The pronormoblast, or erythroblast, is the earliest stage in erythroid maturation. It is a very round cell that is about the same size as a myeloblast. It has a distinctive deeply basophilic, velvety cytoplasm that does not have the fine background grittiness found in the myeloblast. A pronormoblast typically has a round, centrally-located nucleus , unlike a myeloblast that typically has an eccentric nucleus.
The chromatin texture is coarser than myeloid chromatin and is more reticular and bumpy, almost like beads on a string. The pronormoblast will have multiple prominent nucleoli. The nuclear membrane appears highlighted compared to other cell types and there will be small breaks in the membrane that are known as nuclear pores. The erythroid lineage is the only cell line that has nuclear pores, which can help to distinguish intermediate erythroid precursors from lymphocytes.
The upper image on the right shows a pronormoblast (red arrow) adjacent to a few monocytes (blue arrows). Notice that the pronormoblast is round and regular and the cytoplasm is intensely basophilic. Observe the central placement of the round nucleus and the nucleoli. Notice the coarse and grainy chromatin texture as well.
The lower image on the right shows a late pronormoblast (red arrow) with a few later stage erythrocyte precursors (blue arrows). While the overall size of the late pronormoblast shown in this image is similar to the cell in the upper image, notice the less prominent nucleoli with the classic reticular grainy pattern of the chromatin. The cytoplasm still has the midnight-blue, velvety-look of a pronormoblast.