A morphological study of nonrandom senescence in a colonial urochordate.

Botryllus schlosseri is a clonally modular ascidian, in which individuals (zooids) have a finite life span that is intimately associated with a weekly budding process called blastogenesis. Every blastogenic cycle concludes with a synchronized phase of regression called takeover, during which all zooids in a colony die, primarily by apoptosis, and are replaced by a new generation of asexually derived zooids. We have previously documented that, in addition to this cyclical death phase, entire colonies undergo senescence during which all asexually derived individuals in a colony, buds and zooids, die in concert. In addition, when a specific parent colony (genet) is experimentally separated into a number of clonal replicates (ramets), ramets frequently undergo senescence simultaneously, indicating that mortality can manifest itself in nonrandom fashion. Here, we document a morphological portrait of senescence in laboratory-maintained colonies from Monterey Bay, California, that exhibit nonrandom mortality. Nonrandom senescence proceeded according to a series of characteristic changes within the colony over a period of about one week. These changes included systemic constriction and congestion of the vasculature accompanied by massive accumulation of pigment cells in the zooid body wall (mantle), blood vessels, and ampullae; gradual shrinkage of individual zooids; loss of colonial architecture, and ultimately death. At the ultrastructural level, individual cells exhibited changes typical of ischemic cell death, culminating in necrotic cell lysis rather than apoptosis. Collectively, these observations indicate that senescence is accompanied by unique morphological changes that occur systemically, and which are distinct from those occurring during takeover. We discuss our findings in relation to current experimental models of aging and the possible role of a humoral factor in bringing about the onset of senescence.

A morphological and immunohistochemical study of programmed cell death in Botryllus schlosseri (Tunicata, Ascidiacea).

The blastogenic cycle of the colonial ascidian Botryllus schlosseri concludes in a phase of selective cell and zooid death called takeover. Every week, all asexually derived parental zooids synchronously regress over a 30-h period and are replaced by a new generation. Here we document the sequential ultrastructural changes which accompany cell death during zooid degeneration. The principal mode of visceral cell death during takeover occurred by apoptosis, the majority of cells condensing and fragmenting into multiple membrane-bounded apoptotic bodies. Cytoplasmic organelles (mitochondria, basal bodies, striated rootlets) within apoptotic bodies retained ultrastructural integrity. Dying cells and fragments were then swiftly ingested by specialized blood macrophages or intraepithelial phagocytes and subsequently underwent secondary necrotic lysis. Certain organs (stomach, intestine) displayed a combination of necrotic and apoptotic changes. Lastly, the stomach, which demonstrated some of the earliest regressive changes, exhibited intense cytoplasmic immunostaining with a monoclonal antibody to ubiquitin at the onset of takeover. Affinity-purified rabbit antiserum against sodium dodecyl sulfate-denatured ubiquitin detected a characteristic 8.6-kDa mono-ubiquitin band by Western blot analysis. Collectively, these findings raise the possibility that cell death during takeover is a dynamic process which requires active participation of cells in their own destruction.