7-3 Bone

CELLS OF THE BONE TISSUE – 1

The bone cells arise from two lineages: the osteoblastic lineage and the osteoclastic lineage.
The osteoclastic lineage is composed of osteoclasts. They originate from stem cells produced in the bone marrow that reach the bone tissue through the blood circulation. The fusion of these stem cells results in the osteoclasts, large multinucleated cells placed on bone surfaces. They act on the resorption of the bone tissue in several situations, such as during bone growth, bone remodeling or repair.
The osteoblastic lineage originates from mesenchymal cells and is constituted by the osteoprogenitor cells also called osteoprogenitors, osteoblasts and osteocytes.
An important fact about bones: all of its surfaces, either outer or inner surfaces, are covered by cells.
The osteoprogenitor cells, as their name implies, genetrate the other blood cells. They rest on bone surfaces and have the potential to differentiate into osteoblasts, also located in bone surfaces.
The active osteoblasts are cuboidal cells and in histological sections resemble a simple cuboidal epithelium. They are bone-forming cells because they synthesize and secrete the peculiar bone extracellular matrix. After being entirely surrounded by the matrix the become incorporated into the already formed bone tissue and are recognised as osteocytes.
Resting osteoblasts, that are not producing bone tissue, convert to flattened cells and together with osteoprogenitor cells form the so-called bone lining cells.
The osteocytes, the mature bone cells are the majority of bone cells. They are positioned inside the bone tissue, surrounded by the mineralized bone matrix. The osteocytes occupy small cavities of the matrix called lacunae. Cytoplasmic extensions of the osteocytes occupy very narrow tunnels placed within the bone matrix and connect with extensions of other osteocytes.
Osteoblasts and osteocytes are easily recognizable in histological sections, but not so the osteoprogenitor cells.

The bone tissue shown in the figures is still being formed, meaning that it is not a mature bone.

Upper figure
The short tissue stretches stained bluish/pink that merge with similar stretches are bone trabeculae. They become highlighted pink after using the mouse cursor or clicking on the image.
Most of the nuclei seen as rows apposed on the trabecular surfaces belong to osteoblasts whereas the nuclei present inside the trabeculae belong to osteocytes.
The less stained areas placed between trabeculae, highlighted in light blue, are loose connective tissue.

Lower figure
Osteocytes inside the bone tissue (highlighted in light green) are surrounded by the bone extracellular matrix.
Most of the nuclei disposed in rows and placed on trabecular surfaces belong to osteoblasts (highlighted in orange).
Observe in the lower right corner of the figure a large osteoclast placed on the surface of a trabecula (highlighted in blue and indicated by an arrow).