Sulcus Limitans Research Articles

Regionalized expression of the Dbx family homeo☐ genes in the embryonic CNS of the mouse

Here we report the identification of a novel homeo☐ gene family Dbx in mouse, which consists of Dbx and Dbx2. The two genes share similar structural organization and are encoded by different chromosomes. The predicted Dbx and Dbx2 proteins share 85% identity in their homeodomain amino acid sequences, but otherwise showed no significant similarity. Characterization of the expression of these two genes in the embryos suggested their role in the development of the CNS. In the forebrain, Dbx is expressed in various regions, while Dbx2 showed a more restricted pattern of expression. In the midbrain, the expression domains of Dbx and Dbx2 overlap along the dorso-lateral wall of the ventricle. In the hindbrain and spinal cord, both genes are expressed in the boundary separating the basal and alar plates, which seems to correspond to the sulcus limitans. Expression of the Dbx/Dbx2 genes is restricted to the ventricular region of the embryonic CNS except for that of Dbx in the septum of the telencephalon. Together these observations indicate possible participation of the members of the Dbx family in regionalization of the CNS. While the expression of Dbx was restricted to the CNS, Dbx2 was also expressed in some of the mesenchymal cells, such as limb buds and tooth germs.

Development and cell differentiation of the vestibulocochlear nuclei in cattle

Based upon light- and electron microscopic examinations (50 embryos ranging from 1 to 53 cm crown-rump-length, CRL) the origin of the nuclei of the cranial nerve VIII is described with special regard to neurogenesis. The ventricular matrix lateral to the sulcus limitans represents the alar plate with its sensory areas. Up to 2.7 cm CRL migrating neurons from the vestibular nuclei can be detected, the bigger neuronal elements of which are the early formed lateral vestibular nucleus. From 6.7 cm CRL onward all nuclear groups of the vestibular nerve can be identified. At 1 cm CRL the recess plate represents the primordium of the cochlear nuclear complex. Identification of the definitive nuclei is possible at 3.8 cm CRL. Subsequently from 7.6 cm CRL onward the process of lamination can be observed in the dorsal cochlear nucleus. Due to the proceeding maturation the nuclei of the cranial nerve VIII correspond at 53 cm CRL topographically and cytologically to the characteristics of adult animals. Electron microscopic examinations are documenting characteristic features of cytogenesis of sensory neurons (vestibular nucleus) during early embryonic stages (2.5 cm and 3.6 cm CRL). At 2.5 cm CRL the elements exhibiting features of migrating neurons are predominating, whereas at 3.6 cm CRL an increased differentiation is typical for neurons localized in their ultimate position. At this stage synapses can be identified for the first time.

Topological analysis of the brainstem of the bowfin, Amia calva

This paper presents a survey of the cell masses in the brainstem of the generalized actinopterygian fish Amia calva, based on transversely cut Nissl-, Klüver-Barrera-, and Bodian-stained serial sections. This study is intended to serve a double purpose. First it forms part of a now almost complete series of publications on the structure of the brainstem in representative species of all groups of vertebrates. Within the framework of this comparative program the cell masses in the brainstem and their positional relations are analyzed in the light of the Herrick-Johnston concept; according to this the brainstem nuclei are arranged in four longitudinal, functional zones or columns, the boundaries of which are marked by ventricular sulci. The procedure employed in this analysis essentially involves two steps: first, the cell masses and large individual cells are projected upon the ventricular surface, and next, the ventricular surface is flattened out, that is, subjected to a one-to-one continuous topological transformation (Nieuwenhuys [1974] J. Comp. Neurol. 156:255-267). The second purpose of the present paper is to provide a cytoarchitectonic basis for experimental analysis of the fiber connectivity in the brainstem of Amia. Five longitudinal sulci--the sulcus medianus inferior, the sulcus intermedius ventralis, the sulcus limitans, the sulcus intermedius dorsalis, and the sulcus lateralis mesencephali--could be distinguished. Some shorter grooves, present in the isthmal region, clearly deviate from the overall longitudinal pattern of the other sulci. Although in Amia most neuronal perikarya are contained within a diffuse periventricular gray, 40 cell masses could be delineated: Eight of these are primary efferent or motor nuclei, 10 are primary afferent or sensory centers, seven are considered to be components of the reticular formation, and the remaining 15 may be interpreted as "relay" nuclei. The topological analysis yielded the following results. In the rhombencephalon the gray matter is arranged in four longitudinal columns or areas, termed area ventralis, area intermedioventralis, area intermediodorsalis, and area dorsalis. The sulcus intermedius ventralis, the sulcus limitans, and the sulcus intermedius dorsalis mark the boundaries between these morphological entities. These longitudinal areas coincide largely, but not entirely, with the functional columns of Herrick and Johnston. The most obvious incongruity is that the area intermediodorsalis contains, in addition to the viscerosensory nucleus of the solitary tract, several general somatosensory and special somatosensory nuclei.(ABSTRACT TRUNCATED AT 400 WORDS)

XASH-3, a novel Xenopus achaete-scute homolog, provides an early marker of planar neural induction and position along the mediolateral axis of the neural plate

We have isolated a novel Xenopus homolog of the Drosophila achaete-scute genes, called XASH-3. XASH-3 expression is neural specific and is detected as early as stage 11 1/2, making it one of the earliest markers of neural induction so far described. Moreover, XASH-3 expression within the neural plate is regionally restricted. Transverse bands of XASH-3 mRNA mark discrete positions along the anteroposterior axis, while longitudinal bands mark a discrete position along the mediolateral axis. This latter site of XASH-3 expression appears to demarcate the prospective sulcus limitans, a boundary zone that later separates the functionally distinct dorsal (alar) and ventral (basal) regions of the spinal cord. In sandwich explants lacking any underlying mesoderm, XASH-3 is expressed in longitudinal stripes located lateral to the midline. This provides the first indication that planar or midline-derived inductive signals are sufficient to establish at least some aspects of positional identity along the mediolateral axis of the neural plate. By contrast, the transverse stripes of XASH-3 expression are not detected, suggesting that this aspect of anteroposterior neural pattern is lost or delayed in the absence of vertically passed signals. The restricted mediolateral expression of XASH-3 suggests that mediolateral patterning of the neural plate is an early event, and that this regionalization can be achieved in the absence of inducing signals derived from underlying mesoderm.

Spatially and temporally restricted expression of Pax2 during murine neurogenesis

The expression of the murine paired-box-containing gene, Pax2, is examined in the developing central nervous system by in situ hybridization. Pax2 expression is detected along the boundaries of primary divisions of the neural tube. Initially, Pax2 is expressed in the ventricular zone in two compartments of cells on either side of the sulcus limitans and along the entire rhombencephalon and spinal cord. At later times, Pax2 is restricted to progeny cells that have migrated to specific regions of the intermediate zone. In the eye, Pax2 expression is restricted to the ventral half of the optic cup and stalk and later to the optic disc and nerve. In the ear, expression is restricted to regions of the otic vesicle that form neuronal components. The transient and restricted nature of Pax2 expression suggests that this murine segmentation gene homologue may also establish compartmental boundaries and contribute to the specification of neuronal identity, as do certain Drosophila segmentation genes.

A SEM study on the development of the ventricular surface morphology in the diencephalon of the rat.

The morphogenesis of the ventricular surface of the diencephalon of the rat was studied using scanning electron microscopy, cryostat serial sections and direct observations under a dissection microscope. Based on these observations a description is given of the neuromeres present within the prosencephalon and of the termination of the sulcus limitans. Two conclusions are reached. First, three neuromeres are present in the prosencephalon. Neuromere I consists of the telencephalon, the hypothalamic regions and the parencephalon anterius. Neuromere II is the parencephalon posterius, neuromere III the synencephalon. Second, the sulcus limitans terminates ventrally in the parencephalon posterius and does not continue towards the preoptic recess. No exact termination point of the sulcus limitans could be delineated.

The development of the human brain, including the longitudinal zoning in the diencephalon at stage 15.

Twenty-six embryos (6-11 mm) of stage 15 (approximately 33 days) were studied in detail and graphic reconstructions of three of them were prepared. Characteristic features of this stage include closed lens vesicles, presence of nasal pits, and retinal pigment. The neuromeric pattern is still visible. Each cerebral hemisphere is limited by the torus hemisphericus internally and by the di-telencephalic sulcus externally. The medial (diencephalic) eminence of the basal nuclei (previously misinterpreted by others as the lateral) had appeared in stage 14, and the lateral eminence, which is telencephalic, is now distinguishable. The amygdaloid body in stages 14 and 15 is derived from the medial eminence. The hippocampal thickening is identifiable in the dorsomedial part of the cerebral hemisphere. Medial and basal forebrain bundles are developing. The olfactory eminence is visible. Future olfactory bulb and tubercle possess an intermediate layer. The wall of the diencephalon presents five longitudinal zones: epithalamus, dorsal thalamus, ventral thalamus, subthalamus, and hypothalamus. The primordium of the epiphysis cerebri is beginning in the more advanced embryos. The sulcus limitans ends rostrally at the midbrain (M1) and is not continuous with the hypothalamic sulcus. Hence the alar/basal distinction does not arise in the forebrain. In the roof of the midbrain (M2) the mesencephalic evagination already noticed at stage 14 is characteristic. It is suggested that it may function as a temporary circumventricular organ. The precursors of some new tracts are identifiable: habenulo-interpeduncular, medial tectobulbar, and mamillotegmental fibres. Commissures include the supramamillary, that of the superior colliculi, and (in some embryos) the first fibres of the posterior commissure. Nuclei include the habenular, mamillary, and probably subthalamic. The cerebellum, the beginning of which was already noted at stages 13 and 14, consists of (1) a rostral part that arises from the alar plate of the isthmic segment and will form the superior medullary velum and part of the corpus cerebelli; and (2) a caudal part that develops from rhombomere 1. The involvement of the isthmic segment, first elucidated with stage 14, has not been observed in previous reports. All cranial nerves except the olfactory and optic are present in the more advanced embryos.

Central distribution of the efferent cells and the primary afferent fibers of the trigeminal nerve in Pleurodeles waltlii (Amphibia, Urodela).

As part of a study on the organization of the brainstem in a primitive group of vertebrates, the efferent cells and primary afferent fibers of the urodele amphibian Pleurodeles waltlii were examined by means of retrograde and anterograde axonal transport and anterograde degeneration. The trigeminal motor nucleus is located in the periventricular gray just medial to the sulcus limitans. Its rostral part is a band of pear-shaped cells lying parallel to the wall of the ventricle, whereas its caudal part is a round mass consisting of polygonal cells. In addition, a small group of scattered neurons is situated ventral to the rostral part of the nucleus. The primary afferent fibers enter the brainstem in the dorsal two-thirds of the trigeminal root. They diverge into a short ascending and a long descending tract. The former distributes its axons to the principal sensory trigeminal nucleus, which is an ill-defined cell group located at the ventrolateral edge of the periventricular gray. In the descending tract, the fibers of the ophthalmic nerve are predominantly located ventromedially, and those of the maxillomandibular nerve dorsolaterally. A fascicle of the ophthalmic nerve leaves the descending tract and, apparently, makes contact with the accessory abducens nucleus. The descending tract extends caudally into the three upper cervical segments of the spinal cord. The mesencephalic trigeminal nucleus consists of conspicuous large cells, which are scattered through the tectum of the mesencephalon. The cells with peripheral branches in the ophthalmic nerve are mainly located in the caudal half of the tectum, and those with peripheral branches in the maxillomandibular nerve in the rostral half. Collaterals of the central branches of the mesencephalic trigeminal system were traced to an area of the periventricular gray situated between the motor nucleus and the principal sensory nucleus of the trigeminus.

The development of the human brain from a closed neural tube at stage 13

Twenty-five embryos of stage 13 (28 days) were studied in detail and graphic reconstructions of seven of them were prepared. Thirty or more somitic pairs are present, and the maximum is possibly 39. The notochord is almost entirely separated from the neural tube and the alimentary epithelium, and its rostral tip is closely related to the adenohypophysial pocket. Caudal to the cloacal membrane, the caudal eminence is the site of secondary neurulation. The eminence, which usually contains isolated somites, is the area where new notochord, hindgut, and neural tube are forming. The neural cord develops into neural tube without the intermediate phase of a neural plate (secondary neurulation). Canalization is regular and the lumen is continuous with the central canal. The neural tube is now a closed system, filled with what may be termed "ependymal fluid." The brain is widening in a dorsoventral direction. Neuromeres are still detectable. The following features are distinguishable: infundibular area of D2, chiasmatic plate of D1, "adult" lamina terminalis, and commissural plate (at levels of nasal plates). The beginning of the synencephalon of D2 can be discerned. The retinal and lens discs are being defined. The mesencephalic flexure continues to diminish. The midbrain possesses a sulcus limitans, and the tegmentum may show the medial longitudinal fasciculus. The isthmic segment is clearly separated from rhombomere 1. Lateral and ventral longitudinal fasciculi are usually present in the hindbrain, and the common afferent tract is beginning. Somatic and visceral efferent fibres are seen in certain nerves: 6, 12; 5, 7, 9-11. The first indication of the cerebellum may be visible in the alar lamina of rhombomere 1. The terminal-vomeronasal crest appears. Various cranial ganglia (e.g., vestibular, superior ganglia of 9, 10) are forming. The trigeminal ganglion may show its three major divisions. Epipharyngeal placodes of pharyngeal arches 2 to 5 contribute to cranial ganglia 7, 9, and 10. The spinal neural crest is becoming segregated, and the spinal ganglia are in series with the somites. Ventral spinal roots are beginning to develop.

The development of the human brain, the closure of the caudal neuropore, and the beginning of secondary neurulation at stage 12.

Twenty-four embryos of stage 12 (26 days) were studied in detail and graphic reconstructions of five of them were prepared. The characteristic features of this stage are 21-29 pairs of somites, incipient or complete closure of the caudal neuropore, and the appearance of upper limb buds. The caudal neuropore closes during stage 12, generally when 25 somitic pairs are present. The site of final closure is at the level of future somite 31, which corresponds to the second sacral vertebral level. Non-closure of the neuropore may be important in the genesis of spina bifida aperta at low levels. The primitive streak probably persists until the caudal neuropore closes, when it is replaced by the caudal eminence or end-bud (Endwulst oder Rumpfknospe). The caudal eminence, which appears at stage 9, gives rise inter alia to hindgut, notochord, caudal somites, and the neural cord. The material for somites 30-34 (which appear in stage 13) is laid down during stage 12, and its absence would be expected to result in sacral agenesis. Aplasia of the caudal eminence results in cloacal deficiency and various degrees of symmelia. The junction of primary and secondary development (primäre und sekundäre Körperentwicklung) is probably at the site of final closure of the caudal neuropore. Secondary neurulation begins during stage 12. The cavity of the already formed spinal cord extends into the neural cord, and isolated spaces are not found within the neural cord. Primary and secondary neurulation are probably coextensive with primary and secondary development of the body, respectively. The telencephalon medium has enlarged, two mesencephalic segments (M1 and M2) are distinguishable, and rhombomere 4 is reduced. The sulcus limitans is detectable in the spinal cord and hindbrain (RhD), and in the mesencephalon and diencephalon, where it extends as far rostrally as the optic sulcus in D1. A marginal layer is appearing in the rhombencephalon and mesencephalon. The first nerve fibres are differentiating, chiefly within the hindbrain (from the nucleus of the lateral longitudinal tract). Optic neural crest is at its maximum, and the otic vesicle is giving crest cells to ganglion 7/8. Neural crest continues to develop in the brain and contributes to cranial ganglia 5, 7/8, and 10/11. The spinal crest extends as far caudally as somites 18-19 but shows no subdivision into ganglia yet. Placodal contribution to the trigeminal ganglion is not certain at stage 12. Such a contribution to ganglion 7/8 is not unlikely.(ABSTRACT TRUNCATED AT 400 WORDS)

Fourth ventricular floor in human embryos: scanning electron microscopic observations.

The ultrastructural surface features of the normal fourth ventricular floor of seven human embryos ranging from Carnegie stage 14 to stage 19 (crown-rump length: 7.6-16.2 mm) were examined by using scanning electron microscopy (SEM). Low-power SEM views showed the median sulcus, sulcus limitans, and neuromeres, transient structures characteristic of the earlier embryonic period. High-power SEM observation revealed supraependymal cells (SE cells) and supraependymal fibers (SE fibers) which exhibited a characteristic localization, as well as generalized surface-membrane modifications such as microvilli and cilia. SE cells could be classified into two major groups. The type 1 SE cells seem to possess neuronal functions, as deduced from morphological similarities to their counterparts in adults and the specialized distribution closely related to neuromeres. The type 2 SE cell morphologically resembled the phagocytic SE cell described in related literature. SE fibers ran a course either rostrocaudally in the median sulcus or mediolaterally on the neuromeres, most frequently near the interneuromeric cleft; they made contact with type 1 SE cells and ependymal surface modifications and then penetrated the ependymal layer.

Ependymal foldings and other related ependymal structures in the cerebral aqueduct and fourth ventricle of man.

The ependymal lining of the cerebral aqueduct and fourth ventricle of 100 normal humans was studied with the light microscope. Ependymal foldings with normal morphology and a constant distribution pattern were detected in all. The most common sites were the median sulcus and sulcus limitans in the fourth ventricle, and the ventral and lateral walls in the cerebral aqueduct. Rows, islands and rosettes of ependymal cells embedded in normal subependyma were present in 25/82 adults (30%) and in 3/18 children (16%) in a similar distribution pattern as that of the ependymal foldings. We illustrate these normal structures which probably result from fusion between the walls of the ependymal foldings and distinguish them from granular ependymitis and postmortem artifact.

The development and migration of large multipolar neurons into the cochlear nucleus of the North American opossum.

We have studied the maturation of the inferior colliculus and cochlear nuclei of the North American opossum with particular emphasis on the large multipolar neurons of the cochlear nucleus. These neurons include the principal and giant cells of the dorsal cochlear nucleus (DCN) and the large neurons of the ventral cochlear nucleus (VCN), all of which can be labelled by horseradish peroxidase (HRP) injections into the contralateral inferior colliculus (IC). The size of these neurons, their characteristic Nissl patterns, and their labelling density after injections into the IC render them distinguishable from other neurons in this nuclei, even in young animals. In Nissl-stained sections of newborn opossums, a band of horizontally oriented neurons can be identified dorsomedial to the vestibular nerve root. This band extends from an apparent cytogenetic zone close to the sulcus limitans, to, but not within, the presumptive cochlear nucleus. Between birth and estimated postnatal day 22 (EPND 22) the band shifts laterally, eventually becoming incorporated into the cochlear nucleus. Many neurons in this band have perinuclear caps of Nissl substance similar to those present in the principal cells of the adult DCN. Injections of HRP into the IC as early as EPND 5 (17 days after conception) labelled neurons in the band referred to above but not in the presumptive cochlear nucleus. By EPND 15, labelled cells were clustered mainly within the nucleus proper. Most of these cells were located in the DCN, but a few were scattered in the dorsocentral VCN. Consistent labelling of small neurons in VCN was not obtained until sometime later. From EPND 15 to EPND 20 most of the labelled cells in DCN reoriented in the vertical plane, aligned in layer II, and differentiated into principal neurons. Some, however, remained deep to layer II and differentiated into giant neurons. The heavily labelled cells in VCN differentiated into large neurons. Our results suggest that the large multipolar neurons of the nucleus are generated in a cytogenetic zone medial to the rhombic lip and that they subsequently migrate laterally, in a band, to reach their adult locations in the nucleus. It is during this migratory stage that their axons reach the inferior colliculus.

Respiratory pattern generation in adult lampreys (Lampetra fluviatilis): interneurons and burst resetting.

The central pattern generator for the respiratory rhythm in adult lampreys was studied in isolated brain preparations. At least three different types of respiratory units were found in intra- and extracellular recordings near the trigeminal nucleus (Fig. 1), including: units that start firing before the motorneurons, recorded from the ventral surface (Figs. 2 and 3), follower cells near rostral nucleus V (Fig. 4), units bursting after the motorneurons, found in the sulcus limitans (Figs. 5 and 6). Transections of the medulla indicated that the rostral half of nucleus V could be removed without reducing the respiratory frequency (Fig. 8). The earliest respiratory events that could be recorded from the ependymal surface or the cranial nerves were observed near nuclei IX-VII-caudal V during quiet breathing. There was a rostrocaudal delay in the onset of bursts of more caudal motorneurons (Figs. 9, 10, and 12). The rostrocaudal delay became reversed, such that bursts started earlier in n.X than n.IX, during episodes of intense breathing that were accompanied by prolonged discharges in the medial reticular formation (Fig. 11). Stimulation of the medulla surface, near the base of nerve V, could trigger bursts prematurely and reset the timing of the respiratory rhythm, yielding a discontinuous phase response curve (Fig. 14). In sum, 6 types of respiratory interneurons can presently be distinguished (Fig. 15). The sulcus limitans near nucleus V is a candidate location for components of the pattern generator.

Central projections of vestibular afferents from the horizontal semicircular canal in the carpet shark Cephaloscyllium isabella.

This study utilizes anterograde axonal transport of cobaltous-lysine and conventional silver-staining techniques to study the central projections of the horizontal semicircular canal branch of the VIII nerve within the vestibular nuclei of the carpet shark Cephaloscyllium isabella. Two major terminating axon fields were observed, one caudal and one rostral to the entrance of the VIII nerve, corresponding to the ventral vestibular nucleus and superior vestibular nucleus, respectively. Both fields appear to be located within the ventral portion of the nuclei indicating an apparent subdivision of the VIII nerve projections within the brainstem. The resolution of the sensitive cobalt tracer indicates the presence of both dendritic and pericellular termination of these primary afferent fibres. In the area immediately caudal to the entrance of the VIII nerve a number of labelled primary afferent fibres project to the ventral region of the intermediate nucleus. Other fibres follow the visceral sensory root VII and terminate proximal to the sulcus limitans of His within the dendritic field of the neurons of the nucleus magnocellularis. Some fibres turn ventromedially from the main group of the ascending fibres and terminate in the area of the inferior reticular formation.

Topological analysis of the brainstem of the reedfish, Erpetoichthys calabaricus.

AbstractThe ventricular sulcal pattern and the cellular structure of the brainstem of the reedfish Erpetoichthys calabaricus were studied in transversely cut Nissl‐, Klüver‐Barrera‐, and Bodian‐stained serial sections. Six longitudinal sulci, the sulcus medianus inferior, the sulcus intermedius ventralis, the sulcus limitans, the sulcus intermedius dorsalis, the sulcus medianus superior, and the sulcus lateralis mesencephali could be distinguished. A seventh groove, the sulcus isthmi, clearly deviates from the overall longitudinal pattern of the other sulci. Although in Erpetoichthys most neuronal perikarya are contained within a diffuse periventricular gray, 32 cell masses could be delineated; six of these are primary efferent or motor nuclei, seven are primary afferent or sensory centers, nine are considered to be components of the reticular formation, and the remaining ten may be interpreted as “relay” nuclei.In order to study the zonal pattern of the brainstem, this structure was subjected to a topological analysis (cf. Nieuwenhuys, '74 and Fig. 15) which yielded the following result. In the rhombencephalon the gray matter is arranged in four longitudinal columns or areas termed area ventralis, area intermedioventralis, area intermediodorsalis, and area dorsalis. In many places the sulcus intermedius ventralis, the sulcus limitans, and the sulcus intermedius dorsalis mark the boundaries between these morphological entities. These longitudinal areas coincide largely, but not entirely, with the so‐called functional columns of Herrick and Johnston. The most obvious incongruity is that the area intermediodorsalis contains, in addition to the nucleus of the solitary tract, three nonviscerosensory cell masses, namely, the nucleus vestibularis magnocellularis and parvocellularis and the nucleus of the tractus descendens of V. The four longitudinal zones cannot be distinguished in the mesencephalon nor can the sulcus limitans be recognized here. Functionally, however, the medial part of the tegmentum mesencephali may be considered the rostral extreme of the somatomotor column, whereas the remainder of the midbrain contains a number of somatosensory centers.

The cell masses in the brainstem of the South African clawed frog Xenopus laevis: a topographical and topological analysis.

AbstractThe ventricular sulcal pattern and the cytoarchitecture of the brainstem of Xenopus laevis, a pipid frog which retains a lateral line system throughout life, were studied in transverse Nissl‐ and Klüver‐Barrera‐stained serial sections. Four distinct longitudinal sulci, the sulcus medianus inferior, the sulcus intermedius ventralis, the sulcus limitans, and the sulcus medianus superior, could be distinguished. With the aid of the usual cytoarchitectonic criteria 42 cell groups were delineated; seven of these are primary efferent or motor nuclei, 13 are primary efferent or sensory centers, seven nuclei are considered to be components of the reticular formation, and the remaining 15 cell masses can be indicated as “relay” nuclei. In order to provide a basis for experimental work, the topographical position of the nuclei is illustrated in photomicrographs of representative levels and in graphical reconstructions.The distribution of the cell masses and their relations to the ventricular sulci were studied with the aid of the reconstruction procedure termed topological analysis (cf. Nieuwenhuys, '74; Fig. 19). This analysis yielded the following results. The sulcus limitans extends throughout almost the entire brainstem, dividing this part of the brain into a motor basal plate and a sensory alar plate. The cell masses in the rhombencephalic basal plate fit into two longitudinal zones, a medial area ventralis and a lateral area intermedioventralis. The former contains somatomotor centers of primary and higher order, whereas the latter is composed of three primary visceromotor nuclei and one visceromotor coordinating center. The rhombencephalic alar plate is occupied by viscerosensory, general somatosensory, and special somatosensory cell masses. Two centers, the nucleus fasciculi solitarii and the nucleus visceralis secundarius, represent together a discontinuous viscerosensory zone, which is situated immediately dorsal to the sulcus limitans. The general somatosensory nuclei, i.e., the gracile and cuneate nuclei, the nucleus tractus descendens of V, and the nucleus princeps of V constitute a zone which largely overlaps the viscerosensory zone. The special somatosensory area, i.e., the area of termination of (1) primary vestibular, (2) primary acoustic, and (3) lateral line nerve fibers, is strongly developed and occupies a considerable part of the alar plate. Presumably, the fibers of each of the three categories mentioned terminate in three separate zones of gray. The midbrain can also be subdivided bilaterally into a basal plate and an alar plate. The former occupies the medial part of the tegmentum mesencephali and may be considered as the rostral extreme of the somatomotor zone. The latter comprises the lateral tegmentum and the tectum, areas which are chiefly occupied by special somatosensory (visual, acoustic, lateral line) centers.

Topological analysis of the brain stem of the lungfish Lepidosiren paradoxa

The ventricular sulcal pattern and the cellular structure of the brain stem of the lungfish Lepidosiren paradoxa have been studied in transversely cut Nissl and Bodian stained sections. Five longitudinal sulci, the sulcus medianus inferior, the sulcus intermedius ventralis, the sulcus limitans, the sulcus intermedius dorsalis and the sulcus medianus superior could be distinguished. In the isthmus region a number of obliquely oriented sulci is present. One of these, designated here as sulcus d, continues as a longitudinal groove into the mesencephalon. In Lepidosiren most neuronal perikarya are contained within a diffuse periventricular gray. However, 24 separate cell masses could be delineated. Six of these are primary efferent nuclei, six are primary afferent center, six nuclei are considered to be components of the reticular formation and the remaining six may be interpreted as "relay" nuclei. The distribution of the cell masses and their relations to the ventricular sulci were studied with the aid of the graphical reconstruction procedure termed tolopogical analysis (cf. Nieuwenhuys, '74, and fig. 11). This analysis yielded the following results. In the rhombencephalon the gray matter is arranged in four longitudinal columns or areas which have been termed the area ventralis, area intermedioventralis, area intermediodorsalis and area dorsalis. In many places the sulcus intermedius ventralis, the sulcus limitans and the sulcus intermedius dorsalis mark the boundaries between these morphological entities. These longitudinal areas coincide largely, but not entirely, with so-called functional columns of Herrick and Johnston. The most obvious incongruity is that the area intermediodorsalis contains, in addition to the nucleus of the solitary tract, two non-visceral sensory cell masses, namely the magnocellular and parvocellular vestibular nuclei. The four longitudinal zones cannot be distinguished in the mesencephalon nor can the sulcus limitans be recognized here. Functionally, however, the medial part of the tegmentum mesencephali may be considered the rostral extreme of the somatic motor column, whereas the remainder of the midbrain contains a number of somatic sensory centers.

Topological analysis of the brain stem of the crossopterygian fish Latimeria chalumnae.

The ventricular sulcal pattern and the cellular structure of the brain stem of the single surviving crossopterygian species Latimeria chalumnae have been studied in transversely cut Nissl, Klüver-Barrera and Bodian stained serial sections. Five longitudinal sulci, the sulcus medianus inferior, the sulcus medianus superior, the sulcus intermedius ventralis, the sulcus limitans and the sulcus intermedius dorsalis could be delimited. The latter three of these sulci are confined to the rhombencephalon. The walls of the mesencephalon also display some longitudinal grooves, but none of these could be traced into continuity with any of the rhombencephalic sulci. Although the neuronal perikarya in many places show a diffuse arrangement, 27 cell masses could be delineated; eight of these are primary efferent nuclei, seven are primary afferent centers, seven nuclei are considered as components of the reticular formation, and the remaining five cell masses may be interpreted as "relay" nuclei. In order to study the zonal pattern of the brain stem, this structure was subjected to a topological analysis (cf., Nieuwenhuys, '74, and fig. 16). This analysis yielded the following results. The sulcus limitans divides the greater part of the rhombencephalon into a basal plate and an alar plate. In the basal plate the sulcus intermedius ventralis marks the boundary between an area ventralis and an area intermedioventralis. The area ventralis contains two somatic motor centers (i.e., the rostral end of the spinal motor column and the nucleus of IV) and most the entire rhombencephalic medial reticular formation. The latter may be primarily considered as a somatic motor coordinating center. The area intermedioventralis contains the visceral motor nuclei of V, VII, IX and X. However, the basal plate also harbours a number of non-motor centers, for example the sensory princeps nucleus of V and the inferior olive. The alar plate is subdivided by the sulcus intermedius dorsalis into an area intermediodorsalis and an area dorsalis. The area intermediolateralis is largely occupied by the common visceral sensory center of VII, IX and X; however, this area also contains a number of somatic sensory cell masses, as e.g. the nucleus descendens of V and the magnocellular vestibular nucleus. The area dorsalis is entirely occupied by two large lateral line centers. The cell masses in the isthmus region do not exhibit a clear-cut morphological pattern. As regards the mensencephalon, the medial part of the tegmentum, which contains a primary somatic motor center (the nucleus of III) and a somatic motor coordination center (the nucleus of the f.l.m.) may be considered a direct rostral continuation of the area ventralis. The remainder of the midbrain contains a number of somatic sensory centers of primary and higher order.

Topological analysis of the brain stem of the axolotl Ambystoma mexicanum.

The ventricular sulcal pattern and the cellular structure of the brain stem of the axolotl Ambystoma mexicanum have been studied in transversely cut Nissl and Bodian stained serial sections. Six longitudinal sulci, the sulcus medianus inferior, the sulcus intermedius ventralis, the sulcus limitans, the sulcus intermedius dorsalis, the sulcus medianus superior and the sulcus lateralis mesencephali could be distinguished. A seventh groove, the sulcus isthmi, clearly deviates from the overall longitudinal pattern of the other sulci. Although most neuronal perikarya are contained within a diffuse periventricular gray, 19 cell masses could be delineated; seven of these are primary efferent or motor nuclei, four are primary afferent or sensory centers, four nuclei are considered as components of the reticular formation, and the remaining four cell masses can be interpreted as "relay" nuclei. In order to study the zonal pattern of the brain stem, this structure was subjected to a topological analysis (cf, Nieuwenhuys, '74 and fig. 13). This analysis yielded the following results. In the rhombencephalon the grisea are arranged in four longitudinal zones which, following Kuhlenbeck, have been termed area ventralis, area intermedioventralis, area intermediodorsalis and area dorsalis. Where present the sulcus intermedius ventralis, the sulcus limitans and the sulcus intermedius dorsalis mark the boundaries between these four morphological entities. The zonal areas in question coincide largely, but not entirely, with the so-called functional columns of Herrick and Johnston. The most obvious incongruity is that the area intermediodorsalis contains, in addition to the nucleus fasciculi solitarii and the nucleus visceralis secundarius, two non-visceral sensory cell masses, namely the nucleus vestibularis magnocellularis and the nucleus cerebelli. The four morphological zones delineated in the rhombencephalon cannot be distinguished in the mesencephalon and it is of particular importance that the sulcus limitans does not extend into this part of the brain. Functionally, however, the medial part of the tegmentum mesencephali may be considered the rostral extreme of the somatic motor column, whereas the tectum primarily represents a somatic sensory correlation area.