Paper - The development of a medial motor nucleus and an accessory abducens nucleus in the pig (1934)

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Shaner RF. The development of a medial motor nucleus and an accessory abducens nucleus in the pig. (1934) J Anat. 68: 314-317. PMID 17104479

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This historic 1934 paper by Shaner describes the neural development of a medial motor nucleus and an accessory abducens nucleus in the pig.

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The Development of a Medial Motor Nucleus and an Accessory Abducens Nucleus in the Pig

By Ralph F. Shaner, Pu.D. University of Alberta, Edmonton, Canada.


The motor nuclei of the cerebral nerves do not conform to the conventional scheme when studied in early embryos which have been treated by silver impregnation methods. Bok (1) and Tello (2) found the cerebral motor nerves of the chick all arising from a medial motor column or nucleus which is superimposed upon the medial longitudinal fasciculus. Windle(3, 4) described the same nucleus in early cat embryos.

There is a similar structure in the pig. In the 11 mm. embryo all motor fibres destined for the trigeminal, facial, glosso-pharyngeal, vagus and accessory nerves arise from a medial motor nucleus that lies deeply embedded in the ependymal zone, dorsal to the medial longitudinal fasciculus (Plate I, figs. 1, 2, 8, 4). The nucleus extends without interruption from the trigeminal nerve well down into the cervical cord. There it fades out beneath the usual somatic motor column. It is largest at the facial level. While most fibres from it enter nerves on the same side, a few cross the mid-line, especially at the level of the facial nerve. The medial motor nucleus impregnates easily with silver, and is conspicuous in otherwise poor preparations.

The abducens and hypoglossal nerves arise in part from the same medial motor nucleus (Plate I, figs. 8 and 4) and in part from the lighter stained ependymal zone just lateral to it. Whether the cells of origin all belong to the medial nucleus is a matter of definition; the chief point is that the two nerves arise somewhat lateral to the others of the hind-brain.

The trochlear and oculomotor nerves arise from similar darkly stained groups of cells. These may be fragments of the medial motor nucleus, but are placed slightly lateral in line with the abducens and hypoglossal nerves.

In embryos of between 11 and 16 mm. most cells migrate outwards from the medial motor nucleus. One group follows the trigeminal nerve and settles down as the chief motor nucleus for that nerve. The great migration occurs between the facial and glosso-pharyngeal nerves (Plate I, fig. 5). The cells collect in the ventro-lateral part of the medulla to form the very large facial nucleus and the scattered nucleus ambiguus.

The site of the medial motor nucleus is not abandoned. It continues to be occupied by a slender strand of fibres mingled with a residuum of cells extending from the trigeminal nerve to the upper levels of the hypoglossal nucleus (Plate II, figs. 7, 8, 9). The fibres arise for the most part from the included cells; new ones are added, however, from cells that have begun to proliferate in the area of the chief visceral motor nucleus of the adult (Plate II, fig. 9). The whole region now constitutes a primary viscero-motor nucleus that provides fibres for the trigeminal, nervus intermedius, glosso-pharyngeal, vagus and accessory nerves. Plate I, fig. 5, and Plate I, figs. 7, 9, show the visceromotor fibres leaving the glosso-pharyngeal, trigeminal, and vagus nerves, respectively.

The medial motor nucleus in this modified form is conspicuous in all pig embryos up to 60 mm. It gradually disappears as the permanent visceral motor nuclei develop. It is still traceable in 170 mm. embryos and may persist for a short distance below the genu of the facial nucleus in the adult (Plate II, fig. 10).

At the time of the migration of the facial nucleus a similar phenomenon takes place in the abducens nucleus. In the 16 mm. embryo many abducens fibres fan out laterally (Plate I, fig. 6). Embryos stained with Mallory’s phosphotungstic haematoxylin show cell activity in the ependymal zone just above, indicating that the most lateral cells of the original abducens nucleus are moving ventro-laterally (compare Plate I, figs. 3, 6).

In embryos of from 20 to 30 mm. the migrating cells collect as a ventral accessory abducens nucleus (Plate II, fig. 8). The accessory abducens nucleus lies upon the lateral fillet and rubro-spinal tract, in the plane of the superior olive, just cranial to the facial nucleus. The accessory abducens nucleus has a characteristic reticulated appearance. Fibres (or dendrites?) pass superficially into the fillet area. Wavy bundles of axones run dorso-medially to the abducens nerve. Most of these axones turn down into the nerve rootlets. A few turn up towards the main nucleus, and others still pass by the abducens nucleus towards the mid-line. The last may correspond to the scattered medial rootlets of the abducens of the other side.

The accessory abducens nucleus and its fibres are conspicuous landmarks in all pig embryos up to 100 mm. when prepared by the Cajal silver methods. The nucleus can be identified at birth in both Cajal and Weigert preparations. The axones are traced with greater difficulty as the medulla becomes more densely populated. In the adult a distinct nucleus preserves the characters of the embryonic abducens nucleus (Plate II, fig. 10). It should not be confused with the facial nucleus. The true facial nucleus is cut in sections just caudal to the accessory abducens nucleus illustrated, and is as superficial as the superior olive in Plate II, fig. 10. The cells of the abducens nucleus are slightly smaller than those of the facial.

The accessory abducens nucleus has held an obscure place in neurological literature since its discovery in the fowl in 1898. The two most comprehensive studies upon it are by Van Gehuchten (5) and Terni (6). The nucleus is absent in fishes, doubtful in Amphibia and well-developed in Reptiles (Terni). It occurs in birds and rabbits (Van Gehuchten), in the rat (Terni), but not in the mouse (Cajal(7)). Ziehen (8) considers it doubtful in Man. Mingazzini(9) cites Pacetti as relegating the nucleus to an atavistic status in Man.

Terni regards the accessory abducens nucleus as the innervation of the third eyelid apparatus in Reptiles and Birds. He places the nucleus in proximity to the nucleus of the spinal trigeminal tract, and regards it as the efferent limb of a defence reflex are which is initiated by stimuli from the skin through the trigeminal nerve. In Mammals the nucleus is associated with the retractor bulbi muscle, which is especially developed in Ungulates. The reported absence of the accessory nucleus in Man would harmonise with this conception, for the muscle is rare in man (Whitnall (10)).

While there is nothing in the pig to contradict such conceptions of the significance of the accessory abducens nucleus, it should be noted that the nucleus is rather intimately connected with the lateral fillet, and would seem therefore a part of the co-ordinating mechanism between the auditory tract and the medial longitudinal fasciculus. The superior olive of the pig has the usual peduncle, which ends obscurely.

The accessory abducens nucleus falls into the group of special visceral motor nuclei in Herrick’s scheme of nerve components, and adds one more complication. The nucleus offers another example of neurobiotaxis. It deserves more attention by those investigating the auditory mechanism in lower forms. It might be looked for in human embryos successfully treated with silver impregnation methods.


  1. The motor cerebral nerves of the hind-brain of the early pig embryo arise from a medial motor nucleus buried in the ependymal zone over the medial longitudinal fasciculus. The abducens and hypoglossal nerves arise partly from the nucleus and partly from the adjacent ependyma. The oculomotor and trochlear nerves arise from isolated fragments of the same nucleus placed in line with the abducens and hypoglossal centres.
  2. After the migration of the facial and chief motor trigeminal nuclei the rest of the medial motor nucleus persists as the primary source of visceromotor fibres for the trigeminal, facial, glosso-pharnygeal, vagus and accessory nerves. The nucleus diminishes with the advent of the adult visceral motor nuclei. Remnants may appear in the adult.
  3. At the time of the migration of the facial nucleus a lesser migration of abducens cells occurs. The cells establish an accessory abducens nucleus just rostral to the facial, in the plane of the superior olive and close to the lateral fillet. The nucleus can be found in the adult brain.

I am indebted to Mr A. G. Fairall for the preparation of material used in this study, and to Dr D. G. Revell, who has kindly read the manuscript. I wish to record my thanks to the Banting Research Foundation for a grant for the purchase of materials used in this work.


Box, S. T. (1915). ‘Die Entwicklung der Hirnnerven und ihrer zentralen Bahnen. Die stimulogene Fibrillation.” Folia neuro-biologica, Bd. 1x, SS. 475-565.

TELLO, J. F. (1923). “Les différenciations neuronales dans l’embryon du poulet, pendant les premiers jours de l’incubation.” Travaux du Laboratoire de Recherches Biologiques de U Université de Madrid, xxi, pp. 1-94.

Winnie, W. F. (1932). “The neurofibrillar structure of the 7 mm. cat embryo.” J. Comp. Neurol. vol. tv, pp. 99-138.

—— (1932). ‘‘The neurofibrillar structure of the five-and-one-half-millimeter cat embryo.” J. Comp. Neurol. vol. Lv, pp. 315-332.

Van GenHuCcHTEN, A. (1898). ‘‘ Recherches sur l’origine réelle des nerfs craniens. I. Les nerfs moteurs oculaires.”” J. de neurologie et @hypnologie, 1898 Mars. Abstracted in Neurologisches Centralblatt, Bd. xvi, 8. 503, 1899.

Terni, T. (1921). “Ricerche sul nervo abducente e in special modo intorno al significato del suo nucleo accessorio d’origine.” Folia neuro-biologica, Bd. xu, SS. 278-312.

Casa, S. R. (1909). Histologie du systéme nerveux.

ZIEHEN, TH. (1899). ‘“‘Nervensystem.” In Bardeleben’s Handbuch der Anatomie des Menschen, Bd. rv, T. 2.

Mineazzint, G. (1928). In Mollendorf’s Handbuch der Mikroskopischen Anatomie des Menschen, Bd. rv, T. 1.

Warrnatt, S. E. (1911). ‘An instance of the retractor bulbi muscle in Man.” J. Anat. and Physiol. vol. XLv1, p. 36.

SHaner, R. F. (1932). “The development of the nuclei and tracts of the mid-brain.” J. Comp. Neurol. vol. Lv, pp. 493-512.



Cr. Restiform body. C.t. Trapezoid body. Ll. Lateral lemniscus.

M1. Medial longitudinal fasciculus. M.mmn. Medial motor nucleus. N.c.VIII. Cochlear nucleus.

S.o. Superior olive.

Sp.v. Spinal tract of trigeminal nerve. Sp.VIII. Spinal tract of vestibular nerve. T.s. Tractus solitarius.

V.-XII. Cerebral nerves.

Vim. Motor nucleus of trigeminal nerve.

VI.Acc. Accessory nucleus of abducens nerve.

VIII.v. Vestibular nerve.

IX.v.-m. Viscero-motor nucleus and fibres of glosso-pharyngeal nerve. X.v.-m. Viscero-motor nucleus and fibres of vagus nerve.

Plate I

Figs. 1-4. Sections from an 1] mm. pig embryo brain, prepared by Ranson’s pyridine-silver

method. x 66.

Figs. 5, 6. Sections from a 16 mm. pig embryo brain, prepared by Ranson’s pyridine-silver

method. x 66.

Plate II

Figs. 7-9. Sections from a 28 mm. pig embryo brain prepared by a modification of the Campbell method (Shaner(11)). x 55.

Fig. 10. Section from an adult hog brain. Weigert method. x 6.

Cite this page: Hill, M.A. (2021, April 12) Embryology Paper - The development of a medial motor nucleus and an accessory abducens nucleus in the pig (1934). Retrieved from

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