Bone marrow was a biological tissue found within the larger bones of many species. Generally, bone marrow produced oxygen-carrying blood cells (erythrocytes), among other types, or cells that could become fat, cartilage, or bone.
In 2267, Pavel Chekov was subjected to a variety of medical tests, including sampling of his bone marrow, following the discovery of his apparent immunity to the radiation which caused rapid aging of all members of a landing party except him. (TOS: "The Deadly Years")
In 2371, Kes discovered the genetic markers in Seska's blood did not match that of a Bajoran, but rather appeared to be Cardassian. When confronted with this evidence by Chakotay, Seska claimed to have survived an outbreak of Orkett's disease due to a bone marrow transplant from a sympathetic Cardassian woman named Kattell. However, The Doctor knew that a bone marrow transplant could not explain the genetic markers, and Seska was exposed as a Cardassian spy. (VOY: "State of Flux")