Regeneration in the lower metazoa
Goffin, Mary Frances
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The problem of regeneration has to do with the phenomenon of growth resulting in a restoration of an old form of a mutilated organism. The process, the stimuli and factors causing it, and the effect of agents upon regeneration in the lower metazoa were discussed. The sponger, the simpxest multicellular animals, can after dissociation reconstitute the whole organism again. The arcnaeocytes were found to have tne most important role in regeneration, differentiating into gonocytes, scieroblasts, colienocytes and desmacytes, thus giving rise to the mesenchyme ana skeleton of the new sponge. The pinacocytes formed the dermal membrane and lining of the canals, while the choanocytes gave rise to the flagellated chamoers. Aggregates were formed by the amoeboid activity of the arcnaeocytes within eighteen to twenty-four hours after dissociation. On the third day after dissociation the flagellated chambers appeared, and on the fourth and fifth aays, canals appeared. Development after dissociation was then completed. Dissociated cells from different species (Microciona and Clicna) coalesce and form aggregates only with cells of their own species. The dissociated cells will not coalesce and form aggregates in pure sodium or potassium chloride solutions. The cells were also found to be sensitive to increases in osmotic pressure. Hydra, turned inside out, will regain their normal organization of the rearrangement of endoaermal ana ectodermal cells migrating in opposite directions through the mesoglea. Ectodermal or endodermal layers of hydra cultured alone do not regenerate due to the inability of one cell type to differentiate into the cell type of the other layer. Dissociated cells of hydra will fuse and form aggregates only when the three body layers are present. The pieces of a hydra must measure more than one-sixth of a millimeter in diameter to regenerate. Size also determines the number of tentacles, and the size of the hypostome formed on the regenerant. Various sections of hydra and grafts also form regenerated individuals, although in some cases the results are abnormal. The determination and organization of new polarities depended upon the rate of metabolism incorporated in the regenerating mass. In Tubularia, removal of the perisarc stimulates regeneration, as the cut end is then cathed with more oxygen. The more coenosarc exposed to oxygen the greater is the activation to form hydranth tissue. When there is an increase in the oxygen consumption there is also an increase in the rate of regeneration. The hydranth forms in relation to the oxygen supply with the oral end of the regenerant at the point of highest oxygen tension. The rate of regeneration can be measured at any level under various conditions wnen the formula R=πr^2n/t is used. Exposure of the coenosarc to sea water obliterates the already existing gradients in the stem and thus the regenerating coenosarc fragments form new polarities. The theory of dominance seems to be best explained by the transportation of substances theory, in whicn there are differences in the stem, "E", present in highest concentration at the distal end and lowest at the proximal end. There is also another factor in the stem "S", which may or may not inhibit regeneration depending on the rate of regeneration when "E" is forming new hydranths. Eudendrium and Pennaria cells fuse after dissociation in much the same manner as sponges and hydra. Light is essential for the regeneration of hydranths in Pennaria, but not for Eudendrium. Direct x-radiation inhibited regeneration of Pennaria, but screened colonies connected to those colonies radiated, did regenerate new hydranths. Podocoryne and Hydractinia are also capable of regenerating polyps. Gonionemus shows incomplete regeneration. Mnemiopsis leidyi, a ctenophore, regenerated plate rows, canal connections, and apical organs, depending on the manner in which they were cut. There was found to be no physiological gradient in Mnemiopsis. Polychoerus is the simplest flatworm to stimulate to regenerative activity as these worms have no central nervous system to influence regeneration. Many types of regeneration have been produced from stimulation of planarians to regenerate. The smallest and most irregular pieces have the capability of forming new individuals. The extensive powers of regeneration of this group are due to the localization and differentiation of the formative cells, which are found in the mesoderm. The polyclads will regenerate complete individuals from pieces taken from any part of the oody. The triclads will restore missing anterior parts onxy when the cephalic ganglia are present. Anesthetics and strychnine inhioit regeneration in the flatworms as doeo exposure to x-radiation.
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