Normal Hematopoiesis And Cell Morphology

Hematopoiesis: From Stem Cell To Mature Cell

Hematopoiesis is the production of blood cells. In adults it occurs in the red marrow of the axial skeleton (sternum, ribs, vertebrae, pelvis, proximal femur/humerus). It begins with the pluripotent hematopoietic stem cell (HSC), which gives rise to two pathways: the common myeloid progenitor (red cells, granulocytes, monocytes, platelets) and the common lymphoid progenitor (T cells, B cells, NK cells).

Growth factors drive lineage commitment. The MLS exam expects you to pair the cytokine to its target: erythropoietin (EPO) from the kidney stimulates red cell production in response to hypoxia; thrombopoietin (TPO) from the liver drives megakaryocytes and platelets; G-CSF stimulates neutrophils; GM-CSF stimulates granulocytes and monocytes; IL-5 supports eosinophils.

Erythrocyte Maturation Sequence

Memorize this order (oldest mnemonic: "Please Be Polite Or Pay Real-money Reticulocyte"):

Reticulocytes stained with new methylene blue show reticulum (precipitated RNA). Normal reticulocyte count is 0.5-2.5% in adults. An exam stem describing polychromasia on a Wright stain is describing reticulocytes. The overall maturation theme is consistent across every lineage: as a cell matures, the nucleus condenses and shrinks (chromatin clumps, nucleoli disappear), the N:C ratio falls, and the cytoplasm shifts from deeply basophilic to its mature tint. Basophilia reflects abundant ribosomal RNA in young cells; as protein synthesis declines and specific products (hemoglobin, granules) accumulate, the blue fades.

Tying any stage on the exam back to these two trends - nucleus shrinking, cytoplasm maturing - lets you order unfamiliar cells correctly.

The entire erythroid maturation series from pronormoblast to mature red cell takes roughly 3-5 days in the marrow, and the cell typically divides about four times, so one pronormoblast yields about 16 mature red cells. The marrow normally holds far more developing cells than the blood, and only the reticulocyte and mature red cell normally leave the marrow. Seeing nucleated red blood cells (NRBCs) in the peripheral blood of an adult is therefore abnormal and signals marrow stress, severe hemolysis, or a marrow-replacing process.

Granulocyte Maturation

The neutrophil sequence is: myeloblast -> promyelocyte -> myelocyte -> metamyelocyte -> band -> segmented neutrophil. The promyelocyte makes primary (azurophilic) granules; the myelocyte makes secondary (specific) granules. The metamyelocyte is the last stage that cannot divide and has an indented (kidney-shaped) nucleus. A common trap: the myelocyte is the last dividing granulocyte stage, not the metamyelocyte.

Granulocytes mature through a mitotic pool (myeloblast, promyelocyte, myelocyte - cells still dividing) and a maturation/storage pool (metamyelocyte, band, segmented - no division). The marrow holds a large storage reserve of segmented neutrophils and bands that can be released rapidly during infection, which is why a bacterial infection produces a left shift so quickly. Eosinophils and basophils follow the same myeloblast-to-myelocyte framework but acquire their characteristic orange (eosinophil) or dark blue-purple (basophil) specific granules at the myelocyte stage.

Lymphocytes and monocytes do not pass through a granulocytic series; lymphoblasts mature to lymphocytes, and monoblasts mature through promonocytes to monocytes, which become tissue macrophages.

Reference Ranges And Normal Morphology

Know these adult values cold; the exam uses them as the baseline for every calculation and abnormal-flagging question:

• WBC: 4.5-11.0 x10^9/L
• RBC: 4.2-5.4 x10^12/L (female), 4.7-6.1 x10^12/L (male)
• Hemoglobin: 12-16 g/dL (F), 14-18 g/dL (M)
• Hematocrit: 36-46% (F), 41-53% (M)
• Platelets: 150-450 x10^9/L
• MCV: 80-100 fL; MCH: 27-31 pg; MCHC: 32-36 g/dL

A normal RBC is 6-8 micrometers in diameter with a central pallor occupying about one-third of the cell. The normal differential is roughly: neutrophils 50-70%, lymphocytes 20-40%, monocytes 2-8%, eosinophils 1-4%, basophils 0-1%. A handy size reference used constantly on the bench and on the exam: a normal red cell is about the size of a small lymphocyte's nucleus. Cells clearly larger than that nucleus are macrocytes; cells clearly smaller are microcytes.

The central pallor estimate translates directly to hemoglobin content - a pallor larger than one-third diameter is hypochromia (low MCH), while loss of central pallor suggests spherocytes or high hemoglobin concentration.

The exam also expects you to recognize normal inclusions and their abnormal counterparts. Reticulocytes (residual RNA) are normal in small numbers, but Howell-Jolly bodies (DNA remnants), basophilic stippling (precipitated ribosomes, seen in lead poisoning), Pappenheimer bodies (iron), and Heinz bodies (denatured hemoglobin) are abnormal. The spleen normally removes inclusion-bearing cells, so finding Howell-Jolly bodies points to absent or non-functioning splenic activity (post-splenectomy or functional asplenia in sickle cell disease).

Worked Example

A peripheral smear shows red cells the size of a small lymphocyte nucleus with central pallor exceeding one-third diameter. This is microcytic, hypochromic morphology, correlating with a low MCV and low MCH, classically iron deficiency. The exam wants you to connect the morphologic word to the index, not just label the picture: microcytic = low MCV, hypochromic = low MCH/MCHC, macrocytic = high MCV. Conversely, if a stem gives you an MCV of 110 fL, you should already be picturing oval macrocytes and possibly hypersegmented neutrophils before the question finishes.

Building this two-way mapping between numeric index and morphologic word is one of the highest-yield skills for the hematology portion of the exam, because nearly every disease-correlation item depends on it.

Common Traps

• Confusing the metamyelocyte (cannot divide, indented nucleus) with the myelocyte (last dividing stage).
• Forgetting that the orthochromic normoblast still has a nucleus but cannot divide.
• Treating reticulocytes as nucleated; they are anucleate but RNA-positive.