srp transcript expression seen in this study agrees with the results previously documented in FBrf0065324. It is proposed that one of the mesodermal patches of srp expression seen at the blastoderm stage is the hemocyte primordium. After invagination with the ventral furrow, these primordial cells become located laterally to the stomodeum, and later differentiate into prohemocytes which migrate into the head. The cells are subsequently distributed throughout the body and differentiate into mature hemocytes, at which time srp expression is downregulated.

srp transcripts are detected throughout development on northern blots with the highest levels in 6-12hr embryos. In embryos, they are initially detected in four regions of the cellular blastoderm, the anterior and posterior midgut primordia, the primordium of the cephalic mesoderm, and the vitellophages. As gastrulation begins, transcripts are seen in the amnioserosa. During germ band extension, expression diminishes in the midgut but remains strong in the cephalic mesoderm. Later in embryogenesis, srp is expressed in the developing fat body. By stage 15, transcripts are observed exclusively in the fat body.

srp is a marker for all the hematopoietic cells of the lymph gland.

srp protein is first detected in fat cell precursors at embryonic stage 10, when it is expressed in a cluster of cells in the dorsal mesoderm of parasegment 13, and in segmentally repeated cell clusters in the lateral mesoderm of parasegments 4-9. At embryonic stages 11-12, additional expression is observed in segmentally repeated cell clusters in the ventral mesoderm of parasegments 3-11; these clusters are immediately posterior to the original clusters, and in the equivalent positions of parasegments 10-12.

srp is first detected in hemocyte precursor cells at embryonic stage 5, and is expressed in plasmatocytes through the end of embryogenesis.