7-3 Bone

CELLS OF THE BONE TISSUE – 1

Attention must be paid to the following: besides the cells placed inside the bone tissue, all bone surfaces, either outer or inner surfaces, are covered by bone cells.
Bone cells arise from two lineages: the osteoblastic lineage and the osteoclastic lineage. Their cells have specific distributions and act on different functions.
The cells of the osteoclastic lineage are the osteoclasts. Their stem cells originate from bone marrow cells. They reach the bone tissue through the blood circulation and in the bones they merge forming osteoclasts, large multinucleated cells always placed on bone (outer or inner) surfaces. They act on the resorption of bone matrix in several situations, such as during bone growth, remodeling or repair.
The osteoblastic lineage originates from mesenchymal cells and is constituted by the osteoprogenitor cells also called osteoprogenitors, osteoblasts and osteocytes.
The osteoprogenitor cells, as their name implies, generate the other bone cells. They rest on bone surfaces and have the potential to differentiate into osteoblasts, also placed on bone surfaces.
The active osteoblasts are cuboidal 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 this matrix the become incorporated into the already formed bone tissue and are recognised as osteocytes.
Osteoblasts that ceased originating osteocytes convert to flat cells and are called inactive osteoblasts. Together with osteoprogenitors they form the so-called bone lining cells.
Osteocytes are the mature bone cells and constitute the majority of cells that inhabit the interior of the bone tissue, placed inside small matrix cavities called lacunae and are surrounded by mineralized bone matrix.
The osteocytes emit delicate cytoplasmic extensions. They occupy very narrow tunnels placed within the bone matrix. These extensions connect with extensions of neighbor 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 yet a mature bone.

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

Lower figure
Osteocytes (highlighted in light green) inside the bone tissue 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 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).