Page 101 - Veterinary Histology of Domestic Mammals and Birds, 5th Edition

Connective and supportive tissues (textus connectivus) 83

the extracellular fluid outside the bone. In this way, they This cellular transport system also plays an important
VetBooks.ir perform an important role in regulating mineralisation role in bone metabolism by mediating uptake and release
of calcium and phosphate, as well as intracellular trans-
and bone metabolism.
port of these ions. While osteoblasts and osteoclasts are
OSTEOCYTE (OSTEOCYTUS) responsible for most of the calcium and phosphate trans-
Osteocytes are mature bone cells that develop from osteo- port, osteocytes contribute to the fine-tuning of blood
blasts (Figures 3.32 and 3.33). This transformation process, calcium homeostasis. Changes in the mechanical load on
which occurs in only around 10–20% of the osteoblast bone result in structural adaptations within the intercom-
population, takes approximately 3 days. Osteocytes have municating network of osteoblasts and osteocytes, with
numerous slender cytoplasmic processes (see below). rapid adjustments in the metabolic activity of mature bone
Osteocytes are surrounded by calcified bone matrix. cells. These observations have led to the conclusion that
They are flattened cells, located in narrow lacunae between osteocytes function as mechanosensory cells capable of
lamellar layers of bone. Once osteocytes are completely repairing microscopic bone defects and revitalising dead
embedded in the matrix, their metabolic activity dramati- tissue.
cally decreases due to a reduction in nutrient diffusion and In this scenario, osteocytes are still able to synthesise
gas exchange. Osteocytes are important components of all bone matrix and liberate calcium, as required. If these cells
bones, making a vital contribution to bone maintenance. are released from the matrix by osteoclasts, they can poten-
When osteocytes degenerate, the surrounding matrix tially transform into osteoblasts. The metabolic functions
becomes resorbed. of osteocytes are regulated by calcitropic hormones.
The long, finger-like processes of osteocytes extend
into canaliculi (canaliculi ossei) within the bone matrix OSTEOCLAST (OSTEOCLASTUS)
(Figure 3.33). These processes establish direct contact Osteoclasts are multinuclear giant cells (10–20, maximum
with nearby cells, chiefly other osteocytes, via gap junc- 100, nuclei per cell) (Figures 3.32 and 3.33) derived from
tions. They participate in intercellular transport of ions, pluripotent haemopoietic stem cells of the granulocyte-
low-molecular weight substances, nutrients and waste monocyte line. The mononuclear precursor cells undergo
products. The dense network of cell processes also permits fusion to produce mature, multinucleate osteoclasts.
osteocytes to communicate with osteoblasts, stimulating Osteoclasts appear to extend their life through repeated
these to undergo differentiation. Through induction, a fusion with mononuclear precursors. Under experimen-
pre-osteoblast subsequently develops into a mature osteo- tal conditions, the lifespan of osteoclasts in the absence
blast that takes the place of the initially stimulated cell. of ongoing fusion has been identified as approximately 6
This system of cellular interconnections combines blood weeks.
vessels, interstitial fluid, osteoblasts and osteocytes into a Osteoclasts lie directly on the surface of bone tissue
functional unit. within lacunae formed by the resorption of bone. Resting
During the development of osteoblasts into osteo- and active forms of osteoclasts are recognised.
cytes, the size of the cell decreases by up to 70% and the Active osteoclasts are highly polarised cells that resorb
number of organelles, particularly the rER and Golgi cis- bone. The basolateral cell surface is oriented towards the
ternae, diminishes. At some point, the lacuna in which blood capillaries. At the apical surface of the cell, infold-
osteocytes are located is partially or wholly surrounded ing of the plasmalemma gives rise to numerous finger-like
by matrix that has not yet mineralised. These young cells structures that increase the area of the cell surface (ruf-
are referred to as ‘immature osteocytes’. fled border). These frequently branching projections
Immature osteocytes are located relatively close to the specifically serve to resorb the bone.
surface of the bone, between the osteoblast layer and the At an ultrastructural level, the ruffled border exhibits
calcified bone matrix. With advancing age and maturity, hair-like projections (15–20 nm). Associated with these are
the cell becomes increasingly incorporated into calcified numerous vesicles that secrete acid through the ruffled
bone until, as a mature osteocyte, it is wholly surrounded border into the extracellular space. The resulting low pH
by mineralised matrix. initiates dissolution of the mineral matrix. Digestion of
Osteocytes have a large, typically ovoid nucleus and rel- the organic matrix occurs via lysosomal enzymes released
atively few metabolically active organelles (Figures 3.32 and from the osteoclast.
3.33). Gap junctions connect osteocytes with each other, Adjacent to the ruffled border is the ‘clear zone’ (seal-
and with cells at the bone surface. Their cell processes ing zone), a region in which the cell adheres to the bone
contain microfilaments that may assist in the transport matrix via specific binding structures (podosomes).
of small molecules. Repeated shortening and lengthen- This essentially delineates the micro-environment in
ing of the cell processes facilitates diffusion of nutrients which bone resorption takes place. The cytoplasm of the
and waste products into the extracellular compartment. clear zone is free of organelles but contains cytoskeletal

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