Stem cells in sexual and asexual reproduction of Botryllus schlosseri (Ascidiacea, Tunicata): an overview.

Botryllus schlosseri, a colonial ascidian reproducing both sexually and asexually, is an excellent model for the study of adult stem cells, since blastozooids, produced by budding, derive from a disc of stem cells of the parental atrial chamber and the overlying epidermis. In the colony, adult zooids undergo cyclical generation changes during which the adults die, are resorbed and then replaced by their buds which reach functional maturity and start filtering; at the same time, blood cell renewal takes place thanks to a new haemopoietic wave. Experimental procedures can evaluate the behaviour of stem cells during bud morphogenesis under different developmental constraints. For example, when all the zooids and buds of a colony are extirpated, zooids can derive from totipotent blood cells which aggregate on the walls of the blood vessels of the tunic. Stem cells can also be transferred from one colony to another through blood vessels during fusion of compatible colonies. Germ stem cells persist in a host colony so that heterochtonous offspring can be collected after many blastogenetic generations. The persistence of somatic stem cells alters the fusibility pattern of the colony. Pluripotent stem cells are also involved in the embryonic development of larval and juvenile organs; these embryonic cells were analysed and compared with bud stem cells for their potentials and gene expression.