7-3 Bone

CELLS OF THE BONE TISSUE – 1

The bone cells come from two lineages: the osteoblastic lineage and the osteoclastic lineage.
The osteoclastic lineage is composed of osteoclasts. These cells originate from stem cells produced in the bone marrow that reach the bone tissue through the blood circulation. The fusion of these precursors produce 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, remodeling or repairing.
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 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 are similar to a simple cuboid epithelium. They are bone-forming cells because they synthesize and secrete the peculiar bone extracellular matrix. After being entirely surrounded by the matrix become incorporated into the already formed bone tissue and are then recognised as osteocytes.
Resting osteoblasts, that are not producing bone tissue, transform into flattened cells and together with osteoprogenitor cells constitute the so-called bone lining cells.
The majority of bone cells are the osteocytes, the mature bone cells. They are situated inside the bone, surrounded by the mineralized bone matrix. Osteocytes occupy small cavities of the matrix called lacunae. They have cytoplasmic extensions that occupy very narrow tunnels placed within the bone matrix and establish contacts 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 image is still being formed, hence 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 the rows apposed on the trabecular surfaces belong to osteoblasts whereas the nuclei seen inside the trabeculae belong to osteocytes.
The less stained areas placed between trabeculae, highlighted in light blue, are constituted by loose connective tissue.

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