Foxp2 Regulates Identities and Projection Patterns of Thalamic Nuclei During Development.

The molecular mechanisms underlying the formation of the thalamus during development have been investigated intensively. Although transcription factors distinguishing the thalamic primordium from adjacent brain structures have been uncovered, those involved in patterning inside the thalamus are largely unclear. Here, we show that Foxp2, a member of the forkhead transcription factor family, regulates thalamic patterning during development. We found a graded expression pattern of Foxp2 in the thalamic primordium of the mouse embryo. The expression levels of Foxp2 were high in the posterior region and low in the anterior region of the thalamic primordium. In Foxp2 (R552H) knockin mice, which have a missense loss-of-function mutation in the forkhead domain of Foxp2, thalamic nuclei of the posterior region of the thalamus were shrunken, while those of the intermediate region were expanded. Consistently, Foxp2 (R552H) knockin mice showed changes in thalamocortical projection patterns. Our results uncovered important roles of Foxp2 in thalamic patterning and thalamocortical projections during development.

The Regulation of Corticofugal Fiber Targeting by Retinal Inputs.

Corticothalamic projection systems arise from 2 main cortical layers. Layer V neurons project exclusively to higher-order thalamic nuclei, while layer VIa fibers project to both first-order and higher-order thalamic nuclei. During early postnatal development, layer VIa and VIb fibers accumulate at the borders of the dorsal lateral geniculate nucleus (dLGN) before they innervate it. After neonatal monocular enucleation or silencing of the early retinal activity, there is premature entry of layer VIa and VIb fibers into the dLGN contralateral to the manipulation. Layer V fibers do not innervate the superficial gray layer of the superior colliculus during the first postnatal week, but also demonstrate premature entry to the contralateral superficial gray layer following neonatal enucleation. Normally, layer V driver projections to the thalamus only innervate higher-order nuclei. Our results demonstrate that removal of retinal input from the dLGN induces cortical layer V projections to aberrantly enter, arborize, and synapse within the first-order dLGN. These results suggest that there is cross-hierarchical corticothalamic plasticity after monocular enucleation. Cross-hierarchical rewiring has been previously demonstrated in the thalamocortical system (Pouchelon et al. 2014), and now we provide evidence for cross-hierarchical corticothalamic rewiring after loss of the peripheral sensory input.

Essential role of postsynaptic NMDA receptors in developmental refinement of excitatory synapses.

Neurons in the brains of newborns are usually connected with many other neurons through weak synapses. This early pattern of connectivity is refined through pruning of many immature connections and strengthening of the remaining ones. NMDA receptors (NMDARs) are essential for the development of excitatory synapses, but their role in synaptic refinement is controversial. Although chronic application of blockers or global knockdown of NMDARs disrupts developmental refinement in many parts of the brain, the ubiquitous presence of NMDARs makes it difficult to dissociate direct effects from indirect ones. We addressed this question in the thalamus by using genetic mosaic deletion of NMDARs. We demonstrate that pruning and strengthening of immature synapses are blocked in neurons without NMDARs, but occur normally in neighboring neurons with NMDARs. Our data support a model in which activation of NMDARs in postsynaptic neurons initiates synaptic refinement.

Spatiotemporal molecular dynamics of the developing human thalamus.

The thalamus plays a central coordinating role in the brain. Thalamic neurons are organized into spatially distinct nuclei, but the molecular architecture of thalamic development is poorly understood, especially in humans. To begin to delineate the molecular trajectories of cell fate specification and organization in the developing human thalamus, we used single-cell and multiplexed spatial transcriptomics. We show that molecularly defined thalamic neurons differentiate in the second trimester of human development and that these neurons organize into spatially and molecularly distinct nuclei. We identified major subtypes of glutamatergic neuron subtypes that are differentially enriched in anatomically distinct nuclei and six subtypes of γ-aminobutyric acid-mediated (GABAergic) neurons that are shared and distinct across thalamic nuclei.

Thalamocortical pathfinding defects precede degeneration of the reticular thalamic nucleus in polysialic acid-deficient mice.

The modification of the neural cell adhesion molecule (NCAM) with polysialic acid (polySia) is tightly linked to neural development. Genetic ablation of the polySia-synthesizing enzymes ST8SiaII and ST8SiaIV generates polySia-negative but NCAM-positive (II(-/-)IV(-/-)) mice characterized by severe defects of major brain axon tracts, including internal capsule hypoplasia. Here, we demonstrate that misguidance of thalamocortical fibers and deficiencies of corticothalamic connections contribute to internal capsule defects in II(-/-)IV(-/-) mice. Thalamocortical fibers cross the primordium of the reticular thalamic nucleus (Rt) at embryonic day 14.5, before they fail to turn into the ventral telencephalon, thus deviating from their normal trajectory without passing through the internal capsule. At postnatal day 1, a reduction and massive disorganization of fibers traversing the Rt was observed, whereas terminal deoxynucleotidyl transferase dUTP nick end labeling and cleaved caspase-3 staining indicated abundant apoptotic cell death of Rt neurons at postnatal day 5. Furthermore, during postnatal development, the number of Rt neurons was drastically reduced in 4-week-old II(-/-)IV(-/-) mice, but not in the NCAM-deficient N(-/-) or II(-/-)IV(-/-)N(-/-) triple knock-out animals displaying no internal capsule defects. Thus, degeneration of the Rt in II(-/-)IV(-/-) mice may be a consequence of malformation of thalamocortical and corticothalamic fibers providing major excitatory input into the Rt. Indeed, apoptotic death of Rt neurons could be induced by lesioning corticothalamic fibers on whole-brain slice cultures. We therefore propose that anterograde transneuronal degeneration of the Rt in polysialylation-deficient, NCAM-positive mice is caused by defective afferent innervation attributable to thalamocortical pathfinding defects.

Differential development of myelin in zebra finch song nuclei.

Songbirds learn vocalizations by hearing and practicing songs. As song develops, the tempo becomes faster and more precise. In the songbird brain, discrete nuclei form interconnected myelinated circuits that control song acquisition and production. The myelin sheath increases the speed of action potential propagation by insulating the axons of neurons and by reducing membrane capacitance. As the brain develops, myelin increases in density, but the time course of myelin development across discrete song nuclei has not been systematically studied in a quantitative fashion. We tested the hypothesis that myelination develops differentially across time and song nuclei. We examined myelin development in the brains of the zebra finch (Taeniopygia guttata) from chick at posthatch day (d) 8 to adult (up to 147 d) in five major song nuclei: HVC (proper name), robust nucleus of the arcopallium (RA), Area X, lateral magnocellular nucleus of the anterior nidopallium, and medial portion of the dorsolateral thalamic nucleus (DLM). All of these nuclei showed an increase in the density of myelination during development but at different rates and to different final degrees. Exponential curve fits revealed that DLM showed earlier myelination than other nuclei, and HVC showed the slowest myelination of song nuclei. Together, these data show differential maturation of myelination in different portions of the song system. Such differential maturation would be well placed to play a role in regulating the development of learned song.

Early development of the thalamic inhibitory feedback loop in the primary somatosensory system of the newborn mice.

Spontaneous neuronal activity plays an important role during the final development of the brain circuits and the formation of the primary sensory maps. In young rats, spindle bursts have been recorded in the primary somatosensory cortex. They are correlated with spontaneous muscle twitches and occur before active whisking. They bear similarities with the spindles recorded in adult brain that occur during early stages of sleep and rely on a thalamic feedback loop between the glutamatergic nucleus ventroposterior medialis (nVPM) and the GABAergic nucleus reticularis thalami (nRT). However, whether a functional nVPM-nRT loop exists in newborn rodents is unknown. We studied the reciprocal synaptic connections between nVPM and nRT in thalamic acute slices from mice from birth [postnatal day 0 (P0)] until P9. We first demonstrated that nVPM-to-nRT EPSCs could be distinguished from corticothalamic EPSCs by their inhibition by 5-HT attributable to the transient expression of functional presynaptic serotonin 1B receptors. The nVPM-to-nRT EPSCs and nRT-to-nVPM IPSCs were both detected the first day after birth; their amplitude near 2 nS was relatively stable until P5. At P6-P7, there was a rapid and simultaneous increase of both nVPM-to-nRT EPSCs and nRT-to-nVPM IPSCs that reached 8 and 9 nS, respectively. Our results show that the thalamic synapses implicated in spindle activity are functional shortly after birth, suggesting that they could already generate spindles during the first postnatal week. Our results also suggest an inhibitory action of 5-HT on the spindle bursts of the newborn mice.

MeCP2 is required for normal development of GABAergic circuits in the thalamus.

Methyl-CpG binding protein 2 (MeCP2) is highly expressed in neurons in the vertebrate brain, and mutations of the gene encoding MeCP2 cause the neurodevelopmental disorder Rett syndrome. This study examines the role of MeCP2 in the development and function of thalamic GABAergic circuits. Whole cell recordings were carried out in excitatory neurons of the ventrobasal complex (VB) of the thalamus and in inhibitory neurons of the reticular thalamic nucleus (RTN) in acute brain slices from mice aged P6 through P23. At P14-P16, the number of quantal GABAergic events was decreased in VB neurons but increased in RTN neurons of Mecp2-null mice, without any change in the amplitude or kinetics of quantal events. There was no difference between mutant and wild-type mice in paired-pulse ratios of evoked GABAergic responses in the VB or the RTN. On the other hand, unitary responses evoked by minimal stimulation were decreased in the VB but increased in the RTN of mutants. Similar changes in the frequency of quantal events were observed at P21-P23 in both the VB and RTN. At P6, however, quantal GABAergic transmission was altered only in the VB not the RTN. Immunostaining of vesicular GABA transporter showed opposite changes in the number of GABAergic synaptic terminals in the VB and RTN of Mecp2-null mice at P18-P20. The loss of MeCP2 had no significant effect on intrinsic properties of RTN neurons recorded at P15-P17. Our findings suggest that MeCP2 differentially regulates the development of GABAergic synapses in excitatory and inhibitory neurons in the thalamus.

A developmental redox dysregulation leads to spatio-temporal deficit of parvalbumin neuron circuitry in a schizophrenia mouse model.

The fast-spiking parvalbumin (PV) interneurons play a critical role in neural circuit activity and dysfunction of these cells has been implicated in the cognitive deficits typically observed in schizophrenia patients. Due to the high metabolic demands of PV neurons, they are particularly susceptible to oxidative stress. Given the extant literature exploring the pathological effects of oxidative stress on PV cells in cortical regions linked to schizophrenia, we decided to investigate whether PV neurons in other select brain regions, including sub-cortical structures, may be differentially affected by redox dysregulation induced oxidative stress during neurodevelopment in mice with a genetically compromised glutathione synthesis (Gclm KO mice). Our analyses revealed a spatio-temporal sequence of PV cell deficit in Gclm KO mice, beginning with the thalamic reticular nucleus at postnatal day (P) 20 followed by a PV neuronal deficit in the amygdala at P40, then in the lateral globus pallidus and the ventral hippocampus Cornu Ammonis 3 region at P90 and finally the anterior cingulate cortex at P180. We suggest that PV neurons in different brain regions are developmentally susceptible to oxidative stress and that anomalies in the neurodevelopmental calendar of metabolic regulation can interfere with neural circuit maturation and functional connectivity contributing to the emergence of developmental psychopathology.

Neurogenic development of the auditory areas of the midbrain and diencephalon in the Xenopus laevis and evolutionary implications.

To study whether the core-versus-shell pattern of neurogenesis occurred in the mesencephalic and diencephalic auditory areas of amniotes also appears in the amphibian, [(3)H]-thymidine was injected into tadpoles at serial developmental stages of Xenopus laevis. Towards the end of metamorphism, [(3)H]-thymidine labeling was examined and led to two main observations: 1) neuron generation in the principal nucleus (Tp) started at stage 50, and peaked at stage 53, whereas it began at stage 48.5, and peaked around stage 49 in the other two mesencephalic auditory areas, the laminar nucleus (Tl) and the magnocellular nucleus (Tmc). 2) Neuron generation appeared at stage 40, and peaked around stage 52 in the posterior thalamic nucleus (P) and the central thalamic nucleus (C). Our study revealed that, like the cores of mesencephalic auditory nuclei in amniotes, Tp showed differences from Tl and Tmc in the onset and the peak of neurogenesis. However, such differences did not occur in the P and C. Our neurogenetic data were consistent with anatomical and physiological reports indicating a clear distinction between the mesencephalic, but not the diencephalic auditory areas of the amphibian. Our data are helpful to get insights into the organization of auditory nuclei and its evolution in vertebrates.

Short-term deep brain stimulation of the thalamic reticular nucleus modifies aberrant oscillatory activity in a neurodevelopment model of schizophrenia.

Dysfunction of thalamo-cortical networks involving particularly the thalamic reticular nucleus (TRN) is implicated in schizophrenia. In the neonatal ventral hippocampal lesion (NVHL), a heuristic animal model of schizophrenia, brain oscillation changes similar to those of schizophrenic patients have been reported. The aim of this study was to analyze the effects of short-term deep brain stimulation (DBS) in the thalamic reticular nucleus on electroencephalographic (EEG) activity in the NVHL. Male and female Sprague-Dawley rats were used and the model was prepared by excitotoxicity damage of the ventral hippocampus on postnatal day 7 (PD-7). Chronic bilateral stainless steel electrodes were implanted in the TRN, thalamic dorsomedial nucleus and prelimbic area at PD-90. Rats were classified as follows: sham and NVHL groups, both groups received bilateral DBS in the TRN for one hour (100Hz, 100µs pulses, 200µA). All animals showed a sudden behavioral arrest accompanied by widespread symmetric bilateral spike-wave discharges, this activity was affected by DBS-TRN. Additionally, the power spectra of 0.5-100Hz and the coherence of 0.5-4.5 and 35-55Hz frequencies were modified by DBS-TRN. Our results suggest that DBS in the TRN may modify functional connectivity between different parts of the thalamo-cortical network. Additionally, our findings may suggest a beneficial effect of DBS-TRN on some preclinical aberrant oscillatory activities in a neurodevelopmental model of schizophrenia.

Brn3a-expressing retinal ganglion cells project specifically to thalamocortical and collicular visual pathways.

Retinal ganglion cells (RGCs) innervate several specific CNS targets serving cortical and subcortical visual pathways and the entrainment of circadian rhythms. Recent studies have shown that retinal ganglion cells express specific combinations of POU- and LIM-domain transcription factors, but how these factors relate to the subsequent development of the retinofugal pathways and the functional identity of RGCs is mostly unknown. Here, we use targeted expression of an genetic axonal tracer, tau/beta-galactosidase, to examine target innervation by retinal ganglion cells expressing the POU-domain factor Brn3a. Brn3a is expressed in RGCs innervating the principal retinothalamic/retinocollicular pathway mediating cortical vision but is not expressed in RGCs of the accessory optic, pretectal, and hypothalamic pathways serving subcortical visuomotor and circadian functions. In the thalamus, Brn3a ganglion cell fibers are primarily restricted to the outer shell of the dorsal lateral geniculate, providing new evidence for the regionalization of this nucleus in rodents. Brn3a RGC axons have a relative preference for the contralateral hemisphere, but known mediators of the laterality of RGC axons are not repatterned in the absence of Brn3a. Brn3a is coexpressed extensively with the closely related factor Brn3b in the embryonic retina, and the effects of the loss of Brn3a in retinal development are not severe, suggesting partial redundancy of function in this gene class.

New views of the thalamic reticular nucleus in the adult and the developing brain.

The thalamic reticular nucleus plays a crucial role in modifying the patterns of activity that can reach the cerebral cortex from the thalamus. Although the nucleus is generally viewed as a cell group with widespread and nonspecific thalamic and cortical connections, recent evidence has begun to stress the extent to which at least some of the reticular pathways transmit well-defined maps with a clear local sign from the cortex and the thalamus. Further, evidence from the adult structure of the nucleus and ongoing developmental studies suggest that the reticular nucleus plays an important part in organizing the earliest connections between cortex and thalamus and that the developmental sequence may explain the complex connections formed in the adult.

Distinction in the immunoreactivities of two calcium-binding proteins and neuronal birthdates in the first and higher-order somatosensory thalamic nuclei of mice: Evolutionary implications.

Comparative embryonic studies are the most effective way to discern phylogenetic changes. To gain insight into the constitution and evolution of mammalian somatosensory thalamic nuclei, we first studied how calbindin (CB) and parvalbumin (PV) immunoreactivities appear during embryonic development in the first-order relaying somatosensory nuclei, i.e., the ventral posteromedial (VPM) and posterolateral (VPL) nuclei, and their neighboring higher-order modulatory regions, including the ventromedial or ventrolateral nucleus, posterior, and the reticular nucleus. The results indicated that cell bodies that were immunoreactive for CB were found earlier (embryonic day 12 [E12]) in the dorsal thalamus than were cells positive for PV (E14), and the adult somatosensory thalamus was characterized by complementary CB and PV distributions with PV dominance in the first-order relaying nuclei and CB dominance in the higher-order regions. We then labeled proliferating cells with [(3) H]-thymidine from E11 to 19 and found that the onset of neurogenesis began later (E12) in the first-order relaying nuclei than in the higher-order regions (E11). Using double-labeling with [(3) H]-thymidine autoradiography and CB or PV immunohistochemistry, we found that CB neurons were born earlier (E11-12) than PV neurons (E12-13) in the studied areas. Thus, similar to auditory nuclei, the first and the higher-order somatosensory nuclei exhibited significant distinctions in CB/PV immunohistochemistry and birthdates during embryonic development. These data, combined with the results of a cladistic analysis of the thalamic somatosensory nuclei, are discussed from an evolutionary perspective of sensory nuclei.

Development of connections between the mediodorsal nucleus of the thalamus and the prefrontal cortex in the rat.

The postnatal ingrowth of thalamocortical fibers from the mediodorsal nucleus to the prefrontal cortex was investigated in relation to the development of cortical lamination. Like the dopaminergic fibers in the prefrontal cortex and the thalamic fibers in the visual cortex, the mediodorsal fibers have entered the prefrontal cortex at birth. Most of the fibers are found in the developing layer VI, but, in contrast to the above-mentioned systems, a considerable number of mediodorsal fibers have already penetrated into the upper, most immature part of the cortical plate on postnatal day 1. From day 1 to day 7 an increasing number of mediodorsal fibers reach the upper cortical plate, which by then is developing layer III, the terminal layer of these fibers. The reciprocal connection from the layer VI cells of the prefrontal cortex to the mediodorsal nucleus develops between day 4 and day 9. Finally, the projection from the contralateral prefrontal cortex to the mediodorsal nucleus is established around day 10. The early presence of the mediodorsal fibers in the upper, differentiating cortical plate might indicate an important role for the mediodorsal fibers in the laminar development of the prefrontal cortex.

Development of the thalamic reticular and perireticular nuclei in rats and their relationship to the course of growing corticofugal and corticopetal axons.

This study examines the connections of the thalamic reticular and perireticular nuclei during development. In addition, because these nuclei lie directly in the path of corticofugal and corticopetal axons during development, we have examined the relationship of these growing axons to the reticular and perireticular cell groups. Neurones were labelled by applying DiI, wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP), or HRP to the dorsal thalamus and/or cerebral cortex of rats at different stages of development. The axons of neurons in the reticular nucleus reach the dorsal thalamus as early as embryonic day (E) 14. At this age, and during later prenatal development, a small DiI implant limited to the presumptive lateral geniculate nucleus labels reticulothalamic and thalamocortical axons which travel in a clearly defined bundle through the thalamus. During late gestation, thalamocortical (approximately E15) and corticothalamic (approximately E17) axons pass directly through the reticular nucleus toward their targets. It is not until birth that collaterals are seen extending into the nucleus from the parent axons. Neurones in the perireticular nucleus, in contrast to those in the reticular nucleus, are not labelled from the lateral geniculate nucleus until after birth. The perireticular nucleus is very large at a stage when the first thalamocortical axons leave and when the first corticothalamic axons approach the thalamus. These axons are seen to change course sharply in the region of the internal capsule, where there are many perireticular cells. Corticothalamic axons turn toward the reticular nucleus, and thalamocortical axons turn toward the cortical subplate. Corticospinal and corticobulbar axons, on the other hand, pass directly through the perireticular region toward their more caudal targets. After these axons have reached their targets, the perireticular nucleus reduces dramatically in size.

Posthatching development of synapses in the neuropil of nucleus rotundus of the zebra finch: a quantitative electron microscopic study.

The development and maturation of synapses in the nucleus rotundus of the zebra finch were examined at 1, 5, 10, 20, and 100 days posthatching. Quantitative ultrastructural techniques were applied to investigate synaptic density, size of presynaptic terminals, and length of postsynaptic thickenings. During development there is a steady increases in the number of synapses and an enlargement of the presynaptic terminals. The length of the postsynaptic thickenings, however, decreases significantly during posthatching development. All three parameters reach adult values within 20 days of age. A close relationship was found between the enlargement of the presynaptic terminals and the increase in the number of contact zones up to the 20th day. The general feature of this developmental progress is in good agreement with biochemical and, to a certain extent, behavioral studies.

Postnatal development of afferent projections to the lateral suprasylvian visual area in the cat: an HRP study.

The postnatal development of thalamic and cortical projections to the medial bank of the lateral suprasylvian area was studied in the cat by using the retrograde and orthograde HRP methods. Both projections are already present at birth. In both newborn kittens and adult cats, the thalamic projections arise from the same nuclei. By far the heaviest thalamic projection originates from a relatively lateral portion of the lateral posterior nucleus (the presumed LPl). The cortical laminar distribution of the afferents arising from the presumed LPl changes markedly with aging. In kittens younger than 1 week, the terminals are distributed densely in layer I and sparsely in layer IV. With age, the terminals in layer I become less dense while those in layer IV become denser. By 1 month of age, the terminal distribution is similar to that found in adult cats, in which the terminals are sparse in layer I and dense in depth--particularly, in layer IV. The terminal distribution of the corticocortical projections from areas 17 and 18 also changes with aging. The terminals in kittens younger than 2 weeks are distributed in both superficial and deep cortical layers, whereas those in kittens older than 1 month and in an adult cat are distributed only in deep layers.

Development of anomalous retinal projections to nonvisual thalamic nuclei in Syrian hamsters: a quantitative study.

When two of the principal targets of retinofugal axons, the superior colliculus and dorsal nucleus of the lateral geniculate body, are ablated in newborn hamsters and the somatosensory (ventrobasal) or auditory (medial geniculate) thalamic nuclei are partially deafferented, the optic axons form permanent, abnormal connections in the latter nuclei. The lateral posterior nucleus of the operated hamsters also receives an anomalously large retinal projection. Here, we report on the results of a quantitative study that was undertaken in order to elucidate how these abnormal connections are formed. In normal, newborn hamsters, there is a transient retinal projection to the ventrobasal nucleus that disappears 3-4 days postnatally. Our quantitative data show that postoperatively, the volume of the retino-ventrobasal projection increases proportionately more than the volume of the ventrobasal nucleus so that the retino-ventrobasal projection in operated adult hamsters is due both to the stabilization of the normally transient projection and to a reactive sprouting that increases the size of the projection. The retino-medial geniculate projection arises de novo by reactive sprouting of optic tract fibers that normally pass over and through the nucleus; in unoperated hamsters, terminating projections are never seen at any age. The quantitative data also show that the anomalously large retino-lateral posterior projection is due almost entirely to the reactive sprouting of the normal projection and/or normal fibers of passage that are already present on the day of birth, although it is possible that a minor component can be attributed to the stabilization of a small population of normally transient retino-lateral posterior axons. The present results demonstrate that the transient retino-ventrobasal axons in normal, newborn hamsters are capable of making permanent connections with ventrobasal neurons. This finding raises the important question of the cellular mechanisms that determine whether immature neuronal connections are stabilized or eliminated. The results also suggest that during both normal and abnormal development, the choice of a target by growing axons may depend upon the axons being in proximity to a potential terminal site just at the time when that site is capable of receiving afferents.

Cells of the perireticular nucleus project to the developing neocortex of the rat.

The perireticular nucleus is a recently described thin sheet of small cells among the fibres of the internal capsule, lying lateral to the thalamic reticular nucleus and medial to the globus pallidus (Clemence and Mitrofanis [1992]. J. Comp. Neurol. 322:167-180). During development, the perireticular nucleus is relatively large, lying in the path of the growing corticofugal and thalamocortical axons and filling the area of the internal capsule lateral to the thalamic reticular nucleus. After these axons have formed their connections, the perireticular nucleus rapidly decreases in size, leaving only a few cells in the adult (Mitrofanis [1992] J. Comp. Neurol. 320:161-181). In this study, we aimed to investigate the connections between the developing cortex and thalamus by making injections of tracer into the cortical plate. Injections of Horse Radish Peroxidase (HRP), Wheat Germ Agglutinin bound to HRP (WGA-HRP) and 1'dioctadecyl-3,3,3',3 tetramethycarbocyanine perchlorate (DiI) were made in vivo between embryonic day (E) 18 and adult and DiI was placed in the fixed brains of rats aged between E16 and postnatal day (P)1. Between E17 and P10, the retrograde perikaryal labelling resulting from these injections revealed a transient projection from the perireticular nucleus to the ipsilateral cortical plate. No cells were labelled in the thalamic reticular nucleus. This suggests that the perireticular nucleus must be regarded as a group of cells distinct from the thalamic reticular nucleus and having a separate role in development. Comparisons between the perireticular cells and the cells of the cortical subplate suggest that both may be playing comparable roles in early development, possibly guiding fibres towards their end stations or serving to rearrange the complex mapped projections linking the thalamus and cortex.