Histology of Bone (cortical bone) : Shotgun Histology

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Histology of Bone (cortical bone): Shotgun Histology

Bone Cells and Matrix
Bone is a tissue in which the extracellular matrix has been hardened to accommodate a supporting function. The fundamental components of bone, like all connective tissues, are cells and matrix. There are three key cells of bone tissue. They each have unique functions and are derived from two different cell lines.

Osteoblasts synthesize the bone matrix and are responsible for its mineralization. They are derived from osteoprogenitor cells, a mesenchymal stem cell line.
Osteocytes are inactive osteoblasts that have become trapped within the bone they have formed.
Osteoclasts break down bone matrix through phagocytosis. Predictably, they are derived from the monocyte (macrophage) cell line. Think of osteoclasts as the "bone version" of the macrophage. Their activity occurs along their ruffled border, and the space between the osteoclast and the bone is known as Howship's lacuna.
The balance between osteoblast and osteoclast activity governs bone turnover and ensures that bone is neither overproduced nor overdegraded. These cells build up and break down bone matrix, which is composed of:

Osteoid, which is the unmineralized matrix composed of type I collagen and glycosaminoglycans (GAGs).
Calcium hydroxyapatite, a calcium salt crystal that gives bone its strength and rigidity.
Bone is divided into two types that are different structurally and functionally. Most bones of the body consist of both types of bone tissue:

Compact bone, or cortical bone, mainly serves a mechanical function. This is the area of bone to which ligaments and tendons attach. It is thick and dense.
Trabecular bone, also known as cancellous bone or spongy bone, mainly serves a metabolic function. This type of bone is located between layers of compact bone and is thin and porous. Located within the trabeculae is the bone marrow.
Macroscopic Bone Structure
Long bones are composed of both cortical and cancellous bone tissue. They consist of several areas:

The epiphyses are at the ends of the long bone and are the parts of the bone that participate in joint surfaces.
The diaphysis is the shaft of the bone and has walls of cortical bone and an underlying network of trabecular bone.
The epiphyseal growth plate lies at the interface between the shaft and the epiphysis and is the region in which cartilage proliferates to cause the elongation of the bone.
The metaphysis is the area in which the shaft of the bone joins the epiphyseal growth plate.
Different areas of the bone are covered by different tissue:

The epiphyses are lined by a layer of articular cartilage, a specialized form of hyaline cartilage, which serves as protection against friction in the joints.
The outside of the diaphysis is lined by periosteum, a fibrous external layer onto which muscles, ligaments, and tendons attach.
The inside of the diaphysis, at the border between the cortical and cancellous bone and lining the trabeculae, is lined by endosteum.
Microscopic Bone Structure
Compact bone is organized as parallel columns, known as Haversian systems, which run lengthwise down the axis of long bones. These columns are composed of lamellae, concentric rings of bone, surrounding a central channel, or Haversian canal, that contains the nerves, blood vessels, and lymphatic system of the bone. The parallel Haversian canals are connected to one another by the perpendicular Volkmann's canals.

The lamellae of the Haversian systems are created by osteoblasts. As these cells secrete matrix, they become trapped in spaces called lacunae and become known as osteocytes. Osteocytes communicate with the Haversian canal through cytoplasmic extensions that run through canaliculi, small interconnecting canals.

The layers of a long bone, beginning at the external surface, are therefore:

Periosteal surface of compact bone
Outer circumferential lamellae
Compact bone (Haversian systems)
Inner circumferential lamellae
Endosteal surface of compact bone
Trabecular bone

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