Putative stem cell origins in solitary tunicates

Abstract

Stem cells are primordial cells in complex metazoans, inside a developing embryo (blastocyst) and are understood to be totipotent (capable of differentiating into every cell type). Hematopoietic stem cells (HSC) are characterized by their capacity of self-renewal, multi-lineage differentiation. Because solitary tunicates are protochordates, they are believed to share certain developmental features of vertebrates and as such they are excellent, inexpensive, non-controversial animal models, when searching for stem cell strategies that are not likely to cause ethical, political or moral concerns. In the pharyngeal region and in the body wall there are collections of cells that form nodules capable of responding to antigenic stimulation. Turning to another approach related to differentiation in nodules vs. circulating cells, lymphocyte-like cells (LLCs) proliferate m response to allogeneic stimuli. In vitro labeling of proliferative hemocytes (blood cells) revealed significantly greater proliferative activity among individuals immunized with allogeneic tissue in contrast to autogeneically primed and naive animals. Enhanced proliferation was restricted to discrete crypts of dividing cells within the body wall of recipients. Here, increased proliferative activity was specifically associated with LLCs. A discrete circulatory hemocyte population mediates the recognition of histocompatibility antigens in Styela plicata. These immunocompetent cells, which bear an overt morphological similarity to vertebrate lymphocytes, specifically infiltrate incompatible tissue transplants prior to obvious allograft rejection. Host lymphocyte-like cells (LLC's) accumulate around and within graft vascular tissue and appear to initiate rejection by means of cytotoxic activity. Yet to be tested are experiments that would show regeneration or transfer of immunological memory by removing these cells, characterizing their appearance with the aim of confirming whether they are terminally differentiated or may be cells with greater potential, i.e. protochordate stem cells. © 2009 Springer Netherlands.