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PubMed2005

A projection-based image registration algorithm and its application.

Proposed is a projection-based image registration technique where, by rearranging the projections of characteristic images, the image registration is implemented with two independent steps - rotation and translation, to perform the two-dimensional or three-dimensional rigid-body image registration addressing the head motion problem in functional magnetic resonance imaging (fMRI). For a 2D problem, the approach is based on a one-dimensional projection of a segmented two-dimensional characteristic image, in which the translation and rotation parameters are obtained with a one-dimensional cross-correlation-based estimator. This is then used to compute the cross-correlation between the projection of an image and a registration table that is created by rearranged projections of a selected two-dimensional image with various rotation angles. In this approach, the translation registration table may be created by rearranged projections of sub-voxel level two-dimensional images with various sub-voxel level parameters, and so it may be applied into a sub-voxel registration. Such an approach replaced the general multi-dimensional optimization procedure with a linear projection calculation and a finite cross-correlation with a registration table, thus the amount of computation is considerably reduced. The performance of this method was confirmed by simulation study different SNRs and applications to 2D and 3D actual functional MRI images.

Algorithms↗
PubMed2004

Ethics in construction project briefing.

The research reported in this paper set out to investigate ethics in the initial stages of construction projects. Briefing is the first real contact stage between the commissioner (client/employer) of a project--at this stage a potential project--and those involved in project realization--the designers and, subsequently, the constructors. It is well known that early decisions are of greatest impact and so, the importance of the initial contacts, communications and consequent decisions are paramount. Different project participants are known to pursue individual objectives to varying degrees as well as possessing different perspectives and perceptions and operating/behaving in different ways. Hence, determination of the appropriate form, content etc. of a project is, inevitably, a matter of exercising value judgements and compromises and so, involves ethical considerations. A case study of a project through the briefing stage is reported and analysed, from initial contacts to scheme approval. It is apparent that a number of ethical concerns are manifest through the various actions of the major participants.

Contracts↗
PubMed1975

Demonstration of a somatotopically organized projection onto the paramedian lobule and the anterior lobe from the lateral reticular nucleus: an experimental study with the horseradish peroxidase method.

Using the retrograde axonal transport of horseradish peroxidase, the projection from the lateral reticular nucleus (NRL) to the cerebellar anterior lobe and paramedian lobule has been studied in 13 cats. Both the anterior lobe and the paramedian lobule receive a somatotopically organized projection from the NRL. The projection to the paramedian lobule is nearly exclusively ipsilateral and originates mainly in the dorsal part of NRL, while the projection to the anterior lobe is bilateral (with ipsilateral predominance) and takes origin from all parts of the NRL. The lateral part of the NRL (closely coinciding with the parvocellular nucleus) projects to the rostral part of the anterior lobe and caudal parts of the paramedian lobule (both representing the hindlimb), while the medial part of the NRL (magnocellular nucleus) projects to the caudal parts of the anterior lobe and rostral parts of the paramedian lobule (representing the forelimb). The small subtrigeminal nucleus projects onto the anterior lobe as well as the paramedian lobule, but apparently mainly to their forelimb areas.

Afferent Pathways↗
PubMed1975

The pontocerebellar projection onto the paramedian lobule in the cat: an experimental study with the use of horseradish peroxidase as a tracer.

Horseradish peroxidase (HRP) was injected into cerebellar cortex of the paramedian lobule in 12 cats, and the ensuing distribution of labeled cells in the pontine nuclei was mapped in some detail. The cells in the pontine gray which give origin to fibers to the paramedian lobule lie together, in part in groups, and in part in columns. The columns are situated both medial and ventrolateral to the peduncle, as well as in the dorsolateral pontine nucleus. The projection is bilateral with a clearcut contralateral preponderance, except in the lateralmost region in the dorsolateral nucleus, which projects mainly ipsilaterally. The column medial to the peduncle projects in a topographical pattern to the paramedian lobule. The dorsal part of this column projects to the rostral folia of the paramedian lobule, while successively more ventral parts in the column project to more caudal paramedian lobules. Within the other columns only a faint sign of a topographical organization is found. The location of the pontine columns projecting onto the paramedian lobule largely corresponds to the pontine terminal areas of fibers from the sensory cerebral cortex (SmI and SmII). The corresponding topography in these parts of the corticopontine and pontocerebellar pathways is suitable for a somatotopical impulse transmission from the sensory cortex to the paramedian lobule, in agreement with the results of physiological investigations. Furthermore, a correlation of the pontine areas projecting onto the paramedian lobule with the terminal areas of pontine afferents shows that the pons may be a relay station in mediating influences from other parts of the cortex (MsI, visual and acoustic), the cerebellar nuclei and the colliculi to the paramedian lobule.

Animals↗
PubMed1975

Cortico-cortical projections from striate cortex of the squirrel monkey (Saimiri sciureus). A radioautographic study.

In 13 squirrel monkeys [3H] leucine was injected into three different portions of the striate cortex: the occipital convexity, the interhemispheric surface and the depth of the calcarine fissure. After survival times between 8.5 h and 23 days. The radioautographic study of the brains revealed terminal fields of projection in various cortical areas outside the striate cortex. Transport of labelled material to cortex close to the striate area and in the superior part of the supratemporal sulcus occurred in all cases. After injection into the striate cortex of the interhemispheric surface, or into the depth of the calcarine fissure, a second projection was found in the prestriate belt situated in the cortex near the parieto-occipital sulcus. The fields of projection in the 3 cortical regions varied systematically with the variation of the injection site indicating a retinotopic organisation. If the cortical labelling was restricted to the supragranular layers, intracortical projection to layer V could be observed. If the cortical injection labelled only the infragranular layers, an intracortical projection appeared clearly at the level of layer IIIc. Labelling restricted to the supragranular layers resulted in cortico-cortical projections whereas labelling of only the infragranular layers did not. In the 3 cases in which the contralateral hemisphere was examined no evidence for transcallosal projections was found.

Animals↗
PubMed1979

A comparison of the organization of the projections of the dorsal lateral geniculate nucleus, the inferior pulvinar and adjacent lateral pulvinar to primary visual cortex (area 17) in the macaque monkey.

Both anterograde and retrograde transport tracing methods were used to study the organization of the projections of the dorsal lateral geniculate (DLG), the inferior pulvinar and subdivisions of the lateral pulvinar to primary visual cortex (striate cortex or area 17). The DLG projects only to striate cortex. These projections are retinotopically organized, and do not extend to any cortical layers above layer IVA. In contrast the inferior pulvinar (PI) and the immediately adjacent portion of the lateral pulvinar (PL alpha 48) project to both striate and prestriate cortex. The projections from these two thalamic areas to the striate cortex are also retinotopically organized and exist in parallel with those from the DLG. In contrast to the DLG, the projections from PI and PL alpha terminate above layer IVA in striate cortex, i.e. layers I, II and III. In prestriate cortex the layers of termination include layers IV, III and I. The pulvinar terminations in layers II and III of area 17 occur in segregated patches as do the geniculate terminations in layers IVC and IVA. On the other hand the pulvinar terminations in layer I which overlie those in layers II and III of area 17 appeared to be continuous. Control studies show that the remainder of the lateral pulvinar overlying PL alpha does not project to striate cortex. It is concluded that there are 3 visuotopically organized inputs from the lateral thalamus to primary visual cortex and that each of these inputs have different layers of termination. The inputs from PI and DLG can convey direct retinal inputs while those from PI and PL alpha can also be involved in intrinsic cortico-thalamocortical connection with prestriate cortex. It remains, then that it cannot be tacitly assumed that the ascending inputs which influence the response properties of the primary cortical neurons arise solely from the dorsal lateral geniculate nucleus. It is also argued that these inputs to the supragranular layers may be excitatory as those from the DLG to the IVth layer.

Animals↗
PubMed1980

Projections of the optic tectum in the longnose gar, Lepisosteus osseus.

Efferent projections of the optic tectum were studied with the anterograde degeneration method in the longnose gar. Ascending projections were found bilaterally to 3 pretectal nuclei -- the superficial pretectal nucleus, nucleus pretectalis centralis and nucleus pretectalis profundus -- and to a number of targets which lie further rostrally -- the central posterior nucleus, dorsal posterior nucleus, accessory optic nucleus, nucleus ventralis lateralis, nucleus of the ventral optic tract, rostral part of the preglomerular complex, suprachiasmatic nucleus, anterior thalamic nucleus, nucleus ventralis medialis, nucleus intermedius, nucleus prethalamicus and rostral entopeduncular nucleus. Projections of the tectum reach the contralateral side via the supraoptic decussation and are less dense contralaterally than ipsilaterally. Descending projections resulting from tectal lesions include: (1) a tectal commissural pathway to the core of the torus longitudinalis bilaterally and the contralateral tectum and torus semicircularis; and (2) a pathway leaving the tectum laterally from which fibers terminate in the ipsilateral torus semicircularis, an area lateral to the nucleus of the medial longitudinal fasciculus, lateral tegmental nucleus, nucleus latealis valvulae, nucleus isthmi and the reticular formation. A component of this bundle decussates at the level of the lateral tegmental nucleus to project to the contralateral reticular formation. On the basis of comparisons of these findings with the pattern of retinal projections in gars and other data, it is argued that the nuclei previously called the lateral geniculate and rotundus in fish are not the homologues of the nuclei of those names in land vertebrates but are rather pretectal cell groups. The overall organization of both retinal and tectal projections in gars is strikingly similar to that in land vertebrates; at present, the best candidate for a rotundal homologue is the dorsal posterior nucleus.

Animals↗
PubMed1980

A qualitative electron microscopic study of the corticopontine projections after neonatal cerebellar hemispherectomy.

The present study shows that 3--5 days following lesions of the dentate and interposed nuclei in normal adult rats degenerating axons and axon terminals can be detected in the contralateral pontine gray. The degenerating axon terminals form Gray's type I axo-dendritic contacts with fine and intermediate dendrites measuring between 0.8--2.4 microns. The present study also investigates, by electron microscopy, the synaptic rearrangement of the sensorimotor corticopontine projections following neonatal left cerebellar hemispherectomy. Following neonatal left cerebellar hemispherectomy, the right sensorimotor and adjacent cortex (SMC) presents a very dense ipsilateral and a modest amount of contralateral corticopontine projections in contrast with a predominantly ipsilateral corticopontine projection seen in the normal adult rat. As with the ipsilateral corticopontine projection seen in the normal adult animal, the bilateral corticopontine projections seen in the experimental animals form contacts with dendrites suggestive of Gray's type I synapses. While the corticopontine projections in normal control animals form synapses with fine dendrites measuring 0.2--1.2 micron the corticopontine projections in the experimental animals form synaptic relations with fine dendrites and with intermediate dendrites measuring 0.2--2.4 microns. As the normal cerebellopontine fibers from the dentate and interposed nuclei also form axo-dendritic synapses on fine and intermediate dendrites and the contracts formed are also of Gray's type I synapses, it is possible that some of the newly formed corticopontine fibers in the experimental animals might have replaced the cerebellopontine fibers synapsing on intermediate dendrites. Synaptic rearrangement appears to take place as suggested by the presence of synaptic complexes in which one axon terminal contacts two or more dendrites or two or more axon terminals contact one dendrite. Such complexes are frequently seen to undergo degeneration following the right SMC lesion in the experimental animals. Other complex synaptic structures are also present in both the right and left pontine gray in the experimental animals. They are not seen to undergo degeneration following the right SMC lesions. Occasional features of neuronal reaction could still be seen in both sides of the pontine gray for as long as 3--6 months after the neonatal cerebellar lesions.

Afferent Pathways↗
PubMed1981

The organization of the efferent projections and striatal afferents of the entopeduncular nucleus and adjacent areas in the rat.

Multiple retrograde fluorescent tracing was employed to investigate the organization of the rat entopeduncular nucleus projections to the lateral habenula, ventral anterior-ventral lateral thalamus, parafasicular-centre median complex, and tegmenti pedunculopontis area of the brain stem. The results indicate that neurons in the rostral 2/3 of the entopeduncular nucleus project to the lateral habenula. In contrast, neurons in the caudal 1/3 of the entopeduncular nucleus project to the ventral anterior-ventral lateral thalamus, parafasicular-centre median complex, and tegmenti pedunculopontis area of the brain stem. The majority of neurons in the caudal 1/3 of the entopeduncular nucleus are retrogradely double-labeled from various combinations of tracer injections into the 3 termination areas to which they projected. Little or no retrograde labeling in the entopeduncular nucleus was produced by tracer injections in the substantia nigra or subthalamic nucleus. Only large injections of tracers in the tegmenti pedunculopontis are and the surrounding brain stem produced retrograde labeling in the entopeduncular nucleus. These brain stem injections also labeled a band of cells surrounding the entopeduncular nucleus in the zona incerta, lateral hypothalamus, of the ansa lenticularis and central nucleus of the amygdala. [3H]Leucine injections in the head of the caudate-putamen complex (but not in the cortex or globus pallidus) produced dense accumulations of silver grains over both the rostral and caudal portions of the entopeduncular nucleus. [3H]Leucine injections in the caudal body of the caudate-putamen complex produced accumulations of silver grains over a "ventral entopeduncular nucleus area' in the nucleus of the ansa peduncularis. It was suggested that the head of the striatum projects both to the neurons of the rostral "limbic' portion of the entopeduncular nucleus, which project to the lateral habenula, and to the neurons of the caudal "motor' portion of the entopeduncular nucleus, which project primarily by way of axon collaterals to the ventral anterior-ventral lateral thalamus, parafascicular-centre median complex, and tegmenti pedunculopontis area of the brain stem.

Afferent Pathways↗
PubMed1983

The cells of origin of cat trigeminothalamic projections: especially in the caudal medulla.

Thalamic projections from the caudal medulla of the cat were examined using the method of retrograde axonal transport of horseradish peroxidase (HRP). Injections were made unilaterally in various thalamic regions. Large injections labeled cells in the subnuclei: zonalis (Vcz), gelatinosus (Vcg), magnocellularis (Vcm), reticularis dorsalis (Vcrd) and ventralis (Vcv) medullae oblongatae. The largest number of labeled cells were in Vcz, Vcrd and Vcrv. Most of the labeled cells in Vcz and Vcrd were contralateral to the injection site, although the labeled cells in the Vcrv were bilateral. Small injections were made into the medial, lateral and dorsal regions of the nucleus ventralis posteromedialis (VPM), rostral regions of the posterior nuclei (POm and PO1), caudal POm, the nucleus centralis lateralis (CL) and the center median-parafascicular nuclear complex (CM-Pf). Most of the neurons in Vcz were found to project to the medial VPM and some to the caudal POm. A small number of cells in the Vcrd project to the medial VPM, but a large number project to the caudal POm and CM-Pf complex. The largest number of neurons projecting to the CM-Pf complex was present in Vcrv, where the labeled cells were bilateral. The types of trigeminothalamic projecting cells and the sizes of their somata were observed for different subnuclei and a considerable difference was found to exist among the subnuclei. This anatomical differentiation of the trigeminothalamic projections probably reflects a functional specialization of neuronal location since the functional properties of neurons vary according to their locations.

Animals↗
PubMed1983

Evidence for an amygdaloid projection to premotor cortex but not to motor cortex in the monkey.

Previous studies in the cat have demonstrated a direct projection from the amygdaloid complex to motor and premotor regions of the neocortex. In the present study both anterograde and retrograde tracer techniques have been used to determine whether a similar projection exists in the monkey brain. We have found that the dorsal, magnocellular division of the basal nucleus of the amygdaloid complex gives rise to a projection to the premotor cortex (Area 6), which terminates principally in layers I and II, and to a lesser extent in layer VI. No component of the amygdaloid complex has been found to project to the motor cortex (Area 4). The amygdaloid projection to Area 6 in the monkey appears to be substantially weaker than other rostrally directed projections from the basal amygdaloid nucleus to orbitofrontal and medial frontal areas, and also relatively weaker than the projection that has been described in the cat.

Afferent Pathways↗
PubMed1984

Autoradiographic demonstration of the projections from the mesencephalic locomotor region.

An autoradiographic tracing technique was used to examine the projections of the classically defined mesencephalic locomotor region (MRL). Injections of [3H]proline and [3H]leucine were made into sites in the caudal mesencephalon which can be stimulated to produce locomotion. The injection sites were confined to the cuneiform nucleus (stereotaxic coordinates P2.0, L4.0, H-1.0). Descending projections were primarily ipsilateral to the gigantocellular and magnocellular reticular formation of the pons and medulla, the dorsal tegmental reticular nucleus, and the nucleus raphe magnus. Some sparse contralateral projections were also observed within the magnocellular and gigantocellular reticular formation. Direct axonal connections with the spinal cord were not consistently observed. Ascending projections were observed to the subthalamic nucleus, caudal hypothalamic nuclei, the centrum medianum nucleus of the thalamus, the ventral tegmental area of Tsai, the superior colliculus, and the periaqueductal gray region. The ascending projections were also ipsilateral, with sparse contralateral labeling confined to areas which received ipsilateral projections. Projections to the contralateral cuneiform nucleus were also consistently observed. The results, when compared to those of another study, suggest that the classical MLR is anatomically distinct from the more medial sites in the mesencephalon which can also induce locomotion.

Animals↗
PubMed1988

Efferent projections from the anterior nucleus of the solitary tract of the hamster.

The efferent projections from the anterior nucleus of the solitary tract (NST) of the golden hamster (Mesocricetus auratus) were determined using both anterograde and retrograde techniques. Injections of [3H]leucine were made into the anterior NST in regions responsive to gustatory stimulation of the anterior tongue. Ascending projections to the parabrachial nuclei (PBN) were evident as were projections within the NST and subjacent reticular formation. The cells of origin for both ascending and descending pathways were characterized by deposits of HRP into the PBN and caudal medulla. Cells projecting to the PBN were located in the dorsal and dorsolateral anterior NST in contrast to cells from the ventral region of the anterior NST which project within the medulla. Neurons in the reticular formation ventral to the anterior NST project to both regions. These local projections adjacent to oral motor nuclei provide an anatomical basis for the anterior nucleus of the solitary tract to influence oro-motor responses.

Animals↗
PubMed1989

Somatostatin-, vasoactive intestinal polypeptide- and neuropeptide Y-like immunoreactivity in eye- and submandibular gland-projecting sympathetic neurons.

Studies combine the use of the retrograde tracer, fluorogold, and immunocytochemical staining to determine whether superior cervical ganglion (SCG) neurons projecting to the iris or submandibular gland (SMG) in adult male and female rats show distinctive immunoreactivity to somatostatin (SS), vasoactive intestinal polypeptide (VIP), or neuropeptide Y. Overall, more SMG-projecting neurons than eye-projecting neurons contain VIP-like immunoreactivity (VIP-LI), and more eye-projecting neurons than SMG-projecting neurons contain SS-LI and VIP-LI. Thus, postganglionic neurons of the SCG that project to specific target tissues are heterogeneous in their peptide content, and there are differences in the pattern of peptide-immunoreactivity between neurons projecting to these two target tissues. In addition, the results indicate that there may be gender differences in the expression of these neuropeptides.

Adrenergic Fibers↗
PubMed1991

Retinal distribution of ganglion cells which project to the ipsilateral optic tectum in Bufo marinus.

The retinotectal projection in anura is mainly crossed, although a small proportion of optic axons projects to the ipsilateral tectum. Using the fluorescent carbocyanide dye, DiI, we mapped the retinal topography of ganglion cells which project to the ipsilateral tectum in adult Bufo marinus. DiI was injected into particular locations in the right tectum. After 10 days survival both the right and the left retinals were wholemounted and the number and retinal position of retrogradely filled ganglion cells were determined. The contralateral and ipsilateral cells were visuotopically distributed in the retina in the majority of experiments. However, in two cases cells were located in visuotopically disparate parts of the retina. The ipsilateral cells represented 3.7% of contralaterally projecting cells in the temporal retina, 0.1% in the nasal and dorsal retina and 0.6% of the ventral retina. The density of ipsilaterally projecting ganglion cells varied from a top of 25 cells/mm2 in the temporal retina, 9 cells/mm2 in the nasal, 3 cells/mm2 in the dorsal to 11 cells/mm2 in the ventral retina. The diversity of size and shape of retrogradely filled ganglion cells indicated that the ipsilateral population corresponded to a heterogeneous class of ganglion cell types. The functional significance of the direct ipsilateral retinotectal projection of the anuran visual system has yet to be elucidated. However, in light of the involvement of the indirect ipsilateral retinotectal projection in binocular vision, the direct pathway is likely to be associated with a retino-tecto-spinal circuit subserving postural adjustment to visually derived stimulation.

Animals↗
PubMed1992

Spinal projections of the locus coeruleus and the nucleus subcoeruleus in the Harlan and the Sasco Sprague-Dawley rat.

The descending projections of the locus coeruleus (LC) and the nucleus subcoeruleus (SC) to the lumbar spinal cord were examined in rats from two vendors using retrograde transport of fluorescent latex beads. There was a vendor difference observed which agrees with previous findings. The differential dorsal horn and ventral horn projections of the Harlan and the Sasco Sprague-Dawley rats, reported by Fritschy and Grzanna, and Clark and Proudfit were confirmed. In the Harlan rat more cells were labeled in the LC following injections in the dorsal horn. In contrast, in the Sasco rat, more cells were labeled in the LC from injections in the ventral horn. Although, in all studies, the LC in rats from these vendors projected to some extent to both the dorsal and the ventral horn. A difference in labeling was noted also for the depth of placement of the tracer in the dorsal horn. When the site of injection was in the nucleus proprius, a predominantly contralateral projection of the LC was noted. In contrast, when horseradish peroxidase (HRP) gel implants were placed to include the superficial laminae, the cells in the LC were labeled predominantly ipsilaterally. The SC has a major projection to the dorsal horn in the Harlan rats while cells in the SC were predominantly labeled following ventral horn injection in the Sasco rats. These cells send mostly ipsilateral projections to the dorsal and ventral horn of the spinal cord. Double labeled studies confirmed that 91% of LC and 86% of SC neurons projecting to the spinal cord were noradrenergic.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1986

The development of sensory projection patterns in embryonic chick hindlimb under experimental conditions.

In the chick, sensory neurons grow to their segmentally appropriate target sites in the hindlimb from the outset during normal development. To elucidate the underlying mechanisms, we performed various manipulations of the neural tube, including the neural crest, or of the hindlimb, before axonal outgrowth and assessed the resulting sensory projections using retrograde and anterograde HRP labeling and electrophysiological techniques. Previous experiments had shown that motoneurons are specified to project to their appropriate target muscles prior to axon outgrowth and that they respond to cues in the limb in order to grow to those targets (C. Lance-Jones and L. Landmesser, 1980, J. Physiol. (London) 302, 559-602; C. Lance-Jones and L. Landmesser, 1981, Proc. R. Soc. London, B 214, 19-52). When several segments of neural tube and neural crest were deleted, sensory neurons in the remaining segments still projected along their correct pathways, as did motoneurons. In situations in which motoneurons grew to their correct targets from altered positions with respect to the limb (e.g., small neural tube reversals), sensory neurons also tended to project along the segmentally appropriate pathways both to skin and to muscle. In situations in which motoneurons were displaced greater distances from their normal point of entry into the limb and made wrong connections (e.g., large neural tube reversals, anterior-posterior limb reversals), sensory neurons also projected incorrectly. The patterns of sensory projections to muscles were, in each situation, generally similar to the motoneuron projections. These results are consistent with the possibility that sensory neurons, like motoneurons, are specified with respect to their peripheral connectivity. Alternatively, the results suggest that motoneurons may play a role in the process of pathway selection by sensory neurons.

Afferent Pathways↗
PubMed1988

The effect of somite manipulation on the development of motoneuron projection patterns in the embryonic chick hindlimb.

Although the formation of motoneuron projections to individual muscles in the embryonic chick hindlimb has been shown to involve the specific recognition of environmental cues, the source of these cues and their mode of acquisition are not known. I show in the accompanying paper (C. Lance-Jones, 1988, Dev. Biol. 126, 394-407) that there is a correlation between the segmental level of origin of motoneurons and the somitic level of origin of the muscle cells of their targets in the chick hindlimb. These data are compatible with the hypothesis that the developmental basis for specific recognition is a positional one. Motoneurons and myogenic cells may be uniquely labeled in accord with their axial level of origin early in development and subsequently matched on the basis of these labels. To test this hypothesis, I have assessed motoneuron projection patterns in the embryonic chick hindlimb after somitic tissue manipulations. In one series of embryos, somitic mesoderm at levels 26-29 or 27-29 was reversed about the anteroposterior axis prior to myogenic cell migration and axon outgrowth. Since previous studies have shown that cells migrate from the somites in accord with their position and that somites 26-29 populate anterior thigh musculature, this operation will have reversed the somitic level of origin of anterior thigh muscles. Retrograde HRP labeling of projections to anterior thigh muscles at stage (st) 30 and st 35-38 showed that motoneuron projections were largely normal. This finding suggests that limb muscle cells or their source, the somites, do not contain the cues responsible for specific recognition prior to myogenic cell migration and axon outgrowth. To confirm that specific guidance cues were still intact after somitic mesoderm reversal, I also assessed motoneuron projections in embryos where somitic tissue plus adjacent spinal cord segments at levels 26-29 were reversed in a similar manner. Analyses of the distribution of retrogradely labeled motoneurons in reversed cord segments at st 35-36 indicated that motoneuron projections were reversed. This finding suggests that motoneurons have altered their course to project to correct targets despite the altered somitic origin of their targets and, thus, that specific guidance cues were intact. I conclude that if cues governing target or pathway choice are encoded positionally then they must be associated with other embryonic tissues such as the connective tissues or that guidance cues are acquired by myogenic cells after the onset of migration and motoneuron specification.

Animals↗