Pepperberg plot: Modeling flash response saturation in retinal rods of mouse.

Retinal rods evolved to be able to detect single photons. Despite their exquisite sensitivity, rods operate over many log units of light intensity. Several processes inside photoreceptor cells make this incredible light adaptation possible.

A 3D mathematical model of coupled stem cell-nutrient dynamics in myocardial regeneration therapy.

Stem cell therapy is a promising treatment for the regeneration of myocardial tissue injured by an ischemic event. Mathematical modeling of myocardial regeneration via stem cell therapy is a challenging task, since the mechanisms underlying the processes involved in the treatment are not yet fully understood. Many aspects must be accounted for, such as the spread of stem cells and nutrients, chemoattraction, cell proliferation, stages of cell maturation, differentiation, angiogenesis, stochastic effects, just to name a few.

A model for enhanced and selective transport through biological membranes with alternating pores.

We investigate the outflux of ions through the channels in a cell membrane. The channels undergo an open/close cycle according to a periodic schedule. Our study is based both on theoretical considerations relying on homogenization theory, and on Monte Carlo numerical simulations.

Monte Carlo study of gating and selection in potassium channels.

The study of selection and gating in potassium channels is a very important issue in modern biology. Indeed, such structures are known in essentially all types of cells in all organisms where they play many important functional roles. The mechanism of gating and selection of ionic species is not clearly understood.

Identification of key factors that reduce the variability of the single photon response.

Rod photoreceptors mediate vision in dim light. Their biological function is to discriminate between distinct, very low levels of illumination, i.e.

Kinetics of rhodopsin deactivation and its role in regulating recovery and reproducibility of rod photoresponse.

The single photon response (SPR) in vertebrate phototransduction is regulated by the dynamics of R* during its lifetime, including the random number of phosphorylations, the catalytic activity and the random sojourn time at each phosphorylation level. Because of this randomness the electrical responses are expected to be inherently variable. However the SPR is highly reproducible.

Dynamics of mouse rod phototransduction and its sensitivity to variation of key parameters.

The deep understanding of the biochemical and biophysical basis of visual transduction, makes it ideal for systems-level analysis. A sensitivity analysis is presented for a self-consistent set of parameters involved in mouse phototransduction. The organising framework is a spatio-temporal mathematical model, which includes the geometry of the rod outer segment (ROS), the layered array of the discs, the incisures, the biochemistry of the activation/deactivation cascade and the biophysics of the diffusion of the second messengers in the cytoplasm and the closing of the cyclic guanosine monophosphate (cGMP) gated cationic channels.

Diffusion of the second messengers in the cytoplasm acts as a variability suppressor of the single photon response in vertebrate phototransduction.

The single photon response in vertebrate phototransduction is highly reproducible despite a number of random components of the activation cascade, including the random activation site, the random walk of an activated receptor, and its quenching in a random number of steps. Here we use a previously generated and tested spatiotemporal mathematical and computational model to identify possible mechanisms of variability reduction. The model permits one to separate the process into modules, and to analyze their impact separately.

Modeling the role of incisures in vertebrate phototransduction.

Phototransduction is mediated by a G-protein-coupled receptor-mediated cascade, activated by light and localized to rod outer segment (ROS) disk membranes, which, in turn, drives a diffusion process of the second messengers cGMP and Ca2+ in the ROS cytosol. This process is hindered by disks-which, however, bear physical cracks, known as incisures, believed to favor the longitudinal diffusion of cGMP and Ca2+. This article is aimed at highlighting the biophysical functional role and significance of incisures, and their effect on the local and global response of the photocurrent.

Mathematical model of the spatio-temporal dynamics of second messengers in visual transduction.

A model describing the role of transversal and longitudinal diffusion of cGMP and Ca(2+) in signaling in the rod outer segment of vertebrates is developed. Utilizing a novel notion of surface-volume reaction and the mathematical theories of homogenization and concentrated capacity, the diffusion of cGMP and Ca(2+) in the inter-disc spaces is shown to be reducible to a one-parameter family of diffusion processes taking place on a single rod cross section; whereas the diffusion in the outer shell is shown to be reducible to a diffusion on a cylindrical surface. Moreover, the exterior flux of the former serves as a source term for the latter, alleviating the assumption of a well-stirred cytosol.

Very high collagen increase in placentae of smoking mothers.

In 55 placentae, collected at term from clinically normal pregnancies, hydroxyproline was assayed. Thirty were from mothers who smoke and 25 from nonsmokers. In placentae of smokers, hydroxyproline concentration was 42.

Short communication: placental collagen and premature rupture of fetal membranes.

Collagen content of 43 placentae from clinically normal females who went into labour spontaneously was determined by assaying hydroxyproline. In 23 cases the membranes ruptured before or at the beginning of labour, and in 20 cases the rupture was intrumentally provoked during the expulsive period. In the group with prematurely ruptured membranes, hydroxyproline concentration was 11.

Reduction of hydroxyproline content in the vessels of the human umbilical cord in premature rupture.

The hydroxyproline content of the amnion and the umbilical vessels obtained from cases in which the fetal membranes were instrumentally or prematurely ruptured was assayed. Hydroxyproline concentration (in micrograms/mg) in the latter group was 23.76, 16.

The low hydroxyproline content of prematurely ruptured human fetal membranes.

Since collagen is one of the main factors responsible for the mechanical properties of soft tissue, we have determined the hydroxyproline content of the placental amnion, the free amnion and the chorion of 32 unruptured and 25 prematurely ruptured human fetal membranes. The hydroxyproline content of prematurely ruptured membranes was approximately 50% lower than in unruptured membranes. Hydroxyproline (microgram/mg lyophilized tissue), reported as mean +/- SD, was: 19.