For the longest time (meaning 2 years with GDD and 12 years with NSF) consideration was made that Gd presence in bone and skin represented simple deposition, simple replacement of Gd for Ca in bone, simple replacement of Gd for Na in sweat. Could it however be something more? At least something in addition to simple deposition.
I relate this back to 2 points, and relate to a third:
1. what are circulating fibrocytes and other bone marrow-derived cells doing as part of the chronic immune reaction?
2. why are intact GBCAs of macrocyclic agents doing in bone, if atomic Gd is replacing atomic Ca in bone?
3. could this relate to the Ghon focus of primary Tb?
Let us start with the Wrath of Ghon. A Ghon focus (part of the Ghon complex - the complex includes hilar adenopathy) is a focus of primary tuberculosis that the host walls off in the lungs, which contains caseated material but also entrapped viable Tb bacteria that can be reactivated in the future in secondary Tb.
Here is my opinion. The Ghon focus represents the host walling off the Tb bacteria (in an Edgar Allen Poe sort of way), because the advance troops and the main army (see the post on the immune system) can't conquer them. The eating cells (neutrophils and macrophages) can't figure out how to eat them, and the knife-poisoning cells (T-cells) can't figure out how to poison them. So the third string, the occupying force (circulating fibrocytes and related cells, ?calcium producing bone marrow-derived cells) contains them behind a wall in the basement (Poe), because they can't figure out what else to do. In my opinion it is circulating fibrocytes doing this.
So the immune system dealing with a hard metal atom like Gd, the eating cells can take them up but cannot break them down (hard atom), so no eating destruction, poisoned knife cells (T cells) can't poison a static hard metal - so what is left but to corral them and wall them off. The bone marrow is a logical place to sequester and imprison foreign invaders who can't be destroyed. So intact GBCAs are likely imprisoned in the marrow because that is the best place to keep troublesome particles away from vital soft organs. In a similar fashion, the skin is a similar type of repository to trap particles that may otherwise injure soft internal organs. Since the deposition is often diffuse with Gd in skin and bone, unlike with Tb that can be corralled into a Ghon focus, thus the fibrosis to contains it becomes amorphous and irregular, matching the type of deposition.
So although in part the Gd in skin and bone follow the classic thinking of passive deposition or substitution of cations, in fact deliberate activity of cells of the chronic immune system to contain the invaders is likely a prominent explanation for the deposition. The bone marrow is an ideal location for bone marrow derived cells to imprison enemies that they can't kill.