Bone and Cartilage – its Structure and Physical Properties - Biomechanics of Hard Tissues - page 38

Each canaliculus carries a fine osteocyte process; at the points of contact of
these processes, gap junctions between the adjacent plasma membranes are the
sites of interchange of materials and information between the cells. This system
is essential to the survival of compact osteonal bone. Canaliculi are not extensively
developed in the flat plates of cancellous (spongy) bone, because the plates are thin
and diffusion transfer of products can occur between the osteocyte and the nearby
marrow space, cf. Figure 1.11. Osteocytes are thought to function as a network of
sensor cells in bones, which can mediate the effects of mechanical loading through
their extensive communication network of Kendall's type, cf. [23, 24], also [26, 27].
It was found by Tanaka et al . that osteocytes from chick embryonic calvariae
stimulated the osteoclast formation and function while maintaining osteocytic
features in vitro . Isolated chick osteocytes stimulate formation and bone-resorbing
activity of osteoclast-like cells. These results suggested that osteocytes may play a
role in osteoclast recruitment [163].
The problem was also studied by Rubinacci et al ., who have proposed a model
system for integrated osseous responses to mechanical, pharmacological, and
endocrine signals [164].
The distribution of the osteocyte processes through the bulk of bone differs
from their distribution on the bone surface. In [161] and [165], Bonewald pre-
sented observations made under a magnification of 3000 × in scanning electron:
micrographs of resin-embedded acid-etched mouse bone samples. It is visible as
to how osteocyte cell processes pass through the bone in thin canals (canaliculi),
connecting osteocytes with each other and with cells on the bone surface. In [165],
Bonewald gives an analysis indicating that the osteocytes (not only osteoclasts) have
both matrix forming and matrix destroying activities and that the osteocytes can
remodel bone's local environment including lacunae and canaliculi, cf. also [101].
1.3.4.5 Osteoclasts
Osteoclasts are specialized cells responsible for bone resorption. They are derived
from the monocyte/macrophage hematopoietic lineage. They develop and adhere
to bone matrix and then secrete acid and lytic enzymes that degrade the bone
matrix in a specialized, extracellular compartment.
Osteoclasts are large multinucleated cells (Figure 1.24). The nuclei resemble
the nuclei of the osteoblasts and osteocytes. The cytoplasm has often a foamy
appearance due to a high concentration of vesicles and vacuoles. An osteoclast cell
frequently has branching processes.
Osteoclasts may arise from stromal cells of the bone marrow, being related
to monocyte/macrophage cells, derived from granulocyte/macrophage-forming
colony units (CFU-GM). They may represent fused osteoblasts or may include
fused osteocytes liberated from resorbing bone. Osteoclasts lie in shallow cavities
(depressions, pits, or irregular grooves) called Howship's lacunae (or resorption
lacuna), formed in the bone that is being resorbed by osteoclasts [166-169].
At the site of active bone resorption, the osteoclast forms a specialized cell
membrane, the ruffled border , that touches the surface of the bone tissue. The
ruffled border increases the surface area interface for bone resorption, facilitates