Biomedical subjects

S Papageorgiou

Publications and source records attributed to S Papageorgiou.

29 records · Page 2Linked to original sources

PubMed1984

A hierarchical polar coordinate model for epimorphic regeneration.

In the framework of polar coordinates three rules are postulated which can describe epimorphic regeneration in amphibian limbs. The rules can be seen as an extension of the polar coordinate model. When cells with different positional values are confronted, cell proliferation at the junction restores the continuity of positional values. Reestablishment of continuity is associated with the eventual congruence of the intercalating cell sequence with the host or graft, or both. The intercalating contours can be simple or twisted. The possible contours are graded within a plausible hierarchical scheme where congruent paths are favored versus non-congruent paths and simple contours are favored versus twisted contours. The model correctly predicts the multiplicity, position and different structures of supernumerary outgrowths resulting from both contralateral graftings and 180 degrees ipsilateral limb rotations. Development and regeneration of mirror-symmetric limbs are also accounted for. Several other experimental results are in agreement with the model. Many model predictions and correlations still remain to be tested.

Amphibians↗

PubMed1983

A reaction-diffusion theory of morphogenesis with inherent pattern invariance under scale variations.

In the framework of reaction-diffusion theory we deal with the problem of pattern regulation in morphogenesis. A generic model is proposed where the kinetic terms follow constraints imposed by scale invariance considerations. These constraints allow a class of kinetic schemes to be formulated so that, starting with an initially homogeneous morphogen distribution in the field, a stable gradient is established of the form: S(chi,L) = Lpf(chi/L). Here L is the length of the morphogenetic field, chi is the position variable and f(chi/L) is some monotonic function of the relative distance. With this distribution a scale invariant gradient can be constructed which leads to pattern regulation. A linear stability analysis of the model permits the definition of the parameter values enabling the system to abandon the homogeneous state spontaneously. Simulations of the evolution of the system towards its final stable state result in approximate pattern invariance for different field lengths. The accuracy of this invariance is in agreement with some recent quantitative experimental findings in both developing and regenerating systems.

Animals↗

PubMed1983

The structure of supernumerary limbs formed after 180 degrees blastemal rotation in the newt Triturus cristatus.

The structure of supernumerary limbs formed following 180 degrees ipsilateral blastema rotations in the arm of the newt Triturus cristatus is analysed. Both the skeletal pattern and the muscle patterns are examined. As is the case after comparable experiments in the axolotl (see, for example, Maden & Mustafa, 12982) the extra limbs which form show a range of anatomies. Limbs symmetrical about the dorsal-ventral and anterior-posterior axis are reported as well as some limbs which were part symmetrical and part asymmetrical. It is clear that newts and axolotls appear to react in similar ways to this particular experimental procedure.

Animals↗

PubMed1980

A morphogen gradient model for pattern regulation. I. Formation of non-repetitive and repetitive structures.

A model for pattern formation is proposed based on two concentration gradients S and Sigma. S is a local morphogen generated by a reaction-diffusion mechanism while Sigma is a by-product of the S-decomposition. Under certain conditions S is well approximated by S(x,L) = alpha(L)f(x L ), where alpha(L) is a scaling function of the length L and f(x L ) is a monotonie function of the relative distance x L from the origin. Sigma degradates and diffuses in the field, reaching a stable L-dependent homogeneous distribution. An allosteric protein P with several active sites reacts with S and Sigma and separates the field into segments. To every segment a corresponding active state of P is dominant. Pattern regulation is automatically achieved since the compartmerttal separation depends explicitly only on x L . For the case of repetitive patterns, a supplementary Gierer-Meinhardt mechanism is introduced for activator X and inhibitor Y. The level of Sigma can affect the decomposition rate of X or Y, e.g. via a second order degradation reaction, hence making the chemical wavelength lambda size-dependent. For a particular decay scheme of Y, a variation of L induces a change of lambda so that finally the number of repetitive structures becomes independent of the field size.

Journal Article↗

PubMed1980

A morphogen gradient model for pattern regulation. II. Time description of global morphogen formation and field compartmentalization.

In a model for pattern regulation, use was made of local and global morphogens S and Sigma. Sigma is produced from the S-degradation and it is decomposed by first order kinetics while it diffuses along the field. We solve exactly the partial differential equation for the distribution of Sigma in one spatial dimension when its source S is monotonie (for simplicity, linear or generally a power function). Assuming that S and Sigma react reversibly with an allosteric protein P according to a sequential scheme, we derive the evolution in time of the field separation into compartments. At equilibrium the relative extent of each compartment is constant (for variable field size) and so pattern regulation is achieved.

Journal Article↗

PubMed1978