nml marrow1 1024x687 What does normal bone marrow look like?

There are several things you should look for when evaluating a bone marrow biopsy specimen – see if you can see them in the image above.

First, take a look at the cellularity. The white spaces are fat cells that have washed out during processing; the cells in between the fat cells are hematopoietic precursors. The ratio of cells to fat is called the “cellularity.” The marrow above is approximately 30-40% cellular. You need to know the age of the patient to estimate whether the cellularity is normal. Here is a rough guide to cellularity by age:

0-3 months: 100%
3 months – 10 years: 80%
20 years: 65%
30 years: 50%
40  years: 45%
50 years: 40%
60 years: 35%
70 years and over: about 30%

Next, take a look at the composition of the marrow. Myeloid cells make up the largest percentage of the normal marrow cellularity; erythroid cells are second most common. The ratio of myeloid to erythroid cells should be about 2:1 to 4:1. It’s easier to see these cells on an aspirate smear, but you can get a pretty good idea on the marrow section too. Neutrophils and precursors often have eosinophilic, granular cytoplasm; if you look closely, you can see the indented nuclei of metamyelocytes and segmented nuclei of mature neutrophils. Erythroblasts generally have very round, dark nuclei; earlier forms are large, and later forms are small. A few megakaryocytes (large cells with abundant eosinophilic cytoplasm and multiple nuclei) should be sprinkled throughout the marrow too. Lymphocytes normally represent about 10-15% of the marrow cellularity. The above marrow appears to have a myeloid:erythroid ratio of 2:1, and megakaryocytes are normal in number.

Finally, take a look through all the sections to see if you see anything weird, like fibrosis, metastatic carcinoma, lymphoid aggregates, or amyloid deposition. As you scan the sections, you should see evenly-distributed cellularity, with evenly-spread hematopoietic precursors. Anything that deviates from this pattern should be investigated on higher power.

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