Intercalary regeneration in legs of crayfish: distal segments

Cytology, function and dynamics of stem and progenitor cells in decapod crustaceans

Stem cells play key roles in development, tissue homeostasis, regeneration, ageing and diseases. Comprehensive reviews on stem cells are available for the determinately growing mammals and insects and some lower invertebrates like hydra but are rare for larger, indeterminately growing invertebrates that can live for many decades. This paper reviews the cytology, function and dynamics of stem and progenitor cells in the decapod crustaceans, a species-rich and ecologically and economically important animal group that includes mainly indeterminate growers but also some determinate growers. Further advantages of decapods for stem cell research are almost 1000-fold differences in body size and longevity, the regeneration of damaged appendages and the virtual absence of age-related diseases and tumours in the indeterminately growing species. The available data demonstrate that the Decapoda possess a remarkable variety of structurally and functionally different stem cells in embryos and larvae, and in the epidermis, musculature, haematopoietic tissue, heart, brain, hepatopancreas, olfactory sense organs and gonads of adults. Some of these seem to be rather continuously active over a lifetime but others are cyclically activated and silenced in periods of days, weeks and years, depending on the specific organ and function. Stem cell proliferation is triggered by signals related to development, moulting, feeding, reproduction, injury, infection, environmental enrichment and social status. Some regulatory pathways have already been identified, including the evolutionarily conserved GATA-binding and runt-domain transcription factors, the widespread neurotransmitter serotonin, the arthropod-specific hormone 20-hydroxyecdysone and the novel astakine growth factors. Knowledge of stem cells in decapods primarily refines our picture on the development, growth and maintenance of tissues and organs in this animal group. Cultured decapod stem cells have good potential for toxicity testing and virus research with practical relevance for aquaculture. Knowledge of stem cells in decapods also broadens our understanding of the evolution of stem cells and regeneration in the animal kingdom. The stem cells of long-lived, indeterminately growing decapods may hold the key to understanding how stem and progenitor cells function into old age without adverse side effects, possibly evoking new ideas for the development of anti-ageing and anti-cancer treatments in humans.

A bioinformatics expert system linking functional data to anatomical outcomes in limb regeneration

Amphibians and molting arthropods have the remarkable capacity to regenerate amputated limbs, as described by an extensive literature of experimental cuts, amputations, grafts, and molecular techniques. Despite a rich history of experimental efforts, no comprehensive mechanistic model exists that can account for the pattern regulation observed in these experiments. While bioinformatics algorithms have revolutionized the study of signaling pathways, no such tools have heretofore been available to assist scientists in formulating testable models of large-scale morphogenesis that match published data in the limb regeneration field. Major barriers preventing an algorithmic approach are the lack of formal descriptions for experimental regenerative information and a repository to centralize storage and mining of functional data on limb regeneration. Establishing a new bioinformatics of shape would significantly accelerate the discovery of key insights into the mechanisms that implement complex regeneration. Here, we describe a novel mathematical ontology for limb regeneration to unambiguously encode phenotype, manipulation, and experiment data. Based on this formalism, we present the first centralized formal database of published limb regeneration experiments together with a user-friendly expert system tool to facilitate its access and mining. These resources are freely available for the community and will assist both human biologists and artificial intelligence systems to discover testable, mechanistic models of limb regeneration.

Ecdysteroid receptor signaling disruption obstructs blastemal cell proliferation during limb regeneration in the fiddler crab, Uca pugilator

To study ecdysteroid signaling during limb regeneration, we have applied RNAi (dsRNA) mediated silencing to EcR/RXR, the genes encoding the ecdysteroid receptor heterodimer, in the fiddler crab Uca pugilator. We injected RNAi into the blastemal chamber during early limb regeneration. Silencing was evaluated by knockdown in receptor transcript abundance, and disruption was evaluated by changes in growth rate and morphology of limb regenerates. q-PCR results indicated a 50% drop in transcript abundance 48h post injection in both RNAi (dsEcR/dsRXR) injected ipsilateral and uninjected contralateral blastemas in experimental animals relative to controls. EcR/RXR transcript levels further decreased over time. Several phenotypes were associated with knockdown. The experimental blastema failed to develop; microscopic examination of the arrested blastema revealed an absence of the cuticular ingrowths characteristic of the beginnings of limb segmentation and cell proliferation assays revealed that the arrested blastema had few dividing cells. Ecdysteroid levels were also lowered in experimental animals; given the bilateral effects of RNAi on limb buds in experimental animals, these results suggest RNAi had a systemic effect. Although hormone titers in experimental animals rose to comparable control levels during the late proecdysial phase of limb regeneration, most experimental crabs failed to molt and died. The overall failure to molt indicates that RNAi receptor knockdown has long-term effects. The combined effects of receptor knockdown indicate that, although circulating ecdysteroid titers are normally low during basal limb bud growth, signaling via the ecdysteroid receptor pathway is necessary for establishment of blastemal cell proliferation and development in the regenerating limbs of U. pugilator.

Morphogenetic fields in embryogenesis, regeneration, and cancer: non-local control of complex patterning

Establishment of shape during embryonic development, and the maintenance of shape against injury or tumorigenesis, requires constant coordination of cell behaviors toward the patterning needs of the host organism. Molecular cell biology and genetics have made great strides in understanding the mechanisms that regulate cell function. However, generalized rational control of shape is still largely beyond our current capabilities. Significant instructive signals function at long range to provide positional information and other cues to regulate organism-wide systems properties like anatomical polarity and size control. Is complex morphogenesis best understood as the emergent property of local cell interactions, or as the outcome of a computational process that is guided by a physically encoded map or template of the final goal state? Here I review recent data and molecular mechanisms relevant to morphogenetic fields: large-scale systems of physical properties that have been proposed to store patterning information during embryogenesis, regenerative repair, and cancer suppression that ultimately controls anatomy. Placing special emphasis on the role of endogenous bioelectric signals as an important component of the morphogenetic field, I speculate on novel approaches for the computational modeling and control of these fields with applications to synthetic biology, regenerative medicine, and evolutionary developmental biology.

A model of morphogenetic pattern formation

A model for the morphogenetic movement of surfaces composed of cellular monolayers is proposed. The cells are presumed joined at their lateral surfaces. An otherwise unspecified substance called a "morphogen" is introduced which is the agent of change in the individual cell (or cell-like region). The distribution of these cellular deformations define a surface (the middle surface, through the middle of the cell heights) via equations given for the Gauss and Mean curvatures of the surface defined at each point. The Gauss curvature as a function of the morphogen level determines the metric of the surface "g(u, v)" in conformal co-ordinates u, v. A unique equation for the morphogen distribution over the survace is presented which has the property of size invariance, that is, the model "regulates" without need of further arguments. The two resulting coupled equations for the metric and the morphogen, eqns (4) and (2), both non-linear equations, are to be solved self-consistently, once the individual cell deformation as a function of morphogen is given. The surface geometry determines the morphogen distribution, and the morphogen distribution in turn affects the surface geometry. Extension of the model to two or more morphogens is straightforward, and the key property of "regulation" or size invariance of the model is retained. Numerical integration of the two coupled equations is carried out in the case of axial symmetry, and the results presented by the case that individual cells deform by changing the ratio of their apical to basal areas, as well as their heights. Gastrulation in small regulating holoblastic eggs (e.g. starfish, sea urchin and amphioxus) is discussed in light of the present model.

Discontinuities of pattern and rules for regeneration in limbs of crayfish

The most caudal limb in crayfish, the uropod, has two rami, the exopodite and the endopodite. Results of earlier experiments (J.E. Mittenthal et al. (1985) W. Roux's Arch. Dev. Biol. 194, 121-130) indicated that ramus morphogenetic fields in the two rami are equivalent and tandem. Thus the proximal (inner) junction of the rami, where intersegmental membrane separates them from each other and from the coxa, is analogous to a boundary between segments of the body or of a leg. In this region a discontinuity in the positional information carried by the ramus fields might occur. To characterize the morphogenetic fields in the region near this junction we have exchanged the medial and lateral margins of the two rami, performing the four possible grafting operations of this kind. While an experimentally generated discontinuity between the lateral margin of the exopodite and the medial margin of the endopodite (outer-to-outer junction) triggers intercalation of supernumerary rami, a discontinuity of pattern between the medial margin of the exopodite and the lateral margin of the endopodite (inner-to-inner junction) is stable despite the absence of intervening intersegmental membrane. Where intercalation does occur, it can proceed in either direction along the margin of a supernumerary ramus. These results suggest that there is no discontinuity of positional value at the boundary between the rami. The results of all of our experiments on the uropod indicate that a conjunction of separate proximodistal, dorsoventral, and mediolateral component fields may give positional information for generating the uropod. Intercalation restores the continuity of pattern in the proximodistal field. In the mediolateral field a discontinuity of pattern may result from a preferred polarity of intercalation: Outer cells may be competent to generate inner cells, but not vice versa. According to this hypothesis the two rami have tandem ramus fields but mirror-symmetric polarity of competence. Alternatively, intercalation may eliminate a mediolateral discontinuity only if the mismatch of mediolateral positional values at the discontinuity exceeds a threshold. The threshold criterion may be a weighted sum of limb field and ramus field positional values.

Intercalary regeneration in legs of crayfish: proximal segments

An arthropod leg represents a protuberance of the body segmental integument which bears distinctive markers in both the mediolateral and the anteroposterior axes. To clarify the biaxial organization of the body segmental morphogenetic field, and to study the relation among the whole-limb, limb segmental, and body segmental fields previously recognized in arthropods, we have grafted a proximal leg segment into the ventral midline in crayfish. After this operation the majority of animals regenerated a mirror-symmetric pair of supernumerary legs at the host site. Some of these legs had the most proximal segment, the coxa, partially fused to the adjacent body surface. Minority patterns of regeneration included one midline leg with a gill, three midline legs with a gill, and two normal legs with a third double-half leg. These results are compatible with the principle that intercalary regeneration restores the continuity of positional information.

A model for shape generation by strain and cell-cell adhesion in the epithelium of an arthropod leg segment

We present a model for the energetic factors determining the most stable shape of a tubular epithelium such as the hypodermis of an arthropod leg segment. The model uses the analysis by Steinberg (1963) of rearrangement of cells in aggregates under the influence of differential adhesion, combining this analysis with the assumption that the epithelium behaves as an elastic sheet. The epithelium is assumed to consist of blocks of cells with different adhesive affinities, which remain unmixed in a quilt pattern. Rearrangement of cells within each block can adjust the shape of the tube by changing the shapes of the blocks. By means of such rearrangements the tube develops that shape which minimizes a free energy. The free energy is the difference between the energy of mechanical strain due to bending of the epithelium and the work of adhesion among cells. Minimization of the free energy for a cylindrical segment yields a scaling relation involving the length and radius of the segment. Leg segments of Drosophila conformed approximately to this relation, with deviations which suggest that a whole-limb pattern of adhesive affinities modulates the shaping effects of an adhesive pattern repeated in each leg segment. The model also predicts a transient deformation in an epithelium following a grafting operation. For example, deleting a slab of tissue from a tubular segment and reuniting the cut ends should produce a constriction of the tube at the host-graft junction. We propose that patterns of strain and adhesion can provide positional information which regulates subsequent development. Local increases in strain or adhesive disparity may stimulate mitoses; the resulting changes in distribution of cells will affect morphogenesis.

Distal regeneration and symmetry

A revision of the "polar coordinate model" shows how pattern formation in diverse regenerating systems can be understood in terms of strictly local cell interactions.