85 research outputs found
Mechanical Properties of Growing Melanocytic Nevi and the Progression to Melanoma
Melanocytic nevi are benign proliferations that sometimes turn into malignant
melanoma in a way that is still unclear from the biochemical and genetic point
of view. Diagnostic and prognostic tools are then mostly based on dermoscopic
examination and morphological analysis of histological tissues. To investigate
the role of mechanics and geometry in the morpholgical dynamics of melanocytic
nevi, we study a computation model for cell proliferation in a layered
non-linear elastic tissue. Numerical simulations suggest that the morphology of
the nevus is correlated to the initial location of the proliferating cell
starting the growth process and to the mechanical properties of the tissue. Our
results also support that melanocytes are subject to compressive stresses that
fluctuate widely in the nevus and depend on the growth stage. Numerical
simulations of cells in the epidermis releasing matrix metalloproteinases
display an accelerated invasion of the dermis by destroying the basal membrane.
Moreover, we suggest experimentally that osmotic stress and collagen inhibit
growth in primary melanoma cells while the effect is much weaker in metastatic
cells. Knowing that morphological features of nevi might also reflect geometry
and mechanics rather than malignancy could be relevant for diagnostic purpose
Volume changes during active shape fluctuations in cells
Cells modify their volume in response to changes in osmotic pressure but it
is usually assumed that other active shape variations do not involve
significant volume fluctuations. Here we report experiments demonstrating that
water transport in and out of the cell is needed for the formation of blebs,
commonly observed protrusions in the plasma membrane driven by cortex
contraction. We develop and simulate a model of fluid mediated membrane-cortex
deformations and show that a permeable membrane is necessary for bleb formation
which is otherwise impaired. Taken together our experimental and theoretical
results emphasize the subtle balance between hydrodynamics and elasticity in
actively driven cell morphological changes.Comment: Phys. Rev. Lett. in press. 13 pages 4 figures, 9 supplementary
figure
Protein-driven lipid domain nucleation in biological membranes
Lipid rafts are heterogeneous dynamic lipid domains of the cell membranes that are involved in several biological processes, such as protein and lipid specific transport and signaling. Our understanding of lipid raft formation is still limited due to the transient and elusive nature of these domains in vivo, in contrast with the stable phase-separated domains observed in artificial membranes. Inspired by experimental findings highlighting the relevance of transmembrane proteins for lipid rafts, we investigate lipid domain nucleation by coarse-grained molecular dynamics and Ising-model simulations. We find that the presence of a transmembrane protein can trigger lipid domain nucleation in a flat membrane from an otherwise mixed lipid phase. Furthermore, we study the role of the lipid domain in the diffusion of the protein showing that its mobility is hindered by the presence of the raft. The results of our coarse-grained molecular-dynamics and Ising-model simulations thus coherently support the important role played by transmembrane proteins in lipid domain formation and stability
Acoustic emission (AE) as a diagnostic tool in geophysics
Acoustic Emissions (AE) are effective for monitoring ground deformation and temporal variation of its porosity. AE are complementary to seismic information, related to the same area, though AE and earthquakes focus on observational evidence concerned with substantially different space- and time-scales. AE information is pertinent (i) either for geodynamically stable areas, where it probes the diurnal thermal and/or tidal deformation, (ii) or for seismic areas where it provides some as yet unexploited precursors, (iii) or for volcanic areas, where it appears capable of recognising precursors originated by some hot fluid that penetrates by diffusion into rock pores, from those associated with eventual plutonic magma intrusions, (iv) and also for monitoring periods of time during which a volcano is «inflated» by underground hot fluids compared to others during which it «deflates». Upon direct comparison between 6 data sets concerned with different physical settings, it seems to be possible (fig. 3 and table II] to distinguish a few significantly different behaviours associated either (i) with a mere compression (such as it occurs for Stromboli, Vesuvius, and a sample compressed in the laboratory), or (ii) with a slip strain, such as it typically occurs in association with faulting or with diurnal thermal rock deformation
OKULASI TANAMAN DURIAN “Durio zibethinus Murr.” DENGAN ASAL TUNAS BATANG ATAS DAN CARA PEMOTONGAN BATANG BAWAH
Durian memiliki prospek ekonomi yang bagus sebab pemasaran meningkat
dari tahun ke tahun, akan tetapi ketersediaan benih bermutu masih terbatas.
Penyediaan benih bermutu perlu dilakukan dengan cara gabungan (vegetatif dan
generatif) salah satunya adalah okulasi. Pertumbuhan okulasi dipengaruhi oleh
asal tunas yang menunjukkan tingkat ketuaan batang dan pemotongan batang
bawah yang akan menekan pertumbuhan tunas lateral, sehingga perlu dilakukan
pemotongan dengan ukuran yang tepat. Tujuan dari penelitian ini untuk
mempelajari asal tunas dan mendapatkan panjang pemotongan batang bawah yang
tepat untuk mempercepat pertumbuhan tunas okulasi serta mendapatkan
kesesuaian antara asal tunas batang atas dengan pemotongan batang bawah
terhadap pertumbuhan tunas okulasi.
Penelitian dilaksanakan bulan Januari sampai Mei 2011 berdasarkan
percobaan faktorial dengan rancangan acak lengkap, diulang empat kali, yang
terdiri atas dua faktor yaitu asal entres (ujung, tengah, pangkal) dan pemotongan
batang bawah (dipotong ½ batang, 10 cm dari ujung, 15 cm dan 5 cm di atas
okulasi), sehingga diperoleh 12 kombinasi perlakuan. Variabel pengamatan
meliputi jumlah keberhasilan okulasi, saat pecah mata entres, saat kemunculan
daun pertama, tinggi tunas okulasi, jumlah daun, keberhasilan okulasi tumbuh.
Hasil penelitian menunjukkan entres ujung diawal pertumbuhan tumbuh lebih
cepat dibandingkan yang lain meskipun selanjutnya entres tengah memberikan
pertumbuhan yang lebih baik. Panjang pemotongan batang bawah yang tepat
adalah 10 cm dari ujung batang bawah. Secara keseluruhan perbanyakan vegetatif
dengan okulasi menggunakan entres tengah dan pemotongan batang bawah 10 cm
dari ujung menghasilkan pertumbuhan yang lebih baik.
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Conformal approach to cylindrical DLA
We extend the conformal mapping approach elaborated for the radial Diffusion
Limited Aggregation model (DLA) to the cylindrical geometry. We introduce in
particular a complex function which allows to grow a cylindrical cluster using
as intermediate step a radial aggregate. The grown aggregate exhibits the same
self-affine features of the original cylindrical DLA. The specific choice of
the transformation allows us to study the relationship between the radial and
the cylindrical geometry. In particular the cylindrical aggregate can be seen
as a radial aggregate with particles of size increasing with the radius. On the
other hand the radial aggregate can be seen as a cylindrical aggregate with
particles of size decreasing with the height. This framework, which shifts the
point of view from the geometry to the size of the particles, can open the way
to more quantitative studies on the relationship between radial and cylindrical
DLA.Comment: 16 pages, 8 figure
Langevin formulation for single-file diffusion
We introduce a stochastic equation for the microscopic motion of a tagged
particle in the single file model. This equation provides a compact
representation of several of the system's properties such as
Fluctuation-Dissipation and Linear Response relations, achieved by means of a
diffusion noise approach. Most important, the proposed Langevin Equation
reproduces quantitatively the \emph{three} temporal regimes and the
corresponding time scales: ballistic, diffusive and subdiffusive.Comment: 9 pages, 5 figures, 1 table, to appear in Physical Review
Correlations in a Generalized Elastic Model: Fractional Langevin Equation Approach
The Generalized Elastic Model (GEM) provides the evolution equation which
governs the stochastic motion of several many-body systems in nature, such as
polymers, membranes, growing interfaces. On the other hand a probe
(\emph{tracer}) particle in these systems performs a fractional Brownian motion
due to the spatial interactions with the other system's components. The
tracer's anomalous dynamics can be described by a Fractional Langevin Equation
(FLE) with a space-time correlated noise. We demonstrate that the description
given in terms of GEM coincides with that furnished by the relative FLE, by
showing that the correlation functions of the stochastic field obtained within
the FLE framework agree to the corresponding quantities calculated from the
GEM. Furthermore we show that the Fox -function formalism appears to be very
convenient to describe the correlation properties within the FLE approach
Fingered growth in channel geometry: A Loewner equation approach
A simple model of Laplacian growth is considered, in which the growth takes
place only at the tips of long, thin fingers. In a recent paper, Carleson and
Makarov used the deterministic Loewner equation to describe the evolution of
such a system. We extend their approach to a channel geometry and show that the
presence of the side walls has a significant influence on the evolution of the
fingers and the dynamics of the screening process, in which longer fingers
suppress the growth of the shorter ones
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