The Lacunocanalicular Network
Bone represents a porous tissue containing a fluid phase, a solid matrix, and cells. Movement of the fluid phase within the pores or spaces of the solid matrix translates endogenous and exogenous mechanobiological,biochemical and electromechanical signals from the system that is exposed to the dynamic external environment to the cells that have the machinery to remodel the tissue from within. Hence, bone fluid serves as a coupling medium, providing an elegant feedback mechanism for functional adaptation.
-Melissa L. Knothe Tate “Whither does the fluid go?”
Bone, has physically, conceptually, and metaphorically been associated with built structures throughout the history of architecture. Its use in common discourse is still pervasive today and critics, jurors, and architects will discuss the “skeleton” of the building. Though this too, is a predominately 19th century phrase, in fact most of building, practice as well as conception, is still about structuring the skeleton first, and wrapping the skin second. A whole host of adjectives and characteristics present themselves along with “structure” and “skeleton,” most of them in the order of rigid, stiff, and static; terms that we have seen are only partially applicable to biological bone. But while the understanding of bone from the bio-mechanical perspective has advanced since the late 19th century, so has our understanding of structure…to a point.
Bone is not a homogenous material
It is a complex matrix of calcified members creating a more or less rigid sponge. Depending on the species, age, and condition of the bone this matrix will be vary widely. For much of the 19th and early 20th century this matrix was the focus of almost all of the related mechano-biological research. What was neglected was the equally complex networks of fluid transport and distribution within the bone matrix. The fluid in bone, marrow and blood (otherwise know as intra-osteous fluid), is responsible for two main roles – the reabsorption and extraction of calcium and other minerals in the bone, and regulating morphology. In short, if one is to begin to understand where and how a bone will deform, morph or grow, one must understand the fluid flow within.
The morphological system of intra-oseous fluid flow:
Bone functions at a range of scales. As a system of bones, the skeleton “fulfills scaffolding, armoring, and damping functions necessary for survival of mobile terrestrial organisms (Tate).” If we zoom in, at the level of individual bone the tissue provides a surface for the exchange and filtering of solutes from the vascular and lymphatic system. At the cellular level, the living cells within the bone tissue provide the “remodeling” machinery necessary for bone to adapt to its dynamic functional demands, and finally, osteocytes, cells deep within bone tissue and osteoblasts, cells that produce osteocytes, form a communication network that controls the remodeling of the bone. The osteoblasts lie on the edges of the bone tissue in contact with the fluid passing in and around the bone and signal back to the osteocytes when there’s a chemical, electrical or mechanical load applied to the bone. Though a mechanical load might effect the entire bone at the tissue level, at the cellular level it is primarily fluid pressure within the lacunocanalicular network that translates mechanical loads to receptor osteoblasts (Tate). Then, once the signal has been received, chemical changes are made in the osteocyte influencing it to develop or to be reabsorbed. The entire process is a cycle of absorption and development, growth and decay. As Tate describes, though complex, the process follows a rigid hierarchical structure. 1) mechanical loads are applied to a bone (i.e. walking) 2)that load causes a pressure gradient inside the fluid network of the bone 3) this pressure difference is detected by receptor osteoblasts lining the inside of the porous spaces within the bone and signaled to cells deep in the structure to either increase development, remain the same, or to begin to degrade, and if to degrade, then to be reabsorbed. The chemical transformations within each osteoblast as it “makes this decision” to grow or die is largely unknown but it is clear that the process of bone growth and adaptation is begun at the cellular level and is inextricable from fluid flow in the lacunocanalicular network. The larger implication here though is that bone, rather than being a static, rigid homogenous material is a constantly changing expression of “endogenous and exogeous mechanobiological signals”, in short a map of internal and external forces.
to be continued…