Skin sensory innervation patterns in embryonic chick hindlimb following dorsal root ganglion reversals

The making of a proprioceptor: a tale of two identities

Proprioception, the sense of body position in space, has a critical role in the control of posture and movement. Aside from skin and joint receptors, the main sources of proprioceptive information in tetrapods are mechanoreceptive end organs in skeletal muscle: muscle spindles (MSs) and Golgi tendon organs (GTOs). The sensory neurons that innervate these receptors are divided into subtypes that detect discrete aspects of sensory information from muscles with different biomechanical functions. Despite the importance of proprioceptive neurons in motor control, the developmental mechanisms that control the acquisition of their distinct functional properties and positional identity are not yet clear. In this review, we discuss recent findings on the development of mouse proprioceptor subtypes and challenges in defining them at the molecular and functional level.

Anatomical coupling of sensory and motor nerve trajectory via axon tracking

It is a long-standing question how developing motor and sensory neuron projections cooperatively form a common principal grid of peripheral nerve pathways relaying behavioral outputs and somatosensory inputs. Here, we explored this issue through targeted cell lineage and gene manipulation in mouse, combined with in vitro live axon imaging. In the absence of motor projections, dorsal (epaxial) and ventral (hypaxial) sensory projections form in a randomized manner, while removal of EphA3/4 receptor tyrosine kinases expressed by epaxial motor axons triggers selective failure to form epaxial sensory projections. EphA3/4 act non-cell-autonomously by inducing sensory axons to track along preformed epaxial motor projections. This involves cognate ephrin-A proteins on sensory axons but is independent from EphA3/4 signaling in motor axons proper. Assembly of peripheral nerve pathways thus involves motor axon subtype-specific signals that couple sensory projections to discrete motor pathways.

Receptor tyrosine phosphatases guide vertebrate motor axons during development

Receptor-type protein tyrosine phosphatases (RPTPs) are required for appropriate growth of axons during nervous system development in Drosophila. In the vertebrate, type IIa RPTPs [protein tyrosine phosphatase (PTP)-delta, PTP-sigma, and LAR (leukocyte common-antigen-related)] and the type III RPTP, PTP receptor type O (PTPRO), have been implicated in the regulation of axon growth, but their roles in developmental axon guidance are unclear. PTPRO, PTP-delta, and PTP-sigma are each expressed in chick motor neurons during the period of axonogenesis. To examine potential roles of RPTPs in axon growth and guidance in vivo, we used double-stranded RNA (dsRNA) interference combined with in ovo electroporation to knock down RPTP expression levels in the embryonic chick lumbar spinal cord. Although most branches of the developing limb nerves appeared grossly normal, a dorsal nerve identified as the anterior iliotibialis was clearly affected by dsRNA knock-down of RPTPs. In experimental embryos treated with dsRNA targeting PTP-delta, PTP-sigma, or PTPRO, this nerve showed abnormal fasciculation, was reduced in size, or was missing entirely; interference with PTPRO produced the most severe phenotypes. Control embryos electroporated with vehicle, or with dsRNA targeting choline acetyltransferase or axonin-1, did not exhibit this phenotype. Surprisingly, embryos electroporated with dsRNA targeting PTP-delta together with PTPRO, or all three RPTPs combined, had less severe phenotypes than embryos treated with PTPRO alone. This result suggests that competition between type IIa and type III RPTPs can regulate motor axon outgrowth, consistent with findings in Drosophila. Our results indicate that RPTPs, and especially PTPRO, are required for axon growth and guidance in the developing vertebrate limb.

The contributions of BMP4, positive guidance cues, and repulsive molecules to cutaneous nerve formation in the chick hindlimb

Our previous surgical manipulations have shown that the target ectoderm is necessary for the initial formation of one of the major cutaneous nerves in the embryonic chick limb (Honig, M.G., Camilli, S.J., Xue, Q.S., 2004. Ectoderm removal prevents cutaneous nerve formation and perturbs sensory axon growth in the chick hindlimb. Dev. Biol. 266, 27-42.). Moreover, the target ectoderm is required during a critical time period, at approximately St. 24, when those axons are about to diverge from the hindlimb plexus. To elucidate the underlying mechanisms, here we examined the effects of removing the ectoderm at St. 24 on a variety of molecules expressed within the limb. We find that, while ectoderm removal is accompanied by changes in the expression of Lmx1, fibronectin, EphA7, cDermo-1, and in the complement of muscle cells, these changes do not account for the cutaneous nerve deficit. In contrast, an upregulation of PNA-binding sites and a downregulation of Bmp4 appear to be associated with this nerve deficit. Exogenous BMP4 reversed the effect of ectoderm removal on cutaneous nerve formation, but did not act as a chemoattractant. Our results suggest that BMP4, together with permissive and repulsive molecules that growing cutaneous axons encounter in the local environment and with signaling molecules, originating from and/or dependent on the ectoderm, work in concert to ensure proper cutaneous nerve formation.

Spinal sensorimotor transformation: relation between cutaneous somatotopy and a reflex network

The projection of primary afferents onto spinal interneurons constitutes the first step in sensorimotor transformations performed by spinal reflex systems. Despite extensive studies on spinal somatotopy, uncertainties remain concerning the extent and significance of representational overlap and relation to spinal reflex circuits. To address these issues, the cutaneous projection from the hindpaw and its relation to the topography of lamina V neurons encoding withdrawal reflex strength ("reflex encoders") was studied in rats. Thin and coarse primary afferent terminations in laminas II and III-IV, respectively, were mapped by wheat germ agglutinin-horseradish peroxidase and choleragenoid tracing. The functional weights of these projections were characterized by mapping nociceptive and tactile field potentials and compared with the topography of reflex encoders. Both anatomical and physiological data indicate that thin and coarse skin afferent input is spatially congruent in the horizontal plane. The representation of the hindpaw in the spinal cord was found to be intricate, with a high degree of convergence between the projections from different skin sites. "Somatotopic disruptions" such as the representation of central pads medial to that of the digits were common. The weight distribution of the cutaneous convergence patterns in laminas III-IV was similar to that of lamina V reflex encoders. This suggests that the cutaneous convergence and features such as somatotopic disruptions have specific relations to the sensorimotor transformations performed by reflex interneurons in the deep dorsal horn. Hence, the spinal somatotopic map may be better understood in light of the topography of such reflex systems.

Peripheral specification of Ia synaptic input to motoneurons innervating foreign target muscles

Muscle sensory neurons, called Ia afferents, make monosynaptic connections with functionally related sets of motoneurons in the spinal cord. Previous work has suggested that peripheral target muscles play a major role in determining the central connections of Ia afferents with motoneurons. Here, we ask whether motoneurons can also be influenced by their target muscles in terms of the monosynaptic input they receive from Ia afferents, by transplanting thoracic motoneurons into the lumbosacral spinal cord so that they innervate foreign muscles. Three or four segments of thoracic neural tube from stage 14-15 chicken embryos were transplanted to the lumbosacral region of stage 16-17 embryos, and electrophysiological recordings were made from transplanted motoneurons after the embryos had reached stage 38-40. Transplanted thoracic motoneurons innervated limb muscles and received monosynaptic inputs from Ia afferents. These connections were not random: Most of the connections were formed between Ia afferents and motoneurons projecting to the same muscle (homonymous connections). Few aberrant connections were found although the anatomical distribution of afferents in the transplant indicated that they had ample opportunity to contact inappropriate motoneurons. We conclude that although peripheral target cues are not sufficient to respecify an already committed motoneuron (turn a thoracic motoneuron into a lumbosacral motoneuron), they do provide sufficient information for Ia afferent input to be functionally correct.

Compartmentalisation of the developing trigeminal ganglion into maxillary and mandibular divisions does not depend on target contact

During development axons contact their target tissues with phenomenal accuracy but the mechanisms that control this homing behaviour remain largely elusive. A prerequisite to the study of the factors involved in hard-wiring the nervous system during neurogenesis is an accurate calendar of developmental events. We have studied the maxillary and mandibular components of the trigeminal system to determine the stages during embryogenesis when a gross somatotopic order is first established within the trigeminal ganglion and the axons projecting to the brainstem. The retrograde transganglionic fluorescent tracers DiO and DiI were injected into the maxillary and mandibular arches or their derivatives in fixed mouse embryos staged between 13 and 40 somites (E9-E11). After 1-4 wk, the distribution of the 2 tracers was determined using confocal laser scanning microscopy. The first maxillary nerve cell bodies and their developing axons were labelled at the 30 somite stage (E10). This was 2 somite stages earlier than the mesencephalic nucleus and the ganglion cell bodies of the mandibular nerve. The gross somatotopic division of cells within the trigeminal ganglion projecting to the maxillary and mandibular targets was established by the 32 somite stage (E10). This arrangement was evident as 2 groups of cell bodies occupying adjacent but separate regions of the trigeminal ganglion. The central branches of the maxillary and mandibular cell bodies entered the metencephalon as 2 distinct bundles at the same stage. The trigeminal motor nucleus was first detected at the 38 somite stage (E10.5). Gross somatotopy in the major divisions of the trigeminal ganglion is established before outgrowing axons have contacted their peripheral target tissue at E10.5. This suggests that target tissues do not induce somatotopy.

Ultrastructural observations on the expression of axonin-1: implications for the fasciculation of sensory axons during axonal outgrowth into the chick hindlimb

To help understand how axons interact as they grow into the developing chick hindlimb, we used electron microscopy in conjunction with immunoperoxidase staining for the cell adhesion molecule axonin-1 to label sensory axons. The results showed that sensory axons travel together in bundles, tightly apposed to one another. In contrast, motoneuron axons are more widely spaced, although motoneuron axons situated at the perimeter of sensory axon bundles are found in close contact with neighboring sensory axons. Sensory growth cones and lamellipodia tend to be located centrally within the bundles, with several lamellipodia typically being found stacked together. Strikingly, regions of close axonal apposition are accompanied by axonin-1 expression, suggesting that such contacts are indeed adhesive. Taken together, these observations suggest that groups of sensory axons of a similar age grow together, with some of the older sensory axons fasciculating along motoneuron axons and younger sensory axons later fasciculating along older sensory axons. Axons situated at the periphery of sensory bundles are typically partly labelled, such that axonin-1 is expressed on membranes apposing other labelled axons but not on those facing unlabelled axons or unlabelled Schwann cells. Thus, axonin-1 appears to become redistributed within the membranes of axons growing into the limb, as it does on cultured neurons. In contrast, the neuron-glia cell adhesion molecule (NgCAM), which binds heterophilically to axonin-1, appears uniformly distributed on even those axons that would have an asymmetric distribution of axonin-1. Thus, the localization of axonin-1 strongly suggests that it plays an important role in sensory axon fasciculation, but the relative contributions of its interactions with various potential ligands are unclear. Finally, we found that some sensory growth cones have lamellipodia that are spread over considerable expanses. This suggests that although fasciculation is important in sensory axon guidance, sensory axons may also explore the local environment.

Independent development of sensory and motor innervation patterns in embryonic chick hindlimbs

Previous studies suggest that sensory axon outgrowth is guided by motoneurons, which are specified to innervate particular target muscles. Here we present evidence that questions this conclusion. We have used a new approach to assess the pathfinding abilities of bona fide sensory neurons, first by eliminating motoneurons after neural crest cells have coalesced into dorsal root ganglia (DRG) and second by challenging sensory neurons to innervate muscles in a novel environment created by shifting a limb bud rostrally. The resulting sensory innervation patterns mapped with the lipophilic dyes DiI and DiA showed that sensory axons projected robustly to muscles in the absence of motoneurons, if motoneurons were eliminated after DRG formation. Moreover, sensory neurons projected appropriately to their usual target muscles under these conditions. In contrast, following limb shifts, muscle sensory innervation was often derived from inappropriate segments. In this novel environment, sensory neurons tended to make more "mistakes" than motoneurons. Whereas motoneurons tended to innervate their embryologically correct muscles, sensory innervation was more widespread and was generally from more rostral segments than normal. Similar results were obtained when motoneurons were eliminated in embryos with limb shifts. These findings show that sensory neurons are capable of navigating through their usual terrain without guidance from motor axons. However, unlike motor axons, sensory axons do not appear to actively seek out appropriate target muscles when confronted with a novel terrain. These findings suggest that sensory neuron identity with regard to pathway and target choice may be unspecified or quite plastic at the time of initial axon outgrowth.

The spatial relationships among cutaneous, muscle sensory and motoneuron axons during development of the chick hindlimb

Previous studies have suggested that interactions with other axons are important in sensory axon pathfinding in the developing chick hindlimb. Yet the nature of these interactions remains unknown, in part because information about the spatial relationships among the different kinds of axons is lacking. To obtain this information, we combined retrograde axonal tracing with an immunofluorescent labelling approach that distinguishes between sensory and motoneuron axons. This allowed us to follow the trajectories of sensory axons having a known destination, while also identifying their neighbors. We found that as sensory and motoneuron axons meet in the spinal nerves and travel into the limb, sensory axons remain bundled together. The large bundles that are present proximally gradually split into smaller bundles as the axons course distally in the spinal nerves; more distally, some bundles join to again form large bundles. Younger, later-growing sensory axons appear to grow primarily along bundles of older sensory axons that grew out earlier. Starting from very proximal levels, axons projecting along an individual cutaneous nerve are found together in bundles that are situated in characteristic regions of each spinal nerve. Some of these bundles are initially interspersed with bundles of axons projecting along other nerves, thereby indicating that the initial position of a cutaneous axon in the spinal nerves does not strictly determine its subsequent trajectory. As they travel distally, bundles of axons projecting along one cutaneous nerve gradually join one another, becoming increasingly separated from axons having different destinations. In contrast, muscle sensory axons are situated adjacent to motoneuron axons innervating the same muscle for much of their course. This suggests that muscle sensory axons may be guided to the appropriate muscles by fasciculating along motoneuron axons. Taken together, the results show that sensory axons projecting along different nerves are different from one another and respond to cues in their environment to navigate through the spinal nerves and plexus. Thus, sensory neurons must be intrinsically specified with respect to their peripheral targets. Sensory axons appear to respond differentially to the axons they encounter, segregating from axons that project along different nerves and often growing with axons destined for the same nerve, suggesting that fasciculation may aid pathfinding.

Neuropilin-semaphorin III/D-mediated chemorepulsive signals play a crucial role in peripheral nerve projection in mice

Neuropilin is a neuronal cell surface protein and has been shown to function as a receptor for a secreted protein, semaphorin III/D, that can induce neuronal growth cone collapse and repulsion of neurites in vitro. The roles of neuropilin in vivo, however, are unknown. Here, we report that neuropilin-deficient mutant mice produced by targeted disruption of the neuropilin gene show severe abnormalities in the trajectory of efferent fibers of the PNS. We also describe that neuropilin-deprived dorsal root ganglion neurons are perfectly protected from growth cone collapse elicited by semaphorin III/D. Our results indicate that neuropilin-semaphorin III/D-mediated chemorepulsive signals play a major role in guidance of PNS efferents.

Muscle sensory innervation patterns in embryonic chick hindlimbs following dorsal root ganglion reversal

Previous studies suggest that sensory innervation of muscles is patterned by motor innervation. Muscle afferent projections mirror motor projections after various experimental manipulations and muscle afferents fail to project to muscle in the absence of motoneurons. It is not known, however, whether muscle afferents are specified with respect to the corresponding motoneurons or target muscles. To test this possibility we rotated three to four segments of neural crest in St. 15-17 chick embryos, leaving motoneurons intact, to reverse the rostrocaudal order of dorsal root ganglia (DRGs) T7/LS1-LS3. This caused sensory neurons derived from one segmental level to grow into the limb with motor axons from a different level. The resulting innervation patterns were assessed at St. 28-37 by injecting DiI and DiA into the sartorius and femorotibialis muscles or into the spinal cord and DRG. DiI labeling of crest prior to rotation showed that DRGs in the operated region were derived primarily from rotated cells. Muscle afferents from rotated DRGs grew to muscles in accord with their new rostrocaudal position, together with "inappropriate" motor axons from the same segmental level. The segmental distribution of sensory neurons innervating each muscle was more widespread in embryos operated at older than at younger stages. In contrast, sensory axons projected to the appropriate muscles in accord with their embryonic origin when segments of the whole neural tube, including motoneurons, were rotated, as reported previously. Thus, sensory neurons do not appear to be selectively matched with motoneurons or target muscles at stages when the corresponding motoneurons have clear identities.

Prenatal development of rat primary afferent fibers: I. Peripheral projections

Development of the peripheral innervation patterns of the L1-S1 lumbosacral ganglia and motor segments in embryonic day 12-17 (E12-17) rat embryos was examined using carbocyanine dyes. Individual dorsal root ganglia (DRGs) and/or isolated ventral horn (VH) segments, or individual peripheral nerves, were isolated in rat embryos fixed at different stages and filled with one of three carbocyanine dyes; DiI, DiA, and DiO. Individual experimental preparations included labeling of 1) single DRGs; 2) multiple DRGs with alternating dyes, DiO, DiI, and DiA; 3) single isolated VH segments; 4) multiple VH segments with alternating dyes; 5) single VH segments and the corresponding segmental DRGs with different dyes; and 6) two or more individual peripheral nerves labeled with different dyes. Results from these preparations have shown that the first fibers exited the lumbar ventral horn and DRGs at E12. At E13 major nerve trunks (e.g., femoral and sciatic) were visible as they exited the plexus region. By E14 afferent fibers were present in the epidermis of the proximal hindlimb, and the major nerve trunks extended into the leg. Fibers originating from L3 to L5 (DRG and VH) reached the paw by E14.5-E15, and the epidermis of the most distal toes was innervated by E16-E16.5. While afferent fibers and motor axons of the same segmental origin mixed extensively in the spinal nerve, fibers of different segmental origin combined in the plexus and major nerve trunks with little or no interfascicular mixing. Dermatomes observed at E14 were in general spotty and non-contiguous. However, by E16 the dermatomes resembled mature forms with substantial overlap only between adjacent ones. Thus the adult pattern of spatial relationships between cutaneous afferent fibers in the periphery is established early in development.

Morphological specification of trigeminal neurites depends on target fields

Primary sensory neurons bridge the sensory periphery to the central nervous system (CNS) via their two axonal processes. The morphological patterning of the terminals of each process in its respective target is unique. Whether the differences between peripheral and central axons result from an intrinsic developmental program of the ganglion cell body, or from target-derived signals is not known. To explore this issue, we have used an explant coculture system in which embryonic (E15) trigeminal ganglion explants were placed between a vibrissa pad and a brainstem explant, but the explants were rotated 180 degrees relative to their normal orientation. In other experiments, individual ganglia were placed between two vibrissa pad explants or between two slices taken through the brainstem. The cultures were fixed after several days and ganglion cell processes were labeled with the lipophilic tracer DiI. Results of the ganglion rotation experiments suggest that trigeminal axons which would be directed centrally in vivo can regenerate into peripheral targets, and peripheral axons can grow into CNS tissue. Similarly, in cocultures with two peripheral or two central targets, both processes of trigeminal ganglion cells can simultaneously invade vibrissa pad explants or project into brainstem slices. Moreover, in all cocultures the differentiation of each set of processes is specific to the target innervated by it. These results show that the axons of embryonic sensory neurons are not selective in their choice of targets, and that their morphological patterning is dictated by target-derived signals.

Normal and abnormal pathfinding of facial nerve fibers in the chick embryo

Development of the facial nerve was studied in normal chicken embryos and after surgical disruption of ingrowing sensory facial nerve fibers at 38-72 h of incubation. Disruption of facial nerve fibers by otocyst removal often induced a rostral deviation of the facial nerve and ganglion to the level of the trigeminal ganglion. Cell bodies of the geniculate ganglion trailed their deviating neurites and occupied an abnormal rostral position adjacent to the trigeminal ganglion. Deviating facial nerve fibers were labeled with the carbocyanine fluorescent tracer DiI in fixed tissue. Labeled fibers penetrated the cranium adjacent to the trigeminal ganglion, but they did not follow the trigeminal nerve fibers into the brain stem. Rather, after entering the cranium, they projected caudally to their usual site of entrance and proceeded towards their normal targets. This rostral deviation of the facial nerve was observed only after surgery at 48-72 h of incubation, but not in cases with early otocyst removal (38-48 h). A rostral deviation of the facial nerve was seen in cases with partial otocyst removal when the vestibular nerve was absent. The facial nerve followed its normal course when the vestibular nerve persisted. We conclude that disruption of the developing facial pathway altered the routes of navigating axons, but did not prevent pathfinding and innervation of the normal targets. Pathfinding abilities may not be restricted to pioneering axons of the facial nerve; later-developing facial nerve fibers also appeared to have positional information. Our findings are consistent with the hypothesis that navigating axons may respond to multiple guidance cues during development. These cues appear to differ as a function of position of the navigating axon.

GAP-43 expression in developing cutaneous and muscle nerves in the rat hindlimb

The expression of the growth associated protein, GAP-43, in developing rat hindlimb peripheral nerves has been studied using immunocytochemistry. GAP-43, is first detected in lumbar spinal nerves at embryonic day (E)12 as the axons grow to the base of the hindlimb. It is expressed along the whole length of the nerves as well as in the growth cones. GAP-43 staining becomes very intense over the next 36 h while the axons remain in the plexus region at the base of the limb bud before forming peripheral nerves at E14. It remains intense along the length of the growing peripheral nerves, the first of which are cutaneous, branching away from the plexus and growing specifically to the skin, their axon tips penetrating the epidermis of the proximal skin at E15 and the toes at E19. GAP-43-containing terminals form a dense plexus throughout the epidermis which subsequently withdraws subepidermally in the postnatal period. GAP-43 staining is also evident along the growing muscle nerves during muscle innervation, which follows behind that of skin. Axons branch over the surface of proximal muscles at E15 but do not form terminals until E17. As target innervation proceeds, GAP-43 staining declines in the proximal part of the nerve but remains intense in the distal portions. Overall GAP-43 expression in the hindlimb decreases in the second postnatal week as axon growth and peripheral terminal formation decline.

Different subsets of axonal guidance cues are essential for sensory neurite outgrowth to cutaneous and muscle targets in the dorsal ramus of the embryonic chick

The dorsal ramus nerve diverges dorsally from each spinal nerve to innervate the epaxial muscle and dermis that are derived in situ from each dermamyotome. The outgrowth of both the sensory and motor components of this nerve are sensitive to the proximity of the dermamyotome. Motoneurons display a direct target response that is not dependent upon the concurrent outgrowth of sensory neurites (Tosney: Dev. Biol. 122:540-588, 1987). Likewise, the outgrowth of sensory neurites could be directly dependent on the dermamyotome. Alternatively, sensory neurites could be dependent on motor axons that in turn require the dermamyotome for outgrowth. To distinguish between these possibilities, motor outgrowth was abolished by unilateral ventral neural tube deletion and the patterns of subsequent sensory neurite outgrowth were assessed. The cutaneous nerve branch formed in all cases. In contrast, neither of the epaxial muscle nerves formed in the absence of epaxial motoneuron outgrowth. Furthermore, sensory neurites could not be detected diverging into muscle from the cutaneous nerve or entering muscle via other novel routes. We conclude that motoneurons are essential for sensory outgrowth to epaxial muscle but not to cutaneous targets. It is clear that different subsets of navigational cues guide sensory afferents to muscle and to cutaneous destinations.

Development of central projections of lumbosacral sensory neurons in the chick

The development of central projections of sensory neurons in lumbosacral dorsal root ganglia (DRGs) was examined by using horseradish peroxidase labeling techniques in chick embryos from stage 23 (E4) to stage 39 (E13). Our results show that primary afferents reach the spinal cord by stage 23. Afferent axons extend in the primordium of the dorsal funiculus for several segments rostral and caudal to their segment of entry for over 24 hours before invading the gray matter at stage 28 (E6). Sensory fibers grow into the vicinity of motoneuron dendrites by stage 32 (E7.5), about the time that reflexes and apparent monosynaptic EPSPs can first be elicited. Dense projections into the dorsal laminae of the spinal cord, presumably representing cutaneous afferents, appear somewhat later, at about stage 39 (E13), when the segmental projection pattern begins to resemble the mature pattern.

Role of neuron-target interactions in the development of a subpopulation of avian sensory neurons

Subpopulations of sensory neurons appear in embryonic dorsal root ganglia during development. In this paper, we examine what role neuron-target interactions play in this process. Previous work has shown that alterations in the environment of developing sensory neurons regulate the proportion of neurons that express a cell surface antigen identified by the monoclonal antibody SN1 (Marusich et al.: Dev. Biol. 118: 505-510, 1986). We now report that neuron-target interactions may also act to stabilize the phenotype of sensory neurons. Thus, older SN1- neurons, which would normally remain SN1- if left undisturbed in vivo, can express SN1 immunoreactivity in vitro when they are deprived of contact with their normal peripheral targets. We also demonstrate that naive sensory neurons, i.e., those that have never made contact with peripheral targets, can be identified and maintained in culture. At least some of these naive neurons (all of which are initially SN1-) can express SN1 immunoreactivity in vitro, in the absence of contact with normal peripheral targets. We conclude that subpopulations of sensory neurons may arise from naive neurons in the absence of neuron-target interactions but that subsequent neuron-target interactions may act to stabilize or modulate subpopulation-specific phenotypes.

Skin sensory innervation patterns in embryonic chick hindlimbs deprived of motoneurons

During normal development and following a variety of experimental manipulations (e.g., neural tube rotations, limb shifts), sensory neurons in the chick grow to their correct targets. L. Landmesser and M. G. Honig (1986, Dev. Biol. 118, 511-531) have suggested that sensory innervation may be precise, not because sensory neurons respond to limb-derived guidance cues, but because sensory neurons interact with motoneurons, which do respond to such cues. To test this hypothesis for skin sensory neurons, the ventral neural tube, including the motoneuron precursors, was removed from chick embryos prior to sensory axon outgrowth and the resulting patterns of dermatomes and axonal projections were mapped physiologically and anatomically. As reported previously, dorsal root ganglia (DRGs) and cutaneous nerves formed in their usual locations following the early removal of motoneurons, while most muscle nerves and the plexus region were reduced substantially (A. C. Taylor, 1944, J. Exp. Zool. 96, 159-185; L. Landmesser and M. G. Honig, 1986, Dev. Biol. 118, 511-531; G. J. Swanson and J. Lewis, 1986, J. Embryol. Exp. Morphol. 95, 37-52). The patterns of axonal projections and dermatomes were surprisingly, although not entirely, normal. In particular, cutaneous nerves in motoneuron-depleted embryos were derived from the same DRGs in approximately the same proportions as normal. Thus, while motoneurons may play a facilitative role in the development of the segmental pattern of skin sensory innervation, they do not appear to be essential.

Proximal tissues and patterned neurite outgrowth at the lumbosacral level of the chick embryo: deletion of the dermamyotome

The role of the dermamyotome (the dorsal portion of the somite which gives rise to muscles and dermis) in the development of patterned axon outgrowth was examined under conditions where limb development was substantially undisturbed. One or more chick dermamyotomes were removed before or during early neurite outgrowth and subsequent development was examined. Several developmental processes suspected to depend on the dermamyotome were not altered by its removal: (1) Neural crest cells that form sensory ganglia migrated and condensed in their normal segmental pattern. (2) The distal progression, dorsal-ventral organization, and segmentation of spinal nerves were unaltered. (3) Motoneuron pathway selection and projection patterns in the limb were normal in all respects. The most interesting finding was that the formation of the dorsal ramus is dependent on the nearby dermamyotome which provides the targets for this nerve. When a single or two adjacent dermamyotomes were removed, the metameric epaxial muscles derived from each dermamyotome were absent and the dorsal ramus extended into epaxial muscle in the closest adjacent segment. However, when dermamyotomes in both adjacent segments had also been removed or substantially reduced, the dorsal ramus did not form. These results strongly suggest that the target provides a chemotactic signal for proper outgrowth of dorsal ramus axons.