Fat tissue occupies about 50% of the space in adult blood-forming bone marrow. There is less fat tissue in childhood and it increases with age. Fat cells are part of the bone marrow stroma.
Staining the bone marrow film, the fat is dissolved. What remains is a large empty and round vacuole with a weakly stained cytoplasm and a small, flat nucleus pressed to the edged. The picture resembles a signet ring ("signet ring cells"). Fat cells have no particular characteristic, and are not specifically assessed in the bone marrow film.