Paper - The early development of the fasciculus longitudinalis medialis and associated secondary neurons in the rat, cat and man

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Ruth Rhines R. and Windle WF. The early development of the fasciculus longitudinalis medialis and associated secondary neurons in the rat, cat and man. (1941) J. Comp. Neuro;. : 165-188.

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This historic 1941 paper by Rhines and Windle described development of the human skeletal muscle fasciculus longitudinalis medialis/




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The Early Development Of The Fasciculus Longitudinalis Medialis And Associated Secondary Neurons In The Rat, Cat And Man

Ruth Rhines And William F. Windle

Anatomical Laboratories, Northwestern University Medical School, Chicago, Illinois

Four Text Figures And Three Plates (Eighteen Figures)


Aided by a grant from The John and Mary R. Markle Foundation.

Introduction

The components of the medial longitudinal fasciculus have been studied rather extensively in the adult, but our knowledge of their early development is fragmentary. Confusion exists in the literature on the path and destination of the descending, diencephalic portion of this complex tract, for the embryology of this fascicle has been reported differently by the several writers who have investigated it in recent years. Interest in this particular group of fibers is enhanced by the fact that it is one of the earliest, if not actually the first tract to arise in the embryo. One may quite reasonably expect it to play an important part in early integrative activities in prenatal life.


An attempt has been made here to determine, as far as microscopic observation will permit, the origin and course of the diencephalic component of the medial. longitudinal fasciculus in early cat, rat and human embryos. Since other longitudinal nerve fibers in the area adjacent to the floor plate are closely related to the diencephalic portion of the medial longitudinal fasciculus, the present study includes all longitudinal nerve fibers in the ventral part of the embryonic basal plate.


It is possible that variations in the terminology used in describing neural structures are partly responsible for lack of agreement concerning the nature of the early longitudinal tracts. The terms used in this report are defined as follows: medial longitudinal fasciculus designates only the primordium of the descending, diencephalic component of the adult medial longitudinal fasciculus (vestibular components are not yet represented) ; ventral longitudinal fasciculus indicates incompletely identifiable fascicles of longitudinal fibers occupying the area just lateral to the floor plate (including the medial longitudinal fasciculus when it is accompanied by other elements and is indistinguishable from them); secondary refers to rhombencephalic interneurons——both associational and commissural. The terms dorsal and ventral are used in reference to planes drawn at right angles to the long axis of the neural tube, regardless of actual curvature of the neural tube. Longitudinal fibers are those coursing parallel to the floor plate.


The diencephalic part of the medial longitudinal fasciculus has been described in the cat embryo in the course of more general studies of neurofibrillar development (Windle, ’32 a, ’32 b, ’33, ’35). In the 5.5 mm. cat embryo, it was seen arising from a nucleus at the junction of the mesencephalon and diencephalon. It decreased from eighty-five fibers at the cephalic fiexure to siXty—five fibers just rostral to the trigeminal motor nucleus, and was practically absent in the fascial region. Below the facial level, a ventral longitudinal tract, at that time thought to be part of the medial longitudinal fasciculus, reappeared and again became prominent. It was observed that: “The size of the medial longitudinal fasciculus seems to vary directly with the number of secondary neuroblasts in the mantle layer and the number of commissural fibers in the lower rhombencephalon and spinal cord” (Windle, ’32 b). In the present study this and another embryo have been reviewed. Little can be added to the earlier description of these two specimens.


The most recent study of the structures in question has been reported by Angulo ( ’39) in rat embryos 2.0 mm. to 5.5 mm. greatest length? This writer expressed the belief that: “. . . . in the stages under consideration the fasciculus longitudinalis medialis and other components that enter into the formation of the ventral longitudinal path are composed entirely of descending fibers.” Angulo saw the first development of the nucleus of the medial longitudinal fasciculus in his 2.0 mm. embryo, the ventral longitudinal fasciculus was said to have reached the lower level of the medulla oblongata at 4.0 mm., at 4.5 mm. it invaded the cervical cord, and at 5.5 mm. it became a well established path throughout this region.


Others who studied the rat had previously stated that the medial longitudinal fasciculus was absent in one of a litter of seven rat embryos 3 to 4 mm. long (fresh measurement), and that its most caudal extent in any specimen was the middle of the trigeminal region (Windle and Baxter, ’36).


Even before this, one investigator suggested that the fibers of the rat embryo ’s medial longitudinal fascicnlus cross in the floor plate of the pons and synapse with floor plate neurons which carry the impulse on caudalward (Hogg, ’30).


Tello (’34) mentions the medial longitudinal fasciculus in an introductory survey of the brainstem of the mouse embryo. His observations on secondary neuroblasts and the ventral commissure are in agreement with those on the cat (Windle, ’32 a, ’33). His study is scarcely applicable to the present report because the 4.0 mm. mouse embryo, his smallest specimen, is comparable with the 7-8 mm. cat and appears to be more advanced in development than those we have used.

Other than the studies on mammals noted above, Mesdag (’O9), Bok (’15), Tello (’23) and Windle and Austin (’36) have presented observations on chick embryos. The development of the nervous system of the chick is somewhat different from that in mammals and comparisons should be drawn cautiously.

‘The author does not say whether the embryos were meaured before or after fixation. If his are postfixation measurements, the embryos correspond to our specimens of approximately 3 mm. to 7.5 mm., fresh measurement.

Materials and Methods

In the present work, methods consisted entirely of direct microscopic observation, supplemented by the construction of a cardboard model of one embryo and nerve fiber counts of developing tracts in nearly all embryos.

All specimens were prepared by the Ranson pyridine-silver technique (Davenport, Windle and Beech, ’34). The cat and rat embryos were obtained from animals of known mating dates. Immediately upon removal from the uterus, they were placed in the fixing fluid. Results of previous work in this laboratory indicate that a minimum amount of delay in fixation and a minimum of handling produce the best histologic picture with the pyridine-silver staining method. For this reason, time was not taken to measure the embryos exactly, but their greatest lengths were estimated while in the amnion and before fixation. Two human specimens were obtained at operations and preserved immediately.3 Details concerning the material used are summarized in table 1.

Observations

The 16—day cat embryos of litter 846 are the smallest speci mens studied. They had between ten and thirten pairs of somites. The brain presents very nearly the beginning of neurofibrillar differentiation.

Embryos 1 and 2 are about alike in appearance and degree of development. There is no neurofibrillar differentiation rostral to the otocysts, and therefore no medial longitudinal fasciculus is present. In fact there are no longitudinal fibers present anywhere in the nervous system of these embryos. Primary motor neuroblasts of the rhombencephalon are clearly distinguishable.‘ At the same levels, and almost as conspicuous as these motor elements, secondary neuroblasts are found near the sulcus limitans. They send their axons ventrad.

3We wish to thank Dr. J. E. Fitzgerald and The Cook County Hospital Staff who made this valuable material available to us.

‘ Motor nuclei of all embryos used in the present study occupy medial positions adjacent to the floor plate (Windle, ’33).


A few reach the floor plate but no Ventral commissure is formed, for none of them has crossed yet. The glossopha— ryngeal-vagus region of embryo number 2 is illustrated in figure 5.

Embryo number 3 was sectioned in the sagittal plane. Differentiation is farther advanced than in 1 and 2. The medial longitudinal fasciculus as well as the oculomotor and trigemi— nal motor nuclei are beginning to form. The medial longitudinal fasciculus originates just rostral to, and extends only as far caudal as the oculomotor nucleus. It consists of five or six fibers on either side. No Ventral longitudinal tract has been formed by the rhombencephalic secondary neuroblasts.

TABLE 1 List of material

PLANE NUMBER

35:55.3. itfiitfi °::,:::r mgfm mm. Rat 633 1 272 hours T 3-4 23 2 272 hours T 3-4 24 3 272 hours S 3-4 24 4 272 hours T 3-4 25 5 272 hours T 3-4 19 6 272 hours T 3-4 23 7 272 hours T 3-4 23 Cat 846 1 16 days T 2-3 10 2 16 days T 2-3 13 3 16 days S 2-3 12 ‘l Cat 841 1 17 days T 3-4 22 Cat 845 1 17+ days T 4-5 33 2 17 + days T 4-5 33 3 17 + days 0 4-5 34 Q 4 17 + days T 4-5 27 Cat 403 1 18 days T 5.5 40 2 18 days T 6.0 40‘! Human 821 26 days 4.0 1' 828 30 days 6.0 38


Plane of section: T=transverse; S=sagitta1; 0=ob1ique, in respect to long axis of rhombencephalon.

Length was based on prefixation estimations. Age of human embryos yvas estimated from size and external morphology. 170 RUTH RHINES AND w. F. WINDLE

One embryo obtained from the cat (no. 841) operated upon 17 days after insemination had twenty—two pairs of somites. Certain advances in development can be clearly distinguished. All primary motor nuclei except the abducens are present, although the trochlear is found on one side only and is composed of but four neuroblasts. The only longitudinal tract in the nervous system is the medial longitudinal fasciculus. It arises from a few lightly stained, bipolar neuroblasts at the junction of the mesencephalon and diencephalon. At its origin it consists of about ten axons coursing caudad inside the external limiting membrane; only two continue for any distance. They reach the trochlear nucleus, a distance of 168 u from the nucleus of origin.

Secondary neuroblasts are more numerous at the level of the Vagus and accessory nerves than elsewhere, but a few have appeared in the trigeminal region. Axons of secondary neuroblasts in the lower rhombencephalon have penetrated the floor plate. Four sections from this embryo are illustrated in figures 6, 7, 14 and 15.


The four embryos of litter 845, obtained about 17 days and 4 hours after insemination, are considerably farther advanced in neurofibrillar development than number 841. Somites vary between twenty—seven and thirty—four. All primary motor nuclei are present and most of them have well-organized nerve roots extending into the mesenchyme. Secondary neuroblasts are numerous; their axons form a prominent ventral commissure and a ventral longitudinal fasciculus in most regions. Secondary neuroblasts are absent at the level of cranial motor nuclei III, IV and VII. The medial longitudinal fasciculus arises from a single group of cells. This lies just rostral to the oculomotor nucleus and contains bipolar and piriform neuroblasts (figs. 11, 19). The axons of these neuroblasts course caudad inside of the external limiting membrane. At the oculomotor level they sweep mesad, spreading out as they pass through the oculomotor nucleus. At this point the tract is full of terminal growth cones, for most of its fibers have progressed no farther caudad (fig. 11). Below the oculomotor nucleus, the few remaining fibers of the medial longitudinal fascic-ulus converge and take up a position adjacent to the floor plate.


More detailed descriptions of the brains of individual embryos of this series are necessary because neurofibrillar development in the mesencephalon and upper rhombencephalon has proceeded at difierent speeds in the several specimens.


In the region between nuclei of the oculomotor and trochlear nerves of embryo number 4, the medial longitudinal fasciculus consists of ten small dark fiber bundles sectioned transversely. Just lateral to it are five lightly stained fibers which have been sectioned more diagonally (they are coursing in a slightly different plane). These form the rostral end of the Ventral longitudinal fasciculus. Farther caudally, the medial longitudinal fasciculus decreases in size, and just before it dis~ appears in the middle trochlear region, its bundles split up into seventeen fine fibers. The more lateral group of fibers (ventral longitudinal fasciculus) increases in size (thirty fibers at the level of IV) as it is followed caudad, but there is considerable fluctuation in the number of its fibers throughout the trigeminal region. In the caudal part of the trochlear region (240u caudal to the first appearance of the ventral longitudinal fasciculus) the first secondary neuroblasts are observed; six sections (72 u) more caudally the first ventral commissure is found. Here one fiber crosses the floor plate. In succeeding sections the ventral commissure enlarges, the average number of fibers per section in the floor plate of the trigeminal region being four (fig. 10).


In embryo number 2 the medial longitudinal fasciculus is composed of twenty fine fibers located in the area just lateral to the floor plate and just caudal to the oculomotor nucleus. Between the trochlear and- trigeminal motor nuclei, a few longitudinal fibers appear lateral to the medial longitudinal fasciculus. They are not organized into at definite tract. These fibers, seen in figure 8, are cut slightly diagonally while those of the medial longitudinal fasciculus are sectioned more transversely at this point. Apparently, they merge with the lateral border of the medial longitudinal fasciculus more caudally, for no separate lateral tract is seen there. The longitudinally coursing fibers increase in number farther caudally, the more medial fibers becoming diagonal, while the lateral ones are cut transversely. At this point, the rostral trigeminal level, neuroblasts of the second order have begun to differentiate and a few sections caudal to this a ventral commissure is first seen.


As they course farther caudad, the medial longitudinal fasciculus fibers tend to form bundles which have the appearance of large darkly stained fibers. These bundles separate into fine fibers just before the medial longitudinal fasciculus ends in the trigeminal region. Fibers from secondary neuroblasts in this region form a homolateral component of the ventral longitudinal fasciculus; they can be clearly seen to enter the tract. Here also a few fibers from the reticular formation appear to shift into the Ventral longitudinal fasciculus (fig. 12). Secondary fibers pass through the trigeminal motor nucleus and the ventral longitudinal fasciculus of the same side to form the ventral commissure and opposite ventral longitudinal fasciculus. A few rostrally directed growth cones were observed in the fasciculus.


The medial longitudinal fasciculus of embryo number 3 contains sixty fibers caudal to the nucleus of origin and twentyfive fibers just caudal to the oculomotor nucleus. No separate lateral group of fibers was observed in this embryo. The lower part of the medial longitudinal fasciculus retains a constant size until after secondary neuroblasts have appeared in the trigeminal region. At this point the tract has decreased to twenty-two fibers, and then immediately increases to twentyseven fibers. This increase is due to the addition of secondary neurons. Near the middle of the trigeminal region the ventral longitudinal fasciculus contains forty fibers (fig. 20). Thereafter, it becomes smaller and disappears in the facial region (fig. 21). However, throughout the facial region a few fibers appear here and there in the position of the ventral longitudinal fascicle; they do not course for more than two or three sections. If these are secondary neurons, they arise on the homolateral side. As in embryo number 2, a ventral commissure begins a few sections caudal to the appearance of secondary neuroblasts. A graphic representation of number of fibers in the longitudinal tracts and the commissures at various levels will be seen in figure 1.

Embryo number 1 is very lightly stained and it is difficult to distinguish secondary neuroblasts. Nerve fiber counts (twenty longitudinal fibers caudal to the oculomotor nucleus)

60 50 40 30 20

ID

Number of fibers


0 mm. 0 .3 .6 .9 J2 J5 .|B .2| .24 .27 .30 .33 .56 .39 .42

IIINXZEHJX X XI

Fig.1 Graph based on the enumeration of longitudinal fibers in the area adjacent to the floor plate on one side and the number of fibers in the ventral commissure of cat embryo 845: 3 beginning at the lower end of the nucleus of the medial longitudinal fasciculus (zero point). The solid line represents the medial and ventral longitudinal fasciculi; the secondary fibers of the ventral commissure are designated by a broken line; the number of motor fibers in the ventral commissure is shown by a dotted line. Levels of motor nuclei are Roman numerals.

The sharp decrease in the number of longitudinal fibers in the rostral part of the brain stem is due to the fact that the medial longitudinal fasciculus ends at the trigeminal level at this stage. Caudal to the lower end of the medial longitudinal fasciculus there is a correlation between the appearance -of the secondary commissure and the increase in number of fibers in the ventral longitudinal fasciculus. In the trigeminal region the peak of the curve of longitudinal fibers is high in proportion to the size of the commissure because of a large homolateral contribution to the ventral longitudinal tract. In the glossopha.ryngeal—vagus region the ventral longitudinal fasciculus is older and longer, and more fibers per section have accumulated; this is reflected in the height of the peaks of the upper curve. The more caudal position of the peaks of the secondary commissure as compared with the peaks formed by plotting the number of longitudinal fibers suggests that the longitudinal tract is predominantly ascending.


cannot be considered as reliable as in the better stained embryos. However, a graph constructed on the basis of these counts, follows the same pattern as in the other embryos of this series. A medial and lateral grouping of fibers is apparent as far as the middle trigeminal region. The ventral longitudinal fasciculus disappears caudal to the emerging trigeminal motor root and rostral to the otocyst.

The lower rhombencephalon caudal to the trigeminal region is similar in all four embryos of litter 845. The ventral longitudinal fasciculus disappears in tlie facial region. The ventral commissure is composed entirely of crossing facial motor axons at this level (fig. 21). These are thick, dark fibers with large club—shaped growth cones. Many of them can be traced directly from motor neuroblasts. At this level, secondary neuroblasts are very scarce and immature. In most instances they have short, ventrally directed axons ending in growth cones a few micra from their origins.

At the caudal end of the facial region, the ventral longitudinal fasciculus reappears or again becomes a prominent tract. A few sections later, more mature secondary neuroblasts, whose axons course ventrad and mingle with the dorsally running intra—medullary motor root fibers, appear. At first there are only one or two per section, but this number rapidly increases to ten or more. Fibers can be seen coursing from the intra—medullary motor bundle through the motor nucleus and into the ventral commissure. There is a correlation between the size of the ventral commissure and the number of fibers in the ventral longitudinal fasciculus. This is shown in figure 1. No homolateral contributions to the ventral longitudinal fasciculus were observed in the lower rhombencephalon. The increase in size of ventral commissure andventral longitudinal fascicle in the vagus region is illustrated in figures 13 and 22.

Seven rat embryos of 19 to 25 somites, obtained 272 hours after insemination, were reexamined. No medial longitudinal fasciculus is present in the smallest specimen (no. 5) nor are there any secondary elements in the rhombencephalon.

embryo number 2, slightly larger, the medial longitudinal fasciculus has just begun to form and extends only as far as the trochlear level (see fig. 5, Windle and Baxter, ’36). The presence of secondary neurons is indeterminable because of a darkly stained background. In embryos numbers 6 and 7, the medial longitudinal fasciculus reaches the rostral trigeminal level and ends there Without decussating. Secondary neuroblasts are beginning to develop in the trigeminal region, but no ventral commissure and no ventral longitudinal fascicu— lus have been formed there. No longitudinal tract is present in the ventral part of the basal plate at other levels. However, a few neuroblasts of the second order can be recognized in the lower rhombencephalon. These appear to be less mature than those at the trigeminal level. The medial longitudinal fasciculus of embryos 1, 3 and 4 extends to the caudal trigeminal region. Embryo 3 is sectioned in a sagittal plane, which makes relationships between the longitudinal tract, ventral commissure and neuroblasts of the second order difiicult to determine.




Figs. 2, 3, 4 Diagrams of brainstems of cat embryos 846, 841 and 845, showing the degree of development of the medial longitudinal fasciculus and the ventral longitudinal fasciculus. Roman numerals designate the levels of the motor nuclei.

In figure 2, the medial longitudinal fasciculus is absent. Secondary neuroblasts have begun to differentiate in the glossopharyngeal-vagusraccessory region, but no ventral commissure has been formed.

In figure 3, the medial longitudinal fasciculus, extends only to the trochlear level. A few secondary neuroblasts have appeared in the trigeminal region. Secondary neurons are most numerous at the glossopharyngeal—vagus level. Some secondary axons in the lower rhombencephalon reach the floor plate, but no ventral longitudinal tract has formed. No secondary neuroblasts are present at the facial level.

In figure 4, the medial longitudinal fasciculus has entered the trigeminal region. It overlaps the ventral longitudinal fasciculus which is formed homolaterally and contralaterally by the ascending axons of secondary neuroblasts. In the rhomben— cephalon, the secondary axons have formed a ventral commissure and a ventral longitudinal fasciculus. At the facial level, the ventral longitudinal fasciculus is interrupted (the commissure is entirely motor) and secondary neuroblasts are just beginning to differentiate.


Embryos 1 and 4 represent stages of development similar in some respects to those in 4 mm. to 5 mm. cat embryos. Between the trochlear and trigeminal regions, the longitudinal tract divides into little bundles. At some levels these bundles are definitely arranged medially and laterally. A medial group of these is more compact and darker than the more lateral bundles and comprises the descending diencephalic component of the medial longitudinal fasciculus. In other regions, however, the fibers are more evenly distributed at the periphery of the area lateral to the floor plate, and it is not possible to say which belong to the medial longitudinal fasciculus. In some sections, particularly in the caudal trigeminal region, fibers can be seen running from one bundle to another, suggesting that an intermingling of fibers takes place.


The total number of fibers in both medial and lateral bundles just caudal to the trochlear level drops below the count of fibers in the medial longitudinal fasciculus at the trochlear level, where only one tract is apparent. A few sections farther caudally, however, the total count of the medial and lateral fibers of the ventral longitudinal tract increases and surpasses that of the medial longitudinal fasciculus fibers at the trochlear level.


Secondary neuroblasts in the trigeminal region send axons ventrad to accompany the intramedullary primary motor roots of the trigeminal nerve. Some of these axons reach the ventral part of the neural tube; a few light fibers can be seen passing ventromesad through the motor nucleus. In embryo number 1, some of these fibers course ventrad from the intra— medullary motor bundle and end at the lateral border of the ventral longitudinal fasciculus of the same side (fig. 9). They enter this tract.


The ventral longitudinal tract ends at the facial region; no longitudinal fibers are found near the floor plate caudal to this in embryos 1 and 4. In both of these embryos, the formation of a ventral commissure is just beginning in the trigeminal region. Three or four fibers in as many sections are seen entering the floor plate and ending there. No fiber crosses to the other side. A few floor plate fibers are encountered in the facial region. These are large, darkly stained axons in contrast with the delicate processes of secondary neuroblasts and are undoubtedly primary motor fibers forming the crossed component of the facial nerve.


The smallest human embryo (no. 821), 26 days’ fertilization age, was dead before abortion. An accurate study of the nervous system is impossible but a few significant neurofibrillar structures are visible. Some of the primary motor nuclei have appeared, and secondary neurons are forming in the rhombencephalon. There is no medial longitudinal fasciculus as yet; in fact no neuroblasts can be seen anywhere in the diencephalon and telencephalon. The secondary neurons of the brain ‘are confined to the levels of the glossopharyngeal and vagus nerves. A few thin fibers cross the floor plate in the lower rhombencephalon and a Ventral longitudinal fasciculus has formed in this region only. Its course is short, for it is seen neither in the trigeminal region nor in the upper cervical spinal cord.


One embryo of 30 days’ fertilization age (no. 828) is our smallest human specimen in which a medial longitudinal fasciculus appears. It is more advanced than the largest rat and cat embryos. The rostral origin of the medial longi— tudinal fasciculus is observed in a group of neuroblasts located in front of and slightly dorsal to the oculomotor nucleus. This cell group is the most highly developed structure rostral to the mesencephalon. It gives rise to several closely associated bundles of fine fibers which course caudad on the same side of the brain. They come to lie just beneath the external limiting membrane at the junction of floor plate and basal plate in the region of the oculomotor nerve (fig. 16). Some of the bundles of this fiber tract are actually in the floor plate. Near its origin it contains more than 450 fibers, but many of these end before the level of the trochlear nerve is reached; less than 300 fibers can be counted at that level. All of the little bundles of fine fibers end by the time the tract reaches the level of the trigeminal nerve roots; at this point they are being replaced by more loosely arranged and coarser fiberssecondary neurons of local origin. Thus the medial longitudinal fasciculus gives way to the ventral longitudinal fasciculus. The total number of fibers in this tract at the level of the trigeminal motor nucleus is less than 150. There is nothing exceptional in the mode of termination of the fine fibers from the nucleus of the medial longitudinal fasciculus. They do not cross to the opposite side for no ventral commissure is present in the floor plate above the trigeminal level. As the bundles break up they simply end on the same side. Below the trigeminal region the ventral longitudinal fasciculus immediately increases in number of fibers (figs. 17, 18). Correlated with this increase, the ventral commissure enlarges.

Discussion

VVith minor species and very slight individual variation, we find that the medial longitudinal fasciculus and its associated tracts develop similarly in the cat, rat and man. VVe find that the tract Which has been assumed to be the primordium of the adult fasciculus longitudinalis medialis is that only in part. There can be no reasonable doubt that the fibers descending from a group of cells at the junction of embryonic mesencephalon and diencephalon constitute the oldest part of the medial longitudinal fasciculus. This is the only part of the medial longitudinal fasciculus present in embryos of the sizes studied here. This group of fibers forms a homolateral descending pathway which takes no part in construction of the ventral commissure in brains of embryos of the size studied here.


A ventral longitudinal fasciculus more caudally occupies a position comparable with that of the medial longitudinal fasciculus. In the trigeminal region this tract is small. In all three species, the medial longitudinal fasciculus is eventually overlapped by the other longitudinal fibers. In most cat embryos, the two tracts are distinguishable from one another because they are separate and because their fibers take a slightly different direction. In the human, the fibers of the medial longitudinal fasciculus are finer than those of the ventral longitudinal fasciculus. The development of the rat rostral to the facial region resembles that in the cat. Neuroblasts of the second order start to form a ventral longitudinal fasciculus by the time the medial longitudinal fasciculus reaches the trigeminal region. If for no other reason, the presence of a ventral longitudinal fasciculus should be indicated in the rat by the fact that the number of longitudinal fibers in the rostral trigeminal region becomes greater than at the trochlear level; fibers are being added from a source below the trochlear level. The ventral longitudinal fasciculus is both crossed and uncrossed in the trigeminal region.


In the younger embryos, both tracts are absent in the facial region, but at the level of the glossopharyngeal and vagus nuclei the ventral longitudinal fasciculus reappears. This tract appears to be formed entirely by secondary neurons whose cell bodies are located at the junction of the alar and basal plates in the lower rhombencephalon. It is predominantly crossed and ascending at this level.


We can be reasonably certain that the ventral longitudinal fasciculus does not represent the vestibular part of the fasciculus longitudinalis medialis, because both primary and secondary vestibular neurons are undeveloped in embryos with well formed ventral longitudinal fasciculi. We suggest that this tract is a compound secondary afferent pathway for trigeminal, glossopharyngeal and vagus nerves (and later, the facial).


We cannot be certain that the ventral longitudinal fascicu— lus contains no caudally directed fibers. Our conclusion that it is predominantly ascending is based partly on relationships like those illustrated in figure 1, i.e., on enumeration of component fibers in the tract and in the commissure at different levels. Furthermore, no caudally directed growth cones were observed in the Ventral longitudinal fasciculus of the lower rhombencephalon although they were present in the medial longitudinal fasciculus at higher levels. Rostrally directed growth cones were seen in the ventral longitudinal fasciculus. In an earlier study it was reported that the first ascending fibers reach the anlage of the thalamus in the 8.0 mm. cat embryo (Windle, ’35). The ventral longitudinal fasciculus appears to be the source of many of these fibers.


These observations do not support the claim that in the rat the medial longitudinal fasciculus and the ventral longitudinal fasciculus descend throughout the nervous system before any ascending secondary elements are contributed (Angulo, ’39). Such a conclusion cannot be reached in objective analysis of adequately stained histologic preparations of normal embryonic material. Angulo (’39) has assigned to the longitudinal fasciculi a function of descending integration, providing an anatomical basis for the theory that mammalian embryos exhibit a “total reaction” motor pattern comparable with that found by Coghill (’29) in Amblystoma. Our present study demonstrates that ascending secondary neurons precede the development of a descending integrating mechanism in cat and human embryos. Differentiation of secondary elements of the lower rhombencephalon takes place relatively earlier in the cat than in the rat, but in neither species is there any doubt regarding the great preponderance of ascending over descending neurons in the embryonic brainstem. We concede that the medial longitudinal fasciculus may ultimately function in motor integration, but it is fallacious to hold that the entire system of ventral longitudinal neurons constitute a great descending pathway in young mammalian embryos.


Our observations do not substantiate the claims that: “During the early stages of genesis the fibers that enter this (ventral) commissure have but one source of origin: the ventral longitudinal path. Moreover this commissure has a sequential order of appearance, beginning in the midbrain and progressively appearing at the more caudal levels with the increase in age of the fetus” (Angulo, ’39). We find that in the rat the ventral commissure begins to form in the trigeminal region (where secondary neuroblasts are already present and quite well advanced) before a ventral commissure appears at the oculomotor and trochlear levels (where the medial longitudinal fasciculus is the only tract present). This is even more striking in the cat where formation of the ventral commissure takes place in areas far removed from the caudal limit of the medial longitudinal fasciculus; in these regions, the commissure develops by growth of secondary axons across the floor plate in the absence of any longitudinal tract.


The diencephalic component of the medial longitudinal fasciculus can be said to be the first tract of nerve fibers in the embryonic brain. However, it does not spring from the first group of interneurons to develop. Before there is any differentiation in the nucleus of the medial longitudinal fasciculus, secondary neuroblasts of the lower rhombencephalon (glossopharyngeal, vagus and accessory region) are sending their axons toward the floor plate. These axons form the /ventral longitudinal fasciculus in this region by crossing the floor plate and turning rostrad only after the first few descending fibers of the medial longitudinal fasciculus appear.

Summary

The early development of the medial longitudinal fasciculus and associated neurons was studied in pyridine silver stained rat, cat and human embryos. The first component of the medial longitudinal fasciculus arises from a nucleus at the region of junction of mesencephalon and diencephalon at approximately 272 hours after insemination in the rat (about 24 somites) , 16 days after insemination in the cat (about 10-13 somites), and between 26 and 30 days (fertilization age) in man. This is the only component of the medial longitudinal fasciculus present in the brains of embryos less than 5 mm. or 6 mm. total length (measured before fixation). It descends homolaterally as far as the trigeminal region without forming a commissure in the specimens studied.


A ventral longitudinal fasciculus, predominantly ascending, develops in the rhombencephalon and comes to overlap (its fibers often mingling with) the diencephalic component of the medial longitudinal fasciculus. This tract arises from secondary neuroblasts at the junction of alar and basal plates in the region of the trigeminal, glossopharyngeal, vagus and accessory nerves. It is deficient in the facial and vestibular regions at first. The portion arising from the lower rhomben— cephalon is older than that arising in the trigeminal region of the cat embryo. The converse appears to be true of the rat embryo. The rhombencephalic component is predominantly a crossed ascending secondary pathway. The portion arising in the trigeminal region has both homolateral and contralateral origins and it courses rostrad.


The ventral commissure is formed mainly by secondary neurons of the rhombencephalon. A few primary motor neurons of thetfacial nerve cross, but no fibers of the diencephalic component of the medial longitudinal fasciculus enter the floor plate.


The ventral longitudinal fasciculus of embryos less than 5 mm. or 6 mm. long is not the primordium of the vestibular portion of the medial longitudinal fasciculus. It may be a compound secondary afferent pathway (partly visceral lemniscus?) which will link afferent neurons of the trigeminal, glossopharyngeal and vagus nerves (later also the facial) with diencephalic centers.


Literature Cited

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Plates

PLATE 1 EXPLANATION or FIGURES

Camera lucida drawings of details of brain structures in pyridine silver stained cat and rat embryos. The rectangles in the small figures (a) indicate the areas drawn iii the large figures. Magnification for figures 5, 6, 8, 9, 10, 11, 12: X 290; for figures 7, 13: X 515.

5 Cat 846: 2; glossopharyngeal level. On the right, the axons of two lightly stained interneurons are directed ventrally through the motor nucleus. All other structures are parts of motor neurons. No ventral tract and no ventral commissure are present.

6 Cat 841; spinal cord. Two spinal motor neuroblasts are present. Due to tl1e absence of fibers of the accessory nerve at this level the course of secondary neurons is unobscured. The one fiber seen in the floor pflate is a commissural secondary axon. N0 ventral longitudinal fasciculus is present.

7 Cat 841; vagus—accessory level. The fibers in the floor plate are axons of secondary neuroblasts. Fibers seen in cross section lateral to the three motor neuroblasts are not forming a ventral longitudinal tract but are diagonally cut motor axons which were traced to the intramedullary motor root of the vagus in other sections.

8 Cat 845: 2; rostral trigeminal level. No ventral commissure is present. The darker medial bundle of fibers (upper) is the medial longitudinal faseieulus. Lateral to and below it, lighter longitudinal fibers represent the rostral part of the ventral longitudinal faseiculus. One trigeminal motor neuroblast is present and parts of another motor axon appear at right angles to the longitudinal tract fibers.

9 Rat 633: 1; trigeminal region. A shrinkage fold in the basal plate has scattered the elements in such a. way that homolateral secondary fibers can be seen entering the ventral longitudinal fascieulus from the region of the motor nucleus. The ventral commissure is just beginning to form. Two rostrally directed growth cones appear in the ventral longitudinal fasciculus.

10 Cat 845: 4; trigeminal level. The ventral longitudinal fasciculus is sectioned diagonally and contains one rostrally directed growth cone. Little of the motor nucleus appears.

11 Cat 845: 2; section through the oeulomotor region at the cephalic flexure. The large neuroblasts at the upper right represent the caudal part of the nucleus of the medial longitudinal fasciculus. The medial longitudinal faseiculus courses caudad near the external limiting membrane. Many of its fibers terminate in growth cones at this level. Figure 19 is a photomicrograph of the same region.

12 Cat 845: 2; middle trigeminal level. Fibers from the small ventral commissure are entering the ventral longitudinal fasciculus. The medial longitudinal faseiculus has ended above this level. Compare with figure 8.

13 Cat 845: 3; vagus level. Ventrally directed growth cones of secondary neurons appear dorsal and ventral to, a11d within the motor nucleus. One dorsally directed growth cone can be seen on a fiber approaching the ventral longitudinal fasciculus of this side.


185 PLATE 2 EXPLANATION or FIGURES

Photoiniorographs (unretouched) from the brains of cat and human eml>r_Vos stained by the pyridineesilver method.

14 Cat 841. Magnification X380. Trigeminnl level. Neither a ventral commissure nor a. longitudinal tract is present. Parts of two motor neuroblasts and a small i11tr:1n1edullary motor root can be seen near the external limiting membrane.

15 Cat 841. Magnification X 380. Lower vagus level. The differentiation at the vagus level is more advanced than in the trigeminal region. No longitudinal tract is present but one fiber appears in the floor plate.

16 Human embryo 828. Magnification X 160. Trochlear level. The medial longitudinal faseieulus is located at the junction of the floor plate and basal plate. No ventral comrnissure is present. The axons of trochlear neuroblasts have reached the dorsal part of the neural tube where they spread out.

17 Human embryo 828. Magnification X 164. Facial level. Motor axons from the facial nucleus are emerging at the sulcus limitans and entering the region of the genieulate ganglion. The ventral eommissure is composed of darkly stained facial motor axons. Near the suleus limitans, some secondary neuroblasts appear. The ventral longitudinal fascieulus consists of few scattered fine fibers.

Photomicrographs (unretouched) from the brains of cat and human embryos stained by the pyridine silver method.

18 Human eml)1'y0 828. Magnification X 164. Hypoglossal region. The vago~ accessory nucleus lies medial to the hypoglossal nucleus. The ventral commissuro, composed of fine secondary nerve fibers, is larger than at the facial level. The ventral longitudinal fascieulus is a. larger and more compact bundle.

19 Cat 845: 2. Magnification X 430. Oeulomotor level (see fig. 11). Near the external limiting membrane, the medial longitudinal fasciculus courses caudad (toward the top of the picture) and crosses oculomotor fibers almost at right angles. In the upper left of the picture, the fibers lying perpendicular to the periphery are medial longitudinal fasciculus axons which have passed through the oculomotor nucleus 21 section or two more rostrally.

20 (fat 845: 3. Trigeminal level. Compared with figure 14, the differentiation of motor elements is greatly advanced, a small ventral eommissure of secondary neurons is present; the ventral longitudinal fasciculus has appeared. X 380.

21 Cat 845: 3. Magnification X 380. Facial level. The facial nucleus shows its characteristic striations. The ventral longitudinal fasciculus is absent (stippled region lateral to the right motor nucleus is not the fasciculus). The ventral eommissure is composed of thick dark axons of facial motor neuroblasts; some motor neuroblusts are situated in the floor plate.

22 Cat 845:3. Magnification X 380. Vagus level. The ventral longitudinal fascieulus has reappeared and the ventral commissure is larger than it is at the trigeminal level. Compare with figures 15 and 20.


ABBREVIATIONS

<=om., ventral secondary commissure X., vagus motor nucleus gen., geniculatc ganglion X'l'I., hypoglossal nucleus 111.0,, primary motor eominissure IVr., intramedullary root of trochloar m.l.f., medial longitudinal fasciculus nerve r.f., reticular formation Vr., intramedullary motor root of tri. v.l.f., ventral longitudinal fasciculus geminal nerve III., oculomotor nucleus VHr., intramedullary motor root of V., trigeminal motor nucleus facial nerve

VIT., facial motor nucleus


Cite this page: Hill, M.A. (2019, November 14) Embryology Paper - The early development of the fasciculus longitudinalis medialis and associated secondary neurons in the rat, cat and man. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_The_early_development_of_the_fasciculus_longitudinalis_medialis_and_associated_secondary_neurons_in_the_rat,_cat_and_man

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