. Between the regions of cranial nerves VII-VIII and IX the ganglionic evagination is interrupted in but two sections. The evagination is much smaller in the glossopharyngeal region than in the facial region. The cavity of the vesicle can be identified, and a11 occasional slight crevice indicates a connection with the cavity of the medullary tube; The evagination in this region has not grown ventrally -far enough to reach the mesodermal somite. The ganglionic evagination is continuous from the glossepharyngeal to the vagus region, and gradually decreases in size. It is larger in the vagus region than in the preceding embryo, but has not yet grown ventrally to the mesodermal somite. The cavity in the vesicle is apparent and here and there a connection with the cavity of the neural tube can be observed.


Proceeding caudally the ganglionic evagination gradually decreases in size, and in the anterior part of the spinal cord is represented only by a slight upwardbulging of the roof of the neural tube in which here and there is present a small cavity connected with the cavity of the neural tube by a slight crevice. In the middle and posterior parts of the medullary tube there is no indication of the neural crest; the roof of the neural tube is thick, and is even in contour. There are no cells between the medullary tube and the ectoderm.


Embryo H. E. C. 213, 5.0 mm. The general appearance of the ganglionic vesicle is illustrated in Scammon’s text figure 9, showing an embryo measuring 5.2 mm. in length. Both the thalamic and maxillo-mandibular parts of the trigeminal portion of the vesicle are connected to the roof of the neural tube by a strand of cells, but there is no longer any trace of a connection between the cavity of the vesicle and the cavity of the neural tube. A proliferation of cells in the_trigeminal vesicle in the region of the hindbrain represents the ganglion, in which the original cavity of the vesicle is now broken up into many small, disconnected spaces between the cells. A cavity is present between tl1e two layers of cells which extend dorsally from the ganglion for a very short distance. This dorsal extension is connected here and there by a single strand of cells to the roof of the neural tube. The thalamic part of the vesicle l1as grown ventrally to the optic vesicle as an elongated pouch in which the cavity is still present. A proliferation of cells above the optic vesicle represents the anlage of the mesoeephalic ganglion, in which the cavity of the vesicle is still present. Numerous mitotic figures tl1roughout tl1e ganglionic vesicle indicate at least one method of growth. The trigeminal ganglion lies alongside the hindbrain, but is separated from the latter by a slight space.


The ventral extension of the trigeminal vesicle which represents the maxillo-niandibular portion has now grown downward between the ectoderm and mandibular somite to the ventral surface of the latter. The cavity of the vesicle is present in this ventral extension to its lowermost limits.


The ganglionic evagination is interrupted between the regions of the trigeminal and facial nerves. A proliferation of cells in the facial-acoustic evagination represents a ganglion and a cavity still remains in Fthc ganglionic mass. All that remains of the original connection of the facial-acoustic evagination with the roof of the medullary tube is a single strand of cells, broken here and there. The bulk of the acoustic ganglion lies on the ventral surface of the otic vesicle and is connected with the vesicle by strands of cells. The cells in the facial and acoustic ganglia are very intimately intermingled. The writer can contribute no evidence as to whether or not any of the cells of the acoustic ganglion are derived from the facial-acoustic portion of the ganglionic evagination. If it is true, as some authors claim, that the ganglion of the acoustic nerve does not receive any cells from the neural crest but derives all cells from the otic vesicle, then the name “facial-acoustic” for this region of the neural crest is a misnomer. The name, however, is used in the present paper without any implication in regard to the question of the source of the cells in the ganglion of the acoustic nerve.


The ganglionic evagination is interrupted between the facial—acoustic and glossopharyngeal regions. The vesicle of the glossopharyngeal nerve is still connected to the roof of the medullary tube by a continuous strand or lamina of cells, but no cavity is present in the strand. The vesicle extends ventrally as a pouch in which there is a continuous cavity. This ventral extension lies between the ectoderm and the fourth mesodermal somite.


Between the glassopharyngeal and vagus regions the ganglionic vesicle is represented by scattered cells. The Vagus vesicle, consisting of a continuous mass, extends ventrally between the mesodermal somite and the ectoderm, and in some places reaches the side of the alimentary canal. A continuous cavity is present in the mass.


The ganglionic evagination is narrow but continuous from the vagus mass to the region of the spinal cord. The vesicle is present in the anterior one-half of the spinal region, diminishing in size as it proceeds caudally, finally disappearing. There is no trace of a differentiated evagination in the posterior part of the neural tube.


Embryo II. E. C. 1503, 7.5 mm. Scammon has described the condition of the neural crest in this embryo, and has illustrated the appearance of the crest in his text figure 10. The cavity of the optic vesicle is still continuous with the cavity of the medullary tube.


All traces of the original connections between the roof of the medullary tube and the ganglionic vesicle of each cranial nerve have disappeared excepting a. short strand of single cells in the thalamic and maxillo-mandibular regions which represent remnants of such original connections of the trigeminal vesicle.


The main ganglion of the trigeminal nerve lies alongside the hindbrain but is separated from the la.tter by a single layer of mesenchymal cells. The ganglion contains small, disconnected, intercellular spaces and many cells in various stages of mitosis.


The ganglion of the facial nerve lies against the lateral margin of the wall of the hindbrain and the ganglion of the acoustic nerve lies in contact with the ventral surface of the otic vesicle. Small, disconnected cavities are present in both ganglia. T The ganglionic evagination is interrupted between the acoustic and glossopharyngeal nerves. Both the dorsal and ventral ganglia of the glossopharyngeal are represented as two slight enlargements connected by a constricted piece of the original vesicle in which the cavity still remains. Each ganglion contains many small, disconnected spaces. The dorsal ganglion is slightly separated from the lateral margin of the hindbrain.


The neural vesicle is interrupted between the glossopharyngeal and vagus nerves in one section only. A slight proliferation of cells in the dorsal part of the vagus at the rostral end of the crest represents a ganglion. Small, disconnected cavities are present in the ganglion. Posterior to this the vagus nerve consists of a pouch seen in the section as two parallel rows of single cells, with a cavity between the rows.


The vagus evagination is continuous with the rostral end of the spinal ganglionic vesicle through a constricted portion of the vesicle called the dorsal ganglionic comrnissure. The dorsal eomniissure is a hollow tube whose wall consists of a single layer of cells. No trace remains of the original connection of the commissure with the roof of the medullary tube. Two enlargements in the dorsal eommissure represent ganglia of the spino-occipital nerves. In each ganglion the single cavity of the tube is broken up into many small, disconnected spaces.


A similar commissure or hollow tube of ganglionic vesicle connects the anlagen of the ganglia of the spinal nerves. These ganglia are likewise proliferations of cells in the vesicle and in each the original cavity is broken up into many disconnected, small spaces between the cells. In the rostral region of the spinal cord the original connection between the ganglionic Vesicle and the roof of the neural tube is represented by single, disconnected cells. Proceeding caudally the anlagen of the spinal ganglia gradually diminish in size until they are finally not diflerentiated but the vesicle is present as a long pouch with a cavity and a wall consisting of a single layer of cells (fig. 15; the photograph shows a section of embryo B. U. 49, 6.58 mm. long). This part of the ganglionic vesicle is still connected to the roof of the neural tube near the midline by a. single strand of cells. More caudally the ganglionic vesicle is but a small evagination of the roof of the medullary tube on each side of the midline, and the cavity of the evagina— tion is continuous with the cavity of the medullary tube as a narrow crevice. In the extreme caudal end of the medullary tube the ganglionic evagination has not yet differentiated.


Embryo B. U. 54, 11.0 mm. The extent of the ganglionic vesicle at this stage is shown in Scammon’s text figure 12, which illustrates an embryo 11.5 mm. long. The original connection of the maxillo-mandibular portion of the vesicle with the dorsal surface of the medullary tube has entirely disappeared. The connection of the thalamic portion with the dorsal surface of the tube is absent in the 9 mm. stage.


The central region of the trigeminal and facial ganglia contain many orange-stained fibers, and a few similar fibers connect each ganglion with the lateral wall of the alar plate in the region of the hindbrain. The ganglia have now thus established their secondary and definitive attachments to the medullary tube. These attachments are ventral to the original points of evagination of the ganglionic vesicle from the roof of the alar plate.


The ganglionic vesicle is interrupted between the trigeminal and facial ganglia, and between the latter and the glossepharyngeal ganglion. The latter has lost its original connection with the dorsal surface of the medullary tube, but has not yet established its definitive connection with the lateral surface of the tube. Both the dorsal and ventral ganglia are differentiated in this nerve, and the two are connected by a strand of the original vesicle in which the cavity is still present.


The ganglionic vesicle is still continuous between the glossopharyngeal and vague nerves as a rod of cells in which a. cavity is present here and there. At the rostral end of the vagus vesicle slight proliferations of cells indicate the anlagen of the dorsal and ventral ganglia in which traces of the original cavity of the neural vesicle can still be seen. The vagus vesicle has lost its original connection with the roof of the medullary tube. The caudal end of the vagus evagination is still a hollow pouch, the ‘wall of Which consists of a single layer of cells.


Between the caudal end of the vagus region and the first spinal nerve a continuous round strand of cells, the dorsal ganglionic cornmissure, in which a cavity is present here and there, represents the original ganglionic vesicle (fig. 16). The cranial nerves are distinguished from the spinal nerves by the fact that the former extend ventrally between the ectoderm and theinesoderinal somites, while the latter extend ventrally medial to the mesodermal somites.


In the rostral two-thirds of the spinal cord segmcntally distributed ganglia have differentiated as proliferations of cells in the ganglionic vesicle. The ganglia are connected by a dorsal ganglionic commissure in which a cavity is occasionally seen. The cominissurc and the ganglia have lost the original connection with the roof of the medullary tube, and the ganglia have not yet gained a fibrous connection with the lateral surface of the alar plate. More posteriorly the ganglia have not differentiated in the ganglionic evagination; the latter gradually diminishes in size, and is not present at all in the most caudal region of the cord.


Torpedo occllata The origin of the ganglionic vesicle by evagination from the extreme dorsal part of the medullary tube is even more evident in Torpedo. figures 17 and 18 are photomicrographs of transverse sections through the hindbrain of Embryo which is 4 mm. long. The wall of the evaginated vesicle consists of a single layer of cells. The cavity of the vesicle is clearly apparent and its connection with the cavity of the medullary tube is very evident. Dorsally the wall of the right and left vesicles is continuous across the midline; ventrally the wall of each vesicle is continuous at the dorsal surface of the corresponding alar plate with the epithelium of the medullary tube.


COMMENT This investigation is concerned only with the earliest origin of the neural crest, and not at all with the ultimate fate of the crest cells or with the question of the contribution of placodal cells to the cranial ganglia. A vast amount of literature has been written regarding the neural crest but only those papers are cited here which are pertinent to the interpretation of its origin as expressed in this paper.


In. Bdellostoma and Selachians the optic vesicle and the ganglia of all the cranial and spinal nerves have a similar origin, and are regional differentiations in a hollow pouch which emerges from the dorsal surface of the alar plate of the medullary folds and tube by a process of evagination proceeding in a cephalocaudal direction. It is hereby proposed that this evaginated pouch be given the name of ganglionic vesicle, and that the regions within it which become differentiated into the definitive ganglia of the cranial and spinal nerves be given the names of the respective nerves, viz., optic, trigeminal, facial-acoustic, glossopharyngeal, vagus, and spinal parts of the ganglionic vesicle. The ganglionic vesicle in its earliest and simplest form is an evaginated pouch whose wall consists of a single layer of cells which are continuous with the epithelium of the medullary tube. Numerous mitotic figures in the vesicle in all the stages discussed here indicate that the evaginated ganglionic vesicle grows in size by this method and that this is at least one method by which the ganglia of the cranial and spinal. nerves are formed. In other Words, the evaginated medullary epithelium is the germinative layer which produces by mitosis a large number, at least, of the cells in the" ganglia of the cranial and spinal nerves, as it does in the eye and in the medullary tube. The ganglion of the a.coustic nerve may be an exception; the method of direct observation is not sufficient for determining‘ whether any of the cells in this ganglion are derived from the neural vesicle. That some of the cells in the acoustic ganglion in Squalus acanthias are derived from the otic vesicle is suggested by the roughened inner surface of the otic vesicle and the subsequent attachment of the ganglion to this surface.


The optic vesicles are the first parts of the ganglionic vesicle to appear in Squalus acanthias. The writer can confirm Scarnmon’s observation that they can first be identified with some degree of certainty in the 2.0 mm. embryo. At this stage they are mere depressions or pits in the alar plate on each side at the anterior end of the medullary plate. Kingsbury ( ’20) has shown that the basal plate of the neural tube ends at the postoptic sulc.us, and that the anterior end of the alar plate projects beyond the end of the basal plate as illustrated in his figure 6, and in Kingsley’s figure 161 (p. 153, ’26). The optic vesicles are evaginations of the alar plate, and they assu1n_e a ventral position when the alar plate bends forward and downward around the anterior end of the basal plate. In Bdellostoma stouti it is quite evident that the optic vesicles are cvaginations of the dorsal surface of the alar plate (fig. 19). In Squalus acanthias, however, the optic part of the ganglionic vesicle includes a greater dorsoventral extent of the alar plate than those parts which form the ganglia of the other cranial and the spinal nerves.


The trigeminal part of the ganglionic vesicle cannot be identified definitely in Squalus acanthias before the embryo has grown to a length of 3.1 mm. At this time the evagination of crest cells involves only the lateral surface of the extreme dorsal part of the alar plate of the medullary fold. A cavity appears in the ganglionic evagination as a slight crcvicc in the 3.25 and 3.3 mm. embryos. In this stage of development a striking resemblance between the cranial ganglionic evagination in Acanthias and the early diiTerentiation of the optic vesicle in human embryos becomes evident upon comparison of figures 2, 3, and 4 in the present paper with figures 20 and 21 on plates 10 and 11, respectively of Bartelmez and Evans (’26). These authors observed a pit (the sulcus cristae) in the neural crest immediately posterior to the primordium of the optic vesicle.


In cat embryos Schulte and Tilney (’15) observed that the trigeminal and acoustico—facial parts of the neural crest first appear as grooves iii the neural fold (’38, figs. 8, 12, 13). In his textbook, Tilney (’38) refers to the neural crest at this stage as the trigeminal and acoustico-facial evaginations, but states that the chief difference between these evaginations and the optic evagination is that, “these outgrowths do not contain any actual extension of the neural groove” (p. 15).


The definitive nerve-fiber connection of each cranial and spinal nerve is established with the lateral wall of the alar plate, i. e., more ventral than the original place of evagination of the ganglionic vesicle.


As the embryo enlarges, the ganglionic vesicle in the region of each cranial nerve from the trigeminal to the vagus extends ventrally between the ectoderm and the mesodermal somites. The ganglionic vesicle for each spinal nerve, however, extends ventrally medial to the corresponding mesodermal somite.


Summary

The neural crest arises as an evagination of the dorsal part of the alar plate of the medullary folds or tube.

This process of evagination occurs in a cephalocaudal direction, the optic vesicle being the first part to appear.

In early stages the evagination is a hollow pouch whose cavity is continuous with the cavity of the medullary tube and whose wall consists of a single layer of cells continuous ventrally with the epithelium of the medullary tube and dorsally with the ectoderm where the tube is still open. "Where the medullary tube is closed the ganglionic evaginations of the two sides fuse across the mid-dorsal line. The optic vesicle and the ganglionic vesicles of the cranial and spinal nerves arise in the same manner from the dorsal part of the alar plate and are, therefore, homologous structures in respect to origin.


The primary attachment of each cranial or spinal ganglion with the dorsal surface of the alar plate disappears and a secondary attachment is established with the lateral surface of the alar plate by means of fibers which grow from the ganglion into the medullary tube.


Literature Cited

Bartelmez GW. and Evans HM. Development of the human embryo during the period of somite formation, including embryos with 2 to 16 pairs of somites. (1926) Contrib. Embryol., Vol. 17, Carn. Inst. Wash. Pub. No. 85, pp. 1-67.

CONEL, J. LEROY 1931 The genital system of the Myxinoidea. Bashford Dean Mem. Vo1., Article III, pp. 67-102. Am. Mus. Nat. Hist, New York.

Kingsbury BF. The extent of the floor-plate of His and its significance. (1920) J. Comp. Neural. 32(1): 113–135.

KINGSLEY, J. S. 1926 Outlines of comparative anatomy of vertebrates. P. Blakiston’s Son and Co., Philadelphia.

KUPFFER, C. VON 1900 Studien zur vergleiclienden E11twi('kelungsgeschicllte des Kopfes der Cranioten. 4 Heft: Zur Entwickelung Von Bdellostoxna. Miinchen und Leipzig.

Sca1nmon, R. E. 1911 Normal plates of the development of Squalus acanthias. Normaltaf. Entwickgesch. d. \Virbelt., F. Keibel, Heft 12. Gustav fischer, Jena.

SCIIULTE, H. VON W., AND FREDERICK TILNEY 1915 Development of the neuraxis in the domestic cat to the stage of 21 somites. Annals of the N. Y. Acad. Sci., vol. 24, pp. 319-346.

TILNEY, F., AND H. A. RILEY 1938 The form and functions of the central nervous system. Third edition. Paul B. Hoeber, N. Y.

Plates

Plate 1

1 A diagrammatic sketch of the ganglionic vesicle in the region of the VII cranial nerve in an early embryo of Bdellostoma stouti. X 300.

Squalus aeanthias

2 The earliest indication of the neural crest (X). Embryo 3.1 mm. X 300.

3 A diagrammatic sketch showing the sulcus cristae in the cephalic region of a 3.2 mm. embryo. X 300.


4 A diagrammatic representation of the ganglionic evagination (vesicle) in the cephalic region of a 3.3 mm. embryo. X 300. 5 A diagrammatic sketch of the ganglionic vesicle in the trigeminal region in 21 3.5 mm. embryo. X 300.


6 and 7 Diagrammatic sketches of the ganglionic vesicle in the facial acoustic region in a 3.5 mm. embryo. X 300.

Plate 2

8 Photograph showing the thalumic region of the g:1ng1ioni<': vesicle in a 4.0 mm. embryo. X 225.


9 Muxillo-111zu1dibu1a1' region of the ganglionic vesicle in :1 4.0 mm. embryo. X 225.


10 Anterior part of the facial-acoustic region of the ganglionic vesicle in a 4.0 mm. embryo. X 225.


11 Posterior part of the facial-acoustic region of the ganglionic vesicle in a 4.0 mm. embryo. X 225.


12 Ganglionic ov.'1gi11ntion in the glossopharnyngeal-vagus region in :14.0 mm. embryo. X 225.


Plate 3

13 Thalmnic region of the ganglionic vesicle in a 4.29 mm. embryo. X 125.


14 Mzixiilo-mamlibular region of the vesicle in the same embryo. X 125.

15 Gauglionic vesicle in the mid-region of the spinal cord in an embryo 6.58 mm. long.


16 Dorsal ganglionic commissure (g. c.) between the vugus and spinal regions i11 an embryo 11.0 mm. long. X 135.

Plate 4

17 and 18 Ganglionic vesicle in the region ofi the lmlclln‘-aiu in am embryo of Torpedo oceliata 4.0 mm. long. X 225. 19 The optic vesicles (op. V.) in an early stage of Bdellostoma stouti.

Reference

Conel JL. The origin of the neural crest. (1942) J Comp. Neurol. 76(2): 191-215.


Cite this page: Hill, M.A. (2018, December 10) Embryology Paper - The origin of the neural crest. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_The_origin_of_the_neural_crest

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