Ventral Islands Research Articles

Phagocyte dynamics in the blastogenetic cycle of the colonial ascidian Botryllus schlosseri: Cell senescence, segregation and clearance after efferocytosis

In the colonial ascidian Botryllus schlosseri, phagocytes are involved in the clearance of apoptotic cells and corpses during the periodical generation changes or takeovers (TOs) that assure the renewal of the colonial zooids. The persistent respiratory burst associated with efferocytosis, leads to the induction of senescence. Indeed, giant, senescent phagocytes are abundant in the colonial circulation at TO, whereas, in the other phases of the colonial blastogenetic cycle, they colonise the ventral islands (VIs), a series of mesenchymal niches located in the lateral lacunae of the mantle, on both sides of the subendostylar sinus. VI phagocytes produce reactive oxygen species probably as a consequence of the massive phagocytosis of effete cells. VIs are progressively dismantled with the progress of the blastogenetic phases and phagocytes are released in the peribranchial chamber via transepithelial expulsion to be definitely expelled with the outflowing water through the cloacal siphon.

Phagocyte dynamics in a highly regenerative urochordate: Insights into development and host defense

Phagocytosis is a cellular process by which particles and foreign bodies are engulfed and degraded by specialized cells. It is functionally involved in nutrient acquisition and represents a fundamental mechanism used to remove pathogens and cellular debris. In the marine invertebrate chordate Botryllus schlosseri, cell corpse engulfment by phagocytic cells is the recurrent mechanism of programmed cell clearance and a critical process for the successful execution of asexual regeneration and colony homeostasis. In the present study, we have utilized a naturally occurring process of vascular parabiosis coupled with intravascular microinjection of fluorescent bioparticles and liposomes as tools to investigate the dynamics of phagocyte behavior in real-time during cyclical body regeneration. Our findings indicate that B. schlosseri harbors two major populations of post-mitotic phagocytes, which display distinct phagocytic specificity and homing patterns: a static population that lines the circulatory system epithelia, and a mobile population that continuously recirculates throughout the colony and exhibits a characteristic homing pattern within mesenchymal niches called ventral islands (VI). We observed that a significant proportion of ventral island phagocytes (VIP) die and are engulfed by other VIP following takeover. Selective impairment of VIP activity curtailed zooid resorption and asexual development. Together, these findings strongly suggest that ventral islands are sites of phagocyte homing and turnover. As botryllid ascidians represent invertebrate chordates capable of whole body regeneration in a non-embryonic scenario, we discuss the pivotal role that phagocytosis plays in homeostasis, tissue renewal and host defense.