Dendritic Orientation Research Articles

Dendroarchitecture of relay cells in thalamic barreloids: a substrate for cross-whisker modulation.

A double-labeling protocol was used to determine how the dendroarchitecture of relay cells relates to the three-dimensional structure of barreloids in the ventral posterior medial nucleus of the rat thalamus. Single barreloids were retrogradely labeled by injecting Fluoro-Gold in identified barrel columns, and single relay cells activated by the same whisker, or by an adjacent whisker located on the same arc, were juxtacellularly labeled with biotinylated dextran. Results show that the dendritic field of relay cells is asymmetric, variously oriented with respect to the geometry of the barreloids, and that all cells extend dendrites in surrounding barreloids. Extrabarreloid dendrites are of small size (<1.5 microm) and represent up to 54% (range, 11-54%) of the total dendritic length. In contrast, the thick proximal dendrites remain confined to the home barreloid of the cell, being directed toward its center or along its margin. There is a trend for cells located dorsally in barreloids to form more elaborate trees with a larger proportion of extrabarreloid dendrites. Electron microscopic examination of labeled cells shows that extrabarreloid dendrites are exclusively contacted by synaptic terminals of cortical and reticular thalamic origin, whereas intrabarreloid dendrites also receive contacts from lemniscal terminals. Because corticothalamic and reticular thalamic cells establish point-to-point connections with homotopic barreloids, it is proposed that the spatial arrangement of dendrites determines the combination of whisker deflection that best modulates cell firing. Because relay cell responses are direction sensitive, maximal modulation would occur if dendritic field orientation relates to the direction selectivity of responses.

Dendroarchitecture of Relay Cells in Thalamic Barreloids: A Substrate for Cross-Whisker Modulation

A double-labeling protocol was used to determine how the dendroarchitecture of relay cells relates to the three-dimensional structure of barreloids in the ventral posterior medial nucleus of the rat thalamus. Single barreloids were retrogradely labeled by injecting Fluoro-Gold in identified barrel columns, and single relay cells activated by the same whisker, or by an adjacent whisker located on the same arc, were juxtacellularly labeled with biotinylated dextran. Results show that the dendritic field of relay cells is asymmetric, variously oriented with respect to the geometry of the barreloids, and that all cells extend dendrites in surrounding barreloids. Extrabarreloid dendrites are of small size (&lt;1.5 μm) and represent up to 54% (range, 11–54%) of the total dendritic length. In contrast, the thick proximal dendrites remain confined to the home barreloid of the cell, being directed toward its center or along its margin. There is a trend for cells located dorsally in barreloids to form more elaborate trees with a larger proportion of extrabarreloid dendrites. Electron microscopic examination of labeled cells shows that extrabarreloid dendrites are exclusively contacted by synaptic terminals of cortical and reticular thalamic origin, whereas intrabarreloid dendrites also receive contacts from lemniscal terminals. Because corticothalamic and reticular thalamic cells establish point-to-point connections with homotopic barreloids, it is proposed that the spatial arrangement of dendrites determines the combination of whisker deflection that best modulates cell firing. Because relay cell responses are direction sensitive, maximal modulation would occur if dendritic field orientation relates to the direction selectivity of responses.

Preferred orientations of primary Al 4Sr dendrites in a rapidly solidified Al–Sr alloy

Preferred crystallographic orientations of primary Al 4Sr dendrites in a rapidly solidified Al–23 Sr (wt.%) alloy have been investigated using transmission electron microscopy (TEM). The Al 4Sr dendrites with 90° branches are dominant in the Al–23 Sr alloy melt-spun at 500 rpm and the dendrite orientation is the 〈110〉 direction. Wheel speed has a significant effect on the morphology and preferred orientation of the Al 4Sr dendrites in the melt-spun Al–23 Sr alloy.

Modeling of magnetostriction in grain aligned terfenol-D and preferred orientation change of terfenol-D dendrites

The alloy Tb 0.3Dy 0.7Fe 2 (terfenol-D) is known to have high magnetostriction and low anisotropy energy. Therefore, comparatively high strain amplitudes, approaching 1000 ppm, can be obtained at low field (<100 kA/m). It was found that terfenol-D zone melted without seed was a polycrystal with preferred orientations. For a polycrystal, where several different fiber textures are mixed, we used an anisotropic domain rotation model to predict its magnetostriction and magnetization. This model, originally proposed by Jiles, assumed that the magnetic moments of domains are constrained by the anisotropy to lie along the 〈1 1 1〉 easy magnetization directions. The magnetostrictive properties of such a polycrystal were estimated by superposing the individual properties of all constituent grains relative to their volume fractions. The volume fractions of the constituent textures were calculated using orientation distribution functions. The results showed that the preferred orientation of terfenol-D dendrites changed from 110 to 112 with the increase of growth velocity. The model showed good qualitative agreement with experimental results, for magnetostriction of grain aligned polycrystalline Tb 0.31Dy 0.69Fe 1.80 with applied magnetic field under a range of constant uniaxial compressive stresses.

Molecular control of cortical dendrite development.

Dendritic morphology has a profound impact on neuronal information processing. The overall extent and orientation of dendrites determines the kinds of input a neuron receives. Fine dendritic appendages called spines act as subcellular compartments devoted to processing synaptic information, and the dendritic branching pattern determines the efficacy with which synaptic information is transmitted to the soma. The acquisition of a mature dendritic morphology depends on the coordinated action of a number of different extracellular factors. Here we discuss this evidence in the context of dendritic development in the cerebral cortex. Soon after migrating to the cortical plate, neurons extend an apical dendrite directed toward the pial surface. The oriented growth of the apical dendrite is regulated by Sema3A, which acts as a dendritic chemoattractant. Subsequent dendritic development involves signaling by neurotrophic factors and Notch, which regulate dendritic growth and branching. During postnatal development the formation and stabilization of dendritic spines are regulated in part by patterns of synaptic activity. These observations suggest that extracellular signals play an important role in regulating every aspect of dendritic development and thereby exert a critical influence on cortical connectivity.

Cell types and response properties of neurons in the ventral division of the medial geniculate body of the rabbit.

Although there is evidence for multiple classes of thalamic relay neurons in the auditory thalamus, correlative anatomical and physiological studies are lacking. We have used the juxtacellular labeling technique, in conjunction with Nissl, Golgi, and immunocytochemical methods, to study the morphology and response properties of cells in the ventral division of the medial geniculate body of the rabbit. Single units in the ventral division of the medial geniculate body (MGV) were characterized extracellularly with monaural and binaural tone and noise bursts (100- to 250-msec duration). Characterized units were filled with biocytin and visualized with an antibody enhanced diaminobenzidine reaction. A total of 31 neurons were physiologically characterized and labeled with the juxtacellular technique. Labeled neurons were fully reconstructed from serial sections by using a computer microscope system. Three subregions of the rabbit MGV were identified, each characterized by differences in Nissl architecture, calcium-binding protein expression, and by the dendritic orientation of tufted relay neurons. In general, the dendritic fields of relay neurons were closely aligned with the cellular laminae. Qualitative and quantitative analyses revealed two types of presumptive relay neurons within the MGV. Type I cells had thick dendrites with a greater total volume and morphologically diverse appendages compared with the Type II cells whose dendrites were thin with a moderate number of small spines. Both classes were acoustically responsive and exhibited a variety of response patterns, including onset, offset, and sustained responses. In terms of binaural characteristics, most (ca. 53%) labeled neurons were of the EE type, with the remaining cells classified as EO (27%) or EI (20%) response types. Two types of presumptive interneurons were also seen: bipolar neurons with large dendritic fields and a small neurogliaform variety. Cell types and dendritic orientation within the MGV are discussed in terms of the physiological organization of the rabbit auditory thalamus.

Wulst Neurons in Nestlings of the Pied Flycatcher Ficedula hypoleuca at the Pre-Visual Stage of Development

Neurons of the Wulst region, an analog of the mammalian visual cortex, were studied in Golgi-impregnated preparations of brain of non-precocial 1-day old nestlings of the pied flycatcher Ficedula hypoleuca. At this age, vision does not function in nestlings, their behavior is provided by an acoustic analyzer. Two populations of nerve cells, large and small juvenile neurons, were differentiated at visual examination. The comparative morphometry has shown these groups to differ significantly from each other by most studied parameters: the area of the profile field of their soma, the total length of dendrites, branching of the cell, the number of the maximal branching foci, the character of distribution of dendrite free endings in three concentric zones of the cell dendritic field. The distribution of dendrites in the dendritic neuron field was similar in the both groups of neurons. An increased density of dendrites was observed from the side of the afferent input. At the same time, study of orientation of the longest dendrite has shown it to be located parallel to the plane of the afferent input practically in all cells of the both groups. It is suggested that such orientation of the longest dendrite broadens the area of cell contacts, which is necessary for search for the maximal number of afferents. The predominant orientation of dendrites in the direction to the afferent input forms foundation for establishing a more extended zone of contacts with growing visual afferents.

Cholecystokinin-immunopositive basket and Schaffer collateral-associated interneurones target different domains of pyramidal cells in the CA1 area of the rat hippocampus

Two types of GABAergic interneurone are known to express cholecystokinin-related peptides in the isocortex: basket cells, which preferentially innervate the somata and proximal dendrites of pyramidal cells; and double bouquet cells, which innervate distal dendrites and dendritic spines. In the hippocampus, cholecystokinin immunoreactivity has only been reported in basket cells. However, at least eight distinct GABAergic interneurone types terminate in the dendritic domain of CA1 pyramidal cells, some of them with as yet undetermined neurochemical characteristics. In order to establish whether more than one population of cholecystokinin-expressing interneurone exist in the hippocampus, we have performed whole-cell current clamp recordings from interneurones located in the stratum radiatum of the hippocampal CA1 region of developing rats. Recorded neurones were filled with biocytin to reveal their axonal targets, and were tested for the presence of pro-cholecystokinin immunoreactivity. The results show that two populations of cholecystokinin-immunoreactive interneurones exist in the CA1 area ( n=15 positive cells). Cholecystokinin-positive basket cells (53%) preferentially innervate stratum pyramidale and adjacent strata oriens and radiatum. A second population of cholecystokinin-positive cells, previously described as Schaffer collateral-associated interneurones [Vida et al. (1998) J. Physiol. 506, 755–773], have axons that ramify almost exclusively in strata radiatum and oriens, overlapping with the Schaffer collateral/commissural pathway originating from CA3 pyramidal cells. Two of seven of the Schaffer collateral-associated cells were also immunopositive for calbindin. Soma position and orientation in stratum radiatum, the number and orientation of dendrites, and the passive and active membrane properties of the two cell populations are only slightly different. In addition, in stratum radiatum and its border with lacunosum of perfusion-fixed hippocampi, 31.6±3.8% (adult) or 26.8±2.9% (postnatal day 17–20) of cholecystokinin-positive cells were also immunoreactive for calbindin. Therefore, at least two populations of pro-cholecystokinin-immunopositive interneurones, basket and Schaffer collateral-associated cells, exist in the CA1 area of the hippocampus, and are probably homologous to cholecystokinin-immunopositive basket and double bouquet cells in the isocortex. It is not known if the GABAergic terminals of double bouquet cells are co-aligned with specific glutamatergic inputs. However, in the hippocampal CA1 area, it is clear that the terminals of Schaffer collateral-associated cells are co-stratified with the glutamatergic input from the CA3 area, with as yet unknown functional consequences. The division of the postsynaptic neuronal surface by two classes of GABAergic cell expressing cholecystokinin in both the hippocampus and isocortex provides further evidence for the uniform synaptic organisation of the cerebral cortex.

Neuroanatomical algorithms for dendritic modelling

The complexity and variability of dendritic morphology constitutes a fascinating challenge to the investigation of the structure-activity-function relationship in the nervous system. Computational modelling has recently emerged as a powerful approach for the quantitative anatomical characterization of dendrites. The key idea is to design a stochastic algorithm to generate digital structures that are statistically indistinguishable from those of real neurons of a given morphological class. The set of parameters used by this algorithm would then constitute a complete and accurate description of that morphological class. We review the strengths and weaknesses of the major types of algorithms used to model dendrogram properties, including those based on branch diameter and on distance from the soma. We also describe some approaches to the simulation of dendritic orientation and three-dimensional geometry. Finally, we discuss the environmental influences on dendritic morphology (especially the presence of axons, other neurons, and anatomical boundaries) and thus the need to include models of the tissue volume in the algorithmic description of dendrites.

Neuronal and Fibre Organization in Neocortical Grafts Placed in Post-ischaemic Adult Rat Brain: a Three-dimensional Confocal Microscopy Study

The dendritic morphology in neocortical grafts was studied with three-dimensional confocal laser scanning microscopy after microinjection of Lucifer Yellow into individual cells. The grafts had been implanted into infarct cavities in the neocortex of hypertensive rats 46 weeks earlier. The carbocyanine dye method was used to identify afferent (host to transplant) and efferent (transplant to host) connections. Pyramidal, nonpyramidal and glial cells were present in the transplants. Some dendrites had an almost normal appearance, but abnormalities (atypical orientation of apical, basal or oblique apical dendrites) were observed. Some bi-apical pyramidal neurons and pyramidal neurons with obliquely oriented apical dendrites were also observed. Carbocyanine dye-labelled fibres of different diameter formed a dense network in the transplant, enabling the border between transplant and host tissue to be clearly recognized. No labelled fibres were observed to enter the host brain. Fibres with ««boutons en passant»» and no preferential orientation were noted. It is proposed that Lucifer Yellow microinjection may be a useful method in studies aimed at improving graft morphology. Failure to demonstrate host to transplant connections with the carbocyanine dye method was contrary to earlier studies in which tracers were applied in vivo. A combined use of in-vivo and post-mortem tracer techniques is needed to establish the reason for the discrepancy.

Abnormal Morphological and Functional Organization of the Hippocampus in a p35 Mutant Model of Cortical Dysplasia Associated with Spontaneous Seizures

Cortical dysplasia is a major cause of intractable epilepsy in children. However, the precise mechanisms linking cortical malformations to epileptogenesis remain elusive. The neuronal-specific activator of cyclin-dependent kinase 5, p35, has been recognized as a key factor in proper neuronal migration in the neocortex. Deletion of p35 leads to severe neocortical lamination defects associated with sporadic lethality and seizures. Here we demonstrate that p35-deficient mice also exhibit dysplasia/ heterotopia of principal neurons in the hippocampal formation, as well as spontaneous behavioral and electrographic seizures. Morphological analyses using immunocytochemistry, electron microscopy, and intracellular labeling reveal a high degree of abnormality in dentate granule cells, including heterotopic localization of granule cells in the molecular layer and hilus, aberrant dendritic orientation, occurrence of basal dendrites, and abnormal axon origination sites. Dentate granule cells of p35-deficient mice also demonstrate aberrant mossy fiber sprouting. Field potential laminar analysis through the dentate molecular layer reflects the dispersion of granule cells and the structural reorganization of this region. Similar patterns of cortical disorganization have been linked to epileptogenesis in animal models of chronic seizures and in human temporal lobe epilepsy. The p35-deficient mouse may therefore offer an experimental system in which we can dissect out the key morphological features that are causally related to epileptogenesis.

Morphological and electrophysiological properties of atypically oriented layer 2 pyramidal cells of the juvenile rat neocortex

We used whole-cell patch clamp recordings combined with intracellular dye-filling to examine the morphological and electrophysiological properties of atypically oriented pyramidal cells located at the layer 1/2 border of the juvenile rat neocortex. Orientation of the apical dendrite varied from oblique (>20° from vertical) to truly horizontal (90° from vertical). The length of the apical dendrite ranged from 150 to 400 μm. The total horizontal domain of the dendritic tree (including basal dendrites) of the longest horizontal pyramids exceeded 500 μm, but we also found short horizontal cells with horizontal dendritic domains of less than 300 μm. In addition, atypically oriented pyramids had long horizontal axon collaterals in layer 1/2. Electrophysiologically, atypically oriented pyramidal cells had intrinsic membrane properties similar to regularly oriented pyramids that have been described in the superficial layers at this age in the rat. Cells that fired repetitively were all regular spiking. In addition, we identified a subgroup of neurons (20%) in this sample, which were unable to fire more than a few spikes at the beginning of the current pulse. We suggest that the unique orientation and size of their dendritic trees and the length and arrangement of their local axons collaterals make atypically oriented pyramids in layer 2 ideally suited to perform horizontal integration of synaptic inputs in the neocortex.

One Signal for Attraction and Repulsion

Pyramidal neurons have an asymmetric morphology with the apical dendrites oriented toward the pial (outer) surface of the cortex and the axon oriented toward the subcortical white matter (see the accompanying news and views article by Strittmatter). Axons of pyramidal neurons are directed away from a gradient of the chemorepellant Semaphorin 3A (Sema3A) acting through the Neuropilin-1 receptor. Polleux, Morrow, and Ghosh show that the same signal, Sema3A, also acts as a chemoattractant to direct the orientation of the dendrites of pyramidal neurons. Antibodies against the Neuropilin-1 receptor blocked the chemoattractive effect of Sema3A for dendrites, suggesting that chemoattraction and chemorepulsion are mediated by the same receptor. Elevation of cyclic guanosine monophosphate converts the Sema3A signal from repulsion to attraction in other systems, thus, the authors investigated whether guanylyl cyclase may be important for mediating the chemoattractive effect in pyramidal neurons. Soluble guanylyl cyclase is asymmetrically oriented with high levels at the pole of the cell where the dendrite is generated. Pharmacological inhibition of guanylyl cyclase or protein kinase G blocked oriented growth of the dendrites, but did not disrupt the oriented growth of the axons in pyramidal neurons. Thus, the same extracellular signal (Sema3A) is used for both chemoattraction and chemorepulsion via a mechanism involving asymmetric distribution of downstream signaling elements Polleux, F., Morrow, T., and Ghosh, A. (2000) Semaphorin 3A is a chemoattractant for cortical apical dendrites. Nature 404 : 567-573. [Online Journal] Strittmatter, S.M. (2000) Dendrites go up, axons go down. Nature 404 : 557-558. [Online Journal]

N-methyl- d-aspartate-type glutamate receptors are found in post-synaptic targets of adrenergic terminals in the thoracic spinal cord

Adrenergic (C1) neurons in the rostral ventrolateral medulla (RVL) are sympathoexcitatory and project directly to sympathetic preganglionic neurons (SPNs) in the thoracic spinal cord. C1 neurons also contain glutamate which may mediate the excitatory effects of RVL stimulation on SPNs through the N-methyl- d-aspartate (NMDA)-type receptor. Dual-labeling immunocytochemistry, combined with electron microscopy, was used to determine if NMDA receptors are located post-synaptic to adrenergic terminals in the spinal cord. Adrenergic terminals were labeled using an antibody directed against phenylethanolamine- N-methyl transferase (PNMT) and the NMDA receptor was identified using an antibody directed against the R1 subunit of the receptor (NMDAR1). NMDAR1 was found primarily in large and small dendrites and a few perikarya. The presence of NMDAR1 in the dendritic targets of PNMT-containing terminals was quantified in spinal cords sectioned either horizontally or coronally. PNMT-labeled terminals formed asymmetric synapses on dendrites containing immunogold labeling for NMDAR1, but NMDAR1 was more often detected in the targets of PNMT terminals when spinal cords were sectioned horizontally (59%) rather than coronally (28%). This difference in prevalence of NMDAR1 in targets of PNMT terminals is likely due to the preferential orientation of SPN dendrites in the horizontal plane, since longer dendritic shafts were visible in horizontal sections. When NMDAR1 was present in the dendritic targets of many adrenergic terminals, it was usually located at sites distal to the adrenergic input. We conclude that NMDA receptor ligands are likely to modulate the activity of dendritic targets of adrenergic terminals in the spinal cord, but are not closely associated with adrenergic synaptic input.

Dynamic mediolateral activation of the pyramidal cell population in human somatosensory 3b area can be visualized by magnetic recordings

Dynamic mediolateral activation in the pyramidal cell population of the human somatosensory 3b area was studied non-invasively. Somatosensory evoked fields (SEFs) were recorded over the hand area contralateral to the right median nerve stimulation at the wrist. Localization of an equivalent current dipole (ECD) for N20m primary cortical response was successively calculated for periods of 2.4 ms around N20m peak where the signal-to-noise ratio exceeded 100 and the spatial resolution was approximately 0.3 mm. The ECD moved toward the anterior, lateral and inferior direction for a distance of 8.7 mm at a propagation velocity of 3.6 m/s. This direction was orthogonal to the orientation of apical dendrites of the pyramidal neurons in area 3b and parallel to the surface of the posterior bank of the central sulcus. These findings suggest that the sequential mediolateral activation of the pyramidal cells in the somatosensory cortex is mediated by horizontal connections running parallel to the cortical surface.

The minimal provision principle of functional systems. Neuronal mechanisms.

An important characteristic of the developing nerve cell is its ability to function long before maturity. At this time, the neuron has the basic properties of an excitable membrane, an incompletely developed dendritic tree, and a low level of afferent input. All of these factors hinder the initiation of synaptic and spike potentials. It is proposed that the functional potential of neurons at the early stages of development are provided by an adaptive system, which may include: 1) the excess phenomenon, i.e., increases in the numbers of nerve cells, their processes, spines, and synapses; 2) factors increasing the chances of meeting the expected afferentation, i.e., the orientation and growth of dendrites towards the afferent input and the structure and localization of branch points; 3) factors facilitating the initiation of nerve spikes i.e., juvenile channels for ion current generation, electrical interactions between cells. and additional trigger zones. These groups of properties solve a single common problem: that of facilitating cells to respond to single or weak signals. This is the basic condition supporting the operation of interneuron interactions at the early stages of brain development.

Origin of Compact Triangular Islands in Metal-on-Metal Growth

The microscopic origin of compact triangular islands on close-packed surfaces is identified using kinetic Monte Carlo simulations with energy barriers obtained from density-functional calculations. In contrast to earlier accounts, corner diffusion anisotropy is found to control the shape of compact islands at intermediate temperatures. We rationalize the correlation between the orientation of dendrites grown at low temperatures and triangular islands grown at higher temperatures, and explain why in some systems dendrites grow fat before turning compact.

Dorsal motor nucleus of the vagus neurons: A multivariate taxonomy

The dorsal motor nucleus of the vagus (DMNX) contains neurons with different projections and discrete functions, but little success has been achieved in distinguishing the cells cytoarchitectonically. The present experiment employed multivariate analytical techniques to evaluate DMNX neuronal morphology. Male Sprague-Dawley rats (n = 77) were perfused, and the brainstems were stained en bloc with a Golgi-Cox protocol. DMNX neurons in each of three planes (coronal, sagittal, and horizontal; total sample = 607) were digitized. Three-dimensional features quantified included dendritic length, number of segments, spine density, number of primary dendrites, dendritic orientation, and soma form factor. Cluster analyses of six independent samples of 100+ neurons and of three composite replicate pools of 200+ neurons consistently identified similar sets of four distinct neuronal profiles. One profile (spinous, limited dendrites, small somata) appears to correspond to the interneuron population of the DMNX. In contrast, the other three distinctive profiles (e.g., one is multipolar, with large dendritic fields and large somata) are different types of preganglionic neurons. Each of the four types of neurons is found throughout the DMNX, suggesting that the individual columnar subnuclei and other postulated vagal motorneuron pools are composed of all types of neurons. Within individual motor pools, ensembles of the different neuronal types must cooperatively organize different functions and project to different effectors within a target organ. By extension, specializations of the preganglionic motor pools are more likely to result from their afferent inputs, peripheral target tissues, neurochemistry, or physiological features rather than from any unique morphological profiles.

Mechanism of single centreline grain formation in titanium alloy welds

The formation of a single through thickness grain found in the centreline of titanium alloy Ti–6Al–4V thin sheet welds was investigated. Based on the numerical and experimental results, a new mechanism is proposed that considers the dendrite growth velocity and orientation and their influences on the solid/liquid interface temperature. This new mechanism, involving dendrite tip undercooling and competitive growth concepts, can be used to explain the formation and stabilisation of the single grain and grain structure in the weld. It was found that the centreline single grain is stabilised by its dendrites along a favourable 〈100〉 direction and its protrusion into the trailing edge of the weld pool, as a consequence of it growing at a lower velocity with less undercooling than the dendrites in the adjacent grains. In addition, the results suggest that the formation of the single through thickness centreline grain is also dependent on the weld geometry controlled by the welding heat input and travel speed. The macroscopic structure and growth directions of the weld pool must be such that the centreline grain can first become established.

Mechanism of single centreline grain formation in titanium alloy welds

The formation of a single through thickness grain found in the centreline of titanium alloy Ti–6Al–4V thin sheet welds was investigated. Based on the numerical and experimental results, a new mechanism is proposed that considers the dendrite growth velocity and orientation and their influences on the solid/liquid interface temperature. This new mechanism, involving dendrite tip undercooling and competitive growth concepts, can be used to explain the formation and stabilisation of the single grain and grain structure in the weld. It was found that the centreline single grain is stabilised by its dendrites along a favourable 〈100〉 direction and its protrusion into the trailing edge of the weld pool, as a consequence of it growing at a lower velocity with less undercooling than the dendrites in the adjacent grains. In addition, the results suggest that the formation of the single through thickness centreline grain is also dependent on the weld geometry controlled by the welding heat input and travel speed. The macroscopic structure and growth directions of the weld pool must be such that the centreline grain can first become established.