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Background: The final size of the apical root canal preparation has been a contraversion issue for a long time. The enlargement of the apical portion of the root canal may severely affect endodontic prognosis. Preparation of the apical zone to a larger file size may negatively affect the ability to control the filling material during root canal obturation. An excessively expanded apical zone increases the risk of overfilling, which has been shown to be associated with a reduced success rate. This article aims to review research papers on the anatomical natural diameter of the apical root canal constriction, which is the endpoint of the endodontic preparation and should determine the appropriate size of the apical and root canal preparation. Review Results: In the review, we have included various scientific articles from recent years that comment on the size and shape of the apical constriction in different groups of teeth. We have summarized the results in graphs that give a visual idea of ​​the differences in the sizes of the different apical constrictions. Conclusion: The root apex is morphologically a challenging zone. The endodontist should have detailed knowledge of anatomic variation and mechanics challenges involved in treating apical third to manage endodontic therapy effectively. More research is needed to elucidate the influence of the size of the apical constriction on the apical stop preparation.

Background: Determining the correct apical limit for root canal treatment is essential for its success. This study evaluates the accuracy of three electronic apex locators (EALs) in locating the apical constriction (AC) in molar canals. Methods: Forty extracted human mandibular molars were scanned using micro-CT, and endodontic access cavities were created. Teeth were mounted in alginate, and three EALs—Root ZX-mini, Root ZX-II, and Sirona integrated apex locator—were used to measure the canal working length in dry canals and with EDTA gel. Micro-CT scans were performed with files in place, and the distance from the AC was calculated. Measurements within 0.1–0.5 mm were categorized as 'close'. Those extending beyond towards the major foramen were categorized as 'beyond', otherwise they were classified as 'far'. Data analysis was conducted with a level of significance set at 5%. Results: Most readings for all EALs were in the 'close' category, with significant differences between devices (p < 0.0001). Root ZX-mini and Root ZX-II had 74.4% and 72.5% 'close' readings, respectively, versus 51% for Sirona integrated. Accuracy did not differ significantly between dry and EDTA-treated canals (p = 0.306). All EALs demonstrated excellent operator reliability (ICC 0.996–1.00). Conclusions: All EALs accurately determined AC, unaffected by lubricants. However, Root ZX-mini and Root ZX-II outperformed Sirona integrated. All EALs showed consistent reliability. [ABSTRACT FROM AUTHOR]

Coordinated cell shape changes are a major driver of tissue morphogenesis, with apical constriction of epithelial cells leading to tissue bending. We previously identified that interplay between the apical-medial actomyosin, which drives apical constriction, and the underlying longitudinal microtubule array has a key role during tube budding of salivary glands in the Drosophila embryo. At this microtubule–actomyosin interface, a hub of proteins accumulates, and we have shown before that this hub includes the microtubule– actin crosslinker Shot and the microtubule minus-end-binding protein Patronin. Here, we identify two actin-crosslinkers, β-heavy (H)-Spectrin (also known as Karst) and Filamin (also known as Cheerio), and the multi-PDZ-domain protein Big bang as components of the protein hub. We show that tissue-specific degradation of β-H-Spectrin leads to reduction of apical-medial F-actin, Shot, Patronin and Big bang, as well as concomitant defects in apical constriction, but that residual Patronin is still sufficient to assist microtubule reorganisation. We find that, unlike Patronin and Shot, neither β-H-Spectrin nor Big bang require microtubules for their localisation. β-H-Spectrin is instead recruited via binding to apicalmedial phosphoinositides, and overexpression of the C-terminal pleckstrin homology domain-containing region of β-H-Spectrin (β-H33) displaces endogenous β-H-Spectrin and leads to strong morphogenetic defects. This protein hub therefore requires the synergy and coincidence of membrane- and microtubule-associated components for its assembly and function in sustaining apical constriction during tubulogenesis. [ABSTRACT FROM AUTHOR]

Information concerning the anatomy of the physiological foramen is still limited. The aim of this study was to investigate the distance between the physiological and anatomic apex, the shape and diameter of the physiological foramen in maxillary (Mx) and mandibular premolars (Mn). The anatomy of the apex of 229 maxillary (first: MxP1; second: MxP2) and 221 mandibular premolars (first: MnP1; second: MnP2) from a mixed Swiss-German population was investigated by means of microcomputed tomography and 3-dimensional software imaging. The following results were obtained in the presence of a main physiological foramen. 1. The distance between the physiological and anatomic foramen was 0.29-0.99 mm (MxP1), 0.21-1.03 mm (MxP2), 0.13-0.8 (MnP1), and 0.15-1.41 (MnP2). 2. The mean narrow and wide diameters of the physiological foramen were 0.19-0.33 mm (MxP1), 0.25-0.42 mm (MxP2), 0.28-0.37 (MnP1), and 0.28-0.40 (MnP2). 3. The most common physiological foramen shape was oval (66.7% MxP1, 89.7% MxP2, 91.8% MnP1, 64.4% MnP2). Considering the recommended preparation sizes based on a size corresponding to the friction, that is at the narrowest point in the area of the apical constriction (physiological foramen), and within the limitations of this ex vivo microcomputed tomography study, a final preparation size could be chosen when considering the pertaining morphologic considerations; yet, to a minimum ISO 30 size. [ABSTRACT FROM AUTHOR]

Abstract The morphogenetic process of apical constriction, which relies on non-muscle myosin II (NMII) generated constriction of apical domains of epithelial cells, is key to the development of complex cellular patterns. Apical constriction occurs in almost all multicellular organisms, but one of the most well-characterized systems is the Folded-gastrulation (Fog)-induced apical constriction that occurs in Drosophila. The binding of Fog to its cognizant receptors Mist/Smog results in a signaling cascade that leads to the activation of NMII-generated contractility. Despite our knowledge of key molecular players involved in Fog signaling, we sought to explore whether other proteins have an undiscovered role in its regulation. We developed a computational method to predict unidentified candidate NMII regulators using a network of pairwise protein–protein interactions called an interactome. We first constructed a Drosophila interactome of over 500,000 protein–protein interactions from several databases that curate high-throughput experiments. Next, we implemented several graph-based algorithms that predicted 14 proteins potentially involved in Fog signaling. To test these candidates, we used RNAi depletion in combination with a cellular contractility assay in Drosophila S2R + cells, which respond to Fog by contracting in a stereotypical manner. Of the candidates we screened using this assay, two proteins, the serine/threonine phosphatase Flapwing and the putative guanylate kinase CG11811 were demonstrated to inhibit cellular contractility when depleted, suggestive of their roles as novel regulators of the Fog pathway.

Epithelial folding is a fundamental process governing the transformation of flat epithelial sheets into intricate three-dimensional structures during morphogenesis. This phenomenon plays a pivotal role in the development of various organs across biological systems. Despite its importance, the underlying mechanisms of epithelial folding remain incompletely understood due to its dynamic and complex nature. In recent years, computational simulations have emerged as powerful tools to study epithelial folding, providing a means to test theories, generate predictions, and integrate data from various sources. The basic workflow of simulation-based research involves formulating hypotheses grounded in insights derived from experimental observations, constructing mechanical models based on these hypotheses, conducting simulations, and subsequently comparing simulation results with experimental observations. This review encompasses studies exploring how spatial distributions of contractile cells and temporal histories of growth and contraction contribute to the three-dimensionalization of epithelial sheets by modeling the mechanics of tissue growth and cell contractile forces. Additionally, it addresses the studies examining the impact of asymmetry in physical constraints imposed by the surrounding structures of epithelial sheets and the non-uniformity of growth on the undulation pattern formation of epithelial sheets by modeling the mechanical interaction between the growing tissue and the surrounding structures. Furthermore, a recent advancement proposes a new framework where computations are employed for initial stages of hypothesis formation by inferring the causality. In this context, we also discuss a recent study that quantitatively infers differential growth, causing the morphogenesis, solely from pre- and post-growth shape data. Through this comprehensive review, we demonstrate the utility of simulations in studying epithelial folding, emphasizing their potential for synergistic integration with various perspectives and exploring synergistic opportunities.

Multicellular rosettes are transient epithelial structures that serve as important cellular intermediates in the formation of diverse organs. Using the zebrafish posterior lateral line primordium (pLLP) as a model system, we investigated the role of the RhoA GEF Mcf2lb in rosette morphogenesis. The pLLP is a group of ~150 cells that migrates along the zebrafish trunk and is organized into epithelial rosettes; these are deposited along the trunk and will differentiate into sensory organs called neuromasts (NMs). Using single-cell RNAsequencing and whole-mount in situ hybridization, we showed that mcf2lb is expressed in the pLLP during migration. Live imaging and subsequent 3D analysis of mcf2lb mutant pLLP cells showed disrupted apical constriction and subsequent rosette organization. This resulted in an excess number of depositedNMs along the trunk of the zebrafish. Cell polarity markers ZO-1 and Par-3 were apically localized, indicating that pLLP cells are properly polarized. In contrast, RhoA activity, as well as signaling components downstream of RhoA, Rock2a and non-muscle Myosin II, were diminished apically. Thus, Mcf2lb-dependent RhoA activation maintains the integrity of epithelial rosettes. [ABSTRACT FROM AUTHOR]

Background and Aim: Determination of the exact working length is one of the most important steps in root canal treatment. Various devices and methods, such as electronic apex locators, have been developed for the determination of the working length. The present study aimed to compare the accuracy of four electronic apex locators, DTE, Denjoy, Woodpecker and Coxo, in determining the actual working length. Materials and Methods: In this descriptive study, fourty mandibular premolar teeth that fulfilled the inclusion criteria were selected and examined. First, each tooth was measured with four apex locators. The electronic measurements were performed following the manufacturers’ instructions within ±0.5 mm, ±1 mm and exactly at apical constriction. To calculate the actual working length, 4 mm of the root end was buccolingually cut from buccal side of the canal, and the distance between the coronal reference point and apical constriction was measured under a microscope. Then, data were analysed with Paired T test. Results:. Within ±0.5 mm from the apical constriction and at the apical constriction, the accuracy of apex locators DTE, Woodpecker, Coxo and Denjoy were 87% and 45%, 85% and 37%, 85% and 45% and 82% and 42%, respectively. All specimens measured with four types of apex locators were within the range of -1 to +1 mm to the apex with an accuracy of 97.5%. Conclusion: None of the four apex locators reported an accuracy of 100% at apical constriction. There was no statistically significant difference between the working length measured by four apex locators and the actual working length. [ABSTRACT FROM AUTHOR]

The morphogenetic process of apical constriction, which relies on non-muscle myosin II (NMII) generated constriction of apical domains of epithelial cells, is key to the development of complex cellular patterns. Apical constriction occurs in almost all multicellular organisms, but one of the most well-characterized systems is the Folded-gastrulation (Fog)-induced apical constriction that occurs in Drosophila. The binding of Fog to its cognizant receptors Mist/Smog results in a signaling cascade that leads to the activation of NMII-generated contractility. Despite our knowledge of key molecular players involved in Fog signaling, we sought to explore whether other proteins have an undiscovered role in its regulation. We developed a computational method to predict unidentified candidate NMII regulators using a network of pairwise protein–protein interactions called an interactome. We first constructed a Drosophila interactome of over 500,000 protein–protein interactions from several databases that curate high-throughput experiments. Next, we implemented several graph-based algorithms that predicted 14 proteins potentially involved in Fog signaling. To test these candidates, we used RNAi depletion in combination with a cellular contractility assay in Drosophila S2R + cells, which respond to Fog by contracting in a stereotypical manner. Of the candidates we screened using this assay, two proteins, the serine/threonine phosphatase Flapwing and the putative guanylate kinase CG11811 were demonstrated to inhibit cellular contractility when depleted, suggestive of their roles as novel regulators of the Fog pathway. [ABSTRACT FROM AUTHOR]

Aim: The aim of this in vitro study is to evaluate the accuracy of Root ZX and Propex Pixi during the retreatment of root canals obturated with different calcium silicate-based root canal sealers. Methodology: Root canals of ninety human premolar teeth were shaped with ProTaper universal rotary files up to size F2 under 5.25% NaOCl irrigation for the present study. Root canal filling was carried out by lateral condensation technique using with gutta-percha and sealers; MTA Fillapex, CeraSeal or AH Plus. After seven days, the obturating materials were removed using D-Race files. While actual working length was recorded by direct observation, electronic working lengths were determined by Propex Pixi or Root ZX for tolerance limits of 0.5 mm and 1.0 mm. Data were analyzed using Shapiro Wilk, Mann-Whitney U, and Kruskal-Wallis tests (p < 0.05). Results: The performance of Propex Pixi and Root ZX was found to be statistically similar and did not differ on the root canals filled with different root canal sealers (p < 0.05). Propex Pixi and Root ZX demonstrated more accurate measurements in the range of ±1 mm compared to ±0.5 mm in all groups, regardless of the type of root canal sealer used when filling the root canals before retreatment (p < 0.05). Conclusion: Root ZX and Propex Pixi can be used confidently in the retreatment of the root canals in which CeraSeal, MTA Fillapex, and AH Plus are used as sealers. [ABSTRACT FROM AUTHOR]

We investigated the interference of apical constriction position and diameter on the accuracy of electronic apex locators using 3D‐printed tooth models. Single‐rooted tooth models with the same length, canal taper and major foramen, but variation in apical constriction position or size, were designed and 3D‐printed. A mounting model was custom‐made for precise measurement of both marks (0.5 and APEX/0.0) of two electronic apex locators. The electronic measurements of both devices were correlated significantly to the major foramen rather than apical constriction. The mean measurements of the group with 0.45 mm in apical constriction width were significantly shorter than those of the other groups for both marks of the two devices (p < 0.05). The variations in apical constriction position and width negatively affected the precision of the 0.5 mark of the tested devices. The 0.0 or APEX mark was consistently located the major foramen. [ABSTRACT FROM AUTHOR]

Background: Determination of working length (WL) is necessary for the successful outcome of root canal treatment (RCT). Common methods in WL determination include tactile, radiographic, and electronic apex locators (EAL). Aim: The aim of this study was to compare three methods of WL determination to the actual visualization of the apical constriction (AC). Materials and Methods: Consecutive patients with indications for extraction of single-rooted single canal teeth at the University of Ghana Dental School clinic were randomly assigned to three groups. In-vivo root canal WL was determined by tactile sensation, digital radiography, and a 5th generation EAL (Sendoline S5). Files were cemented in the canals after the in-vivo measurements. The apical 4-5 mm of the roots was trimmed to expose the inserted files and the AC. Actual WL, as determined by visualization of the AC, was done using a digital microscope. Different WLs were then compared for the various groups, and the mean actual canal lengths were reported. Results: EAL accurately predicted the AC in 31 (96.9%) teeth, while the digital radiographic and tactile sensation methods accurately predicted the constriction in 19 (59.4%) and 8 (25%) teeth, respectively, in the study population. The mean working canal lengths for single-rooted teeth showed no observable difference among sexes, age categories, and side of the jaw. Conclusion: The EAL provided more reliable and accurate WL measurements for single-rooted teeth among Ghanaians, compared to digital radiography and tactile methods. [ABSTRACT FROM AUTHOR]

Epithelial-to-mesenchymal transition (EMT) is a fundamental process whereby epithelial cells acquire mesenchymal phenotypes and the ability to migrate. EMT is the hallmark of gastrulation, an evolutionarily conserved developmental process. In mammals, epiblast cells ingress at the primitive streak to form mesoderm. Cells ingress and exit the epiblast epithelial layer and the associated EMT is dynamically regulated and involves a stereotypical sequence of cell behaviors. 3D time-lapse imaging of gastrulating mouse embryos combined with cell and tissue scale data analyses revealed the asynchronous ingression of epiblast cells at the primitive streak. Ingressing cells constrict their apical surfaces in a pulsed ratchet-like fashion through asynchronous shrinkage of apical junctions. A quantitative analysis of the distribution of apical proteins revealed the anisotropic and reciprocal enrichment of members of the actomyosin network and Crumbs2 complexes, potential regulators of asynchronous shrinkage of cell junctions. Loss of function analyses demonstrated a requirement for Crumbs2 in myosin II localization and activity at apical junctions, and as a candidate regulator of actomyosin anisotropy.

Gastrulation is the first dynamic cell movement during embryogenesis. Endoderm and mesoderm cells are internalized into embryos during this process. Ascidian embryos provide a simple system for studying gastrulation in chordates. Gastrulation starts in spherical late 64-cell embryos with 10 endoderm blastomeres. The mechanisms of gastrulation in ascidians have been investigated, and a two-step model has been proposed. The first step involves apical constriction of endoderm cells, followed by apicobasal shortening in the second step. In this study, isolated ascidian endoderm progenitor cells displayed dynamic blebbing activity at the gastrula stage, although such a dynamic cell-shape change was not recognized in toto. Blebbing is often observed in migrating animal cells. In ascidians, endoderm cells displayed blebbing activity, while mesoderm and ectoderm cells did not. The timing of blebbing of isolated endoderm cells coincided with that of cell invagination. The constriction rate of apical surfaces correlated with the intensity of blebbing activity in each endoderm-lineage cell. Fibroblast growth factor (FGF) signaling was both necessary and sufficient for inducing blebbing activity, independent of cell fate specification. In contrast, the timing of initiation of blebbing and intensity of blebbing response to FGF signaling were controlled by intrinsic cellular factors. It is likely that the difference in intensity of blebbing activity between the anterior A-line and posterior B-line cells could account for the anteroposterior difference in the steepness of the archenteron wall. Inhibition of zygotic transcription, FGF signaling, and Rho kinase, all of which suppressed blebbing activity, resulted in incomplete apical constriction and failure of the eventual formation of cup-shaped gastrulae. Blebbing activity was involved in the progression and maintenance of apical constriction, but not in apicobasal shortening in whole embryos. Apical constriction is mediated by distinct blebbing-dependent and blebbing-independent mechanisms. Surface tension and consequent membrane contraction may not be the sole mechanical force for apical constriction and formation of cup-shaped gastrulae. The present study reveals the hidden cellular potential of endodermal cells during gastrulation and discusses the possible roles of blebbing in the invagination process. [Display omitted] • Endoderm cells have Rho kinase-dependent blebbing activity at the gastrula stage. • Fibroblast growth factor is required and sufficient to induce blebbing activity. • Blebbing activity contributes to the progression and maintenance of apical constriction. • Blebbing intensity can account for A-P difference in steepness of archenteron wall. • Surface tension and contraction may not solely drive apical constriction. [ABSTRACT FROM AUTHOR]

Congenital hydrocephalus is a common condition caused by the accumulation of cerebrospinal fluid in the ventricular system. Four major genes are currently known to be causally involved in hydrocephalus, either isolated or as a common clinical feature: L1CAM, AP1S2, MPDZ and CCDC88C. Here, we report 3 cases from 2 families with congenital hydrocephalus due to bi-allelic variations in CRB2, a gene previously reported to cause nephrotic syndrome, variably associated with hydrocephalus. While 2 cases presented with renal cysts, one case presented with isolated hydrocephalus. Neurohistopathological analysis allowed us to demonstrate that, contrary to what was previously proposed, the pathological mechanisms underlying hydrocephalus secondary to CRB2 variations are not due to stenosis but to atresia of both Sylvius Aqueduct and central medullar canal. While CRB2 has been largely shown crucial for apico-basal polarity, immunolabelling experiments in our fetal cases showed normal localization and level of PAR complex components (PKCι and PKCζ) as well as of tight (ZO-1) and adherens (β-catenin and N-Cadherin) junction molecules indicating a priori normal apicobasal polarity and cell–cell adhesion of the ventricular epithelium suggesting another pathological mechanism. Interestingly, atresia but not stenosis of Sylvius aqueduct was also described in cases with variations in MPDZ and CCDC88C encoding proteins previously linked functionally to the Crumbs (CRB) polarity complex, and all 3 being more recently involved in apical constriction, a process crucial for the formation of the central medullar canal. Overall, our findings argue for a common mechanism of CRB2, MPDZ and CCDC88C variations that might lead to abnormal apical constriction of the ventricular cells of the neural tube that will form the ependymal cells lining the definitive central canal of the medulla. Our study thus highlights that hydrocephalus related to CRB2, MPDZ and CCDC88C constitutes a separate pathogenic group of congenital non-communicating hydrocephalus with atresia of both Sylvius aqueduct and central canal of the medulla. [ABSTRACT FROM AUTHOR]

Neuroepithelial cells balance tissue growth requirement with the morphogenetic imperative of closing the neural tube. They apically constrict to generate mechanical forces which elevate the neural folds, but are thought to apically dilate during mitosis. However, we previously reported that mitotic neuroepithelial cells in the mouse posterior neuropore have smaller apical surfaces than non-mitotic cells. Here, we document progressive apical enrichment of non-muscle myosin-II in mitotic, but not non-mitotic, neuroepithelial cells with smaller apical areas. Live-imaging of the chick posterior neuropore confirms apical constriction synchronised with mitosis, reaching maximal constriction by anaphase, before division and re-dilation. Mitotic apical constriction amplitude is significantly greater than interphase constrictions. To investigate conservation in humans, we characterised early stages of iPSC differentiation through dual SMAD-inhibition to robustly produce pseudostratified neuroepithelia with apically enriched actomyosin. These cultured neuroepithelial cells achieve an equivalent apical area to those in mouse embryos. iPSC-derived neuroepithelial cells have large apical areas in G2 which constrict in M phase and retain this constriction in G1/S. Given that this differentiation method produces anterior neural identities, we studied the anterior neuroepithelium of the elevating mouse mid-brain neural tube. Instead of constricting, mid-brain mitotic neuroepithelial cells have larger apical areas than interphase cells. Tissue geometry differs between the apically convex early midbrain and flat posterior neuropore. Culturing human neuroepithelia on equivalently convex surfaces prevents mitotic apical constriction. Thus, neuroepithelial cells undergo high-amplitude apical constriction synchronised with cell cycle progression but the timing of their constriction if influenced by tissue geometry. [Display omitted] • Posterior neuropore neuroepithelial cells undergo apical constriction in mitosis. • Mitotic apical constriction occurs in mouse and chick embryos, and human cells. • Human iPSC-derived neuroepithelia are pseudostratified and apically constrict. • Mitotic apical constriction does not occur on convex neuroepithelial geometries. [ABSTRACT FROM AUTHOR]

Neural tube closure is a fundamental process during vertebrate embryogenesis, which leads to the formation of the central nervous system. Defective neural tube closure leads to neural tube defects which are some of the most common human birth defects. While the intrinsic morphogenetic events shaping the neuroepithelium have been studied extensively, how tissues mechanically coupled with the neural plate influence neural tube closure remains poorly understood. Here, using Xenopus laevis embryos, live imaging in combination with loss of function experiments and morphometric analysis of fixed samples we explore the reciprocal mechanical communication between the neural plate and the somitic mesoderm and its impact on tissue morphogenesis. We show that although somitic mesoderm convergent extension occurs independently from neural plate morphogenesis neural tube closure depends on somitic mesoderm morphogenesis. Specifically, impaired somitic mesoderm remodelling results in defective apical constriction within the neuroepithelium and failure of neural tube closure. Last, our data reveal that mild abnormalities in somitic mesoderm and neural plate morphogenesis have a synergistic effect during neurulation, leading to severe neural tube closure defects. Overall, our data reveal that defective morphogenesis of tissues mechanically coupled with the neural plate can not only drastically exacerbate mild neural tube defects that may arise from abnormalities within the neural tissue but can also elicit neural tube defects even when the neural plate is itself free of inherent defects.

Embryonic epithelial cells exhibit strong coupling of mechanical responses to chemical signals and most notably to calcium. Recent experiments have shown that the disruption of calcium signals during neurulation strongly correlates with the appearance of neural tube defects. We, thus, develop a multi-dimensional mechanochemical model and use it to reproduce important experimental findings that describe anterior neural plate morphogenetic behaviour during neural tube closure. The governing equations consist of an advectiondiffusion-reaction system for calcium concentration which is coupled to a force balance equation for the tissue. The tissue is modelled as a linear viscoelastic material that includes a calcium-dependent contraction stress. We implement a random distribution of calcium sparks that is compatible with experimental findings. A finite element method is employed to generate numerical solutions of the model for an appropriately chosen range of parameter values. We analyse the behaviour of the model as three parameters vary: the level of IP3 concentration, the strength of the stretchsensitive activation and the maximum magnitude of the calcium-dependent contraction stress. Importantly, the simulations reproduce important experimental features, such as the spatio-temporal correlation between calcium transients and tissue deformation, the monotonic reduction of the apical surface area and the constant constriction rate, as time progresses. The model could also be employed to gain insights into other biological processes where the coupling of calcium signalling and mechanics is important, such as carcinogenesis and wound healing. [ABSTRACT FROM AUTHOR]

Epithelial invagination is a fundamental module of morphogenesis that iteratively occurs to generate the architecture of many parts of a developing organism. By changing the physical properties such as the shape and/or position of a population of cells, invagination drives processes ranging from reconfiguring the entire body axis during gastrulation, to forming the primordia of the eyes, ears and multiple ducts and glands, during organogenesis. The epithelial bending required for invagination is achieved through a variety of mechanisms involving systems of cells. Here we provide an overview of the different mechanisms, some of which can work in combination, and outline the circumstances in which they apply.This article is part of the themed issue 'Systems morphodynamics: understanding the development of tissue hardware'.

Neural tube closure (NTC) is a fundamental process during vertebrate development and is indispensable for the formation of the central nervous system. Here, using Xenopus laevis embryos, live imaging, single-cell tracking, optogenetics and loss-of-function experiments, we examine the roles of convergent extension and apical constriction, and define the role of the surface ectoderm during NTC. We show that NTC is a two-stage process with distinct spatiotemporal contributions of convergent extension and apical constriction at each stage. Convergent extension takes place during the first stage and is spatially restricted at the posterior tissue, whereas apical constriction occurs during the second stage throughout the neural plate. We also show that the surface ectoderm is mechanically coupled with the neural plate and its movement during NTC is driven by neural plate morphogenesis. Finally, we show that an increase in surface ectoderm resistive forces is detrimental for neural plate morphogenesis. [ABSTRACT FROM AUTHOR]

Epithelial folding mediated by apical constriction serves as a fundamental mechanism to convert flat epithelial sheets into multilayered structures. It remains unknown whether additional mechanical inputs are required for apical constriction-mediated folding. Using Drosophila mesoderm invagination as a model, we identified an important role for the non-constricting, lateral mesodermal cells adjacent to the constriction domain ('flanking cells') in facilitating epithelial folding. We found that depletion of the basolateral determinant Dlg1 disrupts the transition between apical constriction and invagination without affecting the rate of apical constriction. Strikingly, the observed delay in invagination is associated with ineffective apical myosin contractions in the flanking cells that lead to overstretching of their apical domain. The defects in the flanking cells impede ventral-directed movement of the lateral ectoderm, suggesting reduced mechanical coupling between tissues. Specifically disrupting the flanking cells in wild-type embryos by laser ablation or optogenetic depletion of cortical actin is sufficient to delay the apical constriction-to-invagination transition. Our findings indicate that effective mesoderm invagination requires intact flanking cells and suggest a role for tissue-scale mechanical coupling during epithelial folding. [ABSTRACT FROM AUTHOR]

Studied 62 teeth removed for medical reasons in 54 adults (32 men and 22 women) of different ages to determine age-related changes in the root tips of teeth.Were used methods of odontoscope, x-ray examination, and prepared and studied thin sections of tooth roots. It is established that with age, in the absence of resorption of the apex and gipertireoze in the apex of the tooth root, in adults there is a reduction of the diameter of the root canal in the apical third of the root, due to the formation of secondary dentine, as well as a slight thickening of the cement in the region of apical hole. Thus, increases with age the distance between apical constriction, apical foramen and the radiographic apex of the tooth root. People young and middle age obliteration of root canal teeth are studied according to x-rays does not occur. In elderly and senile obliteration of the root canals of the teeth in the area of the tops are marked at 75% and 79%, respectively, but in thin sections of the roots, which according to x-ray examination is determined by the obliteration of the cavity of the tooth root, the channels typically have a lumen, that is, they are passable for endodontic instruments.

The present study aimed at the development of an endodontic simulation model, able to implement the electronic method of working length determination (EAL) in a setting of undergraduate learning. The development of the simulation model was accompanied by several investigations, helping to assess products’ potential and aiming to create the sound scientific basis for its future implementation as a learning tool. This investigation was conducted in the framework of the regular phantom-lab course. During the initial implementation 450 student have been trained. The models’ capability to imitate clinical conditions was investigated by means of radiographic, macrophotograpic and CAD/CAM techniques. The EAL-features of the present model allowed not only for precise determination of apical constriction (R²=0,0198) versus radiographic method (R² = 0,0019), but also granted a precise preparation of the apical constriction in the vast majority of cases (up to 83%).

The folding of two-dimensional epithelial sheets into specific three-dimensional structures is a fundamental tissue construction mechanism in animal development. A common mechanism that mediates epithelial folding is apical constriction, the active shrinking of cell apices driven by actomyosin contractions. It remains unclear whether cells outside of the constriction domain also contribute to folding. During Drosophila mesoderm invagination, ventrally localized mesoderm epithelium undergoes apical constriction and subsequently folds into a furrow. While the critical role of apical constriction in ventral furrow formation has been well demonstrated, it remains unclear whether, and if so, how the laterally localized ectodermal tissue adjacent to the mesoderm contributes to furrow invagination. In this study, we combine experimental and computational approaches to test the potential function of the ectoderm in mesoderm invagination. Through laser-mediated, targeted disruption of cell formation prior to gastrulation, we found that the presence of intact lateral ectoderm is important for the effective transition between apical constriction and furrow invagination in the mesoderm. In addition, using a laser-ablation approach widely used for probing tissue tension, we found that the lateral ectodermal tissues exhibit signatures of tissue compression when ablation was performed shortly before the onset of mesoderm invagination. These observations led to the hypothesis that in-plane compression from the surrounding ectoderm facilitates mesoderm invagination by triggering buckling of the mesoderm epithelium. In support of this notion, we show that the dynamics of tissue flow during mesoderm invagination displays characteristic of elastic buckling, and this tissue dynamics can be recapitulated by combining local apical constriction and global compression in a simulated elastic monolayer. We propose that Drosophila mesoderm invagination is achieved through epithelial buckling jointly mediated by apical constriction in the mesoderm and compression from the neighboring ectoderm.

Failures of neural tube closure are common and serious birth defects, yet we have a poor understanding of the interaction of genetics and cell biology during neural tube closure. Additionally, mutations that cause neural tube defects (NTDs) tend to affect anterior or posterior regions of the neural tube but rarely both, indicating a regional specificity to NTD genetics. To better understand the regional specificity of cell behaviors during neural tube closure, we analyzed the dynamic localization of actin and N-cadherin via high-resolution tissue-level time-lapse microscopy during Xenopus neural tube closure. To investigate the regionality of gene function, we generated mosaic mutations in shroom3, a key regulator or neural tube closure. This new analytical approach elucidates several differences between cell behaviors during cranial/anterior and spinal/posterior neural tube closure, provides mechanistic insight into the function of shroom3, and demonstrates the ability of tissue-level imaging and analysis to generate cell biological mechanistic insights into neural tube closure.

Apical constriction driven by actin and non-muscle myosin II (actomyosin) provides a well-conserved mechanism to mediate epithelial folding. It remains unclear how contractile forces near the apical surface of a cell sheet drive out-of-the-plane bending of the sheet and whether myosin contractility is required throughout folding. By optogenetic-mediated acute inhibition of actomyosin, we find that during Drosophila mesoderm invagination, actomyosin contractility is critical to prevent tissue relaxation during the early, ‘priming’ stage of folding but is dispensable for the actual folding step after the tissue passes through a stereotyped transitional configuration. This binary response suggests that Drosophila mesoderm is mechanically bistable during gastrulation. Computer modeling analysis demonstrates that the binary tissue response to actomyosin inhibition can be recapitulated in the simulated epithelium that undergoes buckling-like deformation jointly mediated by apical constriction in the mesoderm and in-plane compression generated by apicobasal shrinkage of the surrounding ectoderm. Interestingly, comparison between wild-type and snail mutants that fail to specify the mesoderm demonstrates that the lateral ectoderm undergoes apicobasal shrinkage during gastrulation independently of mesoderm invagination. We propose that Drosophila mesoderm invagination is achieved through an interplay between local apical constriction and mechanical bistability of the epithelium that facilitates epithelial buckling.

Developmental patterning is thought to be regulated by conserved signalling pathways. Initial patterns are often broad before refining to only those cells that commit to a particular fate. However, the mechanisms by which pattern refinement takes place remain to be addressed. Using the posterior crossvein (PCV) of the Drosophila pupal wing as a model, into which bone morphogenetic protein (BMP) ligand is extracellularly transported to instruct vein patterning, we investigate how pattern refinement is regulated. We found that BMP signalling induces apical enrichment of Myosin II in developing crossvein cells to regulate apical constriction. Live imaging of cellular behaviour indicates that changes in cell shape are dynamic and transient, only being maintained in those cells that retain vein fate competence after refinement. Disrupting cell shape changes throughout the PCV inhibits pattern refinement. In contrast, disrupting cell shape in only a subset of vein cells can result in a loss of BMP signalling. We propose that mechano-chemical feedback leads to competition for the developmental signal which plays a critical role in pattern refinement. [Display omitted] [ABSTRACT FROM AUTHOR]

During organ development, tubular organs often form from flat epithelial primordia. In the placodes of the forming tubes of the salivary glands in the Drosophila embryo, we previously identified spatially defined cell behaviors of cell wedging, tilting, and cell intercalation that are key to the initial stages of tube formation. Here, we address what the requirements are that ensure the continuous formation of a narrow symmetrical tube from an initially asymmetrical primordium whilst overall tissue geometry is constantly changing. We are using live-imaging and quantitative methods to compare wild-type placodes and mutants that either show disrupted cell behaviors or an initial symmetrical placode organization, with both resulting in severe impairment of the invagination. We find that early transcriptional patterning of key morphogenetic transcription factors drives the selective activation of downstream morphogenetic modules, such as GPCR signaling that activates apical-medial actomyosin activity to drive cell wedging at the future asymmetrically placed invagination point. Over time, transcription of key factors expands across the rest of the placode and cells switch their behavior from predominantly intercalating to predominantly apically constricting as their position approaches the invagination pit. Misplacement or enlargement of the initial invagination pit leads to early problems in cell behaviors that eventually result in a defective organ shape. Our work illustrates that the dynamic patterning of the expression of transcription factors and downstream morphogenetic effectors ensures positionally fixed areas of cell behavior with regards to the invagination point. This patterning in combination with the asymmetric geometrical setup ensures functional organ formation.

The developmental strategies used by progenitor cells to allow a safe journey from their induction place towards the site of terminal differentiation are still poorly understood. Here, we uncovered a mechanism of progenitor cell allocation that stems from an incomplete process of epithelial delamination that allows progenitors to coordinate their movement with adjacent extra-embryonic tissues. Progenitors of the zebrafish laterality organ originate from the superficial epithelial enveloping layer by an apical constriction process of cell delamination. During this process, progenitors retain long-lasting apical contacts that enable the epithelial layer to pull a subset of progenitors on their way to the vegetal pole. The remaining delaminated cells follow the movement of apically attached progenitors by a protrusion-dependent cell-cell contact mechanism, avoiding sequestration by the adjacent endoderm, ensuring their collective fate and allocation at the site of differentiation. Thus, we reveal that incomplete delamination serves as a cellular platform for coordinated tissue movements during development.

Morphogenesis of the vertebrate neural tube occurs by elongation and bending of the neural plate, tissue shape changes that are driven at the cellular level by polarized cell intercalation and cell shape changes, notably apical constriction and cell wedging. Coordinated cell intercalation, apical constriction, and wedging undoubtedly require complex underlying cytoskeletal dynamics and remodeling of adhesions. Mutations of the gene encoding Scribble result in neural tube defects in mice, however the cellular and molecular mechanisms by which Scrib regulates neural cell behavior remain unknown. Analysis of Scribble mutants revealed defects in neural tissue shape changes, and live cell imaging of mouse embryos showed that the Scrib mutation results in defects in polarized cell intercalation, particularly in rosette resolution, and failure of both cell apical constriction and cell wedging. Scrib mutant embryos displayed aberrant expression of the junctional proteins ZO-1, Par3, Par6, E− and N-cadherins, and the cytoskeletal proteins actin and myosin. These findings show that Scribble has a central role in organizing the molecular complexes regulating the morphomechanical neural cell behaviors underlying vertebrate neurulation, and they advance our understanding of the molecular mechanisms involved in mammalian neural tube closure. [Display omitted] • Polarized cell intercalation is lost in Scrib mutant embryos. • Scrib mutation has specific effects on rosette formation and resolution. • Scrib mutation disrupts apical constriction and cell shape changes necessary for neural tube closure. • Adherens and tight junction composition is altered in the neural epithelial cells of Scrib mutants. [ABSTRACT FROM AUTHOR]

An electronic apex locator(EAL) is a device used to measure the working length (WL) of the root canal space(RCS) and proved to be a great help to the endodontist reducing radiation exposure of both patients and clinicians and providing accurate reading for the WL. Evolution of different generations of EAL helped to eradicate the shortcomings of the previous generations providing precise measurement of the WL. Electronic Apex locators are used to detect any perforations, furcation involvement, also used to detect horizontal fractures, also to detect internal or external resorption. With so many uses, it is still contraindicated in patients with pace makers and inaccurate reading in canals with common irrigants like Ethylenediamine tetraacetic acid and sodium hypochlorite. It requires dry or partially dry canals to give readings with accuracy. It also gives inaccurate reading in the presence of semi-conductive materials and hence the canals should be free of them. Though it provides almost precise measurement of the WL it cannot be replaced with radiographs but can be used as an adjunct with it. Pre- and post-operative radiographs are still necessary for the success of the endodontic treatment as it provides an idea for any periapical lesion and degree of curvature of roots. Acceptance of an electronic apex locator to determine the correct WL has still not gained approval worldwide. Lack of exposure to the technology by the operator, inadequate knowledge of the root canal anatomy and cost of the instrument are also the factors for it not gaining worldwide acceptance. [ABSTRACT FROM AUTHOR]

Aim: This study aimed to evaluate the accuracy of three apex locators in enlarged teeth with different apical diameters using files that are compatible and incompatible with the apical diameters. Methodology: Sixty lower premolars were used in this study. The actual canal length was determined with a stereo microscope, and the teeth were divided into three different groups (G25, G40 and G50). The teeth in G25, G40 and G50 were enlarged in actual canal length with apical diameters of 0.25, 0.40- and 0.50-mm using hand files and Reciproc R25, R40, and R50, respectively. In the electronic measurements, a #15 hand file was used in each group, and #25, #40 and #50 hand files that were compatible with the apical diameters of the teeth in the groups were used. An alginate model was created with enlarged teeth. The electronic working length was determined for each tooth using Root ZX Mini, Raypex 6, and Propex Pixi electronic apex locators (EALs). Results: Electronic measurement results that were shorter by 0.50 mm or longer by 0.05 mm than the actual length were considered unsuccessful; otherwise, they were deemed successful. Analysis of variance was used to evaluate the accuracy of the EALs with different files in the same apical diameter. For the different apical diameters, the chi-square exact test was used to evaluate the accuracy effect of using files that were incompatible with each apical diameter. The level of significance was p > 0.05. In all the groups, the measured lengths were closer to the actual canal length with the use of #25, #40 and #50 hand files that were compatible with the apical diameters. However, there was no significant difference between the measurements with the #15 hand file (p > 0.05). According to the electronic measurement results made with the #15 hand file in teeth with 0.25, 0.40 and 0.50 apical diameters, the use of a file that was incompatible with the apical diameter did not have a significant effect on accuracy (p > 0.05). Conclusion: The use of a file that is compatible with the apical diameter of the tooth in the determination of the working length with measurement of EALs close to the actual canal length. The accuracy of EALs is not affected by the file size in electronic measurements with file sizes smaller than the apical diameter in teeth with apical diameters up to 0.50 mm. [ABSTRACT FROM AUTHOR]

Cephalization is a major innovation of animal evolution and implies a synchronization of nervous system, mouth, and foregut polarization to align alimentary tract and sensomotoric system for effective foraging. However, the underlying integration of morphogenetic programs is poorly understood. Here, we show that invagination of neuroectoderm through de novo polarization and apical constriction creates the mouth opening in the Caenorhabditis elegans embryo. Simultaneously, all 18 juxta-oral sensory organ dendritic tips become symmetrically positioned around the mouth: While the two bilaterally symmetric amphid sensilla endings are towed to the mouth opening, labial and cephalic sensilla become positioned independently. Dendrite towing is enabled by the pre-polarized sensory amphid pores intercalating into the leading edge of the anteriorly migrating epidermal sheet, while apical constriction-mediated cell–cell re-arrangements mediate positioning of all other sensory organs. These two processes can be separated by gradual inactivation of the 26S proteasome activator, RPN-6.1. Moreover, RPN-6.1 also shows a dose-dependent requirement for maintenance of coordinated apical polarization of other organs with apical lumen, the pharynx, and the intestine. Thus, our data unveil integration of morphogenetic programs during the coordination of alimentary tract and sensory organ formation and suggest that this process requires tight control of ubiquitin-dependent protein degradation.

Cell and tissue shape changes are the fundamental elements of morphogenesis that drive normal development of embryos into fully functional organisms. This requires a variety of cellular processes including establishment and maintenance of polarity, tissue growth and apoptosis, and cell differentiation, rearrangement, and migration. It is widely appreciated that the cytoskeletal networks play an important role in regulating many of these processes and, in particular, that pulsed actomyosin contractions are a core cellular mechanism driving cell shape changes and cell rearrangement. In this review, we discuss the role of pulsed actomyosin contractions during developmental morphogenesis, advances in our understanding of the mechanisms regulating actomyosin pulsing, and novel techniques to probe the role of pulsed actomyosin processes in in vivo model systems.

For over 100 years, the vertebrate eye has been an important model system to understand cell induction, cell shape change, and morphogenesis during development. In the past, most of the studies examined histological changes to detect the presence of induction mechanisms, but the advancement of molecular biology techniques has made exploring the genetic mechanisms behind lens development possible. Despite the particular emphasis given to the induction of the lens placode, there are still many aspects of the cell biology of lens morphogenesis to be explored. Here, we will revisit the classical detailed description of early lens morphological changes, correlating it with the cell biology mechanisms and with the molecules and signaling pathways identified up to now in chick and mouse embryos. A detailed description of lens development stages helps better understand the timeline of the events involved in early lens morphogenesis. We then point to some key questions that are still open. [ABSTRACT FROM AUTHOR]

Neural tube closure defects are a major cause of infant mortality, with exencephaly accounting for nearly one-third of cases. However, the mechanisms of cranial neural tube closure are not well understood. Here, we show that this process involves a tissue-wide pattern of apical constriction controlled by Sonic hedgehog (Shh) signaling. Midline cells in the mouse midbrain neuroepithelium are flat with large apical surfaces, whereas lateral cells are taller and undergo synchronous apical constriction, driving neural fold elevation. Embryos lacking the Shh effector Gli2 fail to produce appropriate midline cell architecture, whereas embryos with expanded Shh signaling, including the IFT-A complex mutants Ift122 and Ttc21b and embryos expressing activated Smoothened, display apical constriction defects in lateral cells. Disruption of lateral, but not midline, cell remodeling results in exencephaly. These results reveal a morphogenetic program of patterned apical constriction governed by Shh signaling that generates structural changes in the developing mammalian brain.

Coordination of actomyosin contraction and cell-cell junctions generates forces that can lead to tissue morphogenetic processes like the formation of neural tube (NT), however, its molecular mechanisms responsible for regulating and coupling this contractile network to cadherin adhesion remain to be fully elucidated. Here, using a gene trapping technology, we unveil the new player in this process, α-catulin, which shares sequence homology with vinculin and α-catenin. Ablation of α-catulin in mouse causes defective NT closure due to impairment of apical constriction, concomitant with apical actin and P-Mlc2 accumulation. Using a 3D culture model system, we showed that α-catulin localizes to the apical membrane and its removal alters the distribution of active RhoA and polarization. Actin cytoskeleton and P-Mlc2, downstream targets of RhoA, are not properly organized, with limited accumulation at the junctions, indicating a loss of junction stabilization. Our data suggest that α-catulin plays an important role during NT closure by acting as a scaffold for RhoA distribution, resulting in proper spatial activation of myosin to influence actin-myosin dynamics and tension at cell-cell adhesion.

Apical constriction is critical for epithelial morphogenesis, including neural tube formation. Vertebrate apical constriction is induced by di‐phosphorylated myosin light chain (ppMLC)‐driven contraction of actomyosin‐based circumferential rings (CRs), also known as perijunctional actomyosin rings, around apical junctional complexes (AJCs), mainly consisting of tight junctions (TJs) and adherens junctions (AJs). Here, we revealed a ppMLC‐triggered system at TJ‐associated CRs for vertebrate apical constriction involving microtubules, LUZP1, and myosin phosphatase. We first identified LUZP1 via unbiased screening of microtubule‐associated proteins in the AJC‐enriched fraction. In cultured epithelial cells, LUZP1 was found localized at TJ‐, but not at AJ‐, associated CRs, and LUZP1 knockout resulted in apical constriction defects with a significant reduction in ppMLC levels within CRs. A series of assays revealed that ppMLC promotes the recruitment of LUZP1 to TJ‐associated CRs, where LUZP1 spatiotemporally inhibits myosin phosphatase in a microtubule‐facilitated manner. Our results uncovered a hitherto unknown microtubule‐LUZP1 association at TJ‐associated CRs that inhibits myosin phosphatase, contributing significantly to the understanding of vertebrate apical constriction. Synopsis: Apical cell constriction drives epithelial morphogenesis and is mediated by junctional actomyosin ring constriction in vertebrates. Here, the leucine zipper motif‐containing protein LUZP1 is found as a new regulator of this process via inhibition of myosin phosphatase activity at tight junction (TJ)‐associated circumferential rings (CRs). The microtubule‐associated protein LUZP1 localizes at TJ‐associated CRs.Di‐phosphorylated myosin light chain (ppMLC) promotes recruitment of LUZP1 to TJ‐associated CRs.LUZP1 inhibits myosin phosphatase to increase myosin light chain phosphorylation and promote CR contraction during apical constriction.Microtubules facilitate LUZP1‐mediated inhibition of myosin phosphatase activity. [ABSTRACT FROM AUTHOR]

Objectives: To obtain radicular measurements of two separate mesiobuccal (MB) root canals in maxillary first molars using micro-computed tomography (μCT) with customized software. Materials and methods: Human maxillary first molar MB roots (N = 36) with two separate canals (MB1, MB2) and apical foramina were scanned by μCT and analyzed with Kappa2 software to reconstruct three-dimensional (3D) surface models of roots and canals. These models were sectioned at 0.1 mm intervals perpendicular to the central axis of each canal. Canal widths, 3D curvatures, and surrounding dentine thicknesses were measured concurrently on each section. Dentine thicknesses were analyzed statistically for differences between each direction and the different levels of both canals. Results: Dentine walls around MB1 were thicker than MB2 (p < 0.05). Thinnest dentine was most often located at disto-inside direction in both canals. Canal widths were significantly smaller in MB2 than MB1 (p < 0.01). Apical constrictions were smaller (p < 0.05) and further (p < 0.05) from the apex in MB2 than MB1. Canal curvatures were greatest in the apical third of both canals (p < 0.001), and they were greater in MB2 than MB1 (p < 0.05). Conclusions: MB2 canals had shorter lengths, smaller widths, and more severe curvatures and were surrounded by thinner dentine walls. In MB2, apical constrictions were between 1 and 2 mm from the apex, compared to about 1 mm for MB1. Clinical relevance: These detailed measurements and in-depth 3D analyses of maxillary first molar MB roots with two separate canals and apical foramina provide morphologic references for root canal therapy. [ABSTRACT FROM AUTHOR]

Actomyosin‐mediated apical constriction drives a wide range of morphogenetic processes. Activation of myosin‐II initiates pulsatile cycles of apical constrictions followed by either relaxation or stabilization (ratcheting) of the apical surface. While relaxation leads to dissipation of contractile forces, ratcheting is critical for the generation of tissue‐level tension and changes in tissue shape. How ratcheting is controlled at the molecular level is unknown. Here, we show that the actin crosslinker βH‐spectrin is upregulated at the apical surface of invaginating mesodermal cells during Drosophila gastrulation. βH‐spectrin forms a network of filaments which co‐localize with medio‐apical actomyosin fibers, in a process that depends on the mesoderm‐transcription factor Twist and activation of Rho signaling. βH‐spectrin knockdown results in non‐ratcheted apical constrictions and inhibition of mesoderm invagination, recapitulating twist mutant embryos. βH‐spectrin is thus a key regulator of apical ratcheting during tissue invagination, suggesting that actin cross‐linking plays a critical role in this process. Synopsis: This study shows that βH‐spectrin is a key regulator of apical ratcheting during tissue invagination, suggesting that actin crosslinking plays a critical role in this process. The actin crosslinker βH‐spectrin is upregulated at the apical surface of invaginating mesodermal cells during Drosophila gastrulation.βH‐spectrin forms a network of filaments which co‐localize with medioapical actomyosin fiber in a Twist and Rho signaling dependent manner.βH‐spectrin knockdown results in non‐ratcheted apical constrictions and inhibition of mesoderm invagination. [ABSTRACT FROM AUTHOR]

A critical juncture in early development is the partitioning of cells that will adopt different fates into three germ layers: the ectoderm, the mesoderm, and the endoderm. This step is achieved through the internalization of specified cells from the outermost surface layer, through a process called gastrulation. In Drosophila, gastrulation is achieved through cell shape changes (i.e., apical constriction) that change tissue curvature and lead to the folding of a surface epithelium. Folding of embryonic tissue results in mesoderm and endoderm invagination, not as individual cells, but as collective tissue units. The tractability of Drosophila as a model system is best exemplified by how much we know about Drosophila gastrulation, from the signals that pattern the embryo to the molecular components that generate force, and how these components are organized to promote cell and tissue shape changes. For mesoderm invagination, graded signaling by the morphogen, Spätzle, sets up a gradient in transcriptional activity that leads to the expression of a secreted ligand (Folded gastrulation) and a transmembrane protein (T48). Together with the GPCR Mist, which is expressed in the mesoderm, and the GPCR Smog, which is expressed uniformly, these signals activate heterotrimeric G-protein and small Rho-family G-protein signaling to promote apical contractility and changes in cell and tissue shape. A notable feature of this signaling pathway is its intricate organization in both space and time. At the cellular level, signaling components and the cytoskeleton exhibit striking polarity, not only along the apical--basal cell axis, but also within the apical domain. Furthermore, gene expression controls a highly choreographed chain of events, the dynamics of which are critical for primordium invagination; it does not simply throw the cytoskeletal "on" switch. Finally, studies of Drosophila gastrulation have provided insight into how global tissue mechanics and movements are intertwined as multiple tissues simultaneously change shape. Overall, these studies have contributed to the view that cells respond to forces that propagate over great distances, demonstrating that cellular decisions, and, ultimately, tissue shape changes, proceed by integrating cues across an entire embryo. [ABSTRACT FROM AUTHOR]

Gastrulation is fundamental to the development of multicellular animals. Along with neurulation, gastrulation is one of the major processes of morphogenesis in which cells or whole tissues move from the surface of an embryo to its interior. Cell internalization mechanisms that have been discovered to date in Caenorhabditis elegans gastrulation bear some similarity to internalization mechanisms of other systems including Drosophila, Xenopus, and mouse, suggesting that ancient and conserved mechanisms internalize cells in diverse organisms. C. elegans gastrulation occurs at an early stage, beginning when the embryo is composed of just 26 cells, suggesting some promise for connecting the rich array of developmental mechanisms that establish polarity and pattern in embryos to the force-producing mechanisms that change cell shapes and move cells interiorly. Here, we review our current understanding of C. elegans gastrulation mechanisms. We address how cells determine which direction is the interior and polarize with respect to that direction, how cells change shape by apical constriction and internalize, and how the embryo specifies which cells will internalize and when. We summarize future prospects for using this system to discover some of the general principles by which animal cells change shape and internalize during development. [ABSTRACT FROM AUTHOR]

Aim: The aim of the present study was to evaluate in vitro the accuracy of the Mini Apex Locator (Sybron-Kerr, Romulus, USA), Novapex (Forum Technologies, Rishon Le-zion, Israel) and ProPex II (Dentsply-Maillefer, Ballaigues, Switzerland) electronic apex locators compared with the radiographic method to determine the working length. Methodology: The mesiobuccal root canals of 30 extracted human mandibular first molars were used. The actual length was determined visually under magnification. A glass container was filled with sterile 0.9% NaCl solution and the specimens were fixed to a plastic bar using acrylic resin. The apical constriction of the teeth was determined with three electronic apex locators and radiographically. The same procedures were performed 1 mm short of the apical foramen. Each measurement was repeated three times and the mean value computed. These values were tabulated for comparison with the measurements obtained by the electronic and radiographic methods with the actual length. Data were analysed statistically by McNemar test at the 5% significance level. Results: The highest coincidence percentage was found with the ProPex II device (83.0%), followed by the radiographic method (46.7%), Novapex (30%), and Mini Apex Locator (13.3%) in relation to the visual determination of the apical constriction. At 1 mm short of the apical foramen, the highest coincidence percentage occurred with the ProPex II device (80.0%), radiographic method (56.7%), Novapex (26.7%), and Mini Apex Locator (6.7%). There were statistically significant differences between the methods used. Conclusions: ProPex II showed the highest accuracy in determining the working length, followed by the radiographic method, Novapex and Mini Apex Locator. [ABSTRACT FROM AUTHOR]