Ultrastructural study of differentiation processes during aggregation of purified sponge archaeocytes

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Abstract
Archaeocytes from the spongeEphydatia fluviatilis were dissociated and then isolated on Ficoll density gradients. Their aggregation and reconstitution processes were studied by transmission electron microscopy to determine their capabilities for differentiation. Archaeocyte aggregates follow a well defined sequence of differentiation to generate the characteristic structures of a sponge. Pinacoderm is the first structure to be regenerated and appears progressively at the surface of the 12 h aggregates. Pinacocytes which have differentiated in archaeocyte aggregates are identical to native ones except that the nucleolus remains in most cells. The choanocytes appear only after 24 h by a two step process. First, small cells (choanoblasts) are formed from archaeocytes by mitosis. These cells then transform into fully differentiated choanocytes possessing collars and flagella. The early choanocyte chambers are small, irregular and randomly dispersed in the aggregates. Finally, collencytes and sclerocytes begin to appear just before the aggregates spread on the substrate. The differentiation of a suspension of pure archaeocytes is a unique model system to study sponge cell differentiation and has allowed us to demonstrate that archaeocytes isolated from developed sponges maintain the capacity to differentiate even though this capacity is not usually expressed.