The haemocyte of the holothurian Eupentacta quinquesemita: ultrastructure and maturation

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Abstract
The ultrastructure of the haemocyte (erythrocyte) is described. A full complement of functional organelles is retained for the cell's lifespan. Sequences in haemocyte maturation are identified. Organelle abundance and metabolic activity appear to peak during maturation. Cell size increases and shape transforms from spheroid to ellipsoid, reverting to spheroid at full maturation; these are accompanied by changes in the configuration of the microtubule cytoskeleton (marginal band). The adaptive significance of haemocyte shape is discussed. Lymphocytes do not appear to be prohaemocytes (stem cells). Very immature haemocytes have protrusions that suggest recent detachment from an epithelial surface. The high frequency of maturing stages, variability of volume, and variability of cell population suggest a frequent turnover of perivisceral coelomic fluid correlated with the species' proneness to evisceration.