SITES OF HEMATOPOIESIS IN THE SEA URCHIN
Courtney Smith, George Washington University
Preethi Golconda, Katherine M. Buckley, Caroline R. Reynolds, Jennifer Romanello, L. Courtney Smith
Department of Biological Sciences, George Washington University, Washington DC
The location of coelomocyte proliferation in adult sea urchins is unknown although this question has been addressed repeatedly since the early 1800s. Immunoquiescent (IQ) purple sea urchins (Strongylocentrotus purpuratus) with down-regulated immune systems have reduced numbers of coelomocytes that increase quickly in response to immune challenge. Whether some or all of these cells are newly proliferated is not known. Furthermore, the gene regulatory network that regulates hematopoiesis in embryonic and larval sea urchins has not been investigated in adults. Hence, cell proliferation was induced in IQ sea urchins either by injection of heat-killed Vibrio diazotrophicus or by aspiration of coelomic fluid. In response, coelomocyte concentration increases, but newly proliferated coelomocytes constitute only about 10% of this cellular increase. In tissues, newly proliferated cells are present in the axial organ, gonad, pharynx, esophagus, and gut with no differences among tissues. Expression of genes encoding transcription factors that regulate hematopoiesis are elevated in the axial organ and the pharynx compared to coelomocytes, esophagus, gut, and gonad. This result is in agreement with an evaluation of an RNAseq dataset for adult sea urchin tissues, which also suggests that the axial organ is a center of apoptosis processes. Results indicate that the axial organ may be a site of coelomocyte proliferation and that it may also be a center for cellular removal and recycling. A second unexpected site, the pharynx, may also have hematopoietic activity, a tissue that has been assumed previously to function only as part of the intestinal tract.