Metamorphosis of calcareous sponges II. Cell rearrangement and differentiation in metamorphosis

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Abstract
The free-swimming larva of the calcareous sponge turns into a sessile juvenile during metamorphosis. Electron microscopic observations of metamorphosing larvae reveal the rearrangement and differentiation of larval cells. About 12 h after the larvae were released from a mother sponge, the settled larvae without flagella consist of an inner cell mass and an enveloping layer of pinacocytes. The inner mass cells have residual flagellar rootlets which clearly show the origin of the cells. On the other hand, the pinacocytes still show the intracellular profile characteristic of the granular cells of the swimming larva. One day after release, scleroblasts and other mesohyl cells differentiate in the peripheral region of the inner cell mass. Two days after release, the central cells of the inner cell mass differentiate into choanocytes. Three days after release, a large gastral cavity is formed and lined by a layer of choanocytes. These results demonstrate the cell lineage in the metamorphosis of the calcareous sponge, that is, the pinacocytes of a young sponge are derived from the granular cells of the swimming larva and the choanocytes from the flagellated cells.