How is examination of bone marrow done?

We take a specimen of bone marrow and inspect it, may be aspiration or biopsy, usually done from pelvis (posterior iliac spine) or sternum (usually only aspiration not biopsy since very painful and risky) We see cytosis of bone marrow and fat tissue, w/ age fat predominates and cytosis is decreased, important to know in order to see any disorder according to the age of presentation