Macrophage involvement for successful degeneration of apoptotic organs in the colonial urochordate<i>Botryllus schlosseri</i>

Abstract

Apoptosis is an important tool for shaping developing organs and for maintaining cellular homeostasis. In the colonial urochordate Botryllus schlosseri, apoptosis is also the hallmark end point in blastogenesis, a cyclical and weekly developmental phenomenon. Then the entire old generation of zooids are eliminated (resorbed) by a process that lasts 24-36 h. Administration of the antioxidant butylated hydroxytoluene (BHT) resulted in resorption being arrested by 1-8 days on average. At high doses (2.5-15.0 mg BHT l(-1)) resorption was completed only after removal of BHT. Colonies that were not removed in time, died. In treated colonies, although DNA fragmentation was high, tissues and organs that would normally have died, survived, and the general oxidative levels of lipids were reduced. Blood vessels were widened, containing aggregates of blood cells with a significantly increased proportion of empty macrophage-like cells without inclusion. In colonies rescued from BHT treatment, resorption of zooids started immediately and was completed within a few days. We propose three possible mechanisms as to how BHT may affect macrophage activity: (1) by interrupting signals that further promote apoptosis; (2) through the respiratory burst initiated following a phagocytic stimulus; and (3) by reducing lipid oxidation and changing cell surface markers of target cells. Our results point, for the first time, to the role of phagocytic cells in the coordination of death and clearance signals in blastogenesis.