Process of Regeneration in Planarians
Regeneration in planarians has been broadly studied and considerable information regarding this phenomenon is now available. Let us see what this tells us about how planarians regenerate. Refer a planaria whose head is cut. The wound formed is basically covered by the epidermis of the skin. The cells move tangentially over the wound surface without proliferating. This process takes place in the first twenty four hours after the cut. Next a blastema is formed beneath the epidermis. Regeneration includes a combination of morphallactic and Epimorphic events as it occurs at either cut end, not just only by migration of coherent sheets as in coelentrates, but as well by proliferation of undifferentiated cells of the blastema. The head and tail blastema are both ascertained from the start. The head blastema makes the head and the tail blastema the tail. The blastema cells also give rise to most of the regenerating organs: the connective tissues, the pharynx, the nervous system, the muscles and still the reproductive organs.
The skin epithelium as well arises from the skin at the edge of the wound. It is believed that the cut edge of the old intestine gives rise to the intestine in the regenerate. For several years regeneration of the missing pans in planarians was believed to be because of neoblast cells. These cells that are distributed throughout the body, are distinguished chiefly by an abundance of cytoplasmic RNA in them. These cells seem to accumulate at the wound surface after an amputation-and appear to form a , regenerating blastema that proliferates and then forms the missing part. The nature and source of the neoblast has aroused some of the controversy that has not been resolved so far. Muscle dedifferentiation throughout regeneration has also been observed in planarian regeneration by some workers. Whereas Hay and Coward (1975) on the basis of electron microscopic studies on neoblasts in planarians have completed that the neoblasts are in reality gland cells and not undifferentiated cells, as earlier studies along with light microscope indicated. These cells have a prominent just a nuclear Golgi zone and an elaborate endoplasmic reticulum as supposed in gland cells. Hay's and Howard's further electron microscopic studies have as well shown the presence of very small cells called be& cells which earlier under the light microscope were poorly visible and resolved. These cells occur in the parenchyma surrounding the several glandular, muscular and digestive tissues, and have the characteristic of undifferentiated cells.
The Beta cells have a nucleus along with small chromatic clump and no nucleolus, whereas their cytoplasm has ribosomes but lacks endoplasmic reticulum. Transitional stages among beta and various differentiated cellular types have also been found. These findings have pointed out that the beta cells constitute progenitor cells for various tissue types with which they are connected, in the same way as cells of the germinal layer of the skin are progenitor cells for the keratinizing cells or cells of the intestinal crypts are the progenitors of mucosal cells. So it appears that beta cells would not essentially be totipotent. A number of studies of planarian regeneration have though still not fully solved the origin of the regenerative cells which may to the neoblasts or the B cells though and the mechanism of regeneration in these animals. some graft experiments have been conducted to see if the regenerative cells are of local origin or capable of migration. Such studies display that if an animal is irradiated by x- rays prior to wounding then its regenerating ability is inhibited. Though this can be restored by a graft of healthy. It has as well been observed that if in the irradiated planaria with a healthy graft an amputation is made at a site remote from the graft, then regeneration takes place, usually after a period of delay that is apparently needed for migration and proliferation of the regenerative cells from the graft to the site of the wound. This points out that the re-genitive cells are capable of long distance migration and the formation of blastema on the cut surface.