Tunable assembly of host-guest colloidal crystals

Entropy compartmentalization provides new self-assembly routes to colloidal host-guest (HG) structures. Leveraging host particle shape to drive the assembly of HG structures has only recently been proposed and demonstrated. However, the extent to which the guest particles can dictate the structure of the porous network of host particles has not been explored. In this work, by modifying only the guest shape, we show athermal, binary mixtures of star-shaped host particles and convex polygon-shaped guest particles assemble as many as five distinct crystal structures, including rotator and discrete rotator guest crystals, two homoporous host crystals, and one heteroporous host crystal. Edge-to-edge alignment of neighboring stars results in the formation of three distinct pore motifs, whose preferential formation is controlled by the size and shape of the guest particles. Finally, we confirm, via free volume calculations, that assembly is driven by entropy compartmentalization, where the hosts and guests contribute differently to the free energy of the system; free volume calculations also explain differences in assembly based on guest shape. These results provide guest design rules for assembling colloidal HG structures, especially on surfaces and interfaces.

Inverse design of triblock Janus spheres for self-assembly of complex structures in the crystallization slot via digital alchemy

The digital alchemy framework is an extended ensemble simulation technique that incorporates particle attributes as thermodynamic variables, enabling the inverse design of colloidal particles for desired behavior. Here, we extend the digital alchemy framework for the inverse design of patchy spheres that self-assemble into target crystal structures. To constrain the potentials to non-trivial solutions, we conduct digital alchemy simulations with constant second virial coefficient. We optimize the size, range, and strength of patchy interactions in model triblock Janus spheres to self-assemble the 2D kagome and snub square lattices and the 3D pyrochlore lattice, and demonstrate self-assembly of all three target structures with the designed models. The particles designed for the kagome and snub square lattices assemble into high quality clusters of their target structures, while competition from similar polymorphs lower the yield of the pyrochlore assemblies. We find that the alchemically designed potentials do not always match physical intuition, illustrating the ability of the method to find nontrivial solutions to the optimization problem. We identify a window of second virial coefficients that result in self-assembly of the target structures, analogous to the crystallization slot in protein crystallization.

Using molecular simulation to understand the skin barrier

Skin's effectiveness as a barrier to permeation of water and other chemicals rests almost entirely in the outermost layer of the epidermis, the stratum corneum (SC), which consists of layers of corneocytes surrounded by highly organized lipid lamellae. As the only continuous path through the SC, transdermal permeation necessarily involves diffusion through these lipid layers. The role of the SC as a protective barrier is supported by its exceptional lipid composition consisting of ceramides (CERs), cholesterol (CHOL), and free fatty acids (FFAs) and the complete absence of phospholipids, which are present in most biological membranes. Molecular simulation, which provides molecular level detail of lipid configurations that can be connected with barrier function, has become a popular tool for studying SC lipid systems. We review this ever-increasing body of literature with the goals of (1) enabling the experimental skin community to understand, interpret and use the information generated from the simulations, (2) providing simulation experts with a solid background in the chemistry of SC lipids including the composition, structure and organization, and barrier function, and (3) presenting a state of the art picture of the field of SC lipid simulations, highlighting the difficulties and best practices for studying these systems, to encourage the generation of robust reproducible studies in the future. This review describes molecular simulation methodology and then critically examines results derived from simulations using atomistic and then coarse-grained models.

Multiscale Simulation of Ternary Stratum Corneum Lipid Mixtures: Effects of Cholesterol Composition

Molecular dynamics simulations of mixtures of the ceramide nonhydroxy-sphingosine (NS), cholesterol, and a free fatty acid are performed to gain molecular-level understanding of the structure of the lipids found in the stratum corneum layer of skin. A new coarse-grained force field for cholesterol was developed using the multistate iterative Boltzmann inversion (MS-IBI) method. The coarse-grained cholesterol force field is compatible with previously developed coarse-grained force fields for ceramide NS, free fatty acids, and water and validated against atomistic simulations of these lipids using the CHARMM force field. Self-assembly simulations of multilayer structures using these coarse-grained force fields are performed, revealing that a large fraction of the ceramides adopt extended conformations, which cannot occur in the single bilayer in water structures typically studied using molecular simulation. Cholesterol fluidizes the membrane by promoting packing defects, and an increase in cholesterol content is found to reduce the bilayer thickness due to an increase in interdigitation of the C₂₄ lipid tails, consistent with experimental observations. Using a reverse-mapping procedure, a self-assembled coarse-grained multilayer system is used to construct an equivalent structure with atomistic resolution. Simulations of this atomistic structure are found to closely agree with experimentally derived neutron scattering length density profiles. Significant interlayer hydrogen bonding is observed in the inner layers of the atomistic multilayer structure that are not found in the outer layers in contact with water or in equivalent bilayer structures. This work highlights the importance of simulating multilayer structures, as compared to the more commonly studied bilayer systems, to enable more appropriate comparisons with multilayer experimental membranes. These results also provide validation of the efficacy of the MS-IBI derived coarse-grained force fields and the framework for multiscale simulation.

Tuning Stoichiometry to Promote Formation of Binary Colloidal Superlattices

The self-assembly of binary nanoparticle superlattices from colloidal mixtures is a promising method for the fabrication of complex colloidal cocrystal structures. However, binary mixtures often form amorphous or metastable phases instead of the thermodynamically stable phase. Here we show that in binary mixtures of differently sized spherical particles, an excess of the smaller component can promote-and, in some cases, may be necessary for-the self-assembly of a binary cocrystal. Using computer simulations, we identify two mechanisms responsible for this phenomenon. First, excess small particles act like plasticizers and enable systems to reach a greater supersaturation before kinetic arrest occurs. Second, they can disfavor competing structures that may interfere with the growth of the target structure. We find the phase behavior of simulated mixtures of nearly hard spheres closely matches published experimental results. We demonstrate the generality of our findings for mixtures of particles of arbitrary shape by presenting a binary mixture of hard shapes that only self-assembles with an excess of the smaller component.

Shape-driven entropic self-assembly of an open, reconfigurable, binary host-guest colloidal crystal

Entropically driven self-assembly of hard anisotropic particles, where particle shape gives rise to emergent valencies, provides a useful perspective for the design of nanoparticle and colloidal systems. Hard particles self-assemble into a rich variety of crystal structures, ranging in complexity from simple close-packed structures to structures with 432 particles in the unit cell. Entropic crystallization of open structures, however, is missing from this landscape. Here, we report the self-assembly of a two-dimensional binary mixture of hard particles into an open host-guest structure, where nonconvex, triangular host particles form a honeycomb lattice that encapsulates smaller guest particles. Notably, this open structure forms in the absence of enthalpic interactions by effectively splitting the structure into low- and high-entropy sublattices. This is the first such structure to be reported in a two-dimensional athermal system. We discuss the observed compartmentalization of entropy in this system, and show that the effect of the size of the guest particle on the stability of the structure gives rise to a reentrant phase behavior. This reentrance suggests the possibility for a reconfigurable colloidal material, and we provide a proof-of-concept by showing the assembly behavior while changing the size of the guest particles in situ. Our findings provide a strategy for designing open colloidal crystals, as well as binary systems that exhibit co-crystallization, which have been elusive thus far.

Examining Tail and Headgroup Effects on Binary and Ternary Gel-Phase Lipid Bilayer Structure

The structural properties of two- and three-component gel-phase bilayers were studied using molecular dynamics simulations. The bilayers contain distearoylphosphatidylcholine (DSPC) phospholipids mixed with alcohols and/or fatty acids of varying tail lengths, with carbon chain lengths of 12, 16, and 24 studied. Changes in both headgroup chemistry and tail length are found to affect the balance between steric repulsion and van der Waals attraction within the bilayers, manifesting in different bilayer structural properties. Lipid components are found to be located at different depths within the bilayer depending on both chain length and headgroup chemistry. The highest bilayer ordering and lowest area per tail are found in systems with medium-length tails. While longer tails can enhance van der Waals attractions, the increased tail-length asymmetry is found to induce disorder and reduce tail packing. Bulkier headgroups further increase steric repulsion, as reflected in increased component offsets and reduced tail packing. These findings help explain how bilayer composition affects the structure of gel-phase bilayers.

Influence of Single-Stranded DNA Coatings on the Interaction between Graphene Nanoflakes and Lipid Bilayers

Using molecular dynamics simulations, it is demonstrated that a partial coating of single-stranded DNA (ssDNA) reduces the penetration depth of a graphene nanoflake (GNF) into a phospholipid bilayer by attenuating the hydrophobic force that drives the penetration. As the GNF penetrates the bilayer, the ssDNA remains adsorbed to the GNF outside of the bilayer where it shields the graphene from the surrounding water. The penetration depth is found to be controlled by the amount of ssDNA coating the GNF, with a sparser coating resulting in a deeper penetration since the ssDNA shields less of the GNF surface. As the coating density is increased, the likelihood of the GNF entering the bilayer is reduced where it instead tends to lie flat on the bilayer surface with the sugar phosphate backbone of ssDNA interacting with the hydrophilic lipid head groups. While no bilayer disruption is observed for a partially inserted ssDNA-coated GNF, a larger, bare, partially inserted GNF is found to preferentially extract phospholipids from the bilayer, offering further evidence of lipid extraction as a main cytotoxicity mechanism of GNFs. Therefore, a coating of ssDNA may reduce the cytotoxicity of GNFs by shielding the unfavorable graphene-water interaction, thus preventing graphene penetration and lipid extraction.

Molecular dynamics simulations of stratum corneum lipid mixtures: A multiscale perspective

The lipid matrix of the stratum corneum (SC) layer of skin is essential for human survival; it acts as a barrier to prevent rapid dehydration while keeping potentially hazardous material outside the body. While the composition of the SC lipid matrix is known, the molecular-level details of its organization are difficult to infer experimentally, hindering the discovery of structure-property relationships. To this end, molecular dynamics simulations, which give molecular-level resolution, have begun to play an increasingly important role in understanding these relationships. However, most simulation studies of SC lipids have focused on preassembled bilayer configurations, which, owing to the slow dynamics of the lipids, may influence the final structure and hence the calculated properties. Self-assembled structures would avoid this dependence on the initial configuration, however, the size and length scales involved make self-assembly impractical to study with atomistic models. Here, we report on the development of coarse-grained models of SC lipids designed to study self-assembly. Building on previous work, we present the interactions between the headgroups of ceramide and free fatty acid developed using the multistate iterative Boltzmann inversion method. Validation of the new interactions is performed with simulations of preassembled bilayers and good agreement between the atomistic and coarse-grained models is found for structural properties. The self-assembly of mixtures of ceramide and free fatty acid is investigated and both bilayer and multilayer structures are found to form. This work therefore represents a necessary step in studying SC lipid systems on multiple time and length scales.

Composition Dependence of Water Permeation Across Multicomponent Gel-Phase Bilayers

The permeability of multicomponent phospholipid bilayers in the gel phase is investigated via molecular dynamics simulation. The physical role of the different molecules is probed by comparing multiple mixed-component bilayers containing distearylphosphatidylcholine (DSPC) with varying amounts of either the emollient isostearyl isostearate or long-chain alcohol (dodecanol, octadecanol, or tetracosanol) molecules. Permeability is found to depend on both the tail packing density and hydrogen bonding between lipid headgroups and water. Whereas the addition of emollient or alcohol molecules to a gel-phase DSPC bilayer can increase the tail packing density, it also disturbed the hydrogen-bonding network, which in turn can increase interfacial water dynamics. These phenomena have opposing effects on bilayer permeability, which is found to depend on the balance between enhanced tail packing and decreased hydrogen bonding.

Investigating the Structure of Multicomponent Gel-Phase Lipid Bilayers

Single- and multicomponent lipid bilayers of 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC), 1,2-distearoyl-sn-glycero-3-phosphatidylcholine (DSPC), isostearyl isostearate, and heptadecanoyl heptadecanoate in the gel phase are studied via molecular dynamics simulations. It is shown that the structural properties of multicomponent bilayers can deviate strongly from the structures of their single-component counterparts. Specifically, the lipid mixtures are shown to adopt a compact packing by offsetting the positioning depths at which different lipid species are located in the bilayer. This packing mechanism affects the area per lipid, the bilayer height, and the chain tilt angles and has important consequences for other bilayer properties, such as interfacial hydrogen bonding and bilayer permeability. In particular, the simulations suggest that bilayers containing isostearyl isostearate or heptadecanoyl heptadecanoate are less permeable than pure 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine or DSPC bilayers. Furthermore, hydrogen-bond analysis shows that the residence times of lipid-water hydrogen bonds depend strongly on the bilayer composition, with longer residence times for bilayers that have a higher DSPC content. The findings illustrate and explain the fundamental differences between the properties of single- and multicomponent bilayers.

A Coarse-Grained Model of Stratum Corneum Lipids: Free Fatty Acids and Ceramide NS

Ceramide (CER)-based biological membranes are used both experimentally and in simulations as simplified model systems of the skin barrier. Molecular dynamics studies have generally focused on simulating preassembled structures using atomistically detailed models of CERs, which limit the system sizes and time scales that can practically be probed, rendering them ineffective for studying particular phenomena, including self-assembly into bilayer and lamellar superstructures. Here, we report on the development of a coarse-grained (CG) model for CER NS, the most abundant CER in human stratum corneum. Multistate iterative Boltzmann inversion is used to derive the intermolecular pair potentials, resulting in a force field that is applicable over a range of state points and suitable for studying ceramide self-assembly. The chosen CG mapping, which includes explicit interaction sites for hydroxyl groups, captures the directional nature of hydrogen bonding and allows for accurate predictions of several key structural properties of CER NS bilayers. Simulated wetting experiments allow the hydrophobicity of CG beads to be accurately tuned to match atomistic wetting behavior, which affects the whole system, since inaccurate hydrophobic character is found to unphysically alter the lipid packing in hydrated lamellar states. We find that CER NS can self-assemble into multilamellar structures, enabling the study of lipid systems more representative of the multilamellar lipid structures present in the skin barrier. The coarse-grained force field derived herein represents an important step in using molecular dynamics to study the human skin barrier, which gives a resolution not available through experiment alone.

Development of a coarse-grained water forcefield via multistate iterative Boltzmann inversion

A coarse-grained water model is developed using multistate iterative Boltzmann inversion. Following previous work, the k-means algorithm is used to dynamically map multiple water molecules to a single coarse-grained bead, allowing the use of structure-based coarse-graining methods. The model is derived to match the bulk and interfacial properties of liquid water and improves upon previous work that used single state iterative Boltzmann inversion. The model accurately reproduces the density and structural correlations of water at 305 K and 1.0 atm, stability of a liquid droplet at 305 K, and shows little tendency to crystallize at physiological conditions. This work also illustrates several advantages of using multistate iterative Boltzmann inversion for deriving generally applicable coarse-grained forcefields.

Derivation of coarse-grained potentials via multistate iterative Boltzmann inversion

In this work, an extension is proposed to the standard iterative Boltzmann inversion (IBI) method used to derive coarse-grained potentials. It is shown that the inclusion of target data from multiple states yields a less state-dependent potential, and is thus better suited to simulate systems over a range of thermodynamic states than the standard IBI method. The inclusion of target data from multiple states forces the algorithm to sample regions of potential phase space that match the radial distribution function at multiple state points, thus producing a derived potential that is more representative of the underlying interactions. It is shown that the algorithm is able to converge to the true potential for a system where the underlying potential is known. It is also shown that potentials derived via the proposed method better predict the behavior of n-alkane chains than those derived via the standard IBI method. Additionally, through the examination of alkane monolayers, it is shown that the relative weight given to each state in the fitting procedure can impact bulk system properties, allowing the potentials to be further tuned in order to match the properties of reference atomistic and/or experimental systems.

Simulation study of the structure and phase behavior of ceramide bilayers and the role of lipid head group chemistry

Ceramides are known to be a key component of the stratum corneum, the outermost protective layer of the skin that controls barrier function. In this work, molecular dynamics simulations are used to examine the behavior of ceramide bilayers, focusing on non-hydroxy sphingosine (NS) and non-hydroxy phytosphingosine (NP) ceramides. Here, we propose a modified version of the CHARMM force field for ceramide simulation, which is directly compared to the more commonly used GROMOS-based force field of Berger (Biophys. J. 1997, 72); while both force fields are shown to closely match experiment from a structural standpoint at the physiological temperature of skin, the modified CHARMM force field is better able to capture the thermotropic phase transitions observed in experiment. The role of ceramide chemistry and its impact on structural ordering is examined by comparing ceramide NS to NP, using the validated CHARMM-based force field. These simulations demonstrate that changing from ceramide NS to NP results in changes to the orientation of the OH groups in the lipid headgroups. The arrangement of OH groups perpendicular to the bilayer normal for ceramide NP, verse parallel for NS, results in the formation of a distinct hydrogen bonding network, that is ultimately responsible for shifting the gel-to-liquid phase transition to higher temperature, in direct agreement with experiment.

Iron-plasma transmission measurements at temperatures above 150 eV

Measurements of iron-plasma transmission at 156+/-6 eV electron temperature and 6.9+/-1.7 x 10(21) cm(-3) electron density are reported over the 800-1800 eV photon energy range. The temperature is more than twice that in prior experiments, permitting the first direct experimental tests of absorption features critical for understanding solar interior radiation transport. Detailed line-by-line opacity models are in excellent agreement with the data.

Hot dense capsule-implosion cores produced by Z-pinch dynamic Hohlraum radiation

Hot dense capsule implosions driven by Z-pinch x rays have been measured using a approximately 220 eV dynamic Hohlraum to implode 1.7-2.1 mm diameter gas-filled CH capsules. The capsules absorbed up to approximately 20 kJ of x rays. Argon tracer atom spectra were used to measure the T(e) approximately 1 keV electron temperature and the n(e) approximately 1-4 x 10(23) cm(-3) electron density. Spectra from multiple directions provide core symmetry estimates. Computer simulations agree well with the peak emission values of T(e), n(e), and symmetry, indicating reasonable understanding of the Hohlraum and implosion physics.

X-ray imaging measurements of capsule implosions driven by a Z-pinch dynamic hohlraum

The radiation and shock generated by impact of an annular tungsten Z-pinch plasma on a 10-mm diam 5-mg/cc CH(2) foam are diagnosed with x-ray imaging and power measurements. The radiative shock was virtually unaffected by Z-pinch plasma instabilities. The 5-ns-duration approximately 135-eV radiation field imploded a 2.1-mm-diam CH capsule. The measured radiation temperature, shock radius, and capsule radius agreed well with computer simulations, indicating understanding of the main features of a Z-pinch dynamic-hohlraum-driven capsule implosion.

Successful use of the "patch, drain, and wait" laparotomy approach to perforated necrotizing enterocolitis: is hypoxia-triggered "good angiogenesis" involved?

The traditional and most frequently employed surgical approach to perforated necrotizing enterocolitis (NEC), laparotomy and bowel resection with enterostomy creation, has been associated with an unacceptably high mortality and major morbidity (sepsis, short-gut syndrome, strictures, long-term total parenteral nutrition (TPN), prolonged and costly hospitalizations with multiple operations, the inevitable open-and-close procedure for "hopeless" extensive gut ischemia in approximately 10% of laparotomy cases, etc.). The use of the laparotomy "patch, drain, and wait" (PD&W) approach to this serious of NEC complication has provided a simple, direct, and effective means of dealing with this problem. The basic principle is to resect no gut and do no enterostomies. The details are presented here as well as the multiple types of "patching" and the importance of use of extensive direct-vision draining with bilateral small Penrose drains from the undersurfaces of both diaphragms into the pelvis with exit sites in both lower quadrants. Proper and effective patching and draining cannot be done blindly,but requires direct vision (laparotomy or laparoscopy). The critical components and timing of the "waiting" are emphasized, including the vital importance of strict avoidance of early post-drainage laparotomy in the 7- to 14-day post-drainage period (whether the drainage is percutaneous, laparotomy PD&W, or laparoscopy PD&W) due to the early, life-threatening-ending hypervascularity that occurs at this time and if left unmolested will function beneficially as life- and gut-saving "good angiogenesis". The bilateral Penrose drains capture fecal fistulas and function quite well as de-facto enterostomies as the peritoneal cavity is rapidly obliterated by adhesions and massive, florid hypervascularity/gut hypoxia triggered "good angiogenesis" (no peritoneal cavity, no peritonitis). Broad-spectrum triple antibiotics and the routine use of TPN contribute to favorable results. The lessons/experiments of nature encountered in newborns with midgut atresia(s) and remarkable levels of gut survival, in the occasional case with only meconium peritonitis and no obstruction ("auto-anastomosis") are pertinent here as the TPN of PD&W is provided in atresia(s) by the maternal-placental circulation and the sterile peritoneal cavity of atresia(s) is simulated by the combination of antibiotics and peritoneal-cavity obliteration. Life- and gut-saving "good angiogenesis" is common to both situations. A 15-year personal experience with the PD&W laparotomy approach to perforated NEC in 23 cases is reported here with no mortality in the initial 60 postoperative days, no major morbidity, and no second operation required in 70% (spontaneous "auto-anastomosis") of cases. All infants with extensive gut ischemia/necrosis (NEC totalis) who would otherwise be classified as "hopeless" and managed by open-and-close only were managed in this experience successfully by PD&W with preservation of both life and an adequate amount of gut, although a second operation was required in these cases to re-establish intestinal continuity. A particularly striking observation was the rapid transition of these infants from profound illness to near-normalcy in a matter of hours after the initiation of PD&W--much like the rapid clinical changes accompanying the lancing of a boil or an abscess. An involvement of hypoxia-induced "good angiogenesis" with marked hypervascularity and involving molecules, genes, and receptors of the vascular endothelial growth factor family of hypoxia-induced angiogenesis molecules is speculated upon, and clinical studies to document these speculations are suggested as well as studies evaluating the potential of laparoscopic PD&W. The usefulness of Argyle chest-tube "venting" and "stenting" by trans-anal passage above colonic "patched" areas as seen in 2 cases is worthy of further study and use.

Pre-term and particularly pre-labor cesarean section to avoid complications of gastroschisis

The marked advantages and merit of pre-term and particularly pre-labor (PTPL) cesarean section (C-section) in the avoidance, and indeed, virtual elimination of severely disabling gastroschisis (GS) complications in infants diagnosed prior to birth by ultrasound has unfortunately remained controversial in the 10 to 12 years since it was first reported and strongly recommended by numerous authors. During this period, GS has remained one of the four major causes of the short-gut syndrome (SGS) in infancy and childhood and a major cause of prolonged, costly, complicated, and hazardous neonatal intensive care unit stays with requirements for total parenteral nutrition (TPN). The most serious and frequent complications of GS in infants born without PTPL C-section are the occurrence of the "peel", which greatly enlarges and rigidifies the eviscerated gut, and of "complicated GS" (intestinal atresia/s, stenosis, necrosis, perforations) (CGS). The "peel" occurs in 100% of these cases and CGS in approximately 20%. "Peel" enlargement and rigidification of eviscerated intestine in the presence of a reduced peritoneal cavity causes great difficulty in covering the eviscerated, enlarged, and rigidified gut with abdominal wall, skin, a prosthesis, etc., and frequently produces gut ischemia from excessive pressure, which may lead to necrotizing enterocolitis (NEC) and SGS as well as prolonged hospital stays. The presence of a "peel" greatly complicates the hazards of dealing with cases of CGS, as resection and anastomosis are virtually impossible in the presence of a "peel." The authors report personal experience with 77 cases of GS dating as far back as 1951; 44 of the infants were born after the onset of labor by vaginal or C-section delivery and all had some degree of "peel" formation. Of 320 cases from the literature (including some of the cases reported here), 61 (19.1%) involved CGS. Of the 33 cases born PT, and especially PL, there were no cases of "peel" and only 1 case of CGS (3.0%). This infant had a single atresia associated with a very small (1 cm) defect in the abdominal wall and no labor-induced "peel," which was easily and successfully repaired by resection and anastomosis. The 6.4-fold reduction in the occurrence of CGS by PTPL C-section (3.0% vs 19.1%) was statistically significant by the chi-square test (P < 0.05), as was the 100% elimination of the disabling "peel." If the single case of CGS associated with a very small defect and no labor or labor-associated "peel" is eliminated, the incidence of CGS in the remaining PTPL group of 32 cases falls to 0 (0% versus 19.1%, P < 0.007). PT and especially PL C-section may be expected to virtually eliminate "peel" formation and CGS and to remove GS as one of the four major causes of SGS. The findings of this report that PT labor prior to PT C-section may result in both "peel" formation and CGS further solidifies the role of labor in the production of both the "peel" and the equally disabling CGS. Failure to appreciate the central role of labor in GS complications has doubtless contributed to the persistent controversy concerning the value and importance of PTPL C-section for gastroschisis diagnosed in utero. The pediatric surgeon has an important responsibility with the obstetrician to monitor the possible occurrence of occult labor in the waning weeks of pregnancy and be prepared to do a prompt C-section if it occurs and there is adequate lung maturity. The achievement of "peel"- and CGS-free gut would greatly facilitate the use of the new Bianchi technique of gut reduction without anesthesia. The combination of the use of epidural anesthesia for the elective PTPL C-section with the Bianchi approach would spare both mother and baby any untoward effects of general anesthesia and present the potential for massive reductions in hospital costs with minimal patient manipulation and disturbance. For infants born with labor-associated "peel," re-evaluation of the suitability and effectiv

Combination of "patch, drain, and wait" and home total parenteral nutrition for midgut volvulus with massive ischemia/necrosis

The successful use of a combination of "patch, drain, and wait" (PDW) and home total parenteral nutrition (TPN) in the management of a case of acute, catastrophic midgut volvulus in a 2-year-11-month-old boy with near-total ischemia/necrosis of his small intestine is reported. The PDW approach to the highly effective management of acute midgut ischemia/necrosis in infancy and childhood (necrotizing enterocolitis and midgut volvulus) involves maximum gut salvage by avoidance of resection, stoma formation, or both through the use of extensive peritoneal cavity drainage by Penrose drains, TPN, and broad-spectrum antibiotics. The extensive peritoneal drainage fosters capture of enteric fistulas with the formation of enterostomies at drain exit sites, while adhesions and ischemia/inflammation-induced hypervascular obliteration of the peritoneal cavity diminish the potential for peritonitis (no peritoneal cavity, no peritonitis) and facilitate impressive salvage of seemingly hopelessly lost ischemic/necrotic gut (a simulation of the in utero ischemic gut process leading to atresias and some varying, but generally mild, gut loss) while simultaneously contributing to the resorption of absolutely non-salvageable gut and the creation of a remarkably clean and adhesion-free peritoneal cavity resembling that of a newborn infant with midgut intestinal atresia.

Metastasis of an intracranial germinoma through a ventriculoperitoneal shunt: recurrence as a yolk-sac tumor

Extraneural metastases of intracranial germinomas, although infrequent, are associated with a generally poor prognosis despite the high radiosensitivity of localized primary tumors. Ventriculoperitoneal shunts have been implicated in facilitating metastatic spread of primary intracranial germinomas. We present a case of a successfully irradiated suprasellar germinoma recurring after 13 months as an intra-abdominal yolk-sac tumor in a young man. The tumor was eradicated with a combination of systemic chemotherapy and local irradiation, with no residual viable tumor cells confirmed at final surgical extirpation. The role of cerebrospinal fluid (CSF) shunts in metastases, mixed germ-cell tumor histology, and tumor markers in recurrences as well as radiation doses and volumes for treating primary tumors are discussed. Systemic chemotherapy may be utilized as prophylaxis against shunt metastases when CSF drainage is necessary.

Omphalomesenteric duct malformations

Omphalomesenteric duct malformations comprise a wide spectrum of anatomic structures and associated symptoms (or no symptoms). They may range from a completely patent omphalomesenteric duct at the umbilicus to a variety of lesser remnants including cysts, fibrous cords connecting the umbilicus to the distal ileum, granulation tissue at the umbilicus, umbilical hernias, and the famous diverticulum of Meckel. Symptoms may involve fecal fistulas at the umbilicus, intussusception/prolapse of ileum at the umbilicus, intestinal obstruction from a variety of causes, melena and anemia, abdominal pain and inflammation, etc. Although symptoms occur most frequently during childhood years (especially in the first 2 years of life), they may occur through adult years as well. Although these malformations are found with equal frequency among the sexes, a significantly greater incidence of symptoms is encountered in males. Although one of the very most frequent malformations to be found (Meckel's diverticulum in 2% to 3% of the population), they are one of the most unlikely to cause symptoms (also Meckel's diverticulum). An awareness of the diversity of these malformations in type and symptomotology is essential to their proper and optimal management.

Possible role of oceanic heat transport in early Eocene climate

Increased oceanic heat transport has often been cited as a means of maintaining warm high-latitude surface temperatures in many intervals of the geologic past, including the early Eocene. Although the excess amount of oceanic heat transport required by warm high latitude sea surface temperatures can be calculated empirically, determining how additional oceanic heat transport would take place has yet to be accomplished. That the mechanisms of enhanced poleward oceanic heat transport remain undefined in paleoclimate reconstructions is an important point that is often overlooked. Using early Eocene climate as an example, we consider various ways to produce enhanced poleward heat transport and latitudinal energy redistribution of the sign and magnitude required by interpreted early Eocene conditions. Our interpolation of early Eocene paleotemperature data indicate that an approximately 30% increase in poleward heat transport would be required to maintain Eocene high-latitude temperatures. This increased heat transport appears difficult to accomplish by any means of ocean circulation if we use present ocean circulation characteristics to evaluate early Eocene rates. Either oceanic processes were very different from those of the present to produce the early Eocene climate conditions or oceanic heat transport was not the primary cause of that climate. We believe that atmospheric processes, with contributions from other factors, such as clouds, were the most likely primary cause of early Eocene climate.

Validity of the Allen Cognitive Levels Assessment: a tri-ethnic comparison

Schizophrenia is a mental illness which is characterized by severe cognitive deficits and impairments in adaptive functioning. The Allen Cognitive Levels (ACL) Assessment is a screening instrument designed to assess cognitive functioning and to aid clinicians in making judgments about how a patient will be able to perform basic activities of daily living. While the ACL has been widely used, the validity of ACL scores for predicting concurrent adaptive functioning has not been established empirically. The present study examined ACL scores in 110 schizophrenic patients. Scores on the ACL were found to be highly related to scores on the Functional Needs Assessment (r = 0.66), which measures a patient's ability to perform basic activities of daily living. Findings provided some of the first strong evidence that ACL scores reflect adaptive functioning. Correlations between the ACL and the Functional Needs Assessment were equally strong in non-Hispanic whites (n = 31, r = 0.67), Mexican-Americans (n = 58, r = 0.60), and African-Americans (n = 21, r = 0.46). Mean scores did not differ between patients from different ethnic groups. In addition, there was no relationship between ACL scores and level of acculturation within the Mexican-American group. Our data strongly support the hypothesis that the ACL provides a valid and culturally unbiased measure of cognitive functioning that can be helpful in determining how a patient is likely to perform activities of daily living.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) distribution and cytochrome P4501A induction in young adult and senescent male mice

While the developmental toxicology of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and its congeners has received considerable attention, the impact of advanced age on the biochemical effects and the pharmacokinetics of dioxins remains largely undetermined. In the present investigation, TCDD tissue distribution and cytochrome P4501A (CYP1A) induction were characterized in male C57BL/6N mice aged 10 weeks and 28 months at 7 days after administration of single oral [3H]TCDD doses ranging from 0.015 to 15 microgram/kg body wt. Determinations of hepatic marker enzyme activities for CYP1A1 (ethoxyresorufin O-deethylation, EROD) and 1A2 (acetanilide-4-hydroxylation, ACOH) indicated that the dose response curves for EROD induction by TCDD were nearly identical for the 2 age groups, but the ACOH induction response was greater in old mice. After receiving the 15 micrograms/kg dose, an increase (approximately 35%) in relative liver weight was observed 7 days after dosing in the 10-week mice, but not in the aged mice, and the hepatic concentration of TCDD was approximately 25% greater in young than old mice. No age difference was found in hepatic nuclear concentrations of TCDD. A dose-dependent increase in liver:fat tissue concentration ratios was noted at both ages, and adipose tissue and blood concentrations of TCDD did not vary significantly with age. In old mice however, TCDD concentrations in skin, kidney and muscle were all approximately twice those of young mice at the 15 micrograms/kg dose. These results suggest that advanced age may have differential effects on Ah receptor-mediated enzyme induction, while increased TCDD concentrations in certain tissues may have toxicological implications for older animals.

Purification and characterization of a multicomponent AP-1.junD complex from T cells. Dependence on a separate cellular factor for enhanced DNA binding activity

TAP-1 (T-cell AP-1) is a previously identified DNA-binding activity that is rapidly induced in activated T cells in the absence of protein synthesis. This activity has been purified over 2,000-fold from the T-cell line MLA144. Purified TAP-1 is a multicomponent complex composed of 38-kDa and 43-kDa junD polypeptides in association with a separate factor(s), distinct from fos, that partly dissociate from the complex during affinity purification but is required for full TAP-1 DNA-binding activity. When reconstituted with TAP-1, this partly dissociated component strongly enhances the DNA-binding activity of the TAP-1 complex. UV-cross-linking analysis identifies the dissociable component of the TAP-1 complex as a separate class of low molecular mass (23-29-kDa) DNA-binding polypeptide(s). 23-29-kDa polypeptides have been partially purified from nuclear extracts derived from MLA144 that enhance TAP-1 DNA-binding activity over 100-fold and increase its contacts with flanking DNA sequence. These results define TAP-1 as a distinct AP-1.junD-containing complex in T cells whose DNA-binding activity is regulated by the interaction of distinct and separate cellular factor(s).

Surveillance of susceptibility testing methodologies for Haemophilus influenzae in Canada, including evaluation of disk diffusion test

We surveyed 75 clinical laboratories to determine if National Committee for Clinical Laboratory Standards (NCCLS) were being used for the susceptibility testing of Haemophilus influenzae. Of the 66 laboratories that performed susceptibility testing, all claimed to follow current NCCLS guidelines. However, upon further questioning, only 23, all of which used disk diffusion testing, accurately interpreted and followed the guidelines. Proficiency testing of 22 of these laboratories found that an unacceptable number of interpretive errors (> 10%) occurred. These results query the merit of routine disk diffusion susceptibility testing of H. influenzae to beta-lactam agents.

Determination of antimicrobial susceptibilities of Canadian isolates of Haemophilus influenzae and characterization of their beta-lactamases. Canadian Haemophilus Study Group

Susceptibility testing of 1,688 Haemophilus influenzae isolates found 484 ampicillin-resistant strains; 474 strains (28.4%) were beta-lactamase positive, and 5 strains (0.4%) were non-beta-lactamase producers. Restriction enzyme digestion of the beta-lactamase amplicon determined that, of 157 strains, 11 (7.0%) contained ROB-1 beta-lactamase and 146 (93.0%) contained a TEM-type beta-lactamase.

Utilization review of the use of BACTEC PLUS high-volume blood culture bottles

The BACTEC PLUS 26 (NR26) (Becton Dickinson, Towson, Md.) high-volume blood culture bottle replaced the less expensive smaller-volume NR6A bottle in our hospital. An audit carried out several months after their introduction revealed that only 17.5% of the NR26 bottles received the required blood volume. Several audits and educational programs were required in order to achieve a compliance rate of > 60%.

Possible methane-induced polar warming in the early Eocene

Reconstructions of early Eocene climate depict a world in which the polar environments support mammals and reptiles, deciduous forests, warm oceans and rare frost conditions. At the same time, tropical sea surface temperatures are interpreted to have been the same as or slightly cooler than present values. The question of how to warm polar regions of Earth without noticeably warming the tropics remains unresolved; increased amounts of greenhouse gases would be expected to warm all latitudes equally. Oceanic heat transport has been postulated as a mechanism for heating high latitudes, but it is difficult to explain the dynamics that would achieve this. Here we consider estimates of Eocene wetland areas and suggest that the flux of methane, an important greenhouse gas, may have been substantially greater during the Eocene than at present. Elevated methane concentrations would have enhanced early Eocene global warming, and also might specifically have prevented severe winter cooling of polar regions because of the potential of atmospheric methane to promote the formation of optically thick, polar stratospheric ice clouds.

Neurovascular immunology: a 25-year odyssey

Small vasoactive neurotransmitter molecules with a long evolutionary history of involvement in biological defense and inflammation play important roles in the up-regulation and down-regulation of the immune response and with similar effects on neuronal and lymphocyte transmembrane signaling molecules and mechanisms. The longest acting of these stimulatory molecules on lymphocyte traffic, substance P and bradykinin, also are transmitters of impulses relating to heat and pain. Heat and pain have been primordial stimuli to learning and memory--immunological as well as CNS.

Atretic, obstructive proximal duodenal mass associated with annular pancreas and malrotation in a newborn male

A case is described of duodenal atresia involving the first portion of the duodenum associated with a partial annular pancreas, complete nonrotation and nonfixation of the intestines, and a congenitally small glottic region in a newborn male. The atretic segment was a 1.3 cm mass adjacent to the pylorus unlike previous reports of duodenal atresia. The diagnosis was delayed for seven days until the child was fed and had bowel movements. Pyloroduodenostomy was performed resulting in cure. An older sibling had a duodenal web requiring previous duodenoduodenostomy.

In vivo influences of phorbol ester and calcium ionophore on lymphocyte traffic, lymph flow and efferent lymph levels of thromboxane B2 in sheep

In vitro systems have provided increasing evidence of significant lymphocyte transmembrane signalling by plasma membrane receptors which utilize antigen and other ligand activation of the inositol phosphate dual second messenger system of intracellular signalling. Elevation of intracellular Ca2+ and activation of protein kinase C are important products of these signals and appear to provide a complete set of mitogenic signals for both T and B cells. Calcium inophore and phorbol ester have been found to mimic these events in vitro and are here employed in vitro to study their effects on lymphocyte traffic and efferent lymph flow through primary peripheral lymph nodes of sheep and on the output into efferent lymph of the arachidonic acid metabolite, thromboxane B2. Calcium ionophore and phorbol ester were given alone or in combination to popliteal lymph nodes of sheep by drainage area injection or by acute infusion into cannulated afferent lympatics of study popliteal lymph nodes whose efferent lymphatic was chronically cannulated for study. The findings resembled those of drainage area immunization with an early increase in efferent lymph flow and prompt and marked depressions in the output into efferent lymph of both small recirculating and blast lymphocytes ('shutdown', 'recruitment'), followed by a marked increase in the output into efferent lymph of both small recirculating and blast lymphocytes. The greatest elevation in both small recirculating and blast lymphocyte outputs was at 24 and 48 h following phorbol ester and calcium ionophore administration. Acute phorbol ester and calcium ionophore administration was associated with a prompt and marked elevation in efferent lymph levels of thromboxane B2 which were of short duration. The findings observed here with lymph node drainage area infusion/injection of both phorbol ester and calcium ionophore are quite similar to those encountered in this sheep lymphocyte traffic model following popliteal lymph node drainage area immunization with killed Salmonella muenchen antigen.

Substance P increases and prolongs increased output of T4 (CD4) lymphocytes from lymph nodes of sheep in vivo: is it a mediator of immunological memory?

There are receptors on lymphocytes for substance P which are found both on small recirculating and on blast lymphocytes. The principal effect of substance P on lymphocytes appears to be a stimulating one, both in vitro and in vivo. The in vivo administration of substance P to sheep by acute infusion into cannulated afferent lymphatics of peripheral lymph nodes has been found to stimulate efferent lymph flow and the output into efferent lymph of both small recirculating and blast lymphocytes. We here report that substance P both enhances and prolongs the enhancement of the output of T4 (CD4) lymphocytes from lymph nodes of sheep in vivo. This output-stimulating effect appears to be specific to T4 (CD4) lymphocytes and is associated with a depressant effect on the output of T8 (CD8) and B lymphocytes. The output-stimulating effect on small T4 (CD4) lymphocytes is quite prolonged, lasting in excess of 96 h after a single 50 micrograms acute infusion. A brief post-infusion depression in T4 (CD4) lymphocyte output is associated with an equally brief, but marked, elevation in the output into efferent lymph of the arachidonic acid metabolite, thromboxane B2. The output-stimulating effect of substance P on blast T lymphocytes is confined to the T4 (CD4) blast lymphocytes. Substance P or a similar molecule may be of value when a specific T4 (CD4) lymphocyte output stimulant effect is desired. A single prior (6 days) acute infusion of substance P into a popliteal lymph node via its cannulated afferent lymphatic produced profound changes in the response to nodal drainage area immunization with killed S. muenchen bacteria. The latent period prior to increased antibody production was abolished, as was the standard post-immunization 'shutdown' period of decreased output of lymphocytes into efferent lymph. These changes were accompanied by a marked and progressive increase in antibody production. The findings reported here suggest substance P-induced long-term potentiation (LTP) of the immune response and raise the question of an involvement of substance P as a major mediator of immunological memory.

On ethylene and stem elongation in green pea seedlings

Maximum elongation of excised internodal stem sections of light-grown pea (Pisum sativum L.) seedlings occurred at 10(-5) molar indoleacetic acid (IAA), with submaximal responses occurring at 10(-4) and 10(-3) molar. Accompanying elongation at concentrations of IAA of 10(-6) to 10(-3) molar was production of ethylene, with the amount increasing up to 10(-4) molar IAA and then becoming nearly constant. Elongation of light-grown sections was not inhibited by exogenous ethylene up to 10,000 ppm in the presence of 10(-5) molar IAA. Marked (up to 50%) inhibition of elongation of internodal segments in situ was observed after treating whole light-grown seedlings with exogenous ethylene for 20 hours. It is concluded that ethylene is not responsible for the submaximal elongation responses of green pea stem sections at high auxin concentrations, but that IAA per se is accountable.

Appendicitis in children aged 13 years and younger

During a recent 4 1/2-year period, an increasing number of young children aged 13 years and younger (230 consecutive patients) came to our institution with signs, symptoms, and findings of acute appendicitis, which was perforated in nearly half of the cases (47%). This was accompanied by a marked increase in the number of very young children aged 6 years and younger (32% of our 230 patients). A prompt response to broad preoperative antibiotic coverage and definitive operative intervention (appendectomy in all cases) was found, with no mortality and essentially no major morbidity (less than 1%). The rapid transition from serious illness to near normalcy in the matter of a few hours with a standardized approach to treatment stimulated this review of appendicitis in this pediatric age group in an effort to define factors influencing these findings.

Recurrent ventriculoperitoneal shunt pseudocyst in a nine-year-old girl

A case of a ventriculoperitoneal shunt pseudocyst in a 9-year-old girl diagnosed using computed tomography is reported. Two attempts at relocation of the shunt failed to prevent reaccumulation of cerebrospinal fluid. A ventriculoatrial shunt was then performed with a successful outcome. Such pseudocysts should be part of the differential diagnosis of abdominal processes in patients with ventriculoperitoneal shunts. If relocation of the shunt outside the pseudocyst is unsuccessful, a ventriculoatrial shunt is generally indicated.

Glass shrapnel injuries to children resulting from "dry ice bomb" explosions: a report of three cases

Glass shrapnel from explosions of "dry ice bombs," created by thrusting dry ice into glass soft-drink bottles and occluding the opening of the bottles, has produced serious and multiple lacerating injuries in three children, which required major operative intervention. We found no previous reports of this grave hazard.

Advantages of performing the sagittal anoplasty operation for imperforate anus at birth

The development of the posterior sagittal anorectoplasty approach has been a major recent advance in the repair of imperforate anus. It has been found that sagittal anoplasty can easily and preferably be carried out in the newborn period without the need for colostomy or "tapering." It is suggested that the perineal sagittal approach be attempted first, with the infant positioned so that the abdominal part of the abdominoperineal approach can be used if necessary--this seldom may be required. Neonatal closure of urinary tract fistulas in boys is an added attractive feature of this approach. The importance of optical magnification (microsurgery) and excellent, intense lighting of the perineal area with a headlamp is stressed for this approach in the newborn, particularly for the management of high pouches and high fistulas. This operation at birth relieves alimentary tract obstruction at birth, eliminates urinary tract contamination (when it exists) at birth, establishes anorectal continuity and maximum potential for "normal" defecation reflexes at birth, and achieves all of this in one rather than three operations.

Colectomy for necrotizing amebic pancolitis in early childhood with survival

A case of necrotizing amebic pancolitis in a 6-year-old boy with asplenia, partial situs inversus, and cyanotic congenital heart disease is reported and the literature is reviewed briefly. Our patient was managed successfully by prompt colectomy, ileostomy, a Stamm gastrostomy, and extensive drainage of the peritoneal cavity with administration of metronidazole postoperatively and prolonged jugular vein Broviac catheter hyperalimentation. This child may be the first survivor of total colonic amebic necrosis in childhood. Necrotizing amebic colitis appears to be more hazardous in infancy and childhood than in adult years. Malnutrition and additional illnesses and malformations may produce greater immunocompromise in the very young, placing them at greater risk for the ultimate of amebic intestinal complications, total colonic necrosis and disintegration.

Substance P increases lymphocyte traffic and lymph flow through peripheral lymph nodes of sheep

Substance P, an 11 amino acid residue vasoactive neurotransmitter peptide, has been found on acute infusion (50 micrograms) into cannulated afferent lymphatics of popliteal lymph nodes of sheep to produce marked elevations in both efferent lymph flow and in the outputs of both blast and small recirculating lymphocytes into popliteal node efferent lymph (chronically cannulated). These elevations were characterized by a delay in the onset of major elevations, a marked prolongation of the elevations and a substantially greater stimulative effect on the output of blast lymphocytes. It is suggested that the number and types of substance P receptors on lymphocytes and in sheep peripheral lymph nodes may be responsible for these observations. Infusion of substance P, known for involvement in pain impulse transmission, was able to briefly overcome anaesthesia-induced depression in lymphocyte traffic. The substance P-induced alterations in lymph flow and lymphocyte traffic in vivo were demonstrated to be due to local rather than systemic effects of substance P.

Prompt elevations of PGE2 and thromboxane A2 metabolites in peripheral node efferent lymph of sheep following drainage area immunization

In the past decade, the main interest in the involvement of prostaglandin E2 (PGE2) in the immune response has been concerned with its role in immunomodulation (suppression) both in vitro and in vivo. Comparatively little attention has been devoted to its immunostimulatory role. It has been suggested that PGE2, like histamine, may function as a 'double agent', initially triggering, facilitating and augmenting a stimulatory immune response and later modulating, limiting and contributing to the turning off of this response. We here report an early (within minutes) immunostimulatory involvement of PGE2 (and thromboxane A2) in the sheep, with prompt elevations in levels of PGE2 and thromboxane B2 in popliteal lymph node efferent lymph following drainage area immunization with killed Salmonella muenchen bacteria. These elevations were associated with an increase in efferent lymph flow and an equally prompt but limited depression of lymphocyte outputs into efferent lymph ('shutdown', 'recruitment'). Local increases in blood flow and vascular permeability probably play important roles in these events.