Bone: a tissue and an organ

This composite image shows a long bone, the femur, together with two light micrographs taken from regions shown by the green boxes. The inset image shows an entire cross section through the bone at the levels of the box in the left image. The higher magnification micrographs show the outer surface of the bone (left) and inner surface (right).

This bone is the femur, a long bone located in the thigh. The bulbous end of the bone is the epiphysis, attached to diaphysis, the cylindrical portion below it. The organ bone is composed of bone tissue along with its surface coverings (periosteum, endosteum), marrow tissue, articular hyaline cartilage, and blood and nerve supplies.

Periosteum is a connective tissue layer that covers the exterior surface of bones, excluding the articular surfaces. The periosteum consists of dense connective tissue plus a layer of bone progenitor cells. Osteoblasts and osteoclasts are also located in the periosteum and rest directly on the bone surface.

The endosteum consists of single layer of cells that cover all internal bone surfaces. In non-active regions, the cells are squamous-shaped and include inactive osteoblasts as well as osteoprogenitor cells, which have the capacity to differentiate into osteoblasts. These two cell types are difficult to distinguish from each other. In areas of bone deposition, inactive osteoblasts transform into active, cuboidal-shaped osteoblasts. In areas of bone resorption, osteoclasts are present.

Bone is a highly vascularized organ and tissue.

The central cavity of the bone is called the marrow cavity (X) and contains either red marrow (blood forming) or yellow marrow (fat cells).

The term “bone” is used to refer to both the organ and the tissue. Bone tissue is composed of cells (osteoblasts, osteocytes, osteoclasts and bone lining cells) and an extracellular matrix with organic (fibers and ground substance) and inorganic (calcium phosphate) components.

Osteoblasts are cells that lie on the surface of bone and produce the matrix (deposition), surrounding themselves during the process. Once entirely enclosed by matrix, these cells are called osteocytes.

Osteocytes are located in lacunae, small spaces within the bone matrix. Osteocytes function to maintain the matrix.

Osteoclasts are also located on a bony surface, where they break down the bony matrix (resorption) during remodeling of the bone as well as to mobilize calcium.

The extracellular matrix of bone has both organic components, consisting of fibers and ground substance as well as inorganic components, primarily calcium phosphate (hydroxyapatite). The matrix of mature bone is highly eosinophilic due to its large content of collagen fibers.

Images from the University of Iowa slide collection.