Cytological analysis of the urn cell complex of Sipunculus nudus before and after serum-induced secretion

This study analyzes the cytology of the urn cell complex (UCC) of Sipunculus nudus, an invertebrate cell model for humorally regulated mucus secretion. An unstimulated UCC is composed of a vesicle cell and a ciliated cell joined together by desmosomes. Another cell population (third-type cells) is frequently associated with ciliated cells. Vesicle cells are thin, have few mitochondria or lipid droplets, and enclose a bubble-like cavity containing microfibrillar material. Ciliated cells contain several rows of cilia that are anchored by prominent rootlets and propel UCCs forward. Five to six concentric bundles of microfilaments are distributed along the outer convexity of ciliated cells and may have a role in the plasticity of the UCC. Many fibrillar deposits that lack a demonstrable limiting membrane are distributed around intracytoplasmic vacuoles facing the mouth-like opening of the UCC. These deposits are reactive to periodic acid-Schiff and resistant to diastase. After stimulation with serum, they appear to migrate through the ciliated cell's plasma membrane, contributing to the formation of a secretory tail. Discharge of secretory material is not observed in third-type cells, which instead contain lysosome-like granules and autophagic-like vacuoles and become displaced distalward by the emerging tail of the UCC. This study indicates that formation and elongation of the UCC secretory tail are functions of ciliated cells.