Cell sociology: a way of reconsidering the current concepts of morphogenesis

A measure of regularity for polygonal mosaics in biological systems

BACKGROUND

The quantification of the spatial order of biological patterns or mosaics provides useful information as many properties are determined by the spatial distribution of their constituent elements. These are usually characterised by methods based on nearest neighbours distances, by the number of sides of cells, or by angles defined by the adjacent cells.

METHODS

A measure of regularity in polygonal mosaics of different kinds in biological systems is proposed. It is based on the condition of eutacticity, expressed in terms of eutactic stars, which is closely related to regularity of polytopes. Thus it constitutes a natural measure of regularity. The proposed measure is tested with numerical and real data. Numerically is tested with a hexagonal lattice that is distorted progressively and with a non-periodic regular tiling. With real data, the distribution of oak trees in forests from three locations in the State of Querétaro, Mexico, and the spiral pattern of florets in a flowering plant are characterised.

RESULTS

The proposed measure performs well and as expected while tested with a numerical experiment, as well as when applied to a known non-periodic tiling of the plane. Concerning real data, the measure is sensitive to the degree of perturbation observed in the distribution of oak trees and detects high regularity in a phyllotactic pattern studied.

CONCLUSIONS

The measure here proposed has a clear geometrical meaning, establishing what regularity means, and constitute an advantageous general purposes alternative to analyse spatial distributions, capable to indicate the degree of regularity of a mosaic or an array of points.

Toward an understanding of immune cell sociology: real-time monitoring of cytokine secretion at the single-cell level

The immune system is a very complex and dynamic cellular system, and its intricacies are considered akin to those of human society. Disturbance of homeostasis of the immune system results in various types of diseases; therefore, the homeostatic mechanism of the immune system has long been a subject of great interest in biology, and a lot of information has been accumulated at the cellular and the molecular levels. However, the sociological aspects of the immune system remain too abstract to address because of its high complexity, which mainly originates from a large number and variety of cell-cell interactions. As long-range interactions mediated by cytokines play a key role in the homeostasis of the immune system, cytokine secretion analyses, ranging from analyses of the micro level of individual cells to the macro level of a bulk of cell ensembles, provide us with a solid basis of a sociological viewpoint of the immune system. In this review, as the first step toward a comprehensive understanding of immune cell sociology, cytokine secretion of immune cells is surveyed with a special emphasis on the single-cell level, which has been overlooked but should serve as a basis of immune cell sociology. Now that it has become evident that large cell-to-cell variations in cytokine secretion exist at the single-cell level, we face a tricky yet interesting question: How is homeostasis maintained when the system is composed of intrinsically noisy agents? In this context, we discuss how the heterogeneity of cytokine secretion at the single-cell level affects our view of immune cell sociology. While the apparent inconsistency between homeostasis and cell-to-cell heterogeneity is difficult to address by a conventional reductive approach, comparison and integration of single-cell data with macroscopic data will offer us a new direction for the comprehensive understanding of immune cell sociology.

Beta-catenin and axis formation in planarians

In three recent articles it was shown that beta-catenin is crucial for the establishment and the maintenance of the overall polarity and especially for the character 'posterior' in planarians. If the transcription of the beta-catenin gene was silenced by RNA interference, the overall polarity is lost, and in regenerating fragments a posterior blastema displays anterior characters by forming eyes and anterior ganglia. An attempt is made to integrate these new data, well-known older observations, and observations from other regenerating systems into an outline of a model that will clarify our current understanding as well as highlight areas still to be developed.

Segmentation of tissue architecture by distance graph matching

BACKGROUND

Characterization of tissues can be based on the topographical relationship between the cells. Such characterization should be insensitive to distortions intrinsic to the acquisition of biological preparation. In this paper, a method for the robust segmentation of tissues based on the spatial distribution of cells is proposed.

MATERIALS AND METHODS

The neighborhood of each cell in the tissue is modeled by the distances to the surrounding cells. Comparison with an example or prototype neighborhood reveals topographical similarity between tissue and prototype. Processing of all cells in the tissue extracts the regions with tissue architecture similar to the given example.

RESULTS

Comparison with other topographical-segmentation methods shows that the proposed method is better suited for partitioning tissue architecture. As an example, the quantification of the structural integrity in rat hippocampi after ischemia is demonstrated. In contrast to other methods, the algorithm correlates well with expert evaluation.

CONCLUSIONS

The present method reduces the nonbiological variation in the analysis of tissue sections and thus improves confidence in the result. The method can be applied to any field where regular patterns have to be detected, as long as the directional distribution of neighbors may be neglected.

Brain growth and differentiation in two fetal bats: qualitative and quantitative aspects

The development of the main brain components in two fetal bats (one insectivorous and one frugivorous type) is studied quantitatively and comparatively. The telencephalon, cerebellum, and diencephalon grow faster in the frugivorous bat. The brainstem maintains a greater relative volume throughout in the insectivorous type. The morphology and density of neurons in the cerebellum, caudate nucleus, olfactory mitral stratum, and neocortical layer II suggest that there exists an initial delay in development in the frugivorous bat; through subsequent reordering, however, it becomes more advanced in development, in accordance with the more progressive status of the adult forms in its category. In this there may be a certain ontogenetic recapitulation of a possible chiropteran phylogeny. These comparisons also point to the precociousness of the functional determination of those structures that are related to future behavior in a purely deterministic, non-teleological sense. The qualitative differentiation appears to be more significant than might be expected in comparison with the quantitative proliferation. Technical problems involved in the study of brain morphogenesis are discussed, especially the difficulty of distinguishing the respective effects of cell multiplication, cell death, and artificial increase in cell density through tissue shrinkage.

From DNA transcription to visible structure: what the development of multicellular animals teaches us

This article is concerned with the problem of the relation between the genetic information contained in the DNA and the emergence of visible structure in multicellular animals. The answer is sought in a reappraisal of the data of experimental embryology, considering molecular, cellular and organismal aspects. The presence of specific molecules only confers a tissue identity on the cells when their concentration exceeds the 'threshold of differentiation'. When this condition is not fulfilled the activity of the genes that code for the specific molecules in question only confers on them a histogenetic potency, i.e. the capacity to form the corresponding tissue in further development (or to trans-differentiate to that tissue). The progressive restriction of histogenetic potencies during development reflects the irreversible repression of more and more genes. The establishment of a given tissue identity under the influence of an inducing tissue (or a morphogenetic hormone) is only possible when the cells have acquired the competence to respond. Tissue differentiation proceeds progressively during development thanks to the cytoplasmic 'memory' that cells retain collectively (or sometimes individually) of the items of information successively registered by their ancestors cells. The increasing complexity of visible structure emerging during development results only from the progression of tissue differentiation. This involves continual exchange of information among the cells and leads to (1) cell displacements and rearrangements, particularly during organogenesis and (2) extreme diversification of cell individualities within tissues, particularly during postembryonic growth. A mutation (just as a teratogenic factor) evokes an anomaly that is localized in both space and time because it alters a certain aspect of cell behaviour (particularly cell surface adhesiveness or mitotic activity) at the time when this is involved in the establishment of a particular structural trait. Neither the organization of the adult nor the modalities of development are encoded in the DNA. The automatic concatenation of cell interactions in the embryo and the structural amplification it entails is conditioned by the specific biochemical composition of the cytoplasm of the egg and by the heterogeneous distribution of its inclusions.

The problem of automation in animal development: confrontation of the concept of cell sociology with biochemical data

The principles of automation in animal development, as previously inferred from the concept of Cell Sociology do not fit in well with the current concept of sequential genet derepression. A more adequate explanation for those principles has been found in the literature dealing with the biochemical aspects of differentiation. Since oocytes and embryonic cells contain a greater variety of mRNAs than differentiated cells, as well as many tissue-specific (luxury) substances, it is concluded that the diversification of tissues consists of a progressive selection of specific metabolic strategies, mediated by cell-to-cell contacts, from a broad range of pre-existing strategies. For each tissue, prior to its final determination, one luxury metabolic strategy is progressively intensified and becomes dominant. The others are either suppressed or maintained as latent metabolic strategies. The latter may on occasion become dominant again (transdifferentiation). These phenomena require a theory which considers gene regulation as the activation of otherwise repressed genes by specific activator RNAs. The high (apparently maximal) transcriptional activity on the lampbrush chromosomes may represent the synthesis of all the kinds of activator RNAs which are required for the reactivation of the genes during early development. A general conception is propounded of the automatism and programming of animal development, as inferred from the confrontation of these ideas with the concept of Cell Sociology.

Cell sociology and the problem of automation in the development of pluricellular animals

The principles of automation (automatism and programming) in the unfolding of spatiotemporal patterns during animal development are deduced from experimental data reconsidered from the point of view of cell sociology. The developmental programme in the egg is not part of the genetic information but a part of the cytoplasmic information. Throughout development cells store extra-cellular information released by their neighbours in the form of cytoplasmic information. Successive determinations cannot be considered as successive reprogrammings of cells: each one consists of a selection of one specific programme from the total information previously stored. This programme specifies cell interactions in the determined population as a whole; it is very imprecise and is progressively completed during the course of further differentiation by information released by neighbouring cell populations. Complicated patterns may emerge from only two homogeneous populations involved in distinct differentiation pathways and confronting each other. Consequently the "egg developmental programme" provides gene effectors and specific physico-chemical conditions necessary for the staring of at least two distinct differentation pathways. Experimental data suggest that there are two components in this programme. One is a molecular machinery which starts at fertilization in the whole cytoplasm. It yields two programmes of differentiation, typically first an endodermal and then an ectodermal one. The other component of the egg developmental programme, which does not require specific information, allows the interception of the first (endodermal) programme. The application of informatics to developmental automatism is discussed in the latter part of the paper.

Cell sociology and the problem of position effect: pattern formation, origin and role of gradients

The control of pattern formation and the significance of gradients is reconsidered on the basis of the concept of cell sociology (which takes into account continuous exchange of information between cells and the possibility of autonomous progression in differentiation). Not all traits of a pattern are imposed by a single prepattern, which would be an organized molecular framework or a gradient. Patterns are unfolded in steps; these are readjustments of a cell population to intrinsic and extrinsic changes in cell activities. Prepatterns are the various components of the programme of every readjustment and are established by information of various origins, which can be dissociated experimentally: determination (elementary social prepattern), preexisting organization (antecedent pp.), surrounding cell populations (environmental pp.), position among other tissues (positional pp.) and the organization of inducers (imprinting pp.). Every transitory pattern formed during a readjustment serves as antecedent pp. during the next readjustment. Covert graded patterns result from various aspects of the social behaviour of cells (growth, aggregation, induction, cell renewal) and may serve as antecedent or imprinting prepatterns. They appear as 'water marks' in the final patterns, or generate overt graded patterns. They also manifest themselves in temporal patterns, particularly in gradients of relative growth.