Organismal differentiation, ageing and rejuvenation.

Abstract An attempt to outline the ageing of multicellular organisms was made by integration of part processes taking place at the molecular, cytological, histological and physiological levels. The correlation of the part processes and the whole with ageing at the cytological level and organismic involution was shown in experiments on the regeneration and rejuvenation of planarians. The primary determinant of their ageing is the change in the ratio of totipotential to postmitotic cells. The relationship between the two cell types and the gradual cessation of the potency to divide was studied by analyzing the myeloid differentiation. Subcellular and cytochemical changes which are presumably responsible for the loss of capacity to divide in postmitotic cells have been described. The gerontological adaptation of the concept of allometry, used in embryology, makes it possible to synthesize the involutional processes taking place at the different levels into a uniform picture. A classification of the allometric changes is best achieved by analyzing the ageing organism as a cybernetic model. It is from this point of view that the alterations in stored information and in the signal-transforming mechanisms related with age, the increase in the noise level of the channels and the effects exerted on the organism by breakdown at the end point of the information system were studied.