A quantitative study of the number and distribution of neoblasts in Dugesia lugubris (Planaria) with reference to size and ploidy

Abstract

ABSTRACT This paper reports observations on the number of neoblasts in diploid and triploid planarian biotypes, the relation of this number to the size of the animal and the distribution of neoblasts with respect to the long axis of the animal. The number of neoblasts is a function of animal length for both diploid and triploid biotypes, but no significant difference was found between the biotypes. The combined data yield a regression equation of log. (number of neoblasts) = 1·90 log. (length) + 3·47. The distribution of neoblasts with respect to the long axis of the planarian was found to have two relative maxima, one anterior and one posterior to the intercalary minimum associated with the pharyngeal region of the animal. The relative position of the pharynx remains unchanged with growth in the triploid animal, in contradistinction to the progressive shift cephalad in the diploid. As statistical analysis showed no significant difference in the effects of the histological methods employed on the two biotypes, this observation is interpreted as a demonstration of different modes of growth in the diploid and the triploid. The volume of the planarian (in the fixed state) was correlated with the animal’s length (when alive) to yield a regression equation of log. (volume) = 2·62 log. (length) + 5·70. The slope of this regression equation is highly significantly different from that of the neoblast regression line. Thus larger animals have a lower density of neoblasts, although more of them. It is suggested that this latter observation may have some bearing on the problem of senescence in planarians. Counts of objects in serial sections are subject to an overestimate which can be corrected by the method of Abercrombie. This method, however, is based on an assumption which is not always valid. Therefore a more rigorous correction formula has been derived. This formula was then applied to the measurement of neoblast nuclear diameters (in the fixed state). No significant difference was found for nuclear diameters of diploid and triploid neoblasts.