The days after: cellular events following the inflammatory allorejection reaction in the colonial ascidian Botryllus schlosseri

Allorecognition between contacting, genetically incompatible colonies of the ascidian Botryllus schlosseri represents a typical inflammatory reaction. It leads to the formation of a series of cytotoxic, necrotic spots along the contact border known as points of rejection (PORs). This is the consequence of the selective recruitment of morula cells (MCs), a peculiar granular, cytotoxic cells present in botryllid ascidians, in the lumen of the ampullae (the peripheral blind termini of the colonial vasculature) facing the alien colony, their migration into the tunic and their degranulation. The released material includes the enzyme phenoloxidase, the activity of which is responsible for the observed cytotoxicity. In the present work, we studied the cellular events in the facing ampullae for 9 days following the initial contact of the colonies. Data confirm that MCs gather inside the lumen of facing ampullae already at day 1 from the contact and start to leak into the tunic at day 2, when the first PORs appear. MCs then decrease progressively in the following days, probably because most of them leave the circulation and enter the tunic whereas, at day 5 from the contact, round phagocytes increase significantly inside facing ampullae, likely deriving from spreading phagocytes having ingested MC corpses in order to prevent a diffuse inflammation in the circulatory network. After the allorejection reaction, colonies orient their growth towards opposite directions and ampullae previously involved in the allorecognition remain in a region of old tunic which is progressively released by the colonies.