The three major cell lines in bone marrow are erythroid (red blood cells), myeloid (white blood cells), and megakaryocytic (platelets). These cells make up what is known as red marrow. The marrow responds to increases and decreases in circulating cell numbers by decreasing or increasing cellular production. It must not be forgotten, however, that a significant portion of the medullary cavity is also made up of adipose tissue (adipocytes), and the cellularity of the cavity shifts towards a greater proportion of fat (yellow marrow) as one ages.
Morphologic variations aid in determining causation for disease. Listed below is a brief discussion of the common morphologic variations seen in red blood cells (RBCs), white blood cells (WBCs) and platelets.
Small erythrocytes are described as microcytic (smaller size of each RBC) and seen with anemias due to iron deficiency and hemoglobinopathies. Such RBCs may also be hypochromic (less hemoglobin in each RBC). There is usually increased anisocytosis (variation in size) and poikilocytosis (variation in shape).
Large erythrocytes are seen with megaloblastic anemias, such as those due to vitamin B12 (cobalamin) and folic acid deficiencies. These cells are classified as macrocytic (larger size of each RBC).
Fragmented erythrocytes (schistocytes, "helmet cells") are seen with microangiopathic hemolytic anemias, such as with disseminated intravascular coagulopathy (DIC), thrombotic thrombocytopenic purpura (TTP) and hemolytic-uremic syndrome (HUS).
Schistocytes are a "critical value" that requires immediate notification, because they indicate a life-threatening condition is present.
An additional test of value in microangiopathic hemolytic anemias (MAHAs) is the D-dimer, a measure of the generation of fibrin degradation products from thrombus formation and lysis. Elevation of the D-dimer supports a diagnosis of MAHA.
Hypersegmentation: Polymorphonuclear leukocytes (PMNs) normally have 3 or 4 lobes, but 5 or 6 or more lobes indicate hypersegmentation. This is seen most often with megaloblastic anemias, myeloproliferative disorders, or following chemotherapy (methotrexate).
Toxic granulations: Increased number and prominence of the azurophilic (primary) granules; seen most often with bacterial infections and myeloproliferative disorders.
Döhle body: Irregularly shaped blue staining area in the cytoplasm due to free ribosomes or RER; seen with infections and myeloproliferative disorders.
Pelger-Huët anomaly: Bilobed neutrophils are seen in 1 in 5000 persons as an autosomal dominant trait. Pseudo-Pelger-Huët anomaly is acquired from underlying hematologic disorders such as leukemia.
The so-called "atypical lymphocytes" are characteristic for infectious mononucleosis from Epstein-Barr virus (EBV) infection.
Smudge cell / Basket cell: A ruptured cell remnant, classically associated with fragile lymphocytes in Chronic Lymphocytic Leukemia (CLL).
Big platelets: may be an adaptation from prolonged thrombocytopenia.
Small platelets: may be seen with iron deficiency or a variety of other conditions.