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Studies on the brown alga Ectocarpus in culture : senescence, an ultrastructural study Oliveira, Luis Augusto Fernandes

Abstract

A study of cell differentiation and senescence on the brown alga Ectocarpus sp. was carried out using standard techniques of light- and electron microscopy as well as cytochemistry. The first 6-8 cells in each filament of Ectocarpus are characterized by a large round and well organized nucleus, a dense cytoplasm rich in ribosomes, endoplasmic reticulum, dictyosomes, mitochondria, and chloroplasts with conspicuous stalked pyrenoids. Few small vacuoles are present in the cytoplasm. New features not previously reported for the brown algae are also described. These include the presence of pore-like interruptions and bridge-like filaments in the cisternae of dictyosomes and chloroplast thylakoids, as well as the formation of concentric bodies of chloroplast origin. Microbody-like organelles are also reported; cytochemical studies have shown catalase activity associated with them. Peroxidase activity is reported in the cell wall, while acid phosphatase activity is found in association with both the endoplasmic reticulum and the Golgi elements, as well as inside the vacuoles. Adenosine triphosphatase is present in mitochondria, chloroplast thylakoids, and plasma membrane. These cells, therefore, can be described as meristematic or the immediate consequence of the differentiation of the meristematic cells. The processes of autophagy and vacuolation are studied in detail and found to be particularly significant during the stages of cell differentiation which lead to the aging of Ectocarpus sporophytic cells. Increases in vacuolation and autophagy are paralleled by an increase in acid phosphatase activity. The ultimate results of these processes are: a reduction in cytoplasmic matrix, a general deterioration of cytoplasmic organelles, and the formation of residual bodies which overcrowd the cytoplasm. Other features are: the irregularity of the nuclear boundary, the disorganization of the E.R. system through vesiculation, the increasing difficulty in detecting mitochondria, the development of large stacks of chloroplast thylakoids as well as numerous patches of electron dense metabolites, distinct from the plastoglobuli, and the formation of conspicuous cell wall ingrowths. Assays of enzyme systems other than acid phosphatase show their distribution to be similar to those reported for young cells. In the final stages of senescence the cells become typically necrotic; no enzymes can be localized, except acid phosphatase whose reaction products are no longer compartmentalized but have become distributed all over the cell cavity. The nucleus and dictyosomes have disintegrated. Only small remnants of the E.R. system remain. Mitochondria and chloroplasts have lost their internal organization, no cytoplasmic matrix can be detected. At the very end even the cell wall shows signs of disorganization. It is concluded that these cells represent the final stage of autolysis.

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