Focused instruction

Granulocyte maturation proceeds from myeloblast through promyelocyte, myelocyte, metamyelocyte, band, and segmented neutrophil. Chromatin becomes coarser, nucleoli disappear, and nuclear shape changes from round to indented, then band-shaped and segmented. Primary granules appear at the promyelocyte stage and lineage-specific secondary granules begin at the myelocyte stage.

Erythroid cells become smaller as ribosomal RNA falls and hemoglobin accumulates. Cytoplasm shifts from blue to gray-blue to pink, chromatin condenses, and an orthochromatic normoblast extrudes its nucleus to form a reticulocyte. A reticulocyte is anucleate but retains residual RNA detectable by supravital staining, an in vitro stain of living cells that precipitates residual RNA into a visible network.

Megakaryocytes enlarge through endomitosis, becoming polyploid with multilobulated nuclei before producing proplatelet extensions. Lymphoid maturation has an important limit: routine light microscopy does not reliably assign every stage or blast lineage. Do not call an immature blast myeloid or lymphoid from morphology alone; use the local escalation pathway for immunophenotypic workup.