Macroscopic Structures of the Skeletal System

You have learned that bone tissue is classified into two types based on structure: compact bone and spongy bone. The parallel arrays of lamellae are organized into different arrangements depending on the bone structure. The lamellae in compact bone form tubular structures, called osteons. The osteons of compact bone are oriented in the direction of the load-bearing axis. The osteons also create a central canal for the passage of blood vessels. Osteocytes in the osteons are embedded in small cavities called lacunae (singular is lacuna), and are oriented around the central canal parallel with the lamellae on the load-bearing axis. The diaphyses of long bones are stronger on their long axis than in any other direction, because of the parallel array of osteons and osteocytes.

The lamellae in spongy bone form a random mesh-like structure of interconnecting plates called trabeculae. Likewise, osteocytes within spongy bone are randomly arranged. The strongest trabeculae in spongy bone are arranged on the bone axis that undergoes the most stress. Flat bones of the skull are primarily made of spongy bone and are good at resisting forces from many directions because of the trabecular arrangement.

In both types of bone tissue, the mineral components, calcium and phosphate, combine with collagen to provide the compressive and tensile strength of bone. Spongy and compact bone tissues are combined to create bones, which store and release calcium and phosphate into the blood through constant resorption and deposition. Many bones then articulate with each other to form the skeletal system.

Ossification Process and Bone Repair Mechanisms

Bones form in two ways. A process known as intramembranous ossification forms bones that develop from layers of connective tissue. Flat bones such as those found in the skull develop through this process. Endochondral ossification (from the word roots endo-, meaning “within,” and chondral, meaning “cartilage”) is bone formation from a hyaline cartilage blueprint or template, which determines the future bone shape. Bones of the limbs and extremities develop through endochondral ossification. For example, an infant’s arm and leg bones contain only small amounts of actual hard bone material; they are primarily made of cartilage. As the child grows, bone replaces the cartilage.

Ossification is the process of forming bone. You learned that there are two types of ossification:

  • intramembranous ossification, which is direct synthesis of bone by specialized stem cells (mesenchymal cells) from fibrous connective tissue; and
  • endochondral ossification, which is synthesis of bone from a (hyaline) cartilage template.

Intramembranous Ossification

Intramembranous ossification is the process that forms and repairs the flat bones of the skull, clavicles and other irregularly shaped bones. In some situations of bone repair and adaptation to excessive force, intramembranous ossification generates new bone.

The process of intramembranous ossification involves multiple steps:

  1. Increased vascularization.
  2. Recruitment of mesenchymal stem cells
  3. Differentiation
  4. Secretion of osteoid
  5. Mineralization
  6. Formation of trabeculae
  7. Formation of outer compact bone

First, the site for future bone formation increases in vascularization—new blood vessels form near the site where the bones will grow. Mesenchymal stem cells, which originate in the embryonic mesoderm, become active and travel through the blood vessels to the future site of bone formation. Chemical messages then cause the mesenchymal stem cells to differentiate: they change into osteoprogenitor cells, which may divide and differentiate into osteoblasts. The osteoblasts deposit osteoid (the unmineralized bone extracellular matrix) and are then trapped in the matrix, where they differentiate into osteocytes. Inorganic salts in the blood travel through the blood vessels to mineralize the bone matrix. As a result, hydroxyapatite crystals form within the osteoid. On the interior of the tissue, small clusters of bone begin to connect with other clusters to form trabeculae. Osteoblasts near the surface of the bone deposit matrix in organized lamellae and form a thin outer layer of compact bone. The periosteum (“peri-” means “surrounding” and “osteum” means “bone”) is living membrane composed of fibrous connective tissue that forms on the outside of the compact bone. Its inside layer has osteoblasts for bone growth and repair.

Endochondral Ossification

Most bones of the skeleton below the skull develop through endochondral ossification.

This process involves the following steps:

  1. Formation of a cartilage template
  2. Growth of the template
  3. Differentiation
  4. Vascularization
  5. Calcification
  6. Bone formation

The first step is formation of a hyaline cartilage template, which is the shape of the desired new bone. The cartilage template grows in size and thickens through the production of new chondroblasts at the perichondrium. The perichondrium is the cartilage equivalent of the periosteum. Chondroblasts differentiate into chondrocytes, which produce chemical messages that stimulate the increase of vascular supply at the perichondrium. This increase in vascular supply brings in inorganic salts, which mineralize the central cartilage matrix.

Cartilage is laid down as a template that provides some mechanical stability. This is like when designers and architects build a template out of balsa wood, clay or foam because it is easy to quickly remodel and manipulate those substances. Then, once the template is worked out, they will remodel it using a stronger material. In bone, the ‘model’ cartilage is remodeled over time and osteoblasts produce a full bone matrix with new collagen and hydroxyapatite. In this way, biology works more efficiently than any engineered tissue graft.

Bone Mechanics, Formation, and Aging

After bone formation, bone resorption and bone deposition occur continually in a process called bone remodeling to allow for skeletal response to mechanical use, nutritional status and as part of the bone repair and healing process. In the absence of malnutrition or disease, this process maintains homeostasis of both total bone mass and inorganic substances such as calcium and phosphate.

As bones age, they tend to decrease in density and, as a consequence, decrease in strength. In some people, especially women, the bones become very brittle and easily broken. The result is a disorder called osteoporosis. Within bones affected by osteoporosis, bone mass and mineral content decrease. As a result, the bones develop canals filled with fibrous and fatty tissues. This leads to an increased risk of bone fracture because the bone organization that is important to weight bearing is lost.

Osteoporosis

Fractures

The microscale structure of a bone gives it significant strength and rigidity, but extreme forces can cause bones to break, or fracture. The table below describes the most common types of fractures.

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Fractures weaken bone, making it less able to perform its functions of support and protection, although once healed, the site of the fracture is stronger than the remaining bone. A fracture may cause a change in the shape of the bone. If that happens, then the way the bone responds to a contracted muscle may change. As a result, fractures can prevent bones from moving correctly when muscles pull on them.A bone fracture affects the bone on several levels of organization. A fracture involves a physical break in the mineral structure of the bone. Fractures typically cause blood vessels in the bone to rupture, reducing the blood flow to the bone tissue. As a result, some of the cells in the surrounding bone die. These dead cells and the related cellular debris are removed by immune cells and osteoclasts. Over time, various cell types—fibroblasts, chondroblasts, and osteoblasts—work together to repair the mineralized bone tissue.

There are pain receptors and nerves in the bone. The pain experienced when a fractured bone moves is one way the body reacts to help itself heal. Bones heal more quickly and thoroughly if they are kept immobilized (which is why the typical treatment for a broken bone is to put it in a cast or other restraint). Because we instinctively avoid actions that cause pain, the pain that occurs when a broken bone is moved causes us to minimize the movement of that bone. This, in turn, helps keep the bone stable while it heals.

Severe fractures, such as compound fractures or comminuted fractures, can cause long-term or permanent disruptions to the body’s homeostasis. Because a compound fracture breaks through the skin, bacteria and other pathogens can enter the body after a compound fracture. Those pathogens can enter the bone, blood, muscles, or other tissues or organs, causing severe infection. Severe fractures are also less likely to heal correctly without medical (typically surgical) intervention. Improperly healed fractures can cause changes in the bone’s reaction to force, leading to changes in body motion (such as a limp).