Structural and ultrastructural aspects of growth in Oscarella lobularis (Porifera, Demonspongiae)

Light and electron microscopy were used to study projections emerging from areas of the sponge Oscarella lobularis. These projections seem to arise from foldings of the exopinacoderm, a process also involving mesohyl cells. Besides folding, exopinacocytes invaginate giving rise to small pockets. These may open directly to the outside and represent the precursors of flagellated chambers. Exopinacocytes lining pockets and spaces between projections transform into endopinacocytelike cells. As pockets become isolated from the exterior, cells further differentiate into choanocytes. Remodeling processes, involving fusion of adjacent protrusions and transformation of the intervening spaces into canals, eventually realize a functional aquiferous system. Allowing the sponge to increase its size, these events represent the steps of a growth mechanism.