A proerythroblast (or rubriblast, or pronormoblast) is the earliest of four stages in development of the normoblast.
In histology, it is very difficult to distinguish it from the other "blast" cells (lymphoblast, myeloblast, monoblast, and megakaryoblast.) The cytoplasm is blue in an H&E stain, indicating that it is basophilic.
Proerythroblasts arise from the CFU-e (colony forming unit erythroid) cells, and give rise to basophilic erythroblasts. In mouse, proerythroblasts are large committed progenitors that express high levels of transferrin receptor (iron acquisition receptor), the erythropoietin receptor (EpoR), some c-Kit (stem cell factor receptor), and are Ter119 (cell surface molecule) positive. Their proliferative capacity is more limited compared to the preceding stage, the CFU-e.
In vivo, starting with the proerythroblast stage, erythroid cells undergo several more cell divisions while at the same time upregulating survival genes such as Bcl-xL, acquiring and storing large amounts of iron, ramping up the synthesis of hemoglobin and other erythroid genes (in large part a GATA-1 dependent process that is augmented by the EpoR signaling) and decreasing in cell size, eventually removing their nucleus and being released into the bloodstream as a reticulocyte.