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The Mars 2020 Perseverance Rover imaged diagenetic textural features in four separate sedimentary units in its exploration of the 25‐m‐thick Shenandoah formation at Jezero Crater, Mars, that we interpreted as probable concretions. These concretions were most abundant in the Hogwallow Flats member of the Shenandoah formation and were restricted to the light‐toned, platy, sulfur‐cemented bedrock at outcrop surfaces, whereas the finely laminated, darker toned, mottled and deformed strata lack concretions. The concretions also had a wide range of morphologies including concentric, oblate, urn, and spheroidal shaped forms that were not clustered, and ranged in size from ∼1 to 16 mm with a median of 2.65 mm. The elemental composition of the concretions compared to the bedrock had greater abundance of magnesium and calcium salts, silicates, and possibly hematite. We compared these Jezero Crater concretions to the geochemistry of concretions from previously published studies and from two new terrestrial analog sites (Gallup Formation, New Mexico and Torrey Pines, California). In addition, we measured organic carbon content of three terrestrial sedimentary analogs of increasing age that contain concretions (Torrey Pines (Pleistocene), Gallup Formation (∼89 Ma), and Moodies Group (∼3.2 Ga)). All measured concretions contained significant concentrations of organic carbon with the maximum organic carbon content (∼2 wt. % Total organic carbon) found in the Moodies Group concretions. Organic carbon abundances in terrestrial concretions was controlled more by the formation mechanism and relative timing of concretion development rather than deposit age. These findings suggested that concretions at Jezero Crater reflect local sites of enhanced biosignature preservation potential. Plain Language Summary: The Perseverance Rover discovered concretions in its exploration of the rock packages at Jezero Crater, Mars and one of the sample return cores was collected from concretion‐rich bedrock. Concretions are resistant cement in the rock that are found in many shapes (usually spherical or oblate) and range from millimeter to meter size scales on Earth; they can be formed through inorganic water‐rock reactions or facilitated by microbial metabolisms. We documented the abundance, size, composition, and shape of the concretions to understand how these features were formed. We found that the concretions are mixtures of salts, clay minerals, and iron oxides. We compared these results to terrestrial concretions with similar mineral compositions and measured the organic carbon in four terrestrial analogs. Comparisons with terrestrial concretions in this study and the literature suggested that the concretion composition in Jezero Crater could have high organic preservation potential. Thus, the concretions in Jezero Crater may retain organic carbon and other biosignatures and might therefore be considered as high priority samples of astrobiological interest out of the current sample suite for return to Earth. Key Points: Jezero Crater concretions are variably enriched in Si, Ca, and Mg salts, and Fe oxidesTerrestrial concretions of similar mineralogy analyzed in this study contain significant organic carbon phasesBased on terrestrial analogs, Jezero Crater concretions may represent sites of enhanced biosignature preservation potential [ABSTRACT FROM AUTHOR]

Equatorial Layered Deposits (ELDs) reveal aspects of past depositional environments useful for timing climatic and geological events on Mars. However, their formation has several contending hypotheses which reflect a diverse range of possible environments. To better constrain their formation mechanism, we analyzed the stratigraphic and mineralogical characteristics and mapped the distribution of ELDs within three close craters, Sera, Jiji, and an unnamed crater in central Arabia Terra. Analyses of craters' geology allowed us to reconstruct the paleo‐environments where ELDs have been deposited. Thinning and thickening sequence trends show shared and repeated variations in the depositional environment, reflecting sediment accumulation strongly controlled by a regional groundwater reservoir. Layered deposits are characterized by monohydrated and polyhydrated sulfate signatures implying an interaction between water and rock in acidic conditions. Our study contributes to the understanding of regional geological processes in Arabia Terra revealing long‐term aqueous activity. Plain Language Summary: Despite years of orbital observations of the surface of Mars, mid‐to low‐latitude layered deposits (LD) in Arabia Terra are not yet fully understood. These deposits record an important geological sequence of the early Martian history, but their exact formation, specifically the putative role of the water in their formation and preservation, remains an unanswered question. This study helps in further understanding these processes by comparing and contrasting LD present in craters in very close proximity to each other. In this way, proposed hypotheses benefit from having a control that is not limited to a singular crater or location. Our study contributes to the understanding of regional geological processes in Arabia Terra revealing long‐term aqueous activity. Results combine various analyses, including measurements of layer thickness and attitudes, orbital spectroscopy, basin geometry, and morphologies aimed at reconstructing the geological evolution of the area. In particular, we interpreted that LD were emplaced in a depositional environment reflecting sediment accumulation strongly controlled by a regional groundwater reservoir in acidic conditions. Key Points: Regional groundwater fluctuations influenced the deposition, mineralogy, and preservation of Equatorial Layered Deposits (ELDs) in Arabia TerraFour different stratigraphic sequences of layered deposits have been identified on the basis of strata thickness and attitudeMineralogical analyses of ELDs suggest the presence of different levels of hydration of sulfates [ABSTRACT FROM AUTHOR]

On Mars, near the equator, much of the terrain in Gale Crater consists of bedrock outcrops separated by relatively smooth, uniform regolith surfaces. In scattered sites, however, distinct patterns—in the form and texture of the ground surface—contrast sharply with the typical terrain and with eolian bedforms. This paper focuses on these diverse, intriguing ground patterns. They include ∼1 to >10 m‐long linear disruptions of uniform regolith surfaces, alignments, and other arrangements of similar‐sized rock fragments and shallow, ∼0.1 m‐wide sandy troughs 1–10 m in length. Similar features were recognized early in the Mars Science Laboratory (MSL) mission, but they received only limited attention until Curiosity, the MSL rover, encountered striking examples in the Glen Torridon region. Herein, the ground patterns are illustrated with rover images. Potential mechanisms are briefly discussed in the context of the bedrock composition and atmospheric conditions documented by Curiosity. The evidence suggests that the patterns are active forms of spontaneous granular organization. It leads to the hypothesis that the patterns arise and develop from miniscule, inferred cyclic expansion and contraction of the bedrock and regolith, likely driven by oscillating transfers of energy and moisture between the atmosphere and the terrain. The hypothesis has significant implications for studies of contemporary processes on Mars on both sides of the atmosphere‐lithosphere interface. The ground patterns, as well as ripples and dunes formed by the wind, constitute remarkable extra‐terrestrial examples of granular self‐organization, complex phenomena well known in diverse systems on Earth. Plain Language Summary: Gale crater, which is near Mars' equator (4.5°S latitude), is generally rocky with sand and rocks scattered between extensive bedrock exposures. However, in a few local sites, distinct natural patterns mark the ground with ∼0.1 m‐wide, sand‐lined troughs and rocks arranged geometrically. We use detailed images from Curiosity, the Mars Science Laboratory rover, to illustrate these intriguing patterns in the Glen Torridon area, where they are widespread. We suggest they are currently active in the dry rock debris at the surface and driven by the sustained shrink‐swell activity of the underlying bedrock. This inferred activity, which likely results from variations in moisture and temperature in the lower Mars atmosphere, could generate these ground patterns, much like vibrations or analogous cyclic disturbances cause patterns to form naturally in diverse settings with granular materials of different sizes and shapes. The inferred cyclic shrink‐swell behavior of the shallow subsurface and underlying transfers of energy and mass may be of broad interest because of their potential implications for studies of processes shaping the Mars landscape and occurring in the lower atmosphere. These ground features, as well as ripples and dunes formed by the wind, are visible examples of extra‐terrestrial self‐organization. Key Points: Near Mars' equator, sandy troughs and organized rock fragments locally form distinct patterns suggestive of active ground deformationUnlike patterned ground on Earth, the ground patterns in Gale Crater form in thin, dry regolith overlying jointed bedrockAtmosphere‐surface exchange of energy and water vapor is inferred to drive self‐organization through cyclic subsurface volume changes [ABSTRACT FROM AUTHOR]

A widely hypothesized but complex transition from widespread fluvial activity to predominantly aeolian processes is inferred on Mars based on remote sensing data observations of ancient landforms. However, the lack of analysis of in situ martian fluvial deposits hinders our understanding of the flow regime nature and sustainability of the martian fluvial activity and the hunt for ancient life. Studying analogs from arid zones on Earth is fundamental to quantitatively understanding geomorphic processes and climate drivers that might have dominated during early Mars. Here we investigate the formation and preservation of fluvial depositional systems in the eastern Sahara, where the largest arid region on Earth hosts important repositories of past climatic changes. The fluvial systems are composed of well‐preserved single‐thread sinuous to branching ridges and fan‐shaped deposits interpreted as deltas. The systems' configuration and sedimentary content suggest that ephemeral rivers carved these landforms by sequential intermittent episodes of erosion and deposition active for 10–100s years over ∼10,000 years during the late Quaternary. Subsequently, these landforms were sculpted by a marginal role of rainfall and aeolian processes with minimum erosion rates of 1.1 ± 0.2 mm/yr, supplying ∼96 ± 24 × 1010 m3 of disaggregated sediment to adjacent aeolian dunes. Our results imply that similar martian fluvial systems preserving single‐thread, short distance source‐to‐sink courses may have formed due to transient drainage networks active over short durations. Altogether, this study adds to the growing recognition of the complexity of interpreting climate history from orbital images of landforms. Plain Language Summary: Mars is currently a dry and cold desert, but rivers preserved in inverted topography suggest that water once flowed during its early history. However, how sustained and how frequently these rivers flowed remains uncertain. Here we study ancient fluvial systems (rivers and deltas) from the eastern Sahara that formed during the late Quaternary, in much wetter conditions than those prevailing today in this desert and which bear striking analogies to martian systems. We find that rivers and deltas, now preserved as ridges, record short distance source‐to‐sink high‐energy systems formed due to heavy rainfall events. Our observations and measurements of the meandering systems within the deltaic features suggest that such wet conditions might have spanned tens to a few hundred years over a total duration of ∼10,000 years. Since the wet conditions ceased, arid conditions prevailed, and the aeolian processes resumed, sculpting ridges out of ancient channels. Our results imply that martian fluvial systems may have been associated with similar local and heavy runoff conditions that lasted 10–100s years over thousands of years, possibly sufficient to support habitability. A shift toward arid environments led to the sculpting of fluvial ridges and the widespread formation of dunes across the modern martian landscape. Key Points: Ancient depositional rivers in southern Egypt record ephemeral fluvial systems formed due to intense rainfall over ∼10 kaThese fluvial systems suggest tens to hundreds of years of river activitySimilar martian systems imply that early Mars's surface was punctuated by local and transient drainage systems fed over short durations [ABSTRACT FROM AUTHOR]

Geochemical models of secondary mineral precipitation on Mars generally assume semiopen systems (open to the atmosphere but closed at the water-sediment interface) and equilibrium conditions. However, in natural multicomponent systems, the reactive surface area of primary minerals controls the dissolution rate and affects the precipitation sequences of secondary phases, and simultaneously, the transport of dissolved species may occur through the atmosphere-water and water-sediment interfaces. Here we present a suite of geochemical models designed to analyze the formation of secondary minerals in basaltic sediments on Mars, evaluating the role of (i) reactive surface areas and (ii) the transport of ions through a basalt sediment column. We consider fully open conditions, both to the atmosphere and to the sediment, and a kinetic approach for mineral dissolution and precipitation. Our models consider a geochemical scenario constituted by a basin (i.e., a shallow lake) where supersaturation is generated by evaporation/cooling and the starting point is a solution in equilibrium with basaltic sediments. Our results show that cation removal by diffusion, along with the input of atmospheric volatiles and the influence of the reactive surface area of primary minerals, plays a central role in the evolution of the secondary mineral sequences formed. We conclude that precipitation of evaporites finds more restrictions in basaltic sediments of small grain size than in basaltic sediments of greater grain size. [ABSTRACT FROM AUTHOR]

We model the fluids involved in the alteration processes recorded in the Sheepbed Member mudstones of Yellowknife Bay ( YKB), Gale crater, Mars, as revealed by the Mars Science Laboratory Curiosity rover investigations. We compare the Gale crater waters with fluids modeled for shergottites, nakhlites, and the ancient meteorite ALH 84001, as well as rocks analyzed by the Mars Exploration rovers, and with terrestrial ground and surface waters. The aqueous solution present during sediment alteration associated with phyllosilicate formation at Gale was high in Na, K, and Si; had low Mg, Fe, and Al concentrations-relative to terrestrial groundwaters such as the Deccan Traps and other modeled Mars fluids; and had near neutral to alkaline pH. Ca and S species were present in the 10−3 to 10−2 concentration range. A fluid local to Gale crater strata produced the alteration products observed by Curiosity and subsequent evaporation of this groundwater-type fluid formed impure sulfate- and silica-rich deposits-veins or horizons. In a second, separate stage of alteration, partial dissolution of this sulfate-rich layer in Yellowknife Bay, or beyond, led to the pure sulfate veins observed in YKB. This scenario is analogous to similar processes identified at a terrestrial site in Triassic sediments with gypsum veins of the Mercia Mudstone Group in Watchet Bay, UK. [ABSTRACT FROM AUTHOR]

Multiple choice questions (MCQs) are one of the most popular tools to evaluate learning and knowledge in higher education. Nowadays, there are a few indices to measure reliability and validity of these questions, for instance, to check the difficulty of a particular question (item) or the ability to discriminate from less to more knowledge. In this work two new indices have been constructed: (i) the no answer index measures the relationship between the number of errors and the number of no answers; (ii) the homogeneity index measures homogeneity of the wrong responses (distractors). The indices are based on the lack-of-fit statistic, whose distribution is approximated by a chi-square distribution for a large number of errors. An algorithm combining several traditional and new indices has been developed to refine continuously a database of MCQs. The final objective of this work is the classification of MCQs from a large database of items in order to produce an automated-supervised system of generating tests with specific characteristics, such as more or less difficulty or capacity of discriminating knowledge of the topic. [ABSTRACT FROM AUTHOR]

- Six large iron meteorites have been discovered in the Meridiani Planum region of Mars by the Mars Exploration Rover Opportunity in a nearly 25 km-long traverse. Herein, we review and synthesize the available data to propose that the discovery and characteristics of the six meteorites could be explained as the result of their impact into a soft and wet surface, sometime during the Noachian or the Hesperian, subsequently to be exposed at the Martian surface through differential erosion. As recorded by its sediments and chemical deposits, Meridiani has been interpreted to have undergone a watery past, including a shallow sea, a playa, an environment of fluctuating ground water, and/or an icy landscape. Meteorites could have been encased upon impact and/or subsequently buried, and kept underground for a long time, shielded from the atmosphere. The meteorites apparently underwent significant chemical weathering due to aqueous alteration, as indicated by cavernous features that suggest differential acidic corrosion removing less resistant material and softer inclusions. During the Amazonian, the almost complete disappearance of surface water and desiccation of the landscape, followed by induration of the sediments and subsequent differential erosion and degradation of Meridiani sediments, including at least 10-80 m of deflation in the last 3-3.5 Gy, would have exposed the buried meteorites. We conclude that the iron meteorites support the hypothesis that Mars once had a denser atmosphere and considerable amounts of water and/or water ice at and/or near the surface. [ABSTRACT FROM AUTHOR]

A wide-ranging study based on compositional and isotopic analyses of minerals and manufactured objects from the north-eastern Iberian Peninsula and their respective archaeological and cultural contexts demonstrates significant lead mineral exploitation in the El Priorat area (Tarragona province) linked to Phoenician trade (seventh–sixth centuries BC). This exploitation continued, despite losing intensity, until the Romanization of the territory. Our project also aims to determine the nature and origin of the lead and silver supply in the northern Iberian territory surrounding the Phocaean enclave of Emporion, especially with regard to the demands of the colonial mint. The behaviour pattern of the circulation of lead, silver and copper in Catalonia in the period studied indicates a plurality of contemporary supply sources, although, at least from the fifth century BC onward, minerals and metals from the south-eastern Iberian Peninsula take on considerable importance. [ABSTRACT FROM AUTHOR]

Relief impostors have been proposed as a compact and high-quality representation for high-frequency detail in 3D models. In this paper we propose an algorithm to represent a complex object through the combination of a reduced set of relief maps. These relief maps can be rendered with very few artifacts and no apparent deformation from any view direction. We present an efficient algorithm to optimize the set of viewing planes supporting the relief maps, and an image-space metric to select a sufficient subset of relief maps for each view direction. Selected maps (typically three) are rendered based on the well-known ray-height-field intersection algorithm implemented on the GPU. We discuss several strategies to merge overlapping relief maps while minimizing sampling artifacts and to reduce extra texture requirements. We show that our representation can maintain the geometry and the silhouette of a large class of complex shapes with no limit in the viewing direction. Since the rendering cost is output sensitive, our representation can be used to build a hierarchical model of a 3D scene. [ABSTRACT FROM AUTHOR]

The exploration of complex walkthrough models is often a difficult task due to the presence of densely occluded regions which pose a serious challenge to online navigation. In this paper we address the problem of algorithmic generation of exploration paths for complex walkthrough models. We present a characterization of suitable properties for camera paths and we discuss an efficient algorithm for computing them with little or no user intervention. Our approach is based on identifying the free-space structure of the scene (represented by a cell and portal graph) and an entropy-based measure of the relevance of a view-point. This metric is key for deciding which cells have to be visited and for computing critical way-points inside each cell. Several results on different model categories are presented and discussed. [ABSTRACT FROM AUTHOR]

Metabotropic γ-aminobutyric acid receptors (GABAB) play modulatory roles in central synaptic transmission and are involved in controlling neuronal migration during development. We used immunohistochemical methods to elucidate the expression pattern as well as the cellular and the precise subcellular localization of the GABAB1a/b and GABAB2 subunits in the rat hippocampus during prenatal and postnatal development. At the light microscopic level, both GABAB1a/b and GABAB2 were expressed in the hippocampal primordium from embryonic day E14. During postnatal development, immunoreactivity for GABAB1a/b and GABAB2 was distributed mainly in pyramidal cells, with discrete GABAB1a/b-immunopositive cell bodies of interneurons present throughout the hippocampus. Using double immunofluorescence, we demonstrated that during the second week of postnatal development, GABAB1a/b but not GABAB2 was expressed in glial cells throughout the hippocampal formation. At the electron microscopic level, GABAB1a/b and GABAB2 showed a similar distribution pattern during postnatal development. Thus, at all ages the two receptor subunits were located postsynaptically in dendritic spines and shafts at extrasynaptic and perisynaptic sites in both pyramidal and nonpyramidal cells. We further demonstrated that the two subunits were localized presynaptically along the extrasynaptic plasma membrane of axon terminals and along the presynaptic active zone in both asymmetrical and, to a lesser extent, symmetrical synapses. These results suggest that GABAB receptors are widely expressed in the hippocampus throughout development and that GABAB1a/b and GABAB2 form both pre- and postsynaptic receptors. © 2004 Wiley-Liss, Inc. [ABSTRACT FROM AUTHOR]

This paper deals with the analysis of stylistic variation between the different rock-art traditions that coexisted in the central Mediterranean coastal area of Spain in the Neolithic period – the Macro Schematic tradition, the Schematic tradition and the Levantine tradition. The stylistic variation is analyzed in different scales from the panel with its compositions and superimpositions, to the shelter – shared or exclusive – and the landscape, focussing on patterns of distribution and relationships with settlement sites. This stylistic analysis enables us to study the chronological aspects of the process of Neolithization; while the symbolic character of these marks on the landscape offers an insight into the perception and use of space by the communities who made them. [ABSTRACT FROM AUTHOR]

Abstract To understand the possible contribution of metabotropic γ-aminobutyric acid receptors (GABAB R) in cortical development, we investigated the expression pattern and the cellular and subcellular localization of the GABAB R1 and GABAB R2 subtypes in the rat neocortex from embryonic day 14 (E14) to adulthood. At the light microscopic level, both GABAB R1 and GABAB R2 were detected as early as E14. During prenatal development, both subtypes were expressed highly in the cortical plate. Using double immunofluorescence, GABAB R1 colocalized with GABAB R2 in neurons of the marginal zone and subplate, indicating that these proteins are coexpressed and could be forming functional GABAB Rs during prenatal development in vivo . In contrast, only GABAB R1 but not GABAB R2 was detected in the tangentially migratory cells in the lower intermediate zone. During postnatal development, immunoreactivity for GABAB R1 and GABAB R2 was distributed mainly in pyramidal cells. Discrete GABAB R1-immunopositive cell bodies of interneurons were present throughout the neocortex. In addition, GABAB R1 but not GABAB R2 was found in identified Cajal-Retzius cells in layer I. At the electron microscopic level, immunoreactivity for GABAB R1 and GABAB R2 was found in dendritic spines and dendritic shafts at extrasynaptic and perisynaptic sites throughout postnatal development. We further demonstrated the presynaptic localization of GABAB R1 and GABAB R2, as well as the association of the receptors with asymmetrical synaptic junctions. These results indicate potentially important roles for the GABAB Rs in the regulation of migratory processes during corticogenesis and in the modulation of synaptic transmission during early development of cortical circuitry. [ABSTRACT FROM AUTHOR]

Abstract Glutamate receptors have been linked to the regulation of several developmental events in the CNS. By using cortical slices of early postnatal mice, we show that in layer I cells, glutamate produces intracellular calcium ([Ca2+]i) elevations mediated by ionotropic and metabotropic glutamate receptors (mGluRs). The contribution of mGluRs to these responses was demonstrated by application of tACPD, an agonist to groups I and II mGluRs, which evoked [Ca2+]i increases that could be reversibly blocked by MCPG, an antagonist to groups I and II mGluRs. In the absence of extracellular Ca2+, repetitive applications of tACPD or quisqualate, an agonist to group I mGluRs, elicited decreasing [Ca2+]i responses that were restored by refilling a thapsigargin-sensitive Ca2+ store. The use of specific group I mGluR agonists CHPG and DHPG indicated that the functional mGluR in layer I was of the mGluR1 subtype. Subtype specific antibodies confirmed the presence of mGlur1α, but not mGluR5, in Cajal-Retzius (Reelin-immunoreactive) neurons. [ABSTRACT FROM AUTHOR]

Abstract Transient pioneer neurons in the neocortical marginal zone generate an early corticofugal axonal projection at E12–E16 (Meyer et al. 1998). We have analysed the functional activity of glutamate and GABA receptors in such cells by measuring changes in intracellular calcium concentrations ([Ca2+]i). The activation of GABAA receptors with muscimol, as well as bath application of glutamate, lead to increases in [Ca2+]i in pioneer neurons. The stimulatory action of glutamate is mostly produced through the NMDA-type of ionotropic receptors. Metabotropic glutamate receptor activation has no effect on [Ca2+]i. Consistent with such results, immunocytochemical studies showed a prominent expression of GABAA and NMDA receptors in pioneer neurons. The activation of such receptors may be implicated in the remodelling of pioneer neurons during development. [ABSTRACT FROM AUTHOR]

Abstract We investigated the substrates supporting neuronal migration, and its routes, during early thalamic development in the rat. Neurons and axonal and glial fibres were identified in embryos with single and double immunohistochemistry; dynamic data were obtained with cell tracers in short-term organotypic cultured slices. The earliest thalamic neurons, originating from the ventricular neuroepithelium between embryonic days 13 and 15, include those of the reticular thalamic nucleus. At this developmental stage, calretinin, calbindin or γ-aminobutyric acid immunostaining revealed both radially and nonradially orientated neurons in the region of reticular thalamic migration, between the dorsal and ventral thalamic primordia. In cultured slices, injections of fluorescent dyes in the neuroepithelium labelled neurons in a migratory stream along radial glia in the same zone. Some labelled fusiform cells departed from this radial trajectory along orthogonal routes within the dorsal thalamus. Confocal microscopy revealed nonradially orientated neurons in close apposition with a fibre system parallel to the lateral thalamic surface. These fibres expressed axonal markers, including the intermediate filament protein α-internexin and a polysialylated form of neuronal cell adhesion molecule. Active migration of nonradially orientated neurons along neuronal substrates was confirmed in living cultured slices. In addition, in vitro and ex vivo experiments revealed neurons migrating tangentially in association with glial fibres. These results provide novel evidence that: (i) early generated thalamic neurons follow nonradial routes in addition to glia-linked radial migration; and (ii), nonradially migrating thalamic neurons move along both glial and axonal substrates, which could represent a distinctive feature of thalamic development. [ABSTRACT FROM AUTHOR]

The distribution of the GABAA receptor/benzodiazepine receptor/chloride channel complex was investigated in the thalamus of the rat by means of immunohistochemistry in adulthood, as well as during embryonic and postnatal development, using a monoclonal antibody. In adults, the immunoreactivity for the GABAA receptor complex was intensely expressed by neuronal processes throughout the dorsal thalamus. Neuronal perikaryal membranes were frequently outlined by punctate immunostaining; cell bodies, intrathalamic fibre bundles and the internal capsule did not display immunoreactivity for the GABAA receptor. Regional differences in the expression of the receptor were consistently observed: the immunostaining was much lighter in the thalamic reticular nucleus than in the dorsal thalamic nuclei and, among the latter, the anteroventral nucleus and the ventral nuclear complex displayed the most intense immunopositivity. Immunostaining for the GABAA receptor was already expressed in embryos at E14, and was homogeneously distributed throughout the neuropil of the dorsal and ventral thalamic primordia. During the first two postnatal weeks, a regional differentiation of the immunopositivity was appreciable in the thalamus, with a progressive reduction in the reticular nucleus and a parallel increase in the dorsal thalamic structures. Immunoreactive neuronal perikarya were not observed in the thalamus at any developmental stage. The expression of the GABAA receptor complex appeared to have reached a mature configuration by the end of the third postnatal week. These findings indicate that in adults the GABAA receptor is differentially expressed by thalamic nuclear structures, including the reticular nucleus. Furthermore, the maturation of the receptor in the thalamus undergoes a rearrangement during the first postnatal weeks that results in a considerable regression within the reticular nucleus.