Resources of regeneration in planarians

We studied the intensity of blastema growth in operated planarians at an early stage of regeneration as a function of the following factors: area of regenerate and its function and number of regeneration foci (volume of regeneration). There was no direct dependence between the intensity of regeneration and the size of regenerating fragment, as well as the volume of regeneration. Some specific features of the early stage of regeneration have been described, which suggest its determinate character. The behavior of neoblasts during formation of blastemas with different localization is discussed.     

Resources of regeneration in planarians

We studied the intensity of blastema growth in operated planarians at an early stage of regeneration as a function of the following factors: area of regenerate and its function and number of regeneration foci (volume of regeneration). There was no direct dependence between the intensity of regeneration and the size of regenerating fragment, as well as the volume of regeneration. Some specific features of the early stage of regeneration have been described, which suggest its determinate character. The behavior of neoblasts during formation of blastemas with different localization is discussed.     

Pharynx regeneration in planarians

The obtained and published data on pharynx regeneration in planarians have been reviewed. Planarians can regenerate from a small body fragment and restore all missing organs including the pharynx. The pharynx is a relatively autonomous organ with a differentiated structure and specialized function. Pharynx regeneration has specific features, and its studies are of considerable theoretical interest. Pharynx regeneration can also be a convenient model to study the molecular mechanisms of regeneration that remain undisclosed. In addition, this model can be used to test biologically active compounds in order to elucidate their effect on morphogenesis. This subject of investigation benefits by a simpler and more adequate analysis as well as a possibility to use large numbers of animals and small quantities of analyzed substances.     

Tissue regeneration during lower jaw restoration in zebrafish shows some features of epimorphic regeneration

Zebrafish have the ability to regenerate skeletal structures, including the fin, skull roof, and jaw. Although fin regeneration proceeds by epimorphic regeneration, it remains unclear whether this process is involved in other skeletal regeneration in zebrafish. Initially in epimorphic regeneration, the wound epidermis covers the wound surface. Subsequently, the blastema, an undifferentiated mesenchymal mass, forms beneath the epidermis. In the present study, we re-examined the regeneration of the zebrafish lower jaw in detail, and investigated whether epimorphic regeneration is involved in this process. We performed amputation of the lower jaw at two different positions; the proximal level (presence of Meckel's cartilage) and the distal level (absence of Meckel's cartilage). In both manipulations, a blastema-like cellular mass was initially formed. Subsequently, cartilaginous aggregates were formed in this mass. In the proximal amputation, the cartilaginous aggregates were then fused with Meckel's cartilage and remained as a skeletal component of the regenerated jaw, whereas in the distal amputation, the cartilaginous aggregates disappeared as regeneration progressed. Two molecules that were observed during epimorphic regeneration, Laminin and msxb, were expressed in the regenerating lower jaw, although the domain of msxb expression was out of the main plain of the aggregate formation. Administration of an inhibitor of Wnt/β-catenin signaling, a pathway associated with epimorphic regeneration, showed few effects on lower jaw regeneration. Our finding suggests that skeletal regeneration of the lower jaw mainly progresses through tissue regeneration that is dependent on the position in the jaw, and epimorphic regeneration plays an adjunctive role in this regeneration.     

Liver regeneration

Liver regeneration after partial hepatectomy is a very complex and well-orchestrated phenomenon. It is carried out by the participation of all mature liver cell types. The process is associated with signaling cascades involving growth factors, cytokines, matrix remodeling, and several feedbacks of stimulation and inhibition of growth related signals. Liver manages to restore any lost mass and adjust its size to that of the organism, while at the same time providing full support for body homeostasis during the entire regenerative process. In situations when hepatocytes or biliary cells are blocked from regeneration, these cell types can function as facultative stem cells for each other.     

Transdifferentiation and regeneration in vitro

Striated muscle tissue and endoderm can be isolated from the anthomedusa Podocoryne carnea. The isolates are uncontaminated by other cell types and can be cultivated in artificial seawater for months without undergoing autonomous regeneration. However, if the endoderm is combined with collagenase-treated striated muscle, a regeneration process is initiated which leads to the formation of the sexual and feeding organ (manubrium) of the medusa. The original endoderm and striated muscle are replaced in the regenerate by at least seven new cell types, including gametes. Labeling experiments with [³H]thymidine and experiments in which mitosis is inhibited in either the striated muscle or the endoderm with mitomycin C demonstrate that the striated muscle is able to transdifferentiate into all the cell types found in the regenerate. With the possible exception of ectodermal smooth muscle this statement is also valid for the endoderm.     

Transdifferentiation in holothurian gut regeneration

It has recently been shown that the whole spectrum of cell types constituting a multicellular organism can be generated from stem cells. Our study provides an example of an alternative mechanism of tissue repair. Injection of distilled water into the coelomic cavity of the holothurian Eupentacta fraudatrix results in the loss of the whole digestive tract, except the cloaca. The new gut reforms from two separate rudiments. One rudiment appears at the anterior end of the body and extends posteriorly. The second rudiment grows anteriorly from the cloaca. In the anterior rudiment, the luminal epithelium (normally derived from endoderm) develops de novo through direct transdifferentiation of the coelomic epithelial cells (mesodermal in origin). In the posterior rudiment, the luminal epithelium originates from the lining epithelium of the cloaca. After 27 days, the two rudiments come into contact and fuse to form a continuous digestive tube lined with a fully differentiated luminal epithelium. Thus in this species, the luminal epithelia of the anterior and posterior gut rudiments develop from two different cell sources-i.e., from the mesodermally derived mesothelium and the endodermally derived epithelium of the cloacal lining, respectively. Our data suggest that differentiated cells of echinoderms are capable of transdifferentiation into other cell types.