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An osteocyte, an oblate shaped type of bone cell with dendritic processes, is the most commonly found cell in mature bone. It can live as long as the organism itself. [1] The adult human body has about 42 billion of them. [2] Osteocytes do not divide and have an average half life of 25 years.
In an ordinary microscopic section, viewed by transmitted light, they appear as fusiform opaque spots. Each lacuna is occupied during life by a branched cell, termed an osteocyte, bone-cell or bone-corpuscle. Lacunae are connected to one another by small canals called canaliculi. A lacuna never contains more than one osteocyte.
Intra-cellular features are characteristic of a synthetically active cell. The cell density of full-thickness, human, adult, femoral condyle cartilage is maintained at 14.5 (±3.0) × 10 3 cells/ mm 2 from age 20 to 30 years. Although chondrocyte senescence occurs with aging, mitotic figures are not seen in normal adult articular cartilage.
Within these spaces are bone marrow and hematopoietic stem cells that give rise to platelets, red blood cells and white blood cells. [16] Trabecular marrow is composed of a network of rod- and plate-like elements that make the overall organ lighter and allow room for blood vessels and marrow.
In embryogenesis, the skeletal system is derived from the mesoderm germ layer. Chondrification (also known as chondrogenesis) is the process by which cartilage is formed from condensed mesenchyme tissue, which differentiates into chondroblasts and begins secreting the molecules (aggrecan and collagen type II) that form the extracellular matrix.
The axial skeleton (80 bones) is formed by the vertebral column (32–34 bones; the number of the vertebrae differs from human to human as the lower 2 parts, sacral and coccygeal bone may vary in length), a part of the rib cage (12 pairs of ribs and the sternum), and the skull (22 bones and 7 associated bones).
Bone is a highly vascular tissue, and active formation of blood vessel cells, also from mesenchymal stem cells, is essential to support the metabolic activity of bone. The balance of bone formation and bone resorption tends to be negative with age, particularly in post-menopausal women, [ 7 ] often leading to a loss of bone serious enough to ...
Bone marrow is a semi-solid tissue found within the spongy (also known as cancellous) portions of bones. [2] In birds and mammals, bone marrow is the primary site of new blood cell production (or haematopoiesis). [3]