Subcuticular Rejection: an Advanced Mode of the Allogeneic Rejection in the Compound Ascidians Botrylloides...

Allogeneic rejection between colonies (colony specificity) was described by electron microscopy in two compound ascidians, Botrylloides simodensis and B. fuscus. When two incompatible colonies are brought into contact at their growing edges, the tunic cuticle dissolves and the tunics of the colonies partially fuse. Alloreactive, humoral factors may diffuse to the opposite colony through the partially fusing tunic, and the tunic cells (free cells distributed in the tunic) possibly recognize these factors and induce a rejection reaction. Then, blood cells--mainly morula cells--infiltrate into the tunic, while tunic cells are disintegrating near where the partial fusion of the tunic is occurring. The infiltrating blood cells aggregate, disintegrate, and discharge electron-dense materials in the tunic at the subcuticular regions where the tunics have partially fused. Since the rejection lesion is restricted to the subcuticular area, some regulatory systems may be involved in this restriction. At the end, new walls are formed in the tunic matrix to separate the rejection lesion from the contacting colonies. The new wall is a continuous layer composed of electron-dense fibers and is structurally identical to the regenerating tunic cuticle. The mode of occurrence of colony specificity (Hirose et al., 1994) and the present results indicate that tunic cells are the only allorecognition sites in B. fuscus.