Seven Types of Tunic Cells in the Colonial Ascidian Aplidium yamazii (Polyclinidae, Aplousobranchia) : Morphology, Classification, and Possible Functions

Seven types of tunic cells and multicellular vesicles are described in the tunic of the polyclinid ascidian Aplidium yamazii : spherical tunic cells (t. c. ), phagocytic t. c. , elongated t. c. , filopodial t. c. , micro-granular t. c. , morula-like t. c. , and macro-granular t. c. The spherical tunic cells are characterized by their spherical cell bodies filled with round granules. Some of these cells are located on the outside of the tunic, and they are probably released from the tunic. The phagocytic tunic cells are irregularly shaped and are motile. They contain phagosomes and vesicles laden with round granules, which are probably derived from the contents of the phagosomes. It is thought that the phagocytic tunic cells filled with the granules become the spherical tunic cells that are released from the tunic. The elongated tunic cells have thin cell bodies with long cellular processes extending from them. These processes appear to connect with each other and possibly form a contractile network in the tunic. The tunic vesicle is a hollow multicellular vesicle composed of thin flattened cells and cuboidal granular cells. The cuboidal cells of the tunic vesicle often contain a large amount of rough endoplasmic reticulum, indicating a high level of protein synthesis, and they may secrete some components of the tunic. The functions of the other tunic cells, i. e. , filopodial tunic cell, micro-granular tunic cell, morula-like tunic cell, and macro-granular tunic cell, are not clear.