CHANGES IN SOMATIC-CELL STRUCTURE DURING SENESCENCE OF VOLVOX CARTERI

Abstract

Senescence of the terminally differentiated somatic cells of the green alga, V. carteri f. weismannia, was investigated by EM, light and fluorescence microscopy. Viability of the somatic cell population, as determined by trypan blue or erythrosin B exclusion, showed a sharp reduction beginning 144 h after the somatic cells had lost the ability to divide. This increased mortality rate was correlated at the light microscopic level with a retraction of the somatic cell cytoplasm, a reduction in chloroplast autofluorescence (and total chlorophyll content), and a decline in the number of vacuoles which could be localized with 9-aminoacridine fluorescence microscopy. Nuclear fluorescence with acridine orange remained unaffected during this time. Lipid bodies increased in older cells, and total lipid analysis showed a sharp increase beginning 96 h after the somatic cells had stopped dividing. EM comparison between young (48-72 h) and old (168 h) somatic cells showed a disorganization of chloroplast structure, a decline in the number of cytoplasmic ribosomes and, substantiating the light microscopy, an accumulation of lipid bodies in the cytoplasm of the older cells. Progressive changes in somatic cell structure with age was demonstrated and suggest cells under nutrient stress even though they are in nutrient medium. Senescence and death of the V. carteri somatic cells may be caused by an inability to take up or utilize nutrients present in the culture medium.