Tectal and related target areas of spinal and dorsal column nuclear projections in hedgehog tenrecs

Cholinergic, catecholaminergic, serotonergic, and orexinergic neuronal populations in the brain of the lesser hedgehog tenrec (Echinops telfairi)

The current study provides an analysis of the cholinergic, catecholaminergic, serotonergic, and orexinergic neuronal populations, or nuclei, in the brain of the lesser hedgehog tenrec, as revealed with immunohistochemical techniques. For all four of these neuromodulatory systems, the nuclear organization was very similar to that observed in other Afrotherian species and is broadly similar to that observed in other mammals. The cholinergic system shows the most variation, with the lesser hedgehog tenrec exhibiting palely immunopositive cholinergic neurons in the ventral portion of the lateral septal nucleus, and the possible absence of cholinergic neurons in the parabigeminal nucleus and the medullary tegmental field. The nuclear complement of the catecholaminergic, serotonergic and orexinergic systems showed no specific variances in the lesser hedgehog tenrec when compared to other Afrotherians, or broadly with other mammals. A striking feature of the lesser hedgehog tenrec brain is a significant mesencephalic flexure that is observed in most members of the Tenrecoidea, as well as the closely related Chrysochlorinae (golden moles), but is not present in the greater otter shrew, a species of the Potomogalidae lineage currently incorporated into the Tenrecoidea. In addition, the cholinergic neurons of the ventral portion of the lateral septal nucleus are observed in the golden moles, but not in the greater otter shrew. This indicates that either complex parallel evolution of these features occurred in the Tenrecoidea and Chrysochlorinae lineages, or that the placement of the Potomogalidae within the Tenrecoidea needs to be re-examined.

The Structure of the Spinal Cord Ependymal Region in Adult Humans Is a Distinctive Trait among Mammals

In species that regenerate the injured spinal cord, the ependymal region is a source of new cells and a prominent coordinator of regeneration. In mammals, cells at the ependymal region proliferate in normal conditions and react after injury, but in humans, the central canal is lost in the majority of individuals from early childhood. It is replaced by a structure that does not proliferate after damage and is formed by large accumulations of ependymal cells, strong astrogliosis and perivascular pseudo-rosettes. We inform here of two additional mammals that lose the central canal during their lifetime: the Naked Mole-Rat (NMR, Heterocephalus glaber) and the mutant hyh (hydrocephalus with hop gait) mice. The morphological study of their spinal cords shows that the tissue substituting the central canal is not similar to that found in humans. In both NMR and hyh mice, the central canal is replaced by tissue reminiscent of normal lamina X and may include small groups of ependymal cells in the midline, partially resembling specific domains of the former canal. However, no features of the adult human ependymal remnant are found, suggesting that this structure is a specific human trait. In order to shed some more light on the mechanism of human central canal closure, we provide new data suggesting that canal patency is lost by delamination of the ependymal epithelium, in a process that includes apical polarity loss and the expression of signaling mediators involved in epithelial to mesenchymal transitions.

Convergence of spinal trigeminal and cochlear nucleus projections in the inferior colliculus of the guinea pig

In addition to ascending auditory inputs, the external cortex of the inferior colliculus (ICX) receives prominent somatosensory inputs. To elucidate the extent of interaction between auditory and somatosensory representations at the level of IC, we explored the dual projections from the cochlear nucleus (CN) and the spinal trigeminal nucleus (Sp5) to the inferior colliculus (IC) in the guinea pig, using both retrograde and anterograde tracing techniques. Injections of retrograde tracers into ICX resulted in cell-labeling primarily in the contralateral DCN and pars interpolaris and caudalis of Sp5. Labeled cells in DCN were either fusiform or multipolar cells, whereas those in Sp5 varied in size and shape. Injections of anterograde tracers into either CN or Sp5 resulted in terminal labeling in ICX primarily on the contralateral side. Most projection fibers from Sp5 terminated in a laminar pattern from ventromedial to dorsolateral within the ventrolateral ICX, the ventral border of IC, and the ventromedial edge of IC (collectively termed "the ventrolateral border region of IC," ICXV). Less dense anterograde labeling was observed in lateral and rostral ICX. Injecting different tracers into both Sp5 and CN confirmed the overlapping areas of convergent projections from Sp5 and CN in IC: The most intense dual labeling was seen in the ICXV, and less intense dual labeling was also observed in the rostral part of ICX. This convergence of projection fibers from CN and Sp5 provides an anatomical substrate for multimodal integration in the IC.

Somatosensory areas S2 and PV project to the superior colliculus of a prosimian primate, Galago garnetti

As part of an effort to describe the connections of the somatosensory system in Galago garnetti, a small prosimian primate, injections of tracers into cortex revealed that two somatosensory areas, the second somatosensory area (S2) and the parietal ventral somatosensory area (PV), project densely to the ipsilateral superior colliculus, while the primary somatosensory area (S1 or area 3b) does not. The three cortical areas were defined in microelectrode mapping experiments and recordings were used to identify appropriate injection sites in the same cases. Injections of wheat germ agglutinin conjugated with horseradish peroxidase (WGA-HRP) were placed in S1 in different mediolateral locations representing body regions from toes to face in five galagos, and none of these injections labeled projections to the superior colliculus. In contrast, each of the two injections in the face representation of S2 in two galagos and three injections in face and forelimb representations of PV in three galagos produced dense patches of labeled terminations and axons in the intermediate gray (layer IV) over the full extent of the superior colliculus. The results suggest that the higher-order somatosensory areas, PV and S2, are directly involved in the visuomotor functions of the superior colliculus in prosimian primates, while S1 is not. The somatosensory inputs appear to be too widespread to contribute to a detailed somatotopic representation in the superior colliculus, but they may be a source of somatosensory modulation of retinotopically guided oculomotor instructions.

Trigeminal projections to thalamus and subthalamus in the hedgehog tenrec

The objective of the present study was the identification and characterization of the trigemino-diencephalic target areas in the Madagascan lesser hedgehog tenrec in order to get a more comprehensive view on the mammalian somatosensory thalamus, its evolution and representation in different species. Such an analysis has been considered important because in lower mammals the head and face are relatively well represented, but their ascending trigeminal projections have scarcely been analysed. Following injections of different tracer substances into the rostral and caudal portions of the trigeminal nuclear complex the most prominent area of termination was found in the medial ventroposterior nucleus. These projections were patchy and scarcely overlapped the region previously shown to receive spinal and dorsal column nuclear afferents. On the basis of the laterality and the intensity of the projections, two subdivisions were distinguished, the principal portion and the accessory portion receiving a dense contralateral and a weak bilateral input, respectively. They were considered equivalents to the magnocellular and parvocellular subdivisions of the medial ventroposterior nucleus in more differentiated mammals. In the latter species, however, the overlap between trigeminal and parabrachial fibres appears less extensive than in the tenrec. In addition, a weak bilateral projection was shown from the caudal trigeminal nucleus to the caudal and dorsal subdivision of the nucleus submedius. There was little, if any evidence for a trigeminal projection to the intralaminar nuclei and we failed to identify a correlate to the posterior nuclear complex of higher mammals. On the other hand, there was a distinct contralateral projection to the ventral portion of the zona incerta. This projection was of similar strength as the projection to the medial ventroposterior nucleus; it supports the notion that the zona incerta may play a crucial role in relaying trigeminal information.

Origin and terminal distribution of the trigeminal projections to the inferior and superior colliculi in the lesser hedgehog tenrec

The trigemino-tectal projections were investigated with anterograde and retrograde tracing techniques in the Madagascan lesser hedgehog tenrec, Echinops telfairi. There were prominent contralateral projections to the inferior colliculus (CoI) and the superior colliculus (CoS), each showing its own characteristic pattern of terminations. While the projections to the CoI were confined consistently to a circumscribed region in its ventrolateral, external portion, the projections from particularly the rostral trigeminal subdivision to the CoS were distributed inhomogenously across almost the entire rostro-caudal and mediolateral extents. Comparing these data with the spino-tectal projections published previously, it demonstrates that the somatotopic organization of ascending tectal afferents is more distinct in the CoI than in the CoS. There were roughly twice as many trigeminal neurones projecting to the CoS than to the CoI. This difference might be due to the fact that the cells projecting to CoS were distributed extensively across the trigeminal nuclear complex (peak densities in the principal and interpolar subdivisions), while the neurones projecting to the CoI were largely confined to the interpolar and caudal trigeminal subdivisions. The latter cells were located adjacent to the spinal trigeminal tract; the neurones projecting to the CoS occupied preferentially the ventral trigeminal regions at rostral levels, while from the interpolar subdivision caudalward the labelled cells shifted dorsolaterally. In comparison to other mammals the trigeminal projection to the tenrec's CoI is unique. There is evidence for such a projection in other species too, but it is poorly documented, presumably due to technical reasons.

Projections of the nucleus of the basal optic root in pigeons (Columba livia) revealed with biotinylated dextran amine

The nucleus of the basal optic root (nBOR) of the accessory optic system is known to be involved in the analysis of the visual consequences of self-motion. Previous studies have shown that the nBOR in pigeons projects bilaterally to the vestibulocerebellum, the inferior olive, the interstitial nucleus of Cajal, and the oculomotor complex and projects unilaterally to the ipsilateral pretectal nucleus lentiformis mesencephali and the contralateral nBOR. By using the anterograde tracer biotinylated dextran amine, we confirmed these projections and found (previously unreported) projections to the nucleus Darkshewitsch, the nucleus ruber, the mesencephalic reticular formation, and the area ventralis of Tsai as well as ipsilateral projections to the central gray, the pontine nuclei, the cerebellar nuclei, the vestibular nuclei, the processus cerebellovestibularis, and the dorsolateral thalamus. In addition to previous studies, which showed a projection to the dorsomedial subdivision of the contralateral oculomotor complex, we found terminal labelling in the ventral and dorsolateral subdivisions. Individual fibers were reconstructed from serial sections, and collaterals to various nuclei were demonstrated. For example, collaterals of fibers projecting to the vestibulocerebellum terminated in the vestibular or cerebellar nuclei; collaterals of fibers to the inferior olive terminated in the pontine nuclei; many individual neurons projected to the interstitial nucleus of Cajal, the nucleus Darkshewitsch, and the central gray and also projected to the nucleus ruber and the mesencephalic reticular formation; collaterals of fibers to the contralateral nucleus of the basal optic root terminated in the mesencephalic reticular formation and/or the area ventralis of Tsai; neurons projecting to the nucleus lentiformis mesencephali also terminated in the dorsolateral thalamus. The consequences of these data for understanding the visual control of eye movements, neck movements, posture, locomotion, and visual perception are discussed.

Connections of the superior colliculus with the tegmentum and the cerebellum in the hedgehog tenrec

Different tracer substances were injected into the superior colliculus (CoS) in order to study its afferents and efferents with the meso-rhombencephalic tegmentum, the precerebellar nuclei and the cerebellum in the Madagascan hedgehog tenrec. The overall pattern of tectal connectivity in tenrec was similar to that in other mammals, as, e.g. the efferents to the contralateral paramedian reticular formation. Similarly the origin of the cerebello-tectal projection in mainly the lateral portions of the tenrec's cerebellar nuclear complex corresponded to the findings in species with little binocular overlap. In comparison to other mammals, however, the tenrec showed a consistent projection to the ipsilateral inferior olivary nucleus, in addition to the classical contralateral tecto-olivary projection. The tenrec's CoS also appeared to receive an unusually prominent monoaminergic input particularly from the substantia nigra, pars compacta. There was a reciprocal tecto-parabigeminal projection, a distinct nuclear aggregation of parabigeminal neurons, however, was difficult to identify. The dorsal lemniscal nucleus did not show perikaryal labeling in contrast to the paralemniscal region. Similar to the cat but unlike the rat there were a few neurons in the nucleus of the central acoustic tract. Unlike the cat, but similar to the rat there was a distinct, predominantly ipsilateral projection to the magnocellular reticular field known to project spinalward.

Cortical and medullary somatosensory projections to the cochlear nuclear complex in the hedgehog tenrec

Various tracer substances were injected into the spinal cord, the dorsal column nuclei, the trigeminal nuclear complex and the somatosensory cortex in Madagascan hedgehog tenrecs. With the exception of the cases injected exclusively into the spinal cord all injections gave rise to sparse, but distinct anterograde projections to the cochlear nuclear complex, particularly the granular cell domain within and outside of the dorsal cochlear nucleus. Among these cochlear afferents the projection from the primary somatosensory cortex is the most remarkable because the hedgehog tenrec has one of the lowest encephalisation indices among mammals and a similar cortico-cochlear connection has not been demonstrated so far in other species.

Efferents from the lateral frontal cortex to spinomedullary target areas, trigeminal nuclei, and spinally projecting brainstem regions in the hedgehog tenrec

This study was done in the Madagascan lesser hedgehog tenrec, an insectivore with a very poorly differentiated neocortex. The cortical region, known to give rise to spinal projections, was injected with tracer, and the cortical efferents to brainstem and spinal cord were analyzed. Bulbar reticular fields, in addition, were identified according to their cells of origin and the laterality of their spinal projections after injection of tracer. Only few cortical fibers could be traced from the bulbar pyramid into the ipsilateral spinal cord, particularly to the lateral funiculus. The projections to the dorsal column nuclei and the classical spinally projecting brainstem regions were also weak. Faint projections were demonstrated to the nucleus of the posterior commissure and the nucleus of Darkschewitsch. In comparison to other mammals, there was no evidence that the contralateral cortico-bulbo-spinal pathway was strengthened, substituting for the almost non-existent contralateral corticospinal projection. Unlike the sensorimotor apparatus controlling limb and body movements, the brainstem regions controlling the head and neck received prominent cortical projections. Direct corticotrigeminal projections and indirect pathways were well represented. The projections to the trigeminal nuclei and the lateral reticular fields were clearly bilateral; those to the superior colliculus were predominantly ipsilateral. The corticobulbar fibers left the pyramid along its entire extent; the principal trigeminal nucleus and the dorsolateral pontine tegmentum were supplied by additional fibers of the corticotegmental tract. The lateral frontal cortex also projected densely to the dorsolateral hypothalamus, the periaqueductal gray, and the adjacent mesencephalic tegmentum, components of the emotional motor system.

Somatovisceral projections from spinal cord and dorsal column nuclei to the thalamus in hedgehog tenrecs

In order first to overcome the difficulties in understanding the increasing amount of information available regarding the mammalian somatosensory thalamus, and then to correlate the findings among different species and integrate them into a general concept of thalamic organization, the present study investigated the spinothalamic and medial lemniscal projections in Madagascan hedgehog tenrecs (Echinops telfairi and Setifer setosus). Tracer substances were injected into the dorsal column nuclei and into spinal segments at various levels; additional injections were made into the inferior colliculus. The ascending somesthetic projections were to predominantly contralateral posterolateral target areas, and were almost mirror-like on both sides to intralaminar and medial thalamic nuclei. The densest and most extensive projections, originating mainly from the high cervical spinal cord and the dorsal column nuclei, reached the posterolateral thalamus caudal to the lateral geniculate nucleus. This region was difficult to subdivide cytoarchitecturally; nevertheless, on the basis of its labeling pattern, several subdivisions could be described and preliminary named. Some of them compared tentatively with the internal portion of the medial geniculate nucleus (GM) and the ventral posterior nuclear complex (VPC) in more differentiated mammals. The most prominent subdivision, however, located subjacent to the lateral surface of the brainstem, was shown to receive additional fibers from the inferior colliculus. This region might be considered a further subdivision of GM, VPC, a perigeniculate area, and/or a region of its own not comparable at present, with thalamic regions in other mammals. On the other hand, it may also be a remnant of the hypothetical, diffuse multimodal region from which GM and VPC have possibly evolved.(ABSTRACT TRUNCATED AT 250 WORDS)