Paper - The Nuclei of Origin of the Cranial Nerves in the 10 mm Human Embryo
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The Nuclei of Origin of the Cranial Nerves in the 10 mm Human Embryo
By G. L. Streeter.
University of Michigan (With two Figures).
Through the kindness of Dr. Huber, the writer had the opportunity of making a study of the nervous system in an unusually good 10 mm. human embryo, for the use of which he takes pleasure in acknowledging his obligation. The embryo had been cut in a faultless series of 5 micra sections and the tissues were in an excellent state of preservation, making it especially well adapted to purposes of reconstruction. The brain and the cranial and cervical nerves were reconstructed in wax after the Born method, and the following paper refers to some of the findings:
This stage in the growth of the brain is particularly interesting in that it represents what might be called a primitive or primary brain. The primary neurones of the cranial and spinal nerves, including their peripheral extensions into the muscle masses and central extensions into the wall of the brain tube, are already well laid down, and, in fact, what we see in brains at this time is almost entirely this primary apparatus. The higher receptive and co-ordinating tracts are still in a rudimentary state. It is before the development of the olive, the pontine nuclei and the cerebellum ; and the forebrain still consists of a thin-walled neural tube, showing l)ut little sign of differentiation.
In contrast to the retarded state of growth of the higher centers, the spinal and the third to twelfth cranial nerves have advanced so far in their differentiation that their appearance closely resembles the features of the adult. Due to this precocious growth of the cranial nerve elements the rhomliencephalon furnishes one-half of the bulk of the entire brain. Peripherally it is possible to trace out the complicated communications between the cranial nerves and the formation of the cervical and brachial plexuses. The characteristic communication, for instance, between the first cervical and hypoglossal nerves, with the consequent descendens liypoglossi, is already established, and it can be seen how the latter unites with the combined branch from the second and third cervical nerves forming a typical ansa liypoglossi, from which branches can be seen entering the muscle masses of the hyoid group. It is likewise possible in this embryo to trace the nerve roots centrally into the brain and outline the nuclei of origin of the motor roots and follow the extension of the sensory roots up and down in the wall of the neural tube.
In the reconstruction it is these structures that have been modelled out, and thus there is represented of the centripetal elements the dorsal funiculi of the spinal cord, and the spinal tract of the trigeminal nerve and the fasciculus solitarius of the seventh, ninth and tenth cranial nerves. Of the motor elements there is the nucleus of origin, which forms a continuous column of cells extending from the spinal cord into the brain. This column is longitudinally subdivided into a median and lateral column. The median column gives off rootlets ventrally, including the ventral spinal roots and hypoglossal nerve, and, placed at intervals more cephalad. the al^ducens. trochlear and oculomotor nerves. The lateral column gives off rootlets leaving the lateral part of the tube, including the motor elements of the spinal accessory, vagus, glossopharyngeal, facial and trigeminal nerves.
The so-called rhombic grooves, or transverse furrows, which are present at this time in the floor of the fourth ventricle, can be definitely made out in the model. That these grooves are a true feature of growth in the mammal and are not artifacts is an opinion that has been accepted with much conservatism ; but, now that they have been reported in the pig, rabbit, dog. sheep, cat and rat, and recently by Mrs. Gage in the human embryo, there can no longer be any doubt as to their reality. The writer gave them an ultimate test in the pig by examining the fresh embryo while still warm in its own amniotic fluid, and under the binocular microscope it could be seen that the grooves had the same characteristics that are present in preserved specimens.
In the model there are six distinct rhombic grooves, which correspond closely with those seen in the living pig embryo by the writer and with the description published by Bradley (1905. Jour. Anat. and Physiol., Yol. XL) of the preserved pig embryo. These commence in the region of the pontine bend and extend eaudad. Their shape, comparative size and arrangement is indicated in the accompanying Fig. 1.
Their relation to the cranial nerves is likewise indicated in the same figure. If the grooves are labelled a, h, c, d, e and /, then it can be seen that we have the following relations: the trigeminal nerve arises conjointly from a and h. the facial nerve runs transversely beneath the floor of groove c, which usually is the deepest and most sharply cut of all six grooves, the acoustic nerve has its attachment to the marginal plate adjoining grooves c and d. the al)ducens nerve arises from d, a shallow and somewhat quadrilateral groove, the glossopharyngeal nerve runs under the floor of the narrow groove e, and the vagus arises from /,, which groove, merges caudally into the general floor of the ventricle.
Fig. 1. Diagram of the floor of the fourth ventricle in the 10 mm. human embryo (Huber embryo No. Ill) illustrating the rhombic grooves and their relations to the cranial nerves. The point of attachment of the acoustic and the sensory root of the trigeminal nerve Is shown by dotted circles ; the motor nuclei are represented by heavy dots.
This nerve distribution is constant in the different mammals, and it is very likely that in this we have an explanation of the significance of these grooves. The predominant view regarding them heretofore has been that they are neuromeric and in a series with the spinal segments and the coarser transverse divisions of the mid- and forebrain. Instead of this, if emphasis is laid on the fact that they stand in constant relation to the lateral gronp of cranial nerves (fifth, seventh, ninth and tenth), then they may be fitted in with and form part of the branchiomeric system. This view has in its favor the fact that they are not only united by nervous trunks, but also numerically correspond to and are embryologically contemporary with the branchial and facial arches, in a manner shown in the following table:
Maxillarv Process ) m â€¢ â€¢ t -v-r ( Groove a.
1V1- Ti, "i AT, - Trigeminal AT. - r^ -l
Mandibular Arch \ Â° I Groove o.
Hyoid Arch Facial X Groove c.
Abducens X Groove d.
Third Branchial Arch Glossophar. X Groove e.
Fourth Branchial Arch Yagus X Groove /.
The one discordant feature is groove d, which has no corresponding branchial arch. It is possible that this should be considered, not as a true branchiomeric groove, but as simply the interval between grooves c and e; or, on the other hand, we may in this instance have to do with a displaced or lost arch, which is suggested by the aberrant course of the abducens nerve which arises from this groove.
Though the scope of the present communication is not intended to include a more detailed description of the cranial nerves and their nuclei, yet there is one feature regarding the facial nerve and its relation to the abducens nerve to which attention should be directed. It is the reversed relative position which these two structures occupy in embryos at this time as compared with the adult condition. Here the sixth nerve is more caudal than the seventh. As seen in Fig. 1, the nucleus of the facial nerve is situated near the median line under the third rhombic groove, groove c. from which nucleus the nerve fibres extend laterally, forming a slightly arched flattened bundle under the floor of this groove, and eventually emerge from the ventro-lateral border of the neural tube near the attachment of the acoustic nerve. The nucleus of the abducens nerve is situated beneath the fourth rhombic groove, groove d^ and in some vertebrates is reported as invading the adjoining caudal groove, groove e. The nerve fibres of the abdncens emerge directly ventral. The entire appearance and behavior of this nerve corresponds with the hypoglossal nerve, with which it is generally considered to stand in serial relation.
Fig. 2. Diagram illustrating the development of the genu of the facial nerve in the human embryo. The drawings show the right facial nerve and its nucleus of origin, in three stages : the youngest, A. being the 10 mm. embryo, and the oldest, C. the new-born child. The relative position of the nucleus of the Abducens nerve is represented in outline. Its nerve trunk could not be shown, as the structures are represented as seen from above.
As development continues, the floor of the neural tube thickens and the different structures shift their relative position. During this process the abducens migrates cephalad, but, like the hypoglossus, it maintains its relation to the floor of the fourth ventricle. The facial nucleus, however, like the motor nuclei of the ninth and tenth nerves, with the development of the formatio reticularis, is crowded ventro-lateralward. The migration of the nuclei of the facial and abducens nerves and the shifting of their relative positions is represented in Fig. 2, in which A corresponds to the 10 mm. embryo and C shows the relation eventually assumed by these two structures in the adult. B represents the intermediate condition, with the directions in which the relative shifting occurs indicated by arrows. It is evident on comparison of the three stages that in case of the formation of the genu of the facial nerve we have to do with a mechanical procedure which is brought about by the forward migration of the abducens nucleus therewith pushing before it the main trunk of the facial nerve. At the same time the genu is extended caudally by a ventro-lateral misjation of the facial nucleus.
Cite this page: Hill, M.A. (2020, June 5) Embryology Paper - The Nuclei of Origin of the Cranial Nerves in the 10 mm Human Embryo. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_The_Nuclei_of_Origin_of_the_Cranial_Nerves_in_the_10_mm_Human_Embryo
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