Growth and shaping of the fin and limb buds

The development of the fin and limb buds involves a balance of centrifugal (active) and centripetal (passive) mechanical forces, the first of which acts to move the walls of these structures away from each other and the second of which holds them together. When the volume of the mesodermal core increases, the generated force meets with the resistance of the basal membrane, and as a result, the limb bud has a tendency to acquire a cylindrical shape. Collagen fibers, individual mesenchymal cells, and their groups hold together the dorsal and the ventral wall of the limb bud, prevent the movement of these walls away from each other, and in this way direct bud growth along the proximodistal and the anteroposterior axes. The balance of the forces which stretch the ectodermal layer and those which constrain it has also been observed in the development of other body parts.     

Kinetics of the reaction of yolk cell surface in the loach to puncture and mechanic deformation

Circumferential and radial components of the yolk cell surface movements were measured in the loach embryos at the late blastula stage within 40–50 min after puncture or indentation by an obliquely directed glass rod. The yolk cell surface was preliminarily marked by coal particles. It was shown that even closely located regions of the surface differed markedly in the rate and direction of their movements. In the vicinity of puncture, the yolk cell surface at first contracted in both circumferential and radial directions and then widened, but did not reach the initial values. In more remote areas, this surface continued to contract in the circumferential direction, but was extended in the radial direction. The degree of its contraction along different radii was unequal. The reaction to oblique indentation was anisotropic: the closest area of the yolk cell surface, located along the direction of indentation, contracted in both circumferential and radial directions and formed a fold “leaking” onto the rod, while the opposite area contracted in the circumferential direction, but extended in the radial direction. A conclusion was drawn that the yolk cell surface is a multivariant mechanosensitive system. Its active responses to mechanical influences obey the same patterns as multicellular embryonic tissues.     

The critical weed-free period in organically-grown winter wheat

Two experiments were conducted in central southern England between September 1994 and August 1996 to identify the critical weed-free period in organically grown winter wheat (Triticum aestivum, cv. Mercia). In competition with a mixed weed infestation of predominately Alopecurus myosuroides and Tripleurospermum inodorum it was found that wheat yield decreased as the duration of the weed-infested period increased and that the crop needed to be kept free of weeds from sowing in order to completely avoid any yield loss. Also, weeds emerging in the wheat crop (predominately T. inodorum) during the growing season had a significant and detrimental effect on yield. The existence of the critical period, therefore, depends on the imposition of an acceptable yield loss. If a 5% yield loss gives a marginal benefit compared with the cost of weed control, the critical period will begin at 506°C days after sowing (November) and end at 1023°C days after sowing (February). This information could be used by farmers to target mechanical weeding operations to control weeds at a time that will have maximum benefit to the crop.     

Biomechanical rationale of coronary artery bypass grafting of multivessel disease

The biomechanical model of human coronary arteries was modified for improving the quality of diagnosis and surgical treatment for coronary heart disease. The problem of hemodynamics in the left coronary artery with multivessel bed disease – 45% stenosis of the anterior descending branch and 75% stenosis of the circumflex branch – was particularly considered. Numerical simulation of the coronary arterial bypass of the main trunk was carried out to estimate the functional condition of the coronary arteries after restoring myocardial blood supply by surgery.     

The effect of excitation on the rate of respiration in the liverwort Conocephalum conicum

Simultaneous measurements were taken of the electrical activity and the rate of respiration of thalli of Conocephalum conicum L. stimulated electrically and mechanically (by cutting). The measurements of the rate of respiration employed a modified Warburg apparatus for O₂ consumption and an infra-red gas analyzer with computer recording and data processing for CO₂ evolution. The action potential, produced by either a cut (a damaging stimulus) or an electrical stimulus (a non-damaging stimulus), caused a transient rise in the rate of respiration. The course of changes in the rate of respiration depends on the character of the excitation and the area of the thallus covered by it. If stimulation does not produce excitation, the increase in the rate of respiration does not take place, regardless of the magnitude and type of the stimulus applied.     

Penetrometer resistance,root penetration resistance and root elongation rate in two sandy loam soils

Root penetration resistance and elongation of maize seedling roots were measured directly in undisturbed cores of two sandy loam soils. Root elongation rate was negatively correlated with root penetration resistance, and was reduced to about 50 to 60% of that of unimpeded controls by a resistance of between 0.26 and 0.47 MPa. Resistance to a 30° semiangle, 1 mm diameter penetrometer was between about 4.5 and 7.5 times greater than the measured root penetration resistance. However, resistance to a 5° semiangle, 1 mm diameter probe was approximately the same as the resistnace to root penetration after subtracting the frictional component of resistance. The diameter of roots grown in the undisturbed cores was greater than that of roots grown in loose soil, probably as a direct result of the larger mechanical impedance in the cores.     

Mechanical impedance increases aluminium tolerance of soybean (Glycine max) roots

The effect of aluminium (Al) on root elongation was studied in solution culture and sand culture. Compared to solution culture, in sand culture a ten times higher Al supply was necessary to inhibit root elongation to a comparable degree. This was due to a much lower Al uptake into the 5 mm root tips in sand culture. Fe concentrations in root tips were also lower in sand culture. Ca concentrations were higher and less depressed by Al, whereas Mg and K concentrations were not affected by the culture substrate. Regressions of Al concentrations in root tips versus inhibition of root elongation by Al revealed root damage at lower Al concentrations in sand culture. The effect of culture substrate on Al tolerance was independent of N source and could also be shown in flowing solution culture with and without sand. The results indicate that mechanical impedance in sand culture decreased Al uptake. This may be due to enhanced exudation of organic complexors thus reducing activites of monomeric Al species.     

Complete mechanical impedance increases the turgor of cells in the apex of pea roots

In this paper we describe an experimental approach which allows turgor (p) in an impeded root to be measured without the need to remove the root from the impeding environment. The maximum axial growth pressure (σ_max) generated by completely impeded pea (Pisum sativum L.) roots was measured using a novel apparatus incorporating a force transducer. The apparatus was designed so that it was possible to gain access to the impeded root with the microcapillary of a pressure probe and so obtain in situ measurements of P. Turgor in cells in the apical region of impeded roots was 0.78 MPa, compared with 0.55 MPa in unimpeded roots. In impeded roots, σ_max was 0.52 MPa, showing that the pressure component resulting from cell wall tension (W, where W=P–σ) decreased from 0.55 to 0.26 MPa as the roots became impeded. When impeded roots were removed from the apparatus, there was no decrease in P over the following 90 min. Impedance did not cause P to change in the non-elongating part of the roots further from the apex.     

Function of osteocytes in bone

Although the structural design of cellular bone (i.e., bone containing osteocytes that are regularly spaced throughout the bone matrix) dates back to the first occurrence of bone as a tissue in evolution, and although osteocytes represent the most abundant cell type of bone, we know as yet little about the role of the osteocyte in bone metabolism. Osteocytes descend from osteoblasts. They are formed by the incorporation of osteoblasts into the bone matrix. Osteocytes remain in contact with each other and with cells on the bone surface via gap junction–coupled cell processes passing through the matrix via small channels, the canaliculi, that connect the cell body–containing lacunae with each other and with the outside world. During differentiation from osteoblast to mature osteocyte the cells lose a large part of their cell organelles. Their cell processes are packed with microfilaments. In this review we discuss the various theories on osteocyte function that have taken in consideration these special features of osteocytes. These are (1) osteocytes are actively involved in bone turnover; (2) the osteocyte network is through its large cell-matrix contact surface involved in ion exchange; and (3) osteocytes are the mechanosensory cells of bone and play a pivotal role in functional adaptation of bone. In our opinion, especially the last theory offers an exciting concept for which some biomechanical, biochemical, and cell biological evidence is already available and which fully warrants further investigations. © 1994 Wiley-Liss, Inc.     

Estimation of disruption of animal cells by turbulent capillary flow

Disruption of animal cells in turbulent capillary flows has been predicted from a model of cell-hydrodynamic interactions using cell mechanical properties determined by micromanipulation. Eddies of sizes similar to or smaller than the cells are presumed to interact with those cells, causing local surface deformations. The proposed mechanism of cell damage is that such deformations result in an increase in membrane tension and surface energy and that a cell disrupts when its bursting membrane tension and bursting surface energy are exceeded. The surface energy of the cells is estimated from the kinetic energy of appropriately sized eddies. To test the model, cells were disrupted in turbulent flows in capillaries at mean energy dissipation rates up to 2 x 10(4) m(2)/s(3). In all cases the model underestimated the cell disruption by about 15%. Such good agreement implies that the approach of the model to the complicated phenomena of cell turbulence interactions is reasonable. (c) 1993 John Wiley & Sons, Inc.