Abstract—This is the second part of a review; it is devoted to the methods of studying ultraweak photon emissions, presents a methodology of biological detection. During the first 25 years of research (1923–1948), almost all experiments in this area were based on biological detection and only the component belonging to the middle ultraviolet region of the spectrum (also called mitogenetic radiation at that time) was studied, because it was observed due to the influence of ultraweak ultraviolet radiation on the mitotic regime of the biological detector. These experiments remain open to debate till present due to the insufficient validation of methods. Many of the results obtained from these investigations were very impressive for that time period and are now verified, the research questions the other results relate to are of considerable interest (for instance, fundamental results concerning cell division and carcinogenesis, early cancer diagnostics). Thus, the analysis of the methodology employed is an emerging issue for the subsequent experimental verification. The details of the methods with the most common biological detectors (onion roots, yeast and bacterial cultures) are analyzed and the onion detection method is compared to the closest analogue (the Allium test).

This review is devoted to the methods for studying the ultraweak luminescence of biological objects; in addition to modern methods aimed at studies in the visible, near-infrared, and near-ultraviolet ranges, considerable attention is paid to the analysis of the methodology for studying the component belonging to the middle ultraviolet range (called mitogenetic radiation in early works). Studies of mitogenetic radiation were carried out in 1923–1948 and remain partially controversial. Many of the results, which were revolutionary for that time, have already been confirmed and the scientific problems of the rest are of significant interest (for example, fundamental results concerning cell division and carcinogenesis and early cancer diagnostics); therefore, the analysis of their methodology is relevant for further experimental verification of these studies. The first part of the review covers the common questions like the history of research and development of methodology, types and properties of ultra-weak photon emission, basic terms, fundamental and application significance of research (the second and the third parts will review biological and physical methods).

We start by comparing two explicatory approaches, which we call law-centered and “instructivistic”. Although the latter dominates in modern biology, we find it inappropriate for treating developmental problems, especially those related to morphogenesis. As an example of a law-centered approach we suggest a simple morphomechanical rule based upon the idea of the hyper-restoration of mechanical stresses. We show that this rule not only provides a general framework for morphogenesis, but can also reproduce, via parametric modulations, quite specific developmental events.

Factors of embryonic polarization in Clava multicornis and Obelia loveni and cellular mechanisms of growth and morphogenesis of vegetal generation mainly in Obelia loveni and Dynamena pumila were studied. Morphological axis of embryo was found to be not determined irreversibly up to morula stage. Unequal distribution of mitotic divisions was demonstrated to be a main cause of embryo elongation. No definite relations were registered between shape and/or growth rate of vegetal rudiments and the characteristics of proliferative processes. Using time-lapse methods, a pulsatory character of growth and shapening was revealed. Growth pulsations are underlined by periodical contractions-extensions of densely packed cells of ectoand entoderm. Some modes of behaviour of a system, composed by contractile, densely packed cells, are formulated. A number of normal and experimentally modified morphogenetic processes in Hydrozoa may be regarded as direct consequences of the properties of such systems.

BACKGROUND:Although the role of endogenous mechanical stresses in regulating morphogenetic movements and cell differentiation is now well established, many aspects of mechanical stress generation and transmission in developing embryos remain unclear and require quantitative studies.RESULTS:By measuring stress-bearing linear deformations (caused by differences in cell movement rates) in the outer cell layer of blastula - early tail-bud Xenopus embryos, we revealed a set of long-term tension-generating gradients of cell movement rates, modulated by short-term cell-cell displacements much increasing the rates of local deformations. Experimental relaxation of tensions distorted the gradients but preserved and even enhanced local cell-cell displacements. During development, an incoherent mode of cell behavior, characterized by extensive cell-cell displacements and poorly correlated cell trajectories, was exchanged for a more coherent regime with the opposite characteristics. In particular, cell shifts became more synchronous and acquired a periodicity of several dozen minutes.CONCLUSIONS:Morphogenetic movements in Xenopus embryos are mediated by mechanically stressed dynamic structures of two different levels: extended gradients and short-term cell-cell displacements. As development proceeds, the latter component decreases and cell trajectories become more correlated. In particular, they acquire common periodicities, making morphogenesis more coherent.

The laboratory is engaged in morphomechanics—the study of self-organization of mechanicalforces that create the shape and structure of the embryonic primordia. As part of its work, the laboratorydescribed pulsating modes of mechanical stresses in hydroids, identified and mapped mechanical stresses inthe tissues of amphibian embryos, and studied morphogenetic reorganization caused by the relaxation andreorientation of tensions. The role of mechanical stresses in maintaining the orderly architectonics of theembryo is shown. Mechano-dependent genes are detected. Microstrains of embryonic tissues and stress gradientsassociated with them are described. A model of hyper-recovery of mechanical stresses as a possibledriving force of morphogenesis is proposed.

Morphogenesis in living tissues is the paramount example of a time-and space-dependent orchestration of living matter where shape and order emerge from undifferentiated initial conditions. The genes encode the protein expression that eventually drives the emergence of the phenotype, while energy supply and cell-to-cell communication mechanisms are necessary to such a process. The overall control of the system likely exploits the laws of chemistry and physics through robust and universal processes. Even if the identification of the communication mechanisms is a question of fundamental nature, a long-standing investigation settled in the realm of chemical factors only (also known as morphogens) faces a number of apparently unsolvable questions. In this paper, we investigate at what extent mechanical forces, alone or through their biological feedbacks, can direct some basic aspects of morphogenesis in development biology. In this branch of mechano-biology, we discuss the typical rheological regimes of soft living matter and the related forces, providing a survey on how local mechanical feedbacks can control global size or even gene expression. We finally highlight the pivotal role of nonlinear mechanics to explain the emergence of complex shapes in living matter.

The presence and morphogenetic role of relatively autonomous pacemakers (“clocks”) during ontogeny is discussed. Although autonomous pacemakers in the strict sense seem nonexistent, the study of temporal processes is of great importance for recognizing the fundamental morphogenetic mechanisms. The most important and insufficiently understood questions in this field include the presence of hierarchies of characteristic times and their smoothing during critical developmental periods and also regulation of spatial processes via temporal delays.

A continuum model of the embryonic epithelial tissue with account for the active deformations and rearrangements of the cells is proposed. The stress tensor is represented as the sum of the stresses undergone by the cell directly and the tensor of active stresses that arise owing to contracting cellular protrusions anchored on the surface of neighboring cells and developing in response to cell reshaping (deformation). The strain rate tensor includes three components: elastic and two inelastic related to the active deformation of the cells and their rearrangement. The first of these components depends on the stresses in the cells and the reached cellular deformation level, whereas the second is determined by the active stresses. The problem of reaction of a thin sheet to a rapid stretching is solved and agreement with experimental data is obtained.

The application of the symmetry theory to the principles of individual development is discussed. Reductions of the symmetry order, relations of different structural levels of symmetry and «symmetry exchange» between symmetry transformations, change of free energy, and entropy in the course of real and simulated morphogenesis are considered. The possibility of parameter regulation of the symmetry order is discussed.

A fundamental problem of morphogenesis is whether it presents itself as a succession of links that are each driven by its own specific cause-effect relationship, or whether all of the links can be embraced by a common law that is possible to formulate in physical terms. We suggest that a common biophysical background for most, if not all, morphogenetic processes is based upon feedback between mechanical stresses (MS) that are imposed to a given part of a developing embryo by its other parts and MS that are actively generated within that part. The latter are directed toward hyper-restoration (restoration with an overshoot) of the initial MS values. We show that under mechanical constraints imposed by other parts, these tendencies drive forth development. To provide specificity for morphogenetic reactions, this feedback should be modulated by long-term parameters and/or initial conditions that are set up by genetic factors. The experimental and model data related to this concept are reviewed.

Active reactions of embryonic tissues to mechanical forces play an important role in morphogenesis. To study these reactions, experimental models that enable to evaluate the applied forces and the deformations of the tissues are required. A model based upon the active intrusion of a living early gastrula Xenopus embryo into a tube half the embryo in diameter is described. The intrusion is initially triggered by a suction force of several dozen Pa but then continues in the absence of external driving force, stopping immediately after the entire embryo has penetrated into the tube. The process can be stopped by cytoskeletal drugs or by the damage of the part of the embryo still non-aspirated and is associated with the transversal contraction and meridional elongation of the non-aspirated part of the embryo surface and quasi-periodic longitudinal contractions/extensions of the cells within the part already aspirated. We suggest that this reaction is an active response to the embryo deformation and discuss its morphogenetic role. The problem of estimating the elastic modules of embryonic tissues is also discussed.

The ideographical approach aimed at detecting specific causative relationships within the process of development prevails in modern embryology. The present work considers the possibilities of using the nomothetic approach aimed at putting forward nonspecific general laws based on the general scientific theory of self-organization and can be formulated in morphomechanical terms based on feedback links between passive and active mechanical stress. The perspectives of this approach and the involvement of genetic factors in the regulation of feedback links are discussed.

With the help of a suction manometric device, the relation between the deformation of Xenonus laevis embryo at the gastrula and neurula stages and the value of the applied force has been studied. Stiffness modules of embryonic tissues were in the order of several dozens of Pascal and they were inversely proportional during deformation from 40 to 20%. At the gastrula stage, a uniform or an increasing rate of expansion of the embryo body in the suction capillary with the diameter of approximately half that of the embryo was observed for 30 min after the action of the suction forces. The length of the stretched portion of the embryo correlates with the value of its deformation at the first minute. As a result of the expansion, the total body surface area of the deformed embryo increases more than twice compared to intact embryos. After expelling the embryo from the capillary, its surface reduced and the deformation became smoothened within 5 min, which indicates the existence of tensional force in the expanded embryo. These data confirm that, at the embryo gastrula stage, external mechanical forces do not only passively deform the embryo but also initiate the active expansion of the embryo which takes place at zero external force and overcomes the tensional resistance of tissues. The mechanism of active expansion and its link with the processes of normal morphogenesis are discussed.

The aim of this essay is to review what we know about the transformation of chemical energy into mechanical, electrical and photonic at the different scales of biological organization. We start from the "classical", short-range mechanoelectrical protein machines emphasizing their capacity to slow down the rate of energy relaxation and to concentrate energy onto a restricted number of freedom degrees. Then we pass to the newly described "low entropy machines" and to the macroscopic electromechanical machines which create circuits of the organismal scales. At last, we come to photonic events, paying a special attention to their regular periodicity within several Hz range and to their relations with cytoskeletal structures and their developmental dynamics. We suggest, that this area of investigations should be related with the theory of self-organization and the notion of coherency.

The mutual arrangement of neural and mesodermal rudiments in artificially bent double explants of Xenopus laevis suprablastoporal areas was compared with that of intact explants. While some of the bent explants straightened or became spherical, most retained and actively reinforced the imposed curvature, creating folds on their concave sides and expanding convex surfaces. In the intact explants, the arrangement of neural and mesodermal rudiments exhibited a distinct antero‐posterior polarity, with some variability. In the bent explants, this polarity was lost: the neural rudiments were shifted towards concave while the mesodermal tissues moved towards the convex side, embracing the neural rudiments in a horseshoe‐shaped manner. We associate these drastic changes in neuro‐mesodermal patterning with the active extension and contraction of the convex and concave sides, respectively, triggered by the imposed deformations. We speculate that similar events are responsible for the establishment of neuro‐mesodermal patterns during normal development. Developmental Dynamics 239:885–896, 2010. © 2010 Wiley‐Liss, Inc.

The paper presents the results of statistical evaluation of the changes of cellular apex connections, apical angles, and apical indices of ventral cells of the epiectodermal gastrula of Xenopus during the first four hours after the relaxation of mechanical tension. In the unrelaxed epithelium, an overwhelming majority of cells have three apical connections, apical angles close to 120°, and apical indices around one (isodiametric cells); after relaxation, the number of cells with more than three connections, the number of apical angles deviating substantially from 120°, and the percentage of columnar cells with high apical index increase. Apices with more than three connections tend to gather in enclosed groups, forming a straightened line of cell walls. The length and curvature of cell walls with four apical connections significantly exceeds those same indicators for cells with three apical connections. The observed changes in topology and geometry of cells correspond to reconstructions observed during normal morphogenesis. They are considered in terms of the hyper-restoration model of mechanical tension in relaxed epithelial layers.

Sandwich explants of the suprablastoporal area of Xenopus early-mid gastrula and same stages of entire embryos were stretched with two needles perpendicular to the direction of natural elongation of the axial rudiments. The changes in the embryonic shape and histological structure were monitored as well as the arrangement of descendants of one of dorsal blastomers labeled with fluorescein-dextran at the 16-cell stage. A substantial fraction of stretched explants reoriented along the applied stretch direction. The arrangement dynamics of fluorescein-dextran-labeled cells and explant shape demonstrate that this is an active response based on convergent intercalation of cells induced by stretching. Stretched gastrulae demonstrated arrested gastrulation, dorsoventral extension of the blastopore, and ventral flow of labeled cells towards the lateral lips of the blastopore, which was also mediated by convergent intercalation and tensotaxis. The obtained data are discussed in terms of the hypothesis of mechanical stress hyper-restoration.