On the robustness of the geometrical model for cell wall deposition.

All plant cells are provided with the necessary rigidity to withstand the turgor by an exterior cell wall. This wall is composed of long crystalline cellulose microfibrils embedded in a matrix of other polysaccharides. The cellulose microfibrils are deposited by mobile membrane bound protein complexes in remarkably ordered lamellar textures. The mechanism by which these ordered textures arise, however, is still under debate. The geometrical model for cell wall deposition proposed by Emons and Mulder (Proc. Natl. Acad. Sci. 95, 7215-7219, 1998) provides a detailed approach to the case of cell wall deposition in non-growing cells, where there is no evidence for the direct influence of other cellular components such as microtubules. The model successfully reproduces even the so-called helicoidal wall; the most intricate texture observed. However, a number of simplifying assumptions were made in the original calculations. The present work addresses the issue of the robustness of the model to relaxation of these assumptions, by considering whether the helicoidal solutions survive when three aspects of the model are varied. These are: (i) the shape of the insertion domain, (ii) the distribution of lifetimes of individual CSCs, and (iii) fluctuations and overcrowding. Although details of the solutions do change, we find that in all cases the overall character of the helicoidal solutions is preserved.

Cellulose microfibril deposition: coordinated activity at the plant plasma membrane.

Plant cell wall production is a membrane-bound process. Cell walls are composed of cellulose microfibrils, embedded inside a matrix of other polysaccharides and glycoproteins. The cell wall matrix is extruded into the existing cell wall by exocytosis. This same process also inserts the cellulose synthase complexes into the plasma membrane. These complexes, the nanomachines that produce the cellulose microfibrils, move inside the plasma membrane leaving the cellulose microfibrils in their wake. Cellulose microfibril angle is an important determinant of cell development and of tissue properties and as such relevant for the industrial use of plant material. Here, we provide an integrated view of the events taking place in the not more than 100 nm deep area in and around the plasma membrane, correlating recent results provided by the distinct field of plant cell biology. We discuss the coordinated activities of exocytosis, endocytosis, and movement of cellulose synthase complexes while producing cellulose microfibrils and the link of these processes to the cortical microtubules.

Rates of exocytosis and endocytosis in Arabidopsis root hairs and pollen tubes.

Exocytosis and endocytosis are pivotal in many biological processes, but remain difficult to quantify. Here we combine a new algorithm for estimating vesicle size with a detailed morphological analysis of tip-growing cells, in which exocytosis is highly localized and therefore more readily quantified. Cell preservation was rendered as life-like as possible by rapid freezing. This allowed us to produce the first estimates of exocytosis rates in the root hairs and pollen tubes of the model plant Arabidopsis. To quantify exocytosis and endocytosis rates during cell growth, we measured the diameter of vesicles located in the tips of Arabidopsis root hairs and pollen tubes and the widths of cell walls and the cell lumen in longitudinal thin transmission electron microscopic sections. In addition, we measured growth velocities of Arabidopsis root hairs and pollen tubes, using video microscopy. The number of exocytotic vesicles required for cell wall expansion, and the amount of excess membrane inserted into the plasma membrane to be internalized, were estimated from the values that were obtained. The amount of excess membrane that is inserted into the plasma membrane during cell growth was estimated as 86.7% in root hairs and 79% in pollen tubes. This membrane has to be recycled by endocytosis. From counting of the total number of vesicles that is present in thin EM sections through the pollen tube tip, we estimated the average number of vesicles that is present in the tip of pollen tubes. By calculating the total amount of membrane and cell wall material that is required for continued cell growth, assuming that all vesicles are exocytotic, we estimated that pollen tubes continue to grow for 33 s when delivery of vesicles to the tip is inhibited. We arrested vesicle delivery to the tip by application of cytochalasin D. After cytochalasin D application, pollen tubes continued to grow for 30-40 s, which is in the same range as the estimated value of 33 s and shows that in this time frame, the availability of exocytotic vesicles is not a limiting factor.

Pulmonary valve insertion late after repair of Fallot's tetralogy.

OBJECTIVES: To analyze the results of pulmonary valve insertion late after initial repair of Fallot's tetralogy. Pulmonary insufficiency (PI) after correction of Fallot's tetralogy is usually well tolerated in the short term, but is associated with symptomatic right ventricular dilatation and an increased risk of ventricular arrhythmias over longer periods of time. METHODS: From 1993 to July 2000, 51 patients were reoperated for PI at a mean age of 25.7+/-11.9 years. The mean age at initial repair was 6.4+/-7.2 years. Patients with a conduit inserted at initial operation, with absent pulmonary valve syndrome or with a more than moderate ventricular septal defect at reoperation were excluded from the study. A cryopreserved pulmonary (96%) or aortic (4%) homograft was implanted in the orthotopic position with the use of cardiopulmonary bypass 19.3+/-9.1 years (2.7-40.3 years) after initial correction. Preoperative symptoms (New York Heart Association, NYHA class), degree of PI (echo-Doppler, MRI), right ventricular dimensions (MRI) and QRS duration were compared to findings at last follow-up. RESULTS: Follow-up is complete and had a mean duration of 1.7+/-1.4 years. Hospital mortality was 2%. No serious morbidity occurred. Severe PI was present preoperatively in all patients. At last follow-up echo-Doppler studies showed PI to be absent or trivial in 96% and mild in 4% of patients. In 13 patients MRI studies were performed both pre- and postoperatively: in this group PI was reduced from a mean of 48 to 4%. After 6 months NYHA capacity class had improved significantly from 2.3+/-0.6 to 1.4+/-0.5. After 1 year end-diastolic and end-systolic right ventricular volumes were reduced significantly. Right ventricular ejection fraction and QRS duration remained unchanged. CONCLUSIONS: PI late after correction of Fallot's tetralogy may lead to serious symptomatic right ventricle dilatation. After pulmonary homograft insertion right ventricular dimensions decrease rapidly and functional improvement is observed in almost all patients.

A dynamical model for plant cell wall architecture formation.

We discuss a dynamical mathematical model to explain cell wall architecture in plant cells. The highly regular textures observed in cell walls reflect the spatial organisation of the cellulose microfibrils (CMFs), the most important structural component of cell walls. Based on a geometrical theory proposed earlier [A. M. C. Emons, Plant, Cell and Environment 17, 3-14 (1994)], the present model describes the space-time evolution of the density of the so-called rosettes, the CMF synthesizing complexes. The motion of these rosettes in the plasma membrane is assumed to be governed by an optimal packing constraint on the CMFs plus adherent matrix material, that couples the direction of motion, and hence the orientation of the CMF being deposited, to the local density of rosettes. The rosettes are created inside the cell in the endoplasmatic reticulum and reach the cell-membrane via vesicles derived from Golgi-bodies. After being inserted into the plasma membrane they are assumed to be operative for a fixed, finite lifetime. The plasma membrane domains within which rosettes are activated are themselves also supposed to be mobile. We propose a feedback mechanism that precludes the density of rosettes to rise beyond a maximum dictated by the geometry of the cell. The above ingredients lead to a quasi-linear first order PDE for the rosette-density. Using the method of characteristics this equation can be cast into a set of first order ODEs, one of which is retarded. We discuss the analytic solutions of the model that give rise to helicoidal, crossed polylamellate, helical, axial and random textures, since all cell walls are composed of (or combinations of) these textures.

On the stall force for growing microtubules.

The assembly of microtubules generates forces that play a role in cellular motility processes such as the motion of chromosomes during mitosis. Recently, Mogilner and Oster proposed a model for the growth of microtubules that agrees quantitatively with the force-velocity relation measured for individual microtubules. In addition, the authors predicted that the stall force for any polymer consisting of N independently growing protofilaments should increase as the square root of N. We simulated this model and found that the stall force increases linearly with N, and is in fact consistent with the maximum force predicted by thermodynamic arguments. We show that this discrepancy can be explained by a more careful treatment of the "off-term" in the Mogilner-Oster model.

How the deposition of cellulose microfibrils builds cell wall architecture.

Cell walls, the extracytoplasmic matrices of plant cells, consist of an ordered array of cellulose microfibrils embedded in a matrix of polysaccharides and glycoproteins. This construction is reminiscent of steel rods in reinforced concrete. How a cell organizes these ordered textures around itself, creating its own desirable environment, is a fascinating question. We believe that nature adopted an economical solution to this design problem: it exploits the geometrical constraints imposed by the shape of the cell and the limited space in which microfibrils are deposited, enabling the wall textures essentially to 'build themselves'. This does not imply that the cell cannot control its wall texture. On the contrary, the cell has ample regulatory mechanisms to control wall texture formation by controlling the insertion of synthases and the distance between individual microfibrils within a wall lamella.

The making of the architecture of the plant cell wall: how cells exploit geometry.

Cell wall deposition is a key process in the formation, growth, and differentiation of plant cells. The most important structural components of the wall are long cellulose microfibrils, which are synthesized by synthases embedded in the plasma membrane. A fundamental question is how the microfibrils become oriented during deposition at the plasma membrane. The current textbook explanation for the orientation mechanism is a guidance system mediated by cortical microtubules. However, too many contraindications are known in secondary cell walls for this to be a universal mechanism, particularly in the case of helicoidal arrangements, which occur in many situations. An additional construction mechanism involves liquid crystalline self-assembly [A. C. Neville (1993) Biology of Fibrous Composites: Development Beyond the Cell Membrane (Cambridge Univ. Press, Cambridge, U.K.)], but the required amount of bulk material that is able to equilibrate thermally is not normally present at any stage of the wall deposition process. Therefore, we have asked whether the complex ordered texture of helicoidal cell walls can be formed in the absence of direct cellular guidance mechanisms. We propose that they can be formed by a mechanism that is based on geometrical considerations. It explains the genesis of the complicated helicoidal texture and shows that the cell has intrinsic, versatile tools for creating a variety of textures. A compelling feature of the model is that local rules generate global order, a typical phenomenon of life.

Excitation-contraction coupling in myocardium: implications of calcium release and Na+-Ca2+ exchange.

In this paper, we present evidence in support of the hypothesis that electrogenic Na+-Ca2+ exchange is responsible for three phenomena in rat cardiac muscle: the slow repolarization phase of the action potential, the time course of the mechanical recovery process, and the development of triggered arrhythmias. It was shown that the duration of the slow phase of repolarization of the action potential varies in proportion to the Na+ concentration gradient and inversely with the Ca2+ concentration gradient over the cell membrane. This suggested that Na+-Ca2+ exchange can generate a current of sufficient magnitude to maintain the membrane depolarized at a level of -60 mV. The mechanical restitution process of rat cardiac trabeculae was shown to exhibit three phase. The first phase, alpha, probably reflects rapid transport of calcium in the sarcoplasmic reticulum from the uptake sites to the release sites. After the initial increase of force during alpha, force rises further during phase beta and then declines during phase gamma. During all phases, force increases with the extracellular calcium concentration. beta is accelerated by preceding extrasystoles, while an increase of the heart rate causes force to increase at approximately the same rate but to a higher level during phase beta. These observations are compatible with a model in which the sarcoplasmic reticulum sequesters calcium from the cytosol, while the membrane of the sarcoplasmic reticulum is assumed to exhibit also a small leak of calcium into the cytosol.(ABSTRACT TRUNCATED AT 250 WORDS)