Ultrastructural and cytochemical studies on the nature and origin of the cytoplasmic inclusions of aging cells of Ectocarpus (Phaeophyta, Ectocarpales)

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Abstract
The origin of vacuoles in young and mature cells of Ectocarpus is described. Vacuoles in young cells are initiated by the fusion of small endoplasmic reticulum-derived provacuoles. In these cells the isolation and sequestration of pockets of cytoplasm by endoplasmic reticulum cisternal elements also lead to the formation of vacuoles, which can, therefore, be readily classified as autophagic vacuoles. In mature cells autophagy is prevalent. It occurs mainly by the invagination of the tonoplast and is similar to the process described by Matile as the sequestration of portions of cytoplasm and specific engulfment of organelles by vacuoles. Autophagy in Ectocarpus is always associated with an intense reaction for acid phosphatase activity and results in the formation of residual inclusions inside the vacuoles. In aging cells of Ectocarpus, the morphology of some of these residual inclusions is reminiscent of that of lipofuscin (aging pigment) of animal and protozoan cells. Cytochemical tests indicate that many of the cytoplasmic inclusions observed in Ectocarpus are, indeed, lipofuscin-related. These findings are discussed in relation to aging and senescence in a brown alga.

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