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| | #REDIRECT [[Paper - The development of the nuclei pontis and the nucleus arcuatus in man (1912)]] |
| {{Ref-Essick1912}}
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| {| class="wikitable mw-collapsible mw-collapsed"
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| | [[File:Mark_Hill.jpg|90px|left]] This historic 1912 paper by Charles R. Essick describes human embryonic neural development using the [[Harvard Collection]] embryos.
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| '''Modern Notes:'''
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| <br>
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| {{Neural Links}}
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| {{Historic Disclaimer}}
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| The Development Of The Nuclei Pontis And The Nucleus Arcuatus In Man
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| Charles R. Essick
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| From the Anatomical Laboratory of The Johns Hopkins University
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| TWELVE fiGURES
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| The following paper shows the origin of the gray matter found
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| on the ventral surface of the adult rhombencephalon from the
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| ‘Rautenlippe’ or rhombic lip, along its attachment to the medulla,
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| and the path of migration of the cells formed here to the position
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| characteristic of the fully developed brain. Almost all of the
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| cells of the arcuate and pontine nuclei arise by karyokinetic division around the attachment of the roof of the fourth ventricle
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| and then Wander to their proper places. The pathway of this
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| cellular migration proves to be a Very superficial one, remarkable
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| both for its constancy and its definite limits. The arcuate nucleus
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| forms by a migration over the surface toward the ventral median
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| fissure; the pontine nuclei choose a path which corresponds in
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| every detail to the fibro—nuclear mass which I described for the
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| adult as the corpus ponto—bulbare.
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| It is Well known that all of the nuclear material in the central
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| nervous system is derived from that portion of the ectoderm which
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| closes in to form the neural tube; and our knowledge of the exact
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| manner of this cellular distribution is due mainly to the researches of His. This author has pointed out that all of the nerve
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| cells in the central nervous system first passed through the stage
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| of neuroblasts and in their development are usually Wont to leave
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| the place of their origin so as to enter into the formation of the
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| gray matter at a distance. The newly formed gray masses may:
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| (1) remain in the neighborhood of the matrix, e.g., motor and
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| arcuate cells of the spinal cord, or, (2) pierce the substance of
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| the medullary tube in a radial direction and collect on the surface
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| into an independent layer. Such wandering of cells from the matrix of the original ventricular gray matter takes place in the
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| formation of the cerebral cortex. (3) Subsequent to a definite
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| bending in the medullary Wall, newly formed gray masses may be
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| transferred from the dorsal to the ventral portion of the brain by
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| a migration of cells in a tangential direction, e.g., arcuate formations, olive and accessory olives, and a part of the nuclei lying in
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| the pons. The third of these processes as described by His is
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| responsible for the development of the gray matter constituting
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| the arcuate nuclei of the medulla and the basilar nuclei of the pons.
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| The compact manner in which the neuroblasts arrange themselves in their migration to the pontine flexure, has attracted the
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| attention of many observers both in macroscopic and microscopic
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| preparations, nevertheless, with the exception of St-reeter (’12)
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| only a casual mention of it has been made by them. Blake C00)
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| in his description of the roof of the fourth ventricle noted cells
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| which were transferred to the ectal surface of the oblongata by
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| the formation of the secondary rhombic lip and he could trace
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| them in many embryos as far cephalad as the trigeminal nerve.
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| He ventured the suggestion that they might be connected with the
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| ganglia of some of the cranial nerves. His (’04) has given a good
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| illustration (Fig. 103) of the rhombencephalon of a 5 cm. fetus
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| and has shown the outlines of the thick mass of cells passing from
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| the rhombic lip to the pontine formation. From its appearance
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| with the naked eye, as Well as in serial sections, he identifies it
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| with the Corpus Trapezoides, which thus occupies a superficial
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| position at this time (p. 163). Streeter CO7) in dissections of the
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| Seventh nerve in pig embryos called attention to the presence of a
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| ganglion mass connected with the pons ganglia which could be
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| traced backward as a surface ridge between the facial and acoustic
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| nerves, to end on the dorso-lateral surface of the restiform body.
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| Since then he has suggested ( ’1?) two possible origins for the
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| pontine nuclei: the corpus ponto—bulbare and the mantle zone‘ of
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| the pontine region. Neuroblasts from the latter source emerge
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| through the marginal zone as happens with the cortical cells of
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| the cerebellum. Orzechowski C08) in human foetus measuring
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| 17 and 23 cm. has described ganglion masses connecting the
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| rhombic lip, lateral recess Wall and pons, which he considers the embryonic corpus ponto-bulbare. He believes that the adult
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| structure may contain portions of undeveloped embryonic tissue
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| and be responsible for the frequent tumors in the cerebe1lo—pon—
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| tine angle. VVith the exception of the mention made by His
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| of a migration of cells to form the arcuate nuclei nothing has
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| been contributed to their development.
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| TABLE 1
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| (.'Ol4L/ECT1()\' NL'J1Bl-37?. PLANE OF .'\‘F4(,'TIU}\' THICKNESS
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| mm. p.
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| 20 128 Coronal 50
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| 20 368 S-agittal 20
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| 20 22 Transverse 50
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| 23 382 Sagittal 50
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| 24 405 Sagittal 40
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| 30 227 Sagittal 50
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| 30 75 Sagittal 50
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| 30 86 Coronal 50
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| 33 211 Sagittal 50-100
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| 33 145 Sagittal 50-100
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| 35 199 Sagittal 50
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| 46 95 Sagittal 100
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| 50 96 Sagittal 100
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| 50 84 Transverse 50
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| 50 184 Sagittal 50~100
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| 80 172 Transverse 100
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| 96 484 Transverse 40
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| 113 -190 Transverse 30
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| 115 219 Sagittal 50-100
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| 143 508 Transverse 40
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| 188 509 Transverse 50
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| 295 491 Transverse 40
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| In carrying out this investigation the writer was given abundant
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| opportunity to examine the large collection of human embryos
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| brought together in this laboratory by Professor Mall. Table
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| 1 gives a list of the embryos used in this study.
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| Wax plate reconstructions Were made after the method of Born.
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| Human, pig and rabbit embryos, stained in toto with alumcochineal, were prepared for dissection as described by Streeter
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| (’04, -p. 87). Whole brains stained in iron—haematoxylin also
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| gave brilliant differentiation. The most instructive specimens of the migrating strands of nuclear material were obtained in embryos stained in methylene blue. After previous hardening
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| (10 per cent formalin is excellent), the brain is carefully taken out
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| of the skull and all of the pia mater dissected off ; very great care
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| must be exercised in removing this vascular membrane in order
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| that the tiny penetrating Vessels do not tear the surface of the
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| brain which then stains Very deeply along the ruptured edges.
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| The specimen is placed into an aqueous solution of methylene
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| blue (saturate aqueous methylene blue and water equal parts)
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| for thirty to sixty seconds, rinsed in Water, and transferred to Water
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| for study. The whole brain is tinted blue but the most prominent
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| parts take a more intense stain so that all of the fine surface
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| irregularities are outlined in great detail. This brings out with
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| remarkable clearness the anastomosing strands of cells converging
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| into the pontinc formation.
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| Confusion might arise out of the terms employed here so that
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| a word may not be out of place concerning their meaning. Inasmuch as the flexures of the brain as well as the position of the head
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| are not fixed, I have used the words ‘cephalad’ (forward, front,
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| anteriorly, cercbrally), ‘caudad’ (backward, behind, spinalward),
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| ‘dorsal,’ ‘ventral,’ ‘lateral’ and ‘mesial’ just as if the central nervous system were a simple straight tube placed in the head as the
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| spinal cord is in the body. This, it seems to me, will facilitate
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| the description of relations of parts which are constantly shifting
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| their positions in relation to the body. In addition, I might state
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| that I have used the term ‘neuroblast’ loosely, so as to include all
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| undifferentiated cells which have not taken on a definite form.
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| In considering the development of these basilar masses it may
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| be of advantage to review briefly some of the relations which exist
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| in the rhombencephalon just before the cells, destined for the
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| pontine and arcuate nuclei, set out from their germ centers. His
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| (’91) has carefully reconstructed some of the intramedullary
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| nuclei and nerve roots with their relations to the surface and brain
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| flexures in an embryo of 22 mm. (figs. 5 and 17). He has called
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| attention to the fact that at this time, towards the end of the
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| second month, the formation of new neuroblasts has ceased in the
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| medulla and it is only with difficulty that a mitotic figure is disDEVELOPMENT OF THE NUCLEI PONTUS IN MAN
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| covered around the ventricular cavity where great numbers Were
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| present in the earlier stages. VVith the cessation of its activity,
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| the epithelium lining the cavity of the fourth ventricle becomes
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| very sharply marked off from the underlying nervous tissue and
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| it would be expected that the various nuclear masses in the me~
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| dulla have received their allotment of cells, further growth consisting of increase in size of individual elements and the addition of
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| nerve fibers.
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|
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| Fig. 1 Profile reconstruction of 22 mm. embryo. X 4.5. Taken from His—
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| Entw. d. mensch. Rautenhirns, Fig. 5. I have drawn in the abdueens nerve (N.
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| VI) and its nucleus (Nu. vi).
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| If now one looks at Fig. 1 (a profile drawing taken from His,
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| (’91), to which I have added the sixth nucleus and its nerve) many
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| differences from the adult are evident, the most striking perhaps
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| being the great flexure in the pontine region—thc cerebellar thickening almost touching the medulla. Just under the floor of the
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| fourth ventricle appears the nucleus 11.. abducentis usually an
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| elongated mass of cells lying immediately behind the ventricular
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| furrow formed by the bend in the neural tube. From this nucleus
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| the axones pass obliquely ventrally through the tegmentum to
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| emerge just behind the summit of the pontine fiexure in a series
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| of rootlets which behave much as the hypoglossal nerve roots.
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| They are quickly gathered together to form a single nerve trunk.
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| The facial nucleus has a remarkably constant form, the outline of
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| which issimilar both in sagittal and coronal sections. It might
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| be compared to a pear, the smaller cephalic extremity tapering
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| off bluntly. The nucleus preserves this constricted end in the
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| adult as has been brought out in a model made by Mr. VVeed in
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| this laboratory. Cephalad through half of its extent it lies dorso—
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| lateral and parallel to the superior olive a.nd extending far in front
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| of the outline of the nucleus 11. abducentis. It will be observed
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| that the relation to the superior olive is that of the adult yet one
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| would miss the familiar appearance of the facial nucleus seen in
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| transverse sections through the cephalic pole of the inferior olive.
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| In other words, the caudal tip of the facial nucleus is distant a
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| considerable interval from the cephalic tip of the inferior olive.
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| The olivary complex, still very incompletely developed, is made
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| up of an elongated mass of cells situated near the raphe. It shows
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| a marked bend conforming to the flexure of the medulla in the
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| neck region. [ts cephalic pole, as projected on the lateral surface, falls behind the transverse level of the seventh nucleus. Of
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| the greatest importance is the histological appearance of the
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| rhombencephalon, the ventral surface of which is made up of the
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| marginal veil (‘Randschleier’ of His) and in its nuclear free network run the fibers comprising the long association tracts. This
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| layer, striking in sections on account of the dearth of nuclear
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| material, forms a brilliant background which permits one to readily
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| outline the nuclei Wandering over its surface later.
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| From the embryologieal series of this laboratory definite evidences of the migration leading up to the formation of pontine
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| nuclei appear in an embryo of 23 mm. (Mall Collection No. 382).
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| Fig. 2 was drawn from a wax-plate reconstruction of the rhombencephalon of this embryo. Here the degree of medullary
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| development corresponds pretty accurately to that of the 22 mm.
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| embryo just described. The behavior of the cells lining the ventricular cavity deserves particular attention, inasmuch as they furnish the neuroblasts for the future pontine nuclei. The epcndyma
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| covering the floor of the fourth ventricle over the basal and alar
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| plates has lost all signs of the great activity which it showed
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| during the formation of the tegmental structures. The cells
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| DEVELOPMENT OF THE NUCLEI PONTUS IN MAN
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| Fig. 2 Ventro-lateral View of an wax-plate model of the Rhombencephalon of a
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| 23 mm. embryo. X 18. (No. 382).
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| composing it are sharply marked out into a definite lamina and
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| only after searching through many microscopic fields is one able
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| to detect evidences of cell division. In marked contrast to this
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| inactive region, the lip-plate which makes up the caudal wall of
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| the lateral recess and the roof of the fourth ventricle just behind
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| it, is found busily engaged in producing new elements. The furrow formed by the attachment of the roof plate to the medulla,
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| contains great numbers of karyokinetic figures in every high power
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| field of the microscope and in this neighborhood the ependymal
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| zone is not so sharply differentiated into such a thin layer as covers
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| the medullary floor nearer the midline. Its cells are more closely
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| packed, its nuclei take on a deeper stain, and the line of demarcation from the subjacent tissue is partly destroyed by the proto—
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| plasmic processes of the new neuroblasts which are beginning to
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| push toward the surface of the brain. The exact manner of arriving at the surface is illustrated by a more fortunate section. (Fig.
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| 3) through a slightly older embryo. Here the deeply staining
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| cells, poor in protoplasmic envelope, may be seen to leave their
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| position near the ventricular cavity, and to come together at the
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| surface where they form a tin sheet of closely arranged cellular
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| material. When once they have gained the surface of the brain
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| they migrate toward the pontine flexure, always preserving their
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| superficial position.
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| By referring to Fig. 2 a very good idea can be obtained of the
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| zone of proliferating cells and the area covered by the migrating
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| neuroblasts that have gained the surface of the rhombencephalon.
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| I shall omit the description of the arcuate formation for the present and consider only that narrow elongated column of cells which
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| is seen to turn toward the pontine fiexure. It is very easy to
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| identify the densely-staining closely-arranged nerve cells in sections and I have imitated the appearance one gets specimens
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| stained in toto by shading this column. The cells, that have left
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| the ventricle, converge into a well-defined band which, as it curves
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| around the restiform body, embraces the more anterior of the rootlets of the glossopharyngeal nerve and passes between the facial
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| and acoustic nerves as far forward as the trigeminal nerve. At
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| this stage the cellular sheet is very thin, being but 4-5 cells deep between the seventh and eighth nerves and where it ends behind
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| the fifth nerve being but a Single cell in depth.
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| We have then a narrow well-defined band of neuroblasts derived
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| from germinal centers situated along the attachment of the roof
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| of fourth ventricle to the medulla, and moving over the surface of
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| the brain toward the pontine flexure. The histological characteristics make it possible to trace them as far as the trigeminal
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| nerve as a sheet which gradually thins out toward its advancing
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| edge. One might well compare the process to ice growing over a
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| pond, yet unlike the latter the new material is formed at the shores
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| only and the whole sheet moves out over the surface its thin
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| advancing edge to meet its fellow from the opposite side.
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| It should be noted that the degree of development of the
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| rhombencephalon does not always correspond absolutely with the
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| measurements of the human embryos given in this table. A
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| priori, it would not be expected that at any given stage each organ
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| would always be found to correspond to those of another embryo
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| of like measurement, but in addition to the personal elements in
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| measuring, the fluid in which they are measured often accounts for
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| the difference of a few millimeters more or less. Embryo No.
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| 405 measuring 24 mm. shows a younger stage from the standpoint
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| of pontine development than No. 382 just described. In the
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| former embryo We can see the same active participation of the
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| ventricular epithelium in the production of new elements and the
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| same distribution of karyokinetic figures, yet the front ranks
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| of advancing neuroblasts have only reached the level of the facial
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| and acoustic nerves. This would give us a stage slightlyyounger
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| than No. 382 where the advancing edge has gained the transverse
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| level of the trigeminal nerve. Furthermore another possible error
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| is introduced by the measurements which I have made to show
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| the growth of the pens and they must be interpreted freely since
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| the plane of section is rarely perfect. Obliqueness of section
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| therefore precludes accurate comparison yet the ‘differences are
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| great enough to draw general conclusions.
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| Nos. 227, 75, and 86 (measuring 30 mm.) furnish valuable
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| steps leading up to the fusion of the columns of the advancing
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| neuroblasts derived from the two halves of the brain. The fre
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| TEE‘. AMERICAN JOURNAL ow ANATOMY, VOL. 13. N0. 1
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| 34 CHARLES R. ESSICK
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| quency and Wide distribution of the karyokinetic figures occurring
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| around the attachment of the ventricular roof as well as the caudal
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| Wall of the lateral recess, speak for the active participation that
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| the rhombic lip is taking in the production of pontine nuclei and
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| as a result the roof and recess wall are thickened perceptibly. The
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| last fetus has been sectioned transversely through the medulla
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| and Fig. 3. shows well the large production of cells along the roof
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| attachment. It is impossible to figure dividing cells at this magnification, yet in this one section which I have illustrated, I was
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| able to count as many as 75 karyokinetic figures immediately
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| beneath the membrana limitans interna. A very few could be
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|
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| Fig. 3 Germinal centers for the basilar nuclei at the roof attachment of the
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| fourth ventricle in a 30 mm. fetus. X 6.0. (No. 86, slide 35, section 1).
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| made out Where the cells converge at the surface, while only an
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| occasional mitotic figure is met with among the cells turning
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| around the restiform body. No evidences of indirect division
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| could be found in the layer when it has arrived in front of the
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| pontine flexure. We see, then, that the mitosis is confined sharply
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| to the central canal. The increased production of new elements
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| is also brought to one’s attention by the increase in depth of the
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| column passing between the facial and acoustic nerves to 10-15
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| superimposed neuroblasts; moreover those which had gained the
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| trigeminal nerve in the 23 mm. embryo have moved forward and
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| mesially in front of the pontine flexure.
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| -The three specimens measuring 30 mm. give us the final steps
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| in the completion of the first anlagen of the nuclei pontis. In
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| No. 227 the cell lamina has not reached the ventral median fissure, it being possible to trace it from the border of the fourth
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| ventricle forward to the fifth nerve where it curves mesially at
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| almost a right angle toward its counterpart from the other side.
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| The sagittal sections containing the sixth nerves mark the thin
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| advancing edges of the cellular sheets approaching each other from
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| the two sides of the brain. In No. 75 the most advanced cells
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| have almost succeeded in gaining the midline, while in No.86
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| the two columns have fused across the raphe. During their
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| entire course from the rhombic lip on the dorsal surface to the
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| raphe on the ventral surface, the wandering neuroblasts have
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| kept a superficial position, only occasionally is there any tendency
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| for any of the cells to penetrate into the clear, almost nuclear-free
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| marginal veil. As they pass between the seventh and eighth
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| nerves the cells are constricted into a narrow band 02 mm. wide,
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| but on reaching the pontine flexure they spread out into a fanshaped layer 0.6 mm. in caudocephalic extent. The lemnisci
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| medialis and lateralis, which up to this stage had occupied a superficial position, are now covered over by a thin bridge of tissue and
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| we can begin to speak of a tegmental and basilar part of the pons.
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| Nos. 211 and 145 (33 mm.) are cut sagitally and give us an
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| opportunity to study the earliest pontine nuclei in their relation
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| to the emergent nervus abducens. Fig. 4 is a camera lucida outline of the section through this nerve. The axones after leaving
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| their nucleus take a ventro-cephalic course through the tegmenturn and emerge from the neural tube just behind the most
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| prominent part of the pontine fiexure. The young pontine neuroblasts,.on the other hand, lie wholly in front of this flexure,
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| spread out into a sheet Whose caudo-cephalic extent is 1.25 mm.
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| and whose depth is 0.057 mm. at its thickest part, tapering down
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| to the thickness of a single cell both caudally and cephalically.
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| Between the most cephalic rootlets of the sixth nerve and the most
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| caudal cells of the pontine nuclei is an appreciable interval
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| (almost 0.5 mm.) so that one is at once reminded of the condition seen in,the adult lower animals where the abducens nerve emerges
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| from the brain some distance behind the pontine formation.
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| figs. 4 to 7 Camera lucida. tracings of sagittal sections through the rhombencephalon of human embryos from Prof. Mal1’s collection. The nucleus facialis
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| (Nu. V11) has been dotted in by profile reconstruction.
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| Fig. 4 33 mm. fetus. X 9.5. (No. 145, s1. 19, sect. 3).
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| Fig. 5 35 mm. fetus. X 9.5. (No. 199, s1. 37, sect. 1).
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| Fig. 6 50 mm. fetus. X 7.2. (No. 96, slide 48).
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| Fig. 7 115 mm. fetus. X 7.7. (No. 219, s1. 40, sect. 4).
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| At 35 mm. (No. 199) the increased cellular activity around the
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| wall of the fourth ventricle is shown by the great numbers of
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| dividing cells and the twofold increase in depth of the migrating
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| column passing between the facial and acoustic nerves. Already
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| the neuroblasts which are crowding from both sides toward the
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| midline have piled up over the ventral surface of the brain, so
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| that, near the raphe, they are new four times (0.22 mm.) as deep
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| as the stage preceding. The free interval between the emergent
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| abducens and the caudal edge of the pons is decreased to half
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| what it was in the 33 mm. embryos. The important contribution which this embryo adds to the development of the basilar
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| part of the pons, consists in a few strands of longitudinal fibers
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| lying near the midline Within the thin sheet of superficial neuroblasts newly descended from the lateral walls of the ventricle.
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| Extending in a direction parallel to the axis of the central nervous
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| system, these inconspicuous fiber bundles separate from the well
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| developed bundle of axones comprising the lemniscus medialis at
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| the level of the cephalic edge of the pontine sheet and plunge into
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| the latter where they take up a middle position as far as its caudal
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| edge. Here again they leave the nuclei pontis and join the fiber
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| mass constituting the medial lemniscus. It is impossible to
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| trace these isolated fibers except where they lie among the pontine nuclei, but, as We shall see When, by continual addition to
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| their number, more of their course can be determined, these few
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| strands represent the first beginnings of the longitudinal fibers
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| which are recognized in the basilar part of the adult pons as the
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| cortical projection system. They are represented in Fig. 5Aby two
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| dotted lines traversing the pontine nuclei. Concerning the first
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| appearance of the pyramidal tract there has been a general unanimity of opinion, the most important work being that of flechsig’s Work on myelinization time. Tiedemann (’16) thought he
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| saw pyramids in fetus of the third month but he was evidently
| |
| looking at the eminences formed by the inferior olives which at
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| this time lie adjacent to the ventral median fissure and cause an
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| elevation in the position occupied by the future pyramidal tract.
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| The latter subsequently forces its way between the olives and
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| gradually displaces them laterally. Certainly at this time the
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| number of fibers making up the pyramidal tract is insufficient
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| to cause the surface markings on the medulla which we know as
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| pyramids. Reasoning back from the interval of time~four
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| months—between the appearance and myelinization of other
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| systems, flechsig came to the conclusion that the pyramids must
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| first be laid down between the middle and the end of the fifth
| |
| month. In cross sections through the olive of an 80 mm.’ (crownrump) fetus he is unable to recognize any tissue which may be
| |
| regarded as matrix for the pyramids but thinks they arise from
| |
| fibers growing down from the cerebral cortex with remarkable
| |
| rapidity in the second half of the fifth month. W. His (’04) has
| |
| given a valuable table (p. 155) showing the various fiber systems
| |
| which he was able to identify at each stage of embryonic growth.
| |
| In a fetus of 83 mm. he was unable to find the pyramidal tract,
| |
| but at a length of 120 mm. he saw evidences of its appearance
| |
| together with cross pontine fibers. He gives us no statement
| |
| as to the part of the brain in which he observed the pyramidal
| |
| tract, merely noting its presence or absence in the various embryos
| |
| in his collection. VVith this statement of the present knowledge
| |
| of the cortical projection system, I shall omit until later the
| |
| reasons for believing that the few axones isolated by the early
| |
| pontine nuclei represent the anlage of the cortico—spinal tract in
| |
| this 35 mm. fetus.
| |
| | |
| | |
| No. 95 (46 mm.) is the youngest stage inwhich I could determine
| |
| cross fibers among the pontine nuclei. They are most conspicuous at the lateral borders where they gather together into compact strands to form the brachia pontis. Here the fibers have
| |
| a superficial position, embracing laterally the corpora restiformia
| |
| as the latter turn sharply into the cerebellar hemispheres. It is
| |
| possible to trace the axones coming from the pontine nuclei for
| |
| some distance into the cerebellum until their course parallels the large mass of fibers coming up in the inferior peduncle. The
| |
| trigeminal nerve in this stage sends its rootlets through these
| |
| cross fibers in an oblique direction to reach its intramedullary
| |
| nuclei. The cerebro—spinal neurones which were seen among the
| |
| relatively thin sheet of nuclei covering the ventral surface of the
| |
| pontine fiexure in the stage just described (35 mm.), are increased
| |
| enormously during the interval left in this series, and we have a
| |
| striking similarity to the picture one gets in sagittal sections
| |
| through the pontine region of the adult brain. The thickened
| |
| layer of cells (now 0.642 mm.) are invaded by large anastomosing
| |
| strands of fibers which collect at the cephalic and caudal border
| |
| of the nuclear sheet into a solid bundle. This behavior is one of
| |
| the peeularities of the cortical projection system as it lies among
| |
| the pontine nuclei where, as is well known, the otherwise compact
| |
| fiber tract is broken up into smaller fasciculi by the cross fibers
| |
| and nuclei of the pons. Caudad the reunited fibers fuse inseparably with the median lemniscus soon after leaving the -nuclei
| |
| pontis; cephalad I am unable to trace them beyond the cephalic
| |
| flexure.
| |
| | |
| | |
| 1 The crown heel measurement which flcchsig used was 11 centimeters. For
| |
| the sake of ready comparison with my study 1 have put his measurements into this
| |
| form from the table given by Mall in Handbuch der Entwicklungsgescliiehte des
| |
| Menschen-Keibel und Mall, Leipzig, 1910, p. 205.
| |
| DEVELOPMENT OF THE NUCLE1 PONTUS IN MAN
| |
| | |
| | |
| | |
| His has suggested that this interweaving of cross fibers of pens
| |
| with pyramidal tract points to an alternating time of deposition
| |
| of the component parts of the two systems—the development
| |
| proceeding in a direction away from the central canal. This for
| |
| the most part is true. We find the new cells which have migrated
| |
| from the ventricular walls, spreading themselves over the surface
| |
| of those ‘already descended to the pontine flexure, and as the new
| |
| axones come from the cortex they tend to grow among the younger
| |
| nuclei, i.e., to grow nearer the surface. Thus each faseiculus
| |
| when it enters the pontine nuclei, pushes along near the surface
| |
| but it is soon deeply buried by new cells which are continually
| |
| streaming down from the ventricle. As a result there is a separation of the pyramidal tract into a series of faseiculi which unite
| |
| again at their exit from the caudal border of the pens. Some of
| |
| the cells, however, after passing between the seventh and eighth
| |
| nerves forsake their superficial position and plunge between the
| |
| cross fibers of the pens. This is well illustrated in Fig. 9 the more
| |
| deeply staining young cells are seen forcing their way between
| |
| 40 CHARLES R. ESSICK
| |
| | |
| the transverse pontine fibers going into the brachium pontis.
| |
| Moreover the fasciculi of the pyramidal tract keep on growing so
| |
| that one must infer that some of the axones coming down from the
| |
| cortex add themselves to the bundles more deeply placed in the
| |
| pons.
| |
| | |
| During the period of growth between 35 mm. and 46 mm.
| |
| enormous numbers of neuroblasts have come down from the lateral
| |
| boundaries of the fourth ventricle. These new cells together with
| |
| their processes sent out transversely and the cortical axones
| |
| threading their way among them have increased the thickness of
| |
| the basilar portion to 0.642 mm. There is a tendency for the cells
| |
| from both sides to crowd toward the midline, thus giving rise to
| |
| the typical crescentic shape of the pontine nuclei which one obtains in transverse sections through the pons. The increase in
| |
| thickness is also accompanied by an increase in caudo-cephalic
| |
| extent. The latter, however, does not proceed with the same
| |
| proportional rate as the former, so that the sagittal sections of
| |
| pontine nuclei are becoming more and more oval. In spreading
| |
| caudad the interval between the nuclei pontis and the abducens
| |
| rootlets has been gradually reduced until in this fetus the more
| |
| cephalic axones are surrounded by pontine nerve cells.
| |
| | |
| In fetus of 50 mm. (Nos. 84, 96, 184) the number of cerebrospinal neurones have increased to such an extent that it is now
| |
| possible to follow them with suflicient accuracy to be certain that
| |
| we are dealing with the axones of the cortical projection system.
| |
| The fibers splitting up among the pontine nuclei already form
| |
| comparatively large bundles (Fig. 6) which are collected together
| |
| into a solid fasciculus at the cephalic end of the pons. Here they
| |
| come into close relationship with the lemniscus medialis, but it
| |
| is not impossible to trace the large fiber mass into the internal capsule. Traced cerebrally the crura gradually diverge from the
| |
| midline and turning around the cephalic fiexure they lie ventral
| |
| and lateral to the nucleus hypothalmicus, while the medial lemniscus has a more dorso—lateral position with regard to this
| |
| nucleus. The fibers making up the pyramidal tract can be traced
| |
| definitely into the internal capsule. Spinal—ward I have been
| |
| unable to differentiate the projection system from the medial lemDEVELOPMENT on THE NUCLEI PONTUS IN MAN 41
| |
| | |
| niseus soon after it has left the caudal border of the pens. There
| |
| is then no question but that we have been dealing with the beginnings of the pyramidal tract as early as 35 mm.; its behavior
| |
| among the pontine nuclei making identification certain. Former
| |
| observers have confined most of their attention to the medulla
| |
| oblongata where it is hopeless to try to pick out the few strands of
| |
| fibers when they first grow down from the cortex. The increase
| |
| in number of these axones is so gradual that it is only in the older
| |
| fetus where enough fibers are collected to form the surface marking on the medulla which we can recognize as pyramids. fleehsig
| |
| is sure that there are no pyramids at 80 mm. and probably “the
| |
| pyramidal tract is completely lacking.” To harmonize the system with other observed facts he assumes that they must grow
| |
| down rather rapidly from the cortex when once they start, since
| |
| their myelinization occurs after birth and the usual interval between the formation of a nerve fiber and its acquirement of a myelin sheath is about four months. This of necessity would have the
| |
| pyramidal tract appear about the middle to the end of the fifth
| |
| month or 14 to 16 cm. To this one must answer that a myelin
| |
| sheath does not appear on every axone of this system simultaneously; it begins rather on isolated fibers and is first complete at the
| |
| age of two years.
| |
| | |
| In these fetus of the eleventh week the basilar part of the pens
| |
| has reached a thickness of 0.7 mm. (Fig. 6). The abducens nerve
| |
| rootlets are almost entirely surrounded by nuclear material after
| |
| they leave the tegmentum, only the caudal two or three fasciculi
| |
| being free. Great numbers of neuroblasts are encountered passing between the seventh and eighth nerves, forming a stream
| |
| 0.16 mm. deep, while the germ centers at the ventricular margin
| |
| are busily producing new cells. A fortunate sagittal section
| |
| through No. 96 has been illustrated to show the participation
| |
| which the greatly thickened lateral recess wall takes in contributing cells to the pons. For purposes of orientation a wax-plate
| |
| reconstruction was n1ade with the section drawn on its cut surface (Fig. 8). As the cerebellum in its growth crowds against the
| |
| medulla, this caudal wall is flattened out and becomes part of the
| |
| mesial wall of the recess. A separation of the cells coming from the recess wall from those of the Ventricular roof is purely arbitrary, since the two origins are really continuous with each other.
| |
| This association in the adult was first pointed out by Orzechowski (’08, p. 41). For embryological reasons the nuclear thickenings of the lateral recess wall with their accompanying fibers
| |
| occurring in the adult should be included in the structure which I
| |
| have termed the corpus ponto-bulbare. At this stage the cavity
| |
| of the fourth ventricle shows a peculiar tendency to formsmall
| |
| outpouchings along the attachment of the roof at the place where
| |
| the pontine nuclei are being formed. From two to four such
| |
| recesses can be made out extending laterally for a considerable
| |
| distance from the main ventricular cavity and causing the external surface to be thrown up into ridges. In section they may be
| |
| round or slit—like and are lined vsdth deeply staining cells, great
| |
| numbers of which are found in process of karyokinetic division.
| |
| The production of neuroblasts at this stage is enormous and these
| |
| lateral extensions from the ventricle furnish a greater expansion
| |
| of ependymal surface and thus increase‘ the germ layer where cell
| |
| division can take place.
| |
| | |
| | |
| | |
| | |
| | |
| Fig. 8 Sagittal section through the cerebellum and lateral recess of a 50 mm.
| |
| | |
| fetus from 9. wax-plate 1'eco11struuti0n. X 15. (No. 96).
| |
| | |
| | |
| | |
| | |
| In the older fetus the system of ventricular outpouchings becomes more complicated and secondary processes are formed
| |
| which are distinctly tubular. The size of the lumen varies, being
| |
| sometimes less than the width of a single nucleus. It is always
| |
| lined with a simple layer of cells which are definitely ependymal
| |
| and as long as pontine nerve cells are being formed these tubules
| |
| can be made out with a little difficulty among the closely packed
| |
| neuroblasts but always the center of mitotic activity. With the
| |
| emigration of the last of the new elements the ventricular prolongations stand out with much greater clearness. This is particularly well shown in the five and eight months fetus_—in the latter,
| |
| one is struck by the greater number of such tubules both in the
| |
| roof attachment and the caudal wall of the lateral recess.
| |
| | |
| At the beginning of the fourth month, as shown by No. 172
| |
| (80 mm.), the evidences of marked cellular activity, i.e., extensive
| |
| mitosis and deeper staining are still present around the roof
| |
| attachment of the fourth ventricle and the caudal wall of the
| |
| lateral recess. This fetus does not illustrate any new principle but furnishes a beautiful single section which includes all of the
| |
| relations of the cells Wandering to the pontine nuclei. Owing to
| |
| the cervical flexure a cut which sections the spinal_ cord transversely passes tangentially through the medulla and pons (Fig.
| |
| 9). The great thickness at which these sections were cut gives
| |
| the migrating column of nuclei a Very deep red stain, almost
| |
| black, and I have represented it as black; the plane of section falls
| |
| behind the roof thickening which marks the true germinal centers,
| |
| but it does show with remarkable clearness the whole path of the
| |
| neuroblasts starting from the Ventricular edge, encircling the
| |
| corpus restiforme, passing between the facial and acoustic nerves,
| |
| to take their place among the cross fibers of the pons.
| |
| | |
| | |
| | |
| | |
| | |
| Fig. 9 Oblique section through the rhornbencephalon of an 80 mm. fetus.
| |
| X 7.5 (No. 172, sl. 200, sect. 2).
| |
| | |
| | |
| | |
| During the period extending through fetus 508 (143 mm.)
| |
| there is a continued addition of cells to the basilar part of the pons.
| |
| In a fetus of the thirteenth week, 96 mm. (N'o. 484), the maximum
| |
| production of new cells has been reached. At this time the Ventricular edges on both sides of the brain are full of karyokinetic
| |
| figures and extending from these places are two thick columns of
| |
| DEVELOPMENT OF THE NUCLEI PONTUS IN MAN 45
| |
| | |
| closely packed young cells which pour into the pons between
| |
| the seventh and eighth nerves. The cells which had already
| |
| descended from the rhombic lip show no tendency to assume the
| |
| ganglionic form. They are still rather closely arranged, the cytoplasm scanty and clear, the nucleus small and containing one or
| |
| two chromatin condensations but no real nucleolus. Two different reactions toward the haematoxylin can be made out among
| |
| the nuclei of these cells-the one quite densely staining and
| |
| usually smaller nucleus, the other slightly larger and more vesicular. The latter form the larger proportion of nuclei. From the
| |
| extensive cross—fiber system already present one must assume that
| |
| great numbers of these pontine cells have sent out nerve processes
| |
| and have taken up their final position. The newly added cells
| |
| tend to remain superficially yet a considerable portion push in
| |
| between the transverse fibers and cells already fixed. Although
| |
| the stain of No. 490 (113 mm.) does not permit of good histo~
| |
| logical study, yet it may be readily seen that the pontine nuclei
| |
| are still receiving great numbers of new elements from the rhom—
| |
| bic lip. At 143 mm. (No. 508) the production of neuroblasts
| |
| destined for the basilar parts of the hind brain has diminished very
| |
| appreciably. The mitoses around the ventricular margin are
| |
| fewer and the ependymal lining has begun to be separated quite
| |
| sharply from the underlying nervous tissue. Many of the cells
| |
| in the path of migration have larger and clearer protoplasmic
| |
| bodies, apparently unwilling to complete their journey to the
| |
| pontine region. Others appear as all of those in earlier stages
| |
| with elongated almost naked nucleus pushing on toward the pons
| |
| before assuming the ganglionic form. Of the cells which have
| |
| long since gained their permanent position in the pons, many can
| |
| now be recognized as ganglion cells. The nucleus is Very much
| |
| larger, although rarely showing a distinct nucleolus——the cytoplasm, paler than the framework in which it is embedded, is also
| |
| increased in amount. The greater number of cells have grown
| |
| slightly——possessing a smaller, more densely staining nucleus
| |
| surrounded by a clear protoplasmic envelope.
| |
| | |
| | |
| During the interval between 143 mm. (N0. 508) and 188 mm.
| |
| (No. 509) the formation of neuroblasts ceases entirely and the further development of the pens consists of an addition of axones
| |
| (with their later myelinization) and the maturity of the nerve
| |
| cells. The rhombic lip has given up all signs of activity in the
| |
| next stage of which I had access to serial sections. In No. 509
| |
| (188 mm.) the lining of the central canal is uniformly at rest in
| |
| the medullary region and is now as sharply demarcated around
| |
| the roof attachment as that covering the basal and alar plates.
| |
| The roof thickening now reminds one of those sections through the
| |
| adult medulla which pass through the ponticulus of Henle. Comparatively few of the cells have not descended into the pontine
| |
| region but are taking on the characters of adult ganglion cells
| |
| along the path where the pontine cells migrated at an earlier period.
| |
| Just how many cells fail to move into the pontine region but take
| |
| up their position around the restiform body varies in the different brains. This helps us to understand the wide differences
| |
| which were noted in the size of the fully developed corpus pontobulbare. With the disappearance of the closely packed nuclei
| |
| around the attachment of the rhombic lip and the consequent
| |
| clearing up of the roof thickening, the ventricular outpouchings
| |
| stand out with great clearness. They are not very unlike tubular
| |
| glands with a single cell lining them and show a tendency to
| |
| branch frequently.
| |
| | |
| The increase in the number and size of the bundles of cross
| |
| fibers in the pens gives the basilar part a greater thickness in
| |
| the adult where one finds the fiber material outweighing the
| |
| nuclear material. In the first beginnings on the contrary just the
| |
| reverse holds true, and the nuclei pontis alone form the protuberances in the pontine region. The individual ganglion cells,
| |
| although larger than the preceding stages, are still immature.
| |
| The protoplasmic bodies do not accept the counter stain and few
| |
| of the nuclei possess a well formed nucleolus.
| |
| | |
| | |
| By the eighth month (No. 491) the protoplasrn of the ganglionic
| |
| cell is no longer clear but takes up the counter stain. Many of
| |
| them now look like the ganglion cells of the adult except for their
| |
| smaller size. The entire migratory path is strewn with cells.
| |
| Passing betweenthe facial and acoustic nerves the column can be
| |
| traced around the restiform body into the roof of the fourth venDEVELOPMENT OF THE NUCLEI PONTUS IN MAN 47
| |
| | |
| tricle and the mesial wall of the lateral recess. A great many of
| |
| the cells can be recognized as ganglion cells of the corpus pontobulbare, but as a whole the appearance is one of immaturity.
| |
| Here and there can be found a cell whose protoplasm stains but as
| |
| a rule the large Vesicular nuclei are surrounded by a colorless
| |
| zone. The migration of nuclear material in the medulla has
| |
| ceased entirely in this stage and one has to expect only the maturity of those elements already present.
| |
| | |
| Throughout the description I have disregarded a very important factor in development which transforms the hind brain of
| |
| an embryo of the second month into the adult form: I refer to the
| |
| obliteration of the pontine flexure. The maximum flexure in the
| |
| neural tube occurs about the time of the appearance of the first
| |
| nerve cells on the ventral surface and then diminishes gradually
| |
| so that at birth there is still an indication of it on the ventricular
| |
| floor in front of the emmentia abducentis by a furrow running
| |
| transversely. The relations of the tegmental nuclei to one
| |
| another as well as to the olivary complex are distorted by the
| |
| extreme flexion’ of the brain in the pontine region. The reduction
| |
| of this may be regarded as taking place around the nucleus 11.
| |
| abducentis as an axis, since it is situated immediately beneath
| |
| the ventricular floor just behind the bend in the brain. The other
| |
| nuclei, superior olive, facial nucleus and olivary complex are distributed around its circumference and are consequently separated
| |
| from one another. A glance at Fig. 1 will show this arrangement.
| |
| figs. 4, 5, 6, and 7, are camera lucida drawings of sections through
| |
| the nucleus 11. abducentis with its emergent root bundles, which
| |
| were selected from such sagittal series as illustrated the change in
| |
| position of the nuclear masses during the obliteration of the pontine fiexure. The nucleus facialis is projected into the section as
| |
| indicated by the broken lines. As the neural tube unbends,
| |
| the olivary complex and pontine nuclei are gradually pushed
| |
| toward one another until the cephalic tip of the former comes to
| |
| be covered by the latter. The abducons nerve which in younger
| |
| stages (figs. 4, 5 and 6) pursues a straight course within the medulla is bent by this process, so that it takes a caudal direction in
| |
| order to reach the surface of the brain (Fig. 7). The facial nucleus at first separated from the olivary body by a considerable
| |
| interval comes to lie in the same transverse section as the latter,
| |
| while the pontine nuclei cover up the cephalic two—thirds of this
| |
| nucleus. We have, then, in addition to the mere increase in size
| |
| of the pontine and olivary nuclei an actual alteration of their positions as a result of the straightening out of the neural tube. As
| |
| a consequence nuclear masses which were separated from one
| |
| another, are crowded together and the course of the sixth cranial
| |
| nerve altered.
| |
| | |
| Having considered the origin of the main mass of nuclei pontis,
| |
| the possibility of cells from other sources must not be overlooked.
| |
| In the region of the pontine flexure near the raphe one can make
| |
| out at an early period collections of cells extending from the ventricular floor to the pontine nuclei with which they are connected.
| |
| They occupy the position which is held by the nuclei reticularis
| |
| tegmenti pontis (flechsig) in the adult. Long before any cells appeared superficially on the pontine flexure the karyokinetic figures
| |
| had disappeared in the ependymal sheet near the raphe, so that it
| |
| is highly improbable that the nuclei pontis depends on this portion
| |
| of the neural tube for many of its elements. In addition these
| |
| cells of the nuclei reticularis tegmenti pontis are evident long
| |
| before the pontine nuclei appear and never have the characteristic
| |
| appearance of young wandering neuroblasts during pontine development. In some of the older embryos a thin sheet (one to two
| |
| cells deep) are migrating from the wall of the lateral recess in front
| |
| of the dorsal cochlear nuclois but the layer is narrow and composed
| |
| of comparatively few cells. These cells join the pontine nuclei
| |
| behind the trigeminal nerve. It is hardly necessary to exclude
| |
| other sources if one considers seriously the great production of
| |
| new cells around the rhombic lip. This begins at 23 mm. and continues incessantly until the fetus has passed 143 mm. in crown:
| |
| rump measurement. Couple with this extensive period the short
| |
| time in which any mitotic division is complete and the great
| |
| numbers met with in every section and it will not take a great
| |
| stretch of imagination to account for all of the cells in the nuclei
| |
| pontis.
| |
| | |
| | |
| ARCUATE NUCLEI
| |
| | |
| Examination of different adult brains in microscopical sections
| |
| reveals a great variation in the amount of nuclear material which
| |
| goes to make up the basilar portions of the brain stem. This
| |
| is especially true of the arcuate nuclei where small, more or less
| |
| isolated patches of nuclear material may often be scattered along
| |
| the ventral and lateral surfaces of the medulla as far as the restiform body. The arcuate nucleus proper, the most constant of
| |
| these masses, lies near the ventral median fissure superficial to
| |
| the pyramidal tract, extending from a point caudal to the olive
| |
| up to and fusing with the pontine nuclei. At its caudal extremity, under the olive, this mass is always of greater dimensions and
| |
| tapers off somewhat as it is followed toward the pons——in some
| |
| brains disappearing here and there for a few sections, in others
| |
| forming a continuous narrow strip under the whole length of the
| |
| medulla. The arcuate nuclei proper, as well as these superficial
| |
| isolated masses lying more laterally, will be shown to have a common origin and at one time to be actually continuous with one
| |
| another. The principles governing their development are identical with those which we have studied in connection with the
| |
| pontine formation. The same germ centers around the attachment of the rhombic lip contribute cells which migrate superficially over the medulla in front of the cervical fiexure.
| |
| | |
| The formation of the arcuate nucleus, unfortunately, is not so
| |
| simple as that of the pontine nuclei, but is complicated by the
| |
| simultaneous development of the olivary complex. As His has
| |
| shown, the latter begins as a migration from the alar plate of the rhombencephalon early in the second month. Toward the end of this month the olive can be outlined readily although it has
| |
| only a small fraction of the cells which it contains in the adult.
| |
| At this time one can make out in embryos of about 20 mm. large
| |
| | |
| elongated nuclei, almost devoid of a protoplasmic body leaving.
| |
| | |
| the ventricular margin along the attachment of the rhombic
| |
| lip. They are arranged in strands of a single cell in depth and two
| |
| or three in width, streaming over the surface of the medulla just
| |
| under the external limiting membrane. This migration is directed
| |
| 50 CHARLES R. ESSICK
| |
| | |
| toward the portion of the medulla which is under the partially
| |
| formed olivary complex and recalls the undifferentiated wandering
| |
| cells seen in connection with the nuclei pontis. A great many
| |
| leave the surface at various points and plunge into the depth to
| |
| join the neuroblasts already massed up in the olivary nuclei. A
| |
| broad sheet, however, remains superficially and can be traced
| |
| from the roof attachment ventrally. In some brains (Nos. 368
| |
| and 453) this sheet has moved among the vagus rootlets and
| |
| advanced almost to the emerging hypoglossal roots. In another
| |
| (N o. 22) the migrating cells cover the entire ventral surface of
| |
| the medulla just in front of the cervical fiexure having met, across
| |
| the raphe, those moving down from the opposite side. In other
| |
| words, there exists at this period a band of superficial undifferentiated cells uniting the roof attachment on both sides which is
| |
| not unlike the early pontine bridge in fetus of 30 mm. The
| |
| former begins just in front of the cervical flexure and subtends
| |
| one—half to tWo—thirds of the olivary complex (Fig. 2), but unlike
| |
| the latter many cells leave it everywhere and make their way into
| |
| the substance of the medulla to form gray matter in the interior.
| |
| It is striking (1) that the neuroblasts of the developing arcuate
| |
| nuclei imitating the pontine formation, pay no attention to the
| |
| raphe‘ but cross it in an uninterrupted sheet; (2) that they appear
| |
| before the anlage of the nuclei pontis, and if one turns to mammalian embryos (I have studied pig and rabbit) which are slightly
| |
| larger than 20 mm. in crown-rump measurement, (3) that a Welldeveloped arcuate formation exists just as in the human material.
| |
| Turning to the adult brain We find each arcuate nucleus a discrete mass which is separated from its counterpart by the raphe.
| |
| Moreover the arcuate nucleus is peculiar to man so that from a‘
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| phylogenetic standpoint We should expect to find it developing
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| later than the pontine nuclei inasmuch as it is last to be acquired.
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| Furthermore, very soon after its formation in pigs and rabbits one
| |
| looks for it in vain. At 51 mm. only comparatively few cells can
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| be found, while the superficial layer of migrating cells has disappeared completely from the subolivary region of a fetal pig
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| of 60 mm. In man, on the other hand, when once there is a collection of cells over the medulla in the subolivary region (as in
| |
| No. 22) all of the later stages invariably show nuclear material
| |
| in the position which we know will be occupied by arcuate nuclei.
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| There appears but one rational explanation which will harmonize
| |
| all of these apparently jarring facts which we have determined.
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| In human embryos at the beginning of the second month there is
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| an intramedullary migration of cells from the rhomboid lip to
| |
| make up the olive, toward the end of the month the path of migration becomes more and more superficial until many of the cells
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| actually cross the raphe before plunging into the medulla. In
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| the lower mammals the comparatively simple olivary complex
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| soon acquires its allotment of cells and when production of olivary neuroblasts ceases in the roof attachment, those on the
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| surface soon find their way into the interior. In man, on the
| |
| contrary, before the olive has received all of its cells and while the
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| migration from the rhombic lip is still proceeding actively, neuroblasts which cannot be differentiated from those destined for the
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| olive, begin to wander over the surface among the vagus roots.
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| These elements stop on the ventral surface near the raphe and
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| constitute the anlage of the arcuate nucleus. Stated differently,
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| we are probably not dealing,with arcuate formation in human
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| embryos of 20 mm. where a cell lamina lies on the surface of the
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| medulla in the place where we know the arcuate nucleus ought to
| |
| be.
| |
| | |
| Just when the arcuate neuroblasts begin to descend from the
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| rhombic lip can only be conjectured; this uncertainty has led me
| |
| to call it ‘olivo-arcuate migration.’ Probably at 30 mm., as
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| exemplified by No. 86, most of the thick superficial sheet of cells
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| in the arcuate region represents a migration of olivary elements
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| (Fig. 10). Here the deeply staining nuclei form a continuous
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| lamina over the ventral surface, the caudo-cephalic extent of
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| which corresponds to the spinal one-half of the olive. Even older
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| embryos present this pons-like structure as figs. 4, 5, and 6 illustrate. first in a fetus of 80 mm. (No'. 172) does one meet with
| |
| any large number of superficial neuroblasts under the cerebral
| |
| one-half of the olive. Here almost the entire surface of the
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| medulla is the seat of cellular migration. From the cervical
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| fiexure almost to the pontine nuclei the pyramidal tract is covered
| |
| | |
| by a superficial sheet of cells many of which are pushing their
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| Way into the medulla near the raphe. The wandering of cells
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| to the region of the cerebral half of the olive fills up, in this embryo, the gap between the pons and the band (olivo arcuate
| |
| migration, (Fig. 2) which was first completed in embryos of 20 mm.
| |
| | |
| The addition of new elements is even more marked in the 96
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| mm. fetus (No. 484) where great numbers of moving cells are
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| directed toward the ventral portion of the medulla immediately
| |
| behind the pons. Here the cells are leaving not only the rhombic
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| lip to pursue a course similar to the earliest olivo-arcuate migration but also from the ventral edge of the thick column of migrating pontine nuclei. All along the corpus ponto—bulbare of this
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| fetus neuroblasts can be seen to leave its ventral edge and migrate
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| directly toward the Ventral median fissure. In the adult it is
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| well known that the arcuate nuclei fuse across the midline as
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| one nears the pons, although at the caudal end of the olive they
| |
| present two ‘discrete swellings which lie some distance from the
| |
| midline. Two mechanical factors are concerned in breaking up
| |
| this uninterrupted sheet of nuclear material which is so striking
| |
| in the younger fetus (Fig. 10). These are the formation of the
| |
| external arcuate fibers and the growth of the pyramidal tract.
| |
| Already in this fetus a considerable number of arcuate axones are
| |
| crossing in the raphe, the main mass of nuclei, however, still lie
| |
| on either side of the midline (Fig. 11). It remains for the constant interstitial addition ofpyramidal axones to bring about the
| |
| further separation of the arcuate nuclei. It is apparent that the
| |
| cortical projection system must occupy a very inconspicuous
| |
| part of the cross sectional area of the medulla of this embryo,
| |
| when one considers the superficial position of the olivary complex
| |
| and their proximity to the ventral medial fissure. Compare with
| |
| this the Fig. 12 which is a camera lucida tracing of No. 508 (143
| |
| mm). The level of this section was made to correspond with that
| |
| of N o. 484 by choosing both about one-tenth of the distance from
| |
| the caudal to the cephalic pole of the olive. The extensive addition to the pyramidal tract has pushed the olives apart as well as
| |
| the arcuates. The latter, remaining superficial to the rapidly
| |
| growing nerve system, have been drawn away from one another.
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| DEVELOPMENT on THE NUCLEI PONTUS IN MAN 53
| |
| | |
| Along the raphe can be found a few cells, the remains of the connecting bridge, and these persist in this position even in the adult.
| |
| Farther laterally one may often find small isolated masses at
| |
| almost any point along the periphery of the medulla, the number
| |
| | |
| R607 Aflachmenl
| |
| | |
| Corpus
| |
| Res}: forms
| |
| | |
|
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|
| |
| | |
| Subilnnlna "Gelsxhnosn
| |
| | |
|
| |
| | |
| Fig. 10 Cross section through the lower olivary region of a. 30 mm. fetus. X
| |
| 16.5. (No. 86, sl. 28, sect. 13).
| |
| | |
| Fig. 11 Camera lucida. tracing of a cross section through the lower olivary
| |
| region of a 96 mm. fetus. X 8.2. (No. 484, sl. 3, row 2, sect. 7).
| |
| | |
| Fig. 12 Camera lueida tracing of a cross section through the lower olivary
| |
| region of a 143 mm. fetus. X 7.3. (No. 508, sl. 3, row 4, sect. 2).
| |
| | |
| and amount varying with different brains. These represent
| |
| portions of the basilar nuclei which have not descended to the
| |
| position of the arcuate nuclei proper. It will be remembered that
| |
| in No. 508 many of the pontine nuclei are assuming ganglionic
| |
| 54 CHARLES R. ESSICK
| |
| | |
| form. This is not true in the arcuate formation where the nuclei
| |
| are still very densely staining and there is very little protoplasm
| |
| in the bodies. In No. 509 (188 mm), however, the protoplasmic
| |
| body is represented by a clear unstained area around a pale
| |
| vesicular nucleus. Here it is possible to speak of young ganglion
| |
| cells with certainty although most of the elements are still undifferentiated.
| |
| | |
| To conclude, then, we have in the rhombic lip or ‘Rautenlippe’
| |
| of His a common ancestor for the olive, pontine nuclei and arcuate
| |
| nuclei——the nuclei pontis being formed by a migration through a
| |
| restricted pathway, the corpus ponto-bulbare; the nuclei arcuati
| |
| along with part of the olive by a superficial migration over the
| |
| ventral surface of the medulla.
| |
| | |
| BIBLIOGRAPHY
| |
| | |
| BLAKE, J. A. 1900 The roof and lateral recesses of the fourth ventricle considered morphologically and embryologieally. Jour. Comp. Neur.,‘
| |
| | |
| vol. 10.
| |
| ESSICK, C. R. 1907 The corpus ponto-bulbare—-a hitherto undeseribed nuclear
| |
| | |
| mass in the human hind brain. Amer. Jour. Anat., vol. 7.
| |
| 1909 On the embryology of the corpus ponto-bulbare and its relation
| |
| to the development of the pons. Anat. Rec., vol. 3.
| |
| | |
| flECHSIG, P. 1876 Die Leitungsbahnen in Gehirn und Ruckenmark des Menschen. Leipzig.
| |
| | |
| HIS, W. 1891 Die Entwicklung des menschlichen Rautenhirns vom -Ende des
| |
| ersten bis zum Beginn des dritten Monats. Leipzig.
| |
| 1904 Die Entwicklung des rnenschlichen Gehirns wahrend der ersten
| |
| Monate. Leipzig.
| |
| | |
| ORZECHOWSKI, K. 1908 Ein Fall von Missbildung des Lateralrecessus. Ein
| |
| Beitrag zur Onkologie des Kleinhirnbrilckenwinkels. Arbeiten aus
| |
| dem Neurol. Instit. Wien, Bd. 14.
| |
| | |
| STREETER, G. L. 1904 The development of the cranial and spinal nerves in the
| |
| occipital region of the human embryo. Amer. Jour. An_a.t., vol. 4.
| |
| 1907 On the development of the membranous labyrinth and the acoustic and facial nerves in the human embryo. Amer. Jour. Anat., vol. 6.
| |
| 1912 The development of the nervous system. Manual of Human
| |
| Embryology, Keibel and Mall, vol. 2.
| |
| | |
| TIEDEMANN, F. 1816 Anatomic und Bildungsgeschichte des Gehirns in Foetus
| |
| des Menschen. Ntirnberg.
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