Anatomical correlates of the forelimb in the ventrobasal complex and the cuneate nucleus of the neonatal rat

Forelimb amputation-induced reorganization in the ventral posterior lateral nucleus (VPL) provides a substrate for large-scale cortical reorganization in rat forepaw barrel subfield (FBS)

In this study, we examined the role of the ventral posterior lateral nucleus (VPL) as a possible substrate for large-scale cortical reorganization in the forepaw barrel subfield (FBS) of primary somatosensory cortex (SI) that follows forelimb amputation. Previously, we reported that, 6 weeks after forelimb amputation in young adult rats, new input from the shoulder becomes expressed throughout the FBS that quite likely has a subcortical origin. Subsequent examination of the cuneate nucleus (CN) 1 to 30 weeks following forelimb amputation showed that CN played an insignificant role in cortical reorganization and led to the present investigation of VPL. As a first step, we used electrophysiological recordings in forelimb intact adult rats (n=8) to map the body representation in VPL with particular emphasis on the forepaw and shoulder representations and showed that VPL was somatotopically organized. We next used stimulation and recording techniques in forelimb intact rats (n=5) and examined the pattern of projection (a) from the forelimb and shoulder to SI, (b) from the forepaw and shoulder to VPL, and (c) from sites in the forepaw and shoulder representation in VPL to forelimb and shoulder sites in SI. The results showed that the projections were narrowly focused and homotopic. Electrophysiological recordings were then used to map the former forepaw representation in forelimb amputated young adult rats (n=5) at 7 to 24 weeks after amputation. At each time period, new input from the shoulder was observed in the deafferented forepaw region in VPL. To determine whether the new shoulder input in the deafferented forepaw VPL projected to a new shoulder site in the deafferented FBS, we examined the thalamocortical pathway in 2 forelimb-amputated rats. Stimulation of a new shoulder site in deafferented FBS antidromically-activated a cell in the former forepaw territory in VPL; however, similar stimulation from a site in the original shoulder representation, outside the deafferented region, in SI did not activate cells in the former forepaw VPL. These results suggest that the new shoulder input in deafferented FBS is relayed from cells in the former forepaw region in VPL. In the last step, we used anatomical tracing and stimulation and recording techniques in forelimb intact rats (n=9) to examine the cuneothalamic pathway from shoulder and forepaw receptive field zones in CN to determine whether projections from the shoulder zone might provide a possible source of shoulder input to forepaw VPL. Injection of biotinylated dextran amine (BDA) into physiologically identified shoulder responsive sites in CN densely labeled axon terminals in the shoulder representation in VPL, but also gave off small collateral branches into forepaw VPL. In addition, microstimulation delivered to forepaw VPL antidromically-activated cells in shoulder receptive field sites in CN. These results suggest that forepaw VPL also receives input from shoulder receptive sites in CN that are latent or subthreshold in forelimb intact rats. However, we speculate that following amputation these latent shoulder inputs become expressed, possibly as a down-regulation of GABA inhibition from the reticular nucleus (RTN). These results, taken together, suggest that VPL provides a substrate for large-scale cortical reorganization that follows forelimb amputation.

Development and critical period plasticity of the barrel cortex

In primary sensory neocortical areas of mammals, the distribution of sensory receptors is mapped with topographic precision and amplification in proportion to the peripheral receptor density. The visual, somatosensory and auditory cortical maps are established during a critical period in development. Throughout this window in time, the developing cortical maps are vulnerable to deleterious effects of sense organ damage or sensory deprivation. The rodent barrel cortex offers an invaluable model system with which to investigate the mechanisms underlying the formation of topographic maps and their plasticity during development. Five rows of mystacial vibrissa (whisker) follicles on the snout and an array of sinus hairs are represented by layer IV neural modules ('barrels') and thalamocortical axon terminals in the primary somatosensory cortex. Perinatal damage to the whiskers or the sensory nerve innervating them irreversibly alters the structural organization of the barrels. Earlier studies emphasized the role of the sensory periphery in dictating whisker-specific brain maps and patterns. Recent advances in molecular genetics and analyses of genetically altered mice allow new insights into neural pattern formation in the neocortex and the mechanisms underlying critical period plasticity. Here, we review the development and patterning of the barrel cortex and the critical period plasticity.

Functional and structural organization of the forelimb representation in cuneate nucleus in rat

We examined the physiological representation of the forelimb in the cuneate nucleus (CN) of forelimb-intact young adult rats (n=38) as the first part in a series of studies aimed at understanding the possible role that CN plays in delayed cortical reorganization that follows forelimb amputation. Metabolic labeling with cytochrome oxidase (CO) and electrophysiological mapping were used to examine the relationship between the structural and functional organization of CN. CN is a cylinder-shaped structure that lies bilaterally in the brainstem and extends nearly 4mm in the rostrocaudal direction. The forelimb is represented along the rostrocaudal extent. CN contains three zones; the rostral and caudal zones receive input largely from deep muscle and joint receptors and a middle zone, in the vicinity of the obex, receives input primarily from cutaneous receptors in the skin. The middle zone is somatotopically organized with the glabrous digits represented centrally, bordered on the medial side by ulnar wrist, ulnar forearm, and posterior upper arm representations; on the lateral side by radial wrist, radial forearm, and anterior upper arm representations; and on dorsal side by the dorsal digits and dorsal hand. The middle zone also contains well-defined CO-filled glomerular structures, called barrelettes, which are located within a homogenously stained field. The barrelettes are associated with the representation of the glabrous digits, with D5 represented most dorsal followed sequentially in a ventral-to-lateral direction by the representation of D4, D3, D2, and D1. The digit representations are topographically organized with the distal digit surface represented laterally with respect to the more medially lying proximal digit surface. The digit and palmar pads are also represented by barrelettes located on the medial side of CN. In contrast, the dorsal digit surfaces are represented dorsally and the dorsal hand is represented directly beneath the cuneate fasciculus, in a region devoid of barrelettes. The representations of the ulnar and radial wrist, forearm, and upper arm also lie within the homogeneously stained field in CN. The forelimb representation is bordered on the medial side by representation of trunk and hindlimb, and on the lateral side by representation of shoulder, ear, and head. While the present findings support and extend previous electrophysiological and anatomical studies of CN in the rat, they also provide a detailed physiological description of the functional organization of CN that is necessary for subsequent understanding of the functional reorganization of CN that may result following forelimb amputation.

Altered parcellation of neocortical somatosensory maps in N-methyl-D-aspartate receptor-deficient mice

The body map in the parietal neocortex is built by inputs from the brainstem and thalamic somatosensory nuclei. Receptor density in the sensory periphery and neural activity play a major role in allocation of cortical tissue to different components of the somatosensory body map. Here we present evidence that neural activity mediated via N-methyl-D-aspartate (NMDA) receptors plays a major role in parcellation of the cortical body map subdivisions. In mice with genetically lowered NMDA receptor function along the trigeminal pathway, subcortical trigeminal nuclei shrink and, consequently, the face representation area of the primary somatosensory cortex diminishes in size. In contrast, dorsal column subcortical paw representation areas that are not as severely affected by the genetic manipulation of NMDA receptors do not show any areal changes, yet their cortical projection zones expand. Our findings indicate that both subcortical and cortical mechanisms contribute to cortical parcellation of body map subdivisions in an NMDA receptor-dependent manner.

The chemo- and somatotopic architecture of the Galago cuneate and gracile nuclei

The pattern of peripheral nerve inputs into the dorsal column nuclei, cuneate and gracile, was investigated in the prosimian Galago garnetti. The major findings were, that there is a greater segregation of the inputs from the fingers/hand within the cuneate compared with input form the toes/foot within the gracile. In both nuclei, cell clusters can be identified as cytochrome oxidase dense blotches, reactive also for the activity-dependent enzyme nitric oxide synthase. In the cuneate, cell clusters were apparent as six main cytochrome oxidase/nitric oxide synthase-reactive ovals arranged in a medial to lateral sequence. In contrast in the gracile, a higher degree of parcellation was noted and several cytochrome oxidase/nitric oxide synthase blotches were distributed along the rostrocaudal axis of the nucleus. This different architecture parallels differences in the organization of the inputs from the hand and from the foot. In the cuneate, cholera toxin B subunit conjugated to horseradish peroxydase labeled terminals from the glabrous and hairy skin of digits d1 to d5 segregated in each of the five most lateral cytochrome oxidase/nitric oxide synthase blotches. Afferents from the thenar, palmar pads and hypothenar overlapped with those from digit 1, digit 2 to digit 4 and digit 5, respectively. Inputs from wrist arm and shoulder were segregated in the most medial blotch. In the gracile, multiple foci of cholera toxin B subunit conjugated to horseradish peroxydase labeled terminals were observed upon injections of single sites in the toes or plantar pads. Although in multiple foci, inputs from different toes segregated from one another as well. Terminals from the plantar pads appeared to converge on the same cytochrome oxidase/nitric oxide synthase blotches targeted by inputs from the toes. In both the cuneate and the gracile, cytochrome oxidase/nitric oxide synthase blotches also presented intense immunoreactivity for GABA, calbindin, parvalbumin, and brain derived neurotrophic factor. Finally, in the cuneate the cell cluster region presented similarities in prosimian galagos and four species of New World monkeys, whereas it appeared more differentiated and complex in the Old Word macaque monkeys. In conclusion, the different pattern of segregation of the inputs from the hand and from the foot can be related to the different metabolic organization of the cuneate and of the gracile, respectively.

Neurotrophin receptor (p75) in the trigeminal thalamus of the rat: development, response to injury, transient vibrissa-related patterning, and retrograde transport

We report on the transient, patterned expression of p75 in the ventrobasal (VB) thalamus, the major thalamic relay for somatosensation. We immunostained the brains of developing rats ranging in age from embryonic day (E) 14.5 to postnatal day (PD) 15 with an antibody against p75. To compare p75 expression with the developing synaptic organization within VB, we also immunolocalized the synaptic-vesicle-associated protein, synaptophysin (SYN), on alternate sections. p75-immunoreactivity (IR) was dense and uniform in the ventroposterior medial nucleus (VPM) in the late embryonic and early postnatal periods (E 16.5 to PD 3). In contrast, from PD 4-10, p75-IR in the VPM was patterned, reminiscent of cytochrome-oxidase-stained barreloids, a characteristic feature of the VB in rodents. By PD 14, p75-IR in the VPM was no longer detectable. The ventroposterior lateral nucleus (VPL), in contrast, exhibited no p75-IR. No p75-IR was detected in the ventroposterior lateral nucleus (VPL) at any developmental stage in which VPM could be distinguished from VPL. Light, but clearly patterned SYN-IR, first detectable on PD 2-3, increased in intensity in both VPL and VPM through PD 15. Sectioning the infraorbital nerve on PD 0 resulted in blurred patterns of p75- and SYN-IR within VPM in PD 7-9 rat pups. Removing large portions of the somatosensory cortex on PD 0 resulted in subsequent greatly reduced p75- and SYN-IR within VB. To specify the source of the p75-IR terminals, we stereotaxically injected into the VPM of PD 4-5 rats a monoclonal antibody to p75. One to 2 days later, IR of retrogradely transported p75 antibodies could be traced within axons and cell bodies of neurons associated with the trigeminothalamic pathway through the caudal diencephalon and mesencephalon; labelling was confined to the contralateral trigeminal principal sensory nucleus. The observed, transiently patterned p75-IR in VPM the early postpartum period suggests a role for p75 in synaptogenesis and pattern formation.

NMDA receptor-dependent refinement of somatotopic maps

We have examined the role of NMDA receptor-mediated neural activity in the formation of periphery-related somatosensory patterns, using genetically engineered mice. We demonstrate that ectopic expression of a transgene of an NMDAR1 splice variant rescues neonatally fatal NMDAR1 knockout (KO) mice, although the average life span varies depending on the level of the transgene expression. In NMDAR1 KO mice with "high" levels of the transgene expression, sensory periphery-related patterns were normal along both the trigeminal and dorsal column pathways. In the KO mice with "low" levels of the transgene expression, the patterns were absent in the trigeminal pathway. Our results indicate that NMDA receptor-mediated neural activity plays a critical role in pattern formation along the ascending somatosensory pathways.

Synaptophysin immunoreactivity and distributions of calcium-binding proteins highlight the functional organization of the rat's dorsal column nuclei

The mammalian dorsal column nuclei (DCN) are principally composed of the cuneate (CN) and gracile (GN) nuclei. Data presented here support previously published anatomical and functional evidence that the longitudinal organization of the CN and GN reflect the complex role of the DCN in somatosensory processing. The CN is organized longitudinally into three parts. Within the middle portion of this nucleus, primary afferent projections and cuneothalamic cells are concentrated. Although traditional cytoarchitectonic analyses had failed to detect this tripartite organization in rats, we found evidence for it, with a functional middle region, extending approximately 0.2-0.9 mm caudal to the obex, characterized by precise somatotopy of primary afferent terminations and corresponding somatotopy of cytochrome oxidase (CO) blotches. Additional evidence is presented here consistent with a functionally distinct middle region within the rat's CN: (1) patches of dense synaptophysin (a synaptic-vesical-associated protein)-immunoreactivity (SYN-IR) are limited to the middle CN region, coincident with the dense CO blotches; (2) neurons immunoreactive for the calcium-binding proteins calbindin-D28 (CB), calretinin (CR) and parvalbumin (PV) are concentrated in the middle CN region. Furthermore, in adult rats subjected to perinatal forepaw removal, (1) the patterns of SYN-IR in the middle region of the CN are disrupted, as had previously been shown for the patterns of CO blotches; (2) in contrast, however, distributions of CN cells with PV-, CB- and CR-IR are unaffected. Evidence for a tripartite division in the GN is also presented, based on the distributions of cells with PV-, CB- and CR-IR.

Timecourse of development of the wallaby trigeminal pathway. I. Periphery to brainstem

The development of the vibrissae and their innervation and the maturation of the brainstem trigeminal sensory nuclei have been studied in the wallaby, Macropus eugenii, from birth to adulthood. At birth, developing vibrissal follicles consist of solid epidermal pegs surrounded by dermal condensations. The developing follicles and adjacent skin are innervated by trigeminal afferents. Ten days after birth the follicle contains a dermal papilla and the deep vibrissal nerve can be recognised. A hair cone is present at postnatal day (P) 30 and hairs are apparent on the skin surface by P35. By P63 the deep vibrissal nerve can be seen innervating Merkel cells in the outer root sheath; in addition, the first signs of the blood sinus can be recognised. Innervation of the inner conical body and lanceolate and lamellated receptors supplying the mesenchymal sheath and waist region are not seen until P119, when the follicle resembles that seen in the adult. At birth, central processes of the trigeminal ganglion cells have entered the trigeminal tract and extend from the rostral pons to the upper cervical cord. Labelling with a carbocyanine dye at P0 shows afferents extending medially from the tract into the trigeminal subnuclei at all levels. At this stage the trigeminal nuclei appear as areas of increased cell density in the lateral brainstem. By P30-40 the four subnuclei can be distinguished on the basis of shape, cytoarchitecture, and succinic dehydrogenase reactivity. Adult morphology is not fully established until P210. In mature animals, nucleus principalis contains closely packed, polymorphic cells, frequently aligned parallel to thick fibre bundles that traverse the nucleus obliquely. Subnuclei oralis and interpolaris contain sparsely distributed, medium to large cells, randomly oriented, as well as prominent rostrocaudally directed fibre bundles. Subnucleus caudalis consists of the marginal layer, substantia gelatinosa, and magnocellular layers as described in other species. Patches of increased succinic dehydrogenase or cytochrome oxidase reactivity, presumably corresponding to the vibrissae, are present in subnuclei principalis, interpolaris, and caudalis in developing and adult animals, although the pattern is less clear than in rats. The brainstem patches are first seen at P40, approximately 6 weeks before the corresponding vibrissal-related pattern develops in the cortex. This suggests that the onset of patch formation may be regulated independently at different levels of the pathway.

The mutability of low-affinity nerve growth factor receptor (p75NGFR) expression in the rat cuneate nucleus following perinatal injury and adult deafferentations: comparisons with cytochrome oxidase

In normal adult rats, intense immunostaining for the 75 kDa low-affinity receptor for nerve growth factor and other neurotrophins (p75NGFR) is concentrated in the middle region of the cuneate nucleus (CN), distributed in a blotchy pattern similar to that of cytochrome oxidase (CO) activity. In the adult rats, partial dorsal rhizotomies (centered around the 7th and 8th cervical spinal segments) resulted in the complete disappearance of p75NGFR-like immunoreactivity within the ipsilateral CN, but did not affect the distribution of the CO blotches. Perinatal (postnatal day 1-8) damage to the ipsilateral forepaw and subsequent rearing to adulthood also resulted in significant disruption of the topographical expression of p75NGFR-like immunoreactivity within the CN, as well as--as previously reported--disruption of the CO blotches. Although the patterns of staining in intact adult rats are similar for CO staining and for p75NGFR-like immunoreactivity within the CN, the CO staining appears to be primarily associated with postsynaptic cells, while the p75NGFR-like immunostaining appears to be associated with primary afferent terminals.

Enhanced cytochrome-oxidase staining of the cuneate nucleus in the rat reveals a modifiable somatotopic map

Existing cytochrome oxidase (CO)-staining techniques were modified to enhance sensitivity and contrast in order to examine patterns of CO-activity in the dorsal column nuclei (DCN) of adult Long-Evans rats. Within a rostrocaudally limited region in the middle of the cuneate nucleus (CN) distinctive blotches of intense CO-activity were observed. The CO-staining was maximally differentiated approximately 0.3-0.7 mm caudal to the obex. No CO-blotches were observed anywhere else in the DCN. Transganglionic labelling (WGA-HRP) demonstrated that some of the CO-blotches in the rat CN are related to the terminal projection fields of primary afferents from the skin of the forepaws. The corresponding location of primary afferent termination fields and CO-staining patterns supports a tripartite rostrocaudal division in the rat CN, similar to that described by other investigators in cats, monkeys and raccoons. Comparing the patterns of CO-staining to (1) the cytoarchitecture (Nissl-stained sections), or to (2) the dendritoarchitecture (distribution of microtubule-associated protein 2 (MAP2) or to (3) the organization of retrogradely labelled (WGA-HRP/HRP) cuneothalamic cells, revealed no topographical organization corresponding to the CO-blotches. Postnatal (at least up to 11 days postpartum) forepaw deafferentation or removal disrupted the CO-staining pattern in the CN.

Current hypotheses of structural pattern formation in the somatosensory system and their potential relevance to humans

Current hypotheses of structural pattern formation in the mammalian somatosensory system are modeled on experimental findings from the trigeminal system of rodents. The present results show that, like rodents, the trigeminal nucleus principalis of humans contains a parcellated pattern of cytochrome oxidase dense patches. These results provide an indication of the potential usefulness of rodent-based hypotheses for understanding pattern formation in human somatosensory connections.

Parcellated organization in the trigeminal and dorsal column nuclei of primates

The subdivisions of the brainstem trigeminal complex in non-primate mammals are characterized by aggregated or parcellated patterns of neural organization. The present studies used cytochrome oxidase histochemistry to test if parcellated organization patterns also occur in the brainstems of primates. The results demonstrate that a parcellated pattern of neural organization exists in the trigeminal nucleus principalis, but not in the spinal trigeminal nuclei, of macaque and squirrel monkeys. The results further suggest that parcellation in the nucleus principalis qualitatively resembles the aggregated organization in dorsal column nuclei. Taken together with previous findings from non-primates, these results indicate that central parcellation is an organizational feature of specific ascending somatosensory projections in many mammals including primates.

Maintenance of discrete somatosensory maps in subcortical relay nuclei is dependent on an intact sensory cortex

Lesions of the rat barrelfield cortex drastically alter the discrete representations of the somatosensory periphery in the central nervous system. We have found that lesions placed in the parietal cortex, after the formation of barrels (postnatal day 5), can irreversibly abolish vibrissae- and extremity-related patterns of cytochrome oxidase activity in the principal sensory nucleus of the trigeminal nerve and in the dorsal column nuclei. Furthermore, abnormal patterns of enzymatic activity occur in the remaining primary somatosensory cortex and the ventrobasal nucleus of the thalamus. We conclude that cortical barrels are essential in maintenance of periphery-related discrete morphological organization in the rodent somatosensory system.

The organization and mutability of the forepaw and hindpaw representations in the somatosensory cortex of the neonatal rat

The present study demonstrates that the primary somatosensory cortex of the rat contains a map of the entire body surface that is discernible with a routine anatomical staining technique, the succinic dehydrogenase reaction. The overall proportions of this map are relatively constant from rat to rat and very similar to those reported in previous physiological investigations (Welker: Brain Res. 26:259-275, '71, J. Comp. Neurol. 166:173-190, '76). We found 67% of the map to be related to the head of the rat, 15% to the forelimb, 14% to the trunk, and 4% to the hindlimb. Within the forelimb and hindlimb representations, there is a consistent internal organization that can be related to specific peripheral structures (digits or palm pads). Further, damage to either the periphery or the nerves innervating these regions on the day of birth produces disruptions in the normal pattern, but damage on day 6 or later does not. We interpret these results as indicating that the role of the periphery in organizing central neuronal structures during development previously demonstrated for the trigeminal system extends to the entire rat somatosensory system. Comparison of the present results with physiological studies of adult cortical maps after peripheral damage suggests to us that different substrates underlie the changes reported in the adult.

The role of the principal sensory nucleus in central trigeminal pattern formation

Complete lesions of the principal sensory nucleus in the neonatal rat disrupts vibrissae-related pattern formation in the ventral posterior nucleus of the dorsal thalamus. Similar lesions of the spinal trigeminal nucleus do not effect pattern formation in the ventral posterior nucleus. The results are interpreted as suggesting that the principal sensory nucleus provides a template for pattern formation in central trigeminal structures.

Ephemeral cellular segmentation in the thalamus of the neonatal rat

The distribution of thalamocortical relay cells in the rat ventrobasal complex was studied during the early postnatal period using the retrograde transport of horseradish peroxidase from the parietal cortex. It was found that the relay cells undergo marked changes in their distribution during the first two postnatal weeks. On postnatal days (PNDs) 0 and 1, the cells are rather homogeneously distributed throughout the ventrobasal complex. However, by PND 2, and more clearly by PND 3, the cells form a distinctly segmented pattern. This pattern consists of discrete curvilinear arrays of cells extending throughout most of the rostrocaudal extent of the nucleus. This distinct cellular pattern is present from PND 2 to about PND 8. In animals sacrificed on PND 15 or as adults, the pattern is no longer obvious. The cellular pattern seen at PND 8 was examined in the 3 standard planes of section and compared to both the somatotopic organization of the nucleus and to the organization of its major ascending and descending inputs. The developmental time course of the cellular segmentation was related to that of the lemniscal and corticothalamic afferents, which also show ephemeral segmentation patterns during the first two postnatal weeks.

Central correlates of peripheral pattern alterations in the trigeminal system of the rat. II. The effect of nerve section

The effect of trigeminal nerve section on pattern formation in the rat central nervous system was investigated. Nerve section was found to abolish all evidence of afferent segmentation in the brain stem. At thalamic and cortical levels the pattern of afferent segmentation was affected in a time-dependent fashion. The later the procedure is performed, the less severe the resultant effect. Early section abolishes all afferent segmentation, later section only within-row segmentation, and still later section has no effect on pattern formation. The results of this experiment support the hypothesis that peripheral spatial information plays an important role in pattern formation in the central trigeminal system.

The sensitive period in the development of the trigeminal system of the neonatal rat

In previous studies we have described vibrissae-related segmentation in the brainstem, thalamus, and cortex of the neonatal rat. Using succinic dehydrogenase (SDH) histochemistry, we delineated the time course in development of the normal segmentation at each level of the trigeminal system and the aberrant segmentation resulting from follicle damage at birth (Killackey and Belford, '79; Belford and Killackey, '79a, b). The present study examines the aberrant patterns that result from damage to the vibrissae follicles at different ages, comparing the patterns at the different levels of the trigeminal system. The present study indicates a number of similarities between the central representations of the vibrissae. First, the patterns are similar within each of the three representations in the trigeminal nuclei for removal at a given age in a particular animal. These changes include a decrease in SDH density but a maintenance of normal row widths. Second, the patterns are similar within both the ventrobasal complex and layer IV of somatosensory cortex for removal at a given age in a particular animal. These changes include a fusion of individual clusters into bands, and a decrease in band width, but maintenance of normal SDH density. Third, the effects of damage to a row of vibrissae follicles at different ages are graded. Earlier damage produces more marked aberrations. Fourth, for all of the structures, the transition between bands and clusters occurs with damage at the same age. Further, the last age at which damage produces aberrant patterns is Day 3 for all of the structures. Thus, the data suggest that there is one sensitive period for pattern alteration in the entire central trigeminal system.

The development of vibrissae representation in subcortical trigeminal centers of the neonatal rat

In every station of the trigeminal system of the young rat, the segmented activity of the mitochondrial enzyme succinic dehydrogenase (SDH) clearly delineates the representation of the mystacial vibrissae. In the trigeminal complex of the medulla, three parallel representation can be seen, two in the spinal trigeminal nucleus and one in the principal trigeminal nucleus. In the next station, the ventrobasal complex of the thalamus, a single representation occurs. Likewise, layer IV of somatosensory cortex contains one representation of the vibrissae. Further, neonatal damage to the mystacial vibrissae results in anomalies within each representation. The present study delineates both the normal development of subcortical trigeminal stations and the aberrant organization seen after vibrisse removal. The results of a similar study on somatosensory cortex (Killackey and Belford, '79) and the present data allow the comparison of the development of each of the five vibrissae representations in the trigeminal system. In the brainstem, each of the three trigeminal complex representations are present at birth, although the pattern becomes more distinct over the first several days of life. Interestingly, vibrissae removal at birth induces an aberrant pattern that is distinct by postnatal Day 3. Although details are not equally discernible in each representation, the abnormalities appear to be similar. The SDH segmentation in the ventrobasal complex develops during postnatal Days 1 through 4. At Day 1, portions of the matrix of high density SDH activity break up into bands. Clusters can be discerned within these bands on Day 2. By Day 4 the pattern is sharply delineated. Vibrissae removal at birth results in anomalies that are a part of the initial development of segmentation, not a later reorganization. Comparison of the present data with that of our previous studies indicates that there is a sequential development of the central somatosensory structures related to the vibrissae, beginning with the most peripheral station. Further, there are many similarities in the development of each station. There are also differences which are particularly important in comparing the trigeminal nuclei with the later stations. The unique features in the abnormal development of the trigeminal nuclei are likely due to their direct connections with the periphery.