Bone marrow is the flexible tissue found in the interior of bones. In humans, bone marrow in large bones produces new blood cells.. It is the engine room of our vascular system producing and releasing red cells, white cells, plasma and platelets into our blood stream.
Funnily enough, marrow was not excluded from the Hebrew diet even though it contained the blood cells :
Insight p 345
In addition it is not excluded from Jehovah's Witnesses medical treatment. They may accept a bone marrow transplant if they wish, even though it contains someone elses blood cells:Animal bone marrow was apparently used for food by the Israelites. (Compare Mic 3:2, 3.) It has a very high nutritional value, being rich in protein, fats, and iron. Jehovah’s banquet for all the peoples, therefore, fittingly includes symbolic “well-oiled dishes filled with marrow.”—Isa 25:6.
Wt 84 5/15 p31
"do the Scriptures class marrow with blood? No ..............................................................................................
Of course, marrow used in human marrow transplants is from live donors, and the withdrawn marrow may have some
blood with it.
Hence, the Christian would have to resolve for himself whether—to him—the bone-marrow graft would amount to simple
flesh or would be unbled tissue. .............................................................................................................................................................................
Though a personal decision has to be made on this matter, .........................................
What does this show us? That it is not the substance of major blood parts that is sacred, but rather the act of pouring out the veinous blood of a killed animal that was required by the Jewish law and carried over to the Christian congregation.
The "Insight Book " explains: (page 344 topic "Blood")
"Every living thing has a purpose and a place in God’s creation. No man has the right to take life except when God permits and in the way that he instructs. After the Flood, Noah and his sons, the progenitors of all persons alive today, were commanded to show respect for the life, the blood, of fellowmen. (Ge 9:1, 5, 6) Also, God kindly allowed them to add animal flesh to their diet. However, they had to acknowledge that the life of any animal killed for food belonged to God, doing so by pouring its blood out as water on the ground. This was like giving it back to God, not using it for one’s own purposes.—De 12:15, 16.Conclusion ? That the reason for the dietry prohibition was just as the Insight Book states - the respect for the life taken, not any "sanctity " of the substance of blood itself, just as their critics have been telling them for years, and in transfusons NO life is taken to get the blood.