Blood flow downstream of a two-dimensional bifurcation

Many cardiovascular lesions such as aneurysms, intimal cushions, and atherosclerotic plaques tend to occur near bifurcations. This suggests that hemodynamic factors may be involved. Since measuring devices (such as anemometers) are still too large to allow local measurements of flow disturbances, we have attempted to predict the nature of these factors mathematically. Biological variables include pulsatile flow of a nonNewtonian fluid in distensible branching vessels with different angles and flow rates. Our initial analysis considers the flow in a two-dimensional bifurcation with a symmetrical flow divider perfused with steady flow at variable Reynolds numbers. At all flows, high shear forces develop on either side of the flow divider (i.e. at the apex of the bifurcation). With high flows, regions of sluggish or reverse flow develop near the outer walls of the bifurcation. The analysis confirms that the flow at the apex is quite different from that at the outer angles and that the latter varies more with flow rate than the former.     

In vitro study of modifications of blood flow patterns induced by a bifurcation

Velocity profiles obtained with atheromatous and normal bifurcation castings in the presence of various types of flows are proposed. In the atheromatous bifurcation, with steady flow, we observe radial positive or negative velocities at distance to the wall smaller than 1 mm, which may be attributed to small local eddy motions. The maximum of velocity needs a larger distance from the apex than in the case of the "normal" bifurcation to be again located on the axes. With periodical flows, the effects are strongly damped. The wall velocity gradients on several geometries of tubings are investigated to separate the effects of the local rugosity of the wall from those incidental to the geometry of the bifurcation. The alterations caused by the atheroma do not seem to be induced by local modifications or rugosity, but by slow modifications of the local diameter. As a consequence, the variations of the velocity gradient caused by atheroma in the total bifurcation, are more likely due to distance effects of the geometry itself than to local effects of rugosity.     

The shoot apex of Podostemaceae: De novo structure or reduction of the conventional type?

The paper reviews and discusses various interpretations of the shoot apex of Podostemaceae with special reference to subfamily Podostemoideae. Main questions concern (1) the proposed absence of a shoot apical meristem (SAM) in apical “meristemless” shoot tips of Podostemoideae and, as the consequence, the endogenous inception of leaf-borne leaves and branches and (2) the predicted stem bifurcation below a “terminal” dithecous (double-sheathed) leaf positioned instead of a shoot apex, as it is reported for subfamily Podostemoideae. Does the “meristemless” shoot apex represent a true evolutionary novelty? Does the view of stem bifurcation represent a new ramification pattern with the consequence that the “classical root–shoot model” of angiosperms is not valid for Podostemaceae? Both interpretations do not conform to previous studies that are complemented here by new data on the SAM of Zeylanidium olivaceum and Thelethylax minutiflora (Podostemoideae). Although a SAM is difficult to observe in the vegetative shoots of many Podostemoideae, it becomes well visible when the shoot passes into the flowering stage approaching the conspicuous shoot apex of floriferous shoots. The arguments of the absence of a SAM in vegetative shoots are not convincing and the endogenous origin of “leaf-borne leaves” appears questionable. Consequently, the “meristemless” shoot apex cannot be considered as a structure having evolved de novo. In the less advanced subfamilies Tristichoideae and Weddellinoideae, the leaf primordia develop only from a few apical cells of the outer shoot layer. This allows the conclusion that the surface layer of the apex in these subfamilies corresponds to the horizontally spread single-layered apical meristem of subfamily Podostemoideae. Similarly, the view of shoot bifurcation does not conform to the diachsial–sympodial branching pattern occurring in the cymose inflorescences of many Podostemoideae. This fact contradicts the presence of a terminal leaf.     

Automated and objective removal of bifurcation aneurysms: Incremental improvements,and validation against healthy controls

Abnormal hemodynamic stresses are thought to correlate with aneurysm initiation, growth, and rupture. We have previously investigated the role of wall shear stress (WSS) and WSS gradients (WSSG) in search for a mechanistic link to formation of sidewall aneurysms using an automated and objective tool for aneurysm removal and arterial reconstruction in combination with computational fluid dynamics (CFD). However, we warned against the use of the tool for bifurcation type aneurysms because of a potential unrealistic reconstruction of the apex. We hypothesized that inclusion of additional morphological features from the surrounding vasculature could overcome these constraints. We extended the previously published method for removal and reconstruction of the bifurcation vasculature based on diverging and converging points of the parent and daughter artery centerlines, to also include two new centerlines between the daughter vessels, one of them passed through the bifurcation center. Validation was performed by comparing the efficacy of the two algorithms, using ten healthy models of the internal carotid artery terminus as ground truth. Qualitative results showed that the bifurcation apexes became smoother relative to the original algorithm; more consistent with the reference models. This was reflected quantitatively by a reduced maximum distance between the reference and reconstructed surfaces, although not statistically significant. Furthermore, the modified algorithm also quantitatively improved CFD derived WSS and WSSG, especially the latter. In conclusion, the modified algorithm does not perfectly reconstruct the bifurcation apex, but provides an incremental improvement, especially important for the derived hemodynamic metrics of interest in vascular pathobiology.     

BIFURCATION OF THE STEM APEX IN ASCLEPIAS SYRIACA

In Asclepias syriaca the overall inflorescence is an anthoclad in which the peduncles are non-axillary, each occurring about 60° away from the axil of a leaf. Ontogenetically, a peduncle is initiated when the stem apex expands laterally and bifurcates into separate apices, neither of which is subtended by any type of organ. One of the two apices continues as the functional apex of the stem (bifurcating again at each subsequent node), and the other functions as the apex of the peduncle. The peduncle first produces a bract and, then, a pedicel in the axil of the bract. Subsequent pedicels are each axillary to separate bracts. The pedicels, therefore, can be interpreted as ordinary lateral branches. However, because the bifurcations of the stem apex are not associated with subtending organs, the branching of the stem does not conform to expected monopodial or sympodial systems in the angiosperms. This suggests the possibility that each bifurcation of the stem apex is a true dichotomy. The anthoclad axis, thus, is a series of dichotomies. Although such a series may have been phylogenetically derived from a monopodial or sympodial ancestor, it is also possible that it may have been retained from a primitive, dichotomizing inflorescence.     

Characteristics of secondary flow in steady and pulsatile flows through a symmetrical bifurcation

Steady and pulsatile flow in a glass model simulating an arterial bifurcation was investigated by flow visualization techniques. Secondary flow generated at the bifurcation has a similar pattern to a vortex, called the horseshoe vortex, produced around a wall-based protuberance in a circular tube. The same flow disturbance was clearly observed during the decelerating phase of pulsatile flow. The vortex produces a stagnation point on the top and bottom wall just upstream from the bifurcation apex. When aluminium dust was suspended in the test fluid perfusing the blood vessel model, particles deposited over an area spreading from the stagnation point to the lateral corners of the bifurcation. Comparison between the present results and topographical patterns of atherosclerosis reported in the literature suggests that it is in such low shear regions that lipid deposition tends to occur most.     

Bending and twisting of an in vivo coronary artery at a bifurcation.

Dynamic changes in the geometric shape and dimensions of a left coronary artery tree were extracted from the computer-tomographically reconstructed three-dimensional images of an in situ beating heart of an anesthetized dog. Wireframe models of the left coronary artery tree at 16 different instants of a cardiac cycle were constructed for the study of its flexing motion. For quantifying the local bending and twisting of the left coronary artery tree, the anatomic landmarks of the bifurcation points are selected as focussed locations. At these points, the space curves of the tree at different cardiac instants were first derived in parametric forms. Curvature and torsion expressions are next obtained in terms of the derivatives with respect to the parameter. This analysis revealed that during the initial contraction of the heart wall, a 2% reduction per millisecond in the radius of curvature occurred near the bifurcation point where the left circumflex coronary artery descends toward the apex of the heart. When the left ventricular chamber reached a maximum value, the radius of curvature was found to decrease at a rate of 2.3% ms-1. At the end of diastole, an increase in the radius of curvature at a rate of 5.7% ms-1 was observed. The twisting rates per unit length of artery near the bifurcation point of the selected artery were found to range from -0.62 to 0.63 degrees ms-1.     

Vortex generation in pulsatile flow through arterial bifurcation models including the human carotid artery

Visualization experiments were performed to elucidate the complicated flow pattern in pulsatile flow through arterial bifurcations. Human common carotid arteries, which were made transparent, and glass-models simulating Y- and T-shaped bifurcations were used. Pulsatile flow with wave forms similar to those of arterial flow was generated with a piston pump, elastic tube, airchamber, and valves controlling the outflow resistance. Helically recirculating flow with a pattern similar to that of the horseshoe vortex produced around wall-based protuberances in circular tubes was observed in pulsatile flow through all the bifurcations used in the present study. This flow type, which we shall refer to as the horseshoe vortex, has also been demonstrated to occur at the human common carotid bifurcation in steady flow with Reynolds numbers above 100. Time-varying flows also produced the horseshoe vortex mostly during the decelerating phase. Fluid particles of dye solution approaching the bifurcation apex diverged, divided into two directions perpendicularly, and then showed helical motion representing the horseshoe vortex formation. While this helical flow was produced, the stagnation points appeared on the wall upstream of the apex. Their position was dependent upon the flow distribution ratio between the branches in the individual arteries. The region affected by the horseshoe vortex was smaller during pulsatile flow than during steady flow. Lowering the Reynolds number together with the Womersley number weakened the intensity of helical flow. A separation bubble, resulting from the divergence or wall roughness, was observed at the outer or inner wall of the branch vessels and made the flow more complicated.     

Mechanical tension in different parts of the left ventricle of the heart exposed to inotropic factors

A study was made of the effects of different inotropic factors on mechanical tension in the left ventricular wall and in the apex of the heart and of the participation of these regions in the formation of hemodynamic characteristics. Adrenaline caused similar effects whereas CaCl2 exerted different inotropic effects on the left ventricular wall and the apex of the heart. Changes in mechanical tension of the wall correlated with variations in the pressure inside the left ventricle. Tension in the apex of the heart produced alterations in the stroke volume.     

Cell polarity: stretching prevents developmental cramps

Initiation and successive development of organs induce mechanical stresses at the cellular level. Using the tomato shoot apex, a new study now proposes that mechanical strain regulates the plasma membrane abundance of the PIN1 auxin transporter, thereby reinforcing a positive feed-back loop between growth and auxin accumulation.     

Early inflorescence and floral development in Zea mays land race Chapalote (Poaceae)

Tassel and ear primordia were collected from greenhouse-grown specimens of the Mexican maize landrace Chapalote and prepared for scanning electron microscopic (SEM) examination. Measurements of inflorescence apices and spikelet pair primordia (spp) were made from SEM micrographs. Correlation of inflorescence apex diameter with number of spikelet ranks showed no significant difference between tassels and ears, except at the two-rank level where the ear apical meristem had a significantly smaller diameter than corresponding two-ranked tassels. Within individual inflorescences, spp in different ranks enlarged at comparable rates, although the rates from one ear to the next along the stem differed. In both tassels and ears, spp divide to form paired sessile and pedicellate spikelet primordia when the spp is 150 μm wide; ear axes are significantly thicker than tassel axes at the time of bifurcation. The similarities in growth between ear and tassel primordia lend further support to the hypothesis that both the maize tassel and ear are derived from a common inflorescence pattern, a pattern shared with teosinte. Inflorescence primordial growth also suggests that a key character difference between teosinte and maize, distichous vs. polystichous arrangement of spikelets, may be related to size of the apical dome and/or rate of primordium production by the apical meristem. There appears to be more than a single morphological event in the shift from vegetative to reproductive growth. The evocation of axillary buds (ears) is independent of, and temporally separated from, the transition to flowering at the primary shoot apex (tassel).     

Diphallia in a mixed-breed dog with multiple anomalies

Complete diphallia was detected in a six-month-old Poodle cross dog. Duplication of the urinary bladder, right renal hypoplasia, bifurcation of the descending colon and bilateral cryptorchidism were also present. The left urethra was patent, and emptied the left urinary bladder; the right urethra ended blindly at a point 2.5 cm from the tip of the right penis. The left urinary bladder was joined medially to a right urinary bladder with no interbladder communication; however, the right bladder opened caudally into a blind sac which drained into the left prostatic urethra. Pyelonephritis of the left kidney was present, and was suspected to be due to ascending bacterial infection of the right urinary bladder and the right segment of the bifurcated descending colon, which terminated at the apex of the right urinary bladder. Diphallia and associated urogenital anomalies are reviewed.     

Nicotine concentration in leaves of flue-cured tobacco plants as affected by removal of the shoot apex and lateral buds

It is believed that the nicotine concentration in tobacco is closely correlated with the amount of nitrogen (N) supplied.On the other hand,N uptake mainly occurs at the early growth stage,whereas nicotine concentration increases at the late growth stage,especially after removing the shoot apex.To identify the causes of the increased nicotine concentration in tobacco plants,and to compare the effects of different ways of mechanical wounding on nicotine concentration,field experiments were carried out in Fuzhou,Fujian Province in 2003 and 2004.Excision of the shoot apex had almost no influence on N content in the plant;however,it caused dramatic increases in nicotine concentration in leaves,especially in the middle and upper leaves.An additional increase of the nicotine concentration was obtained by removal of axillary buds.The wounding caused by routine leaf harvests,however,did not change the leaf nicotine concentration,and neither did reducing leaf harvest times.The present results revealed no direct relationship between N supply and nicotine concentration in tobacco leaves,and indicate that not all kinds of mechanical wounding were capable of stimulating nicotine synthesis in tobacco plants.Since nicotine production is highly dependent on the removal of apical meristems and hence on the major sources of auxin in the plant,and application of 1-naphthylacetic acid onto the cut surface of the stem after removing the shoot apex markedly decreased the nicotine concentration in different leaves and the total nicotine content in the plant,the results suggest that decreased auxin supply caused by removal of the shoot apex as a kind of mechanical wounding might regulate nicotine synthesis in the roots of tobacco plants.     

缅甸北部白垩纪中期克钦琥珀华翅蝉科(昆虫纲, 半翅目)新材料

根据产自缅甸北部白垩纪中期克钦琥珀中的一块昆虫化石标本,建立了1新种——克钦雅翅蝉(Ornatiala kachinensis sp. nov.),归于华翅蝉科(Sinoalidae Wang and Szwedo, 2012)。根据新化石材料,对雅翅蝉属的鉴别特征进行了修订。此外,利用支序分类学分析手段对新种的生物系统学地位进行了确定,并对华翅蝉前翅色型、翅脉变异等问题进行了探讨。新材料的发现进一步证实克钦琥珀生物群中的华翅蝉生物多样性程度较高,为优势昆虫类群。     

Mechanical role of the leaf sheath in rattans

Leaf sheaths of rattans are long, tubular and persistent and unlike many self-supporting palms, extend far from the apex of the plant. The mechanical role of the leaf sheath was investigated in eight rattan species of the subfamily Calamoideae. The main objective was to analyse its influence on the mechanical architecture and contribution to the climbing habit. Bending mechanical properties were measured along climbing axes before and after removal of leaf sheaths. Results were related to stem and leaf sheath geometry and mechanical properties. Contribution of the leaf sheath to axial flexural rigidity was high (c. 90%) in the early stages of growth and towards the apex of older climbing axes for all climbing palms tested. Senescence and loss of the leaf sheath strongly influenced axial stiffness. A nonclimbing species, Calamus erectus, showed a different mechanical architecture. Although lacking secondary growth, palms have been able to develop successful climbers with a mechanical architecture broadly analogous to, although developmentally different from, dicotyledonous lianas. The role of the leaf sheath in modulating mechanical properties during ontogeny ought not to be neglected in studies on monocotyledons, as it possibly contributed significantly to the ways in which different growth forms have evolved in the group.     

Hemodynamics and wall mechanics in human carotid bifurcation and its consequences for atherogenesis: investigation of inter-individual variation

Finite element simulations of fluid-solid interactions were used to investigate inter-individual variations in flow dynamics and wall mechanics at the carotid artery bifurcation, and its effects on atherogenesis, in three healthy humans (normal volunteers: NV1, NV2, NV4). Subject-specific calculations were based on MR images of structural anatomy and ultrasound measurements of flow at domain boundaries. For all subjects, the largest contiguous region of low wall shear stress (WSS) occurred at the carotid bulb, WSS was high (6-10 Pa) at the apex, and a small localized region of WSS > 10 Pa occurred close to the inner wall of the external carotid artery (ECA). NV2 and NV4 had a "spot" of low WSS distal to the bifurcation at the inner wall of the ECA. Low WSS patches in the common carotid artery (CCA) were contiguous with the carotid bulb low WSS region in NV1 and NV2, but not in NV4. In all three subjects, areas of high oscillatory shear index (OSI) were confined to regions of low WSS. Only NV4 exhibited high levels of OSI on the external adjoining wall of the ECA and CCA. For all subjects, the maximum wall shear stress temporal gradient (WSSTG) was highest at the flow divider (reaching 1,000 Pa/s), exceeding 300 Pa/s at the walls connecting the ECA and CCA, but remaining below 250 Pa/s outside of the ECA. In all subjects, (maximum principle) cyclic strain (CS) was greatest at the apex (NV1: 14%; NV2: 11%; NV4: 6%), and a second high CS region occurred at the ECA-CCA adjoining wall (NV1: 11%, NV2: 9%, NV4: 5%). Wall deformability was included in one simulation (NV2) to verify that it had little influence on the parameters studied. Location and magnitude of low WSS were similar, except for the apex (differences of up to 25%). Wall distensibility also influenced OSI, doubling it in most of the CCA, separating the single high OSI region of the carotid bulb into two smaller regions, and shrinking the ECA internal and external walls' high OSI regions. These observations provide further evidence that significant intra-subject variability exists in those factors thought to impact atherosclerosis.     

Effect of mechanical stress on Zea root apex. I. Mechanical stress leads to the switch from closed to open meristem organization

The effect of mechanical stress on the root apical meristem (RAM) organization of Zea mays was investigated. In the experiment performed, root apices were grown through a narrowing of either circular (variant I) or elliptical (variant II) shape. This caused a mechanical impedance distributed circumferentially or from the opposite sides in variant I and II, respectively. The maximal force exerted by the growing root in response to the impedance reached the value of 0.15 N for variant I and 0.08 N for variant II. Significant morphological and anatomical changes were observed. The changes in morphology depended on the variant and concerned diminishing and/or deformation of the cross-section of the root apex, and buckling and swelling of the root. Anatomical changes, similar in both variants, concerned transformation of the meristem from closed to open, an increase in the number of the cell layers at the pole of the root proper, and atypical oblique divisions of the root cap cells. After leaving the narrowing, a return to both typical cellular organization and morphology of the apex was observed. The results are discussed in terms of three aspects: the morphological response, the RAM reorganization, and mechanical factors. Assuming that the orientation of division walls is affected by directional cues of a tensor nature, the changes mentioned may indicate that a pattern of such cues is modified when the root apex passes through the narrowing, but its primary mode is finally restored.     

Wall shear stress at the initiation site of cerebral aneurysms

Hemodynamics are believed to play an important role in the initiation of cerebral aneurysms. In particular, studies have focused on wall shear stress (WSS), which is a key regulator of vascular biology and pathology. In line with the observation that aneurysms predominantly occur at regions of high WSS, such as bifurcation apices or outer walls of vascular bends, correlations have been found between the aneurysm initiation site and high WSS. The aim of our study was to analyze the WSS field at an aneurysm initiation site that was neither a bifurcation apex nor the outer wall of a vascular bend. Ten cases with aneurysms on the A1 segment of the anterior cerebral artery were analyzed and compared with ten controls. Aneurysms were virtually removed from the vascular models of the cases to mimic the pre-aneurysm geometry. Computational fluid dynamics (CFD) simulations were created to assess the magnitude, gradient, multidirectionality, and pulsatility of the WSS. To aid the inter-subject comparison of hemodynamic variables, we mapped the branch surfaces onto a two-dimensional parametric space. This approach made it possible to view the whole branch at once for qualitative evaluation. It also allowed us to empirically define a patch for quantitative analysis, which was consistent among subjects and encapsulated the aneurysm initiation sites in our dataset. To test the sensitivity of our results, CFD simulations were repeated with a second independent observer virtually removing the aneurysms and with a 20 % higher flow rate at the inlet. We found that branches harboring aneurysms were characterized by high WSS and high WSS gradients. Among all assessed variables, the aneurysm initiation site most consistently coincided with peaks of temporal variation in the WSS magnitude.