7.2: Osseous Tissue (Bone Tissue)

Osseous Tissue (Bone Tissue)

Bone tissue (osseous tissue) is a hard and mineralized connective tissue. Bone tissue is made up of different types of bone cells. Osteoblasts and osteocytes are involved in the formation and mineralization of bone; osteoclasts are involved in the resorption of bone tissue. Modified (flattened) osteoblasts become the lining cells that form a protective layer on the bone surface. The mineralized matrix of bone tissue has an organic component of mainly collagen called ossein and an inorganic component of bone mineral made up of various salts. Bone tissue is a mineralized tissue of two types: compact bone (cortical) and spongy bone (trabecular or cancellous).

 

Above: A cross section of bone tissue shows the outer layers are composed of compact bone and the inner layers are composed of spongy bone. A membrane composed of dense connective tissue called the periosteum forms the outermost layer of bone.

 

Above: These illustrations of the structure of bone shows compact bone making up outer layers of bone and spongy bone making up the inner layers of bone. Compact bone is organized into osteons, circular structures surrounding a central canal where blood vessels (red and blue), nerves (yellow), and lymphatic vessels (green) can pass through the bone tissue. Circular rings called lamellae surround the central canals in each osteon. Spongy bone consists of slender bony projections called trabeculae. The trabeculae are surrounded by bone marrow tissues. The outermost layer of compact bone is surrounded by dense connective tissue called the periosteum. Between the periosteum and the osteons of compact bone are layers of circumferential lamellae, tissue that surround the entire bone in rings, similar to how the lamellae of an osteon surrounds the central canal.

 

Bone is not uniformly solid, it consists of a flexible matrix and bound minerals, which are intricately woven and endlessly remodeled by a group of specialized bone cells. Their unique composition and design allow bones to be relatively hard and strong, while remaining lightweight. Bone matrix is 90 to 95% composed of elastic collagen fibers, also known as ossein and the remainder is ground substance. The elasticity of collagen improves fracture resistance. The matrix is hardened by the binding of inorganic mineral salt calcium phosphate in a chemical arrangement known as hydroxyapatite. It is the bone mineralization that give bones rigidity.

Bone is actively constructed and remodeled throughout life by special bone cells known as osteoblasts and osteoclasts. Within any single bone, the tissue is woven into two main patterns, known as compact bone and spongy bone, and each with different appearances and characteristics.

 

Compact Bone

The hard, outer layer of bone is composed of compact bone also called cortical bone being much denser than spongy bone. The cortical bone gives bone its smooth, white, and solid appearance, and accounts for 80% of the total bone mass of an adult human skeleton. It facilitates bone's main functions: to support the whole body, protect organs, provide levers for movement, and store and release minerals, mainly calcium and phosphorous. It consists of multiple microscopic functional units, called an osteon. The osteons (tissue organizational structure known as the Harvesian system) are arrange as multiple layers of osteoblasts and osteocytes around a central canal or Haversian canal where blood vessels, nerves, and lymphatic vessels pass through compact bone. The blood vessels, nerves, and lymphatic vessels passing through the central canals connect with other central canals of other osteons and with the periosteum through channels oriented at right angles known as Volkmann's canals.

 

Above: Compact bone. Tissue is magnified by 40x.

 

Osteons are metabolically active, and as bone is reabsorbed and created the nature and locations of the cells within the bone change. Compact bone is covered by a periosteum on its outer surface, and an endosteum on its inner surface. The endosteum is the boundary between the compact bone and the spongy bone. The periosteum is formed of two layers the outer fibrous layer made of dense irregular connective tissue. The outer layer of periosteum contains blood vessels and nerves and the inner layer of periosteum contains osteogenic cells that become osteoblasts (build the matrix) and osteoclasts (reabsorb the matrix). The fibers of the periosteum extend to form Sharpey’s fibers that anchor the periosteum to the bone.

 

Above: Compact bone. Tissue magnified by 200x.

 

Spongy Bone

Spongy bone, also called trabecular bone or cancellous bone, is the internal tissue of bone and is an open cell porous network. Spongy bone has a higher surface-area-to-volume ratio than compact bone because it is less dense. This makes it weaker and more flexible. The greater surface area also makes it suitable for metabolic activities such as the exchange of calcium ions. Spongy bone is typically found at the ends of long bones, near joints and in the interior of vertebrae. Spongy bone is highly vascular and often contains red bone marrow where hematopoiesis, the production of blood cells, occurs. The primary anatomical and functional unit of spongy bone is the trabecula. The trabeculae are aligned towards the mechanical load distribution that a bone experiences within long bones such as the femur.

Thin formations of osteoblasts, covered in endosteum, create an irregular network of spaces, known as trabeculae. The words spongy bone refers to a tiny lattice of boney extensions (trabeculae) that form the tissue. Within these spaces are bone marrow and hematopoietic stem cells that give rise to platelets, red blood cells (erythrocytes) and white blood cells (leukocytes). Trabecular marrow is composed of a network of rod- and plate-like elements that make the overall organ lighter and allows room for blood vessels and marrow. Trabecular bone accounts for the remaining 20% of total bone mass but has nearly ten times the surface area of compact bone.