The Shaft Of Long Bones Is Called

The diaphysis is the shaft of a long bone, a critical structural element that provides strength and support while housing essential components for bone health and function. Understanding its anatomy, composition, and role is vital for grasping the overall physiology of the skeletal system.

Anatomy of a Long Bone: Setting the Stage

Before diving specifically into the diaphysis, it's helpful to review the general structure of a long bone. These bones, characterized by their length being greater than their width, include the femur, tibia, fibula (in the leg), humerus, radius, ulna (in the arm), and metacarpals, metatarsals, and phalanges (in the hands and feet). A typical long bone consists of the following key parts:

Diaphysis: The long, cylindrical shaft of the bone.
Epiphyses: The expanded ends of the bone. These articulate (form joints) with other bones.
Metaphyses: The regions where the diaphysis and epiphyses meet. In growing bones, this area contains the epiphyseal plate (growth plate).
Articular Cartilage: A thin layer of hyaline cartilage covering the epiphyses where they articulate with other bones. This reduces friction and absorbs shock.
Periosteum: A tough, fibrous membrane covering the outer surface of the bone (except at the articular surfaces). It is rich in blood vessels and nerves and is responsible for bone growth, repair, and nutrition.
Medullary Cavity: A hollow space within the diaphysis that contains bone marrow.
Endosteum: A thin membrane lining the medullary cavity and the inner surfaces of the bone.

The Diaphysis: A Closer Look

The diaphysis, as the main shaft of the long bone, is designed for strength and stability. Its primary functions are to provide leverage for movement and to support the weight of the body. Let's explore its composition and key features:

1. Compact Bone: The Strong Outer Layer

The majority of the diaphysis is composed of compact bone, also known as cortical bone. This type of bone tissue is dense and solid, providing exceptional strength and resistance to bending and twisting forces. Compact bone is organized into structural units called osteons or Haversian systems. Each osteon consists of:

Haversian Canal (Central Canal): A channel running longitudinally through the center of the osteon. It contains blood vessels, nerves, and lymphatic vessels that supply the bone cells.
Lamellae: Concentric layers of bone matrix that surround the Haversian canal. The matrix is composed of collagen fibers and mineral salts, primarily calcium phosphate. The arrangement of collagen fibers in different lamellae provides strength and resilience.
Lacunae: Small spaces located between the lamellae. Each lacuna contains an osteocyte, a mature bone cell.
Canaliculi: Tiny channels that radiate outward from the lacunae, connecting them to the Haversian canal and to other lacunae. These channels allow for the exchange of nutrients and waste products between the osteocytes and the blood supply.
Volkmann's Canals (Perforating Canals): Channels that run perpendicular to the Haversian canals, connecting them to each other and to the periosteum and endosteum. These canals provide routes for blood vessels and nerves to reach the osteons.

The highly organized structure of compact bone makes the diaphysis incredibly strong, allowing it to withstand significant stress and strain.

2. Medullary Cavity: Housing the Marrow

The diaphysis encloses a hollow space known as the medullary cavity (or marrow cavity). In adults, this cavity primarily contains yellow bone marrow, which is rich in fat cells. In children, the medullary cavity contains red bone marrow, which is responsible for hematopoiesis, the production of blood cells.

Even in adults, red bone marrow can be found in certain long bones, particularly in the epiphyses. In cases of severe blood loss or certain medical conditions, yellow bone marrow can convert back to red bone marrow to increase blood cell production.

3. Endosteum: Lining the Inner Surface

The inner surface of the diaphysis, lining the medullary cavity, is covered by a thin layer of connective tissue called the endosteum. The endosteum contains osteoblasts (bone-forming cells) and osteoclasts (bone-resorbing cells), which are essential for bone remodeling and repair. It plays a crucial role in maintaining bone homeostasis and responding to injury.

4. Nutrient Foramen: Gateway for Blood Supply

The diaphysis is supplied with blood vessels through small openings called nutrient foramina. These foramina allow nutrient arteries to enter the bone and provide nourishment to the bone cells. The nutrient arteries branch within the medullary cavity and supply the Haversian canals, ensuring that all parts of the diaphysis receive adequate blood flow.

Development of the Diaphysis: From Cartilage to Bone

The development of the diaphysis is a fascinating process that occurs through endochondral ossification. This process involves the replacement of a cartilage template with bone tissue. Here's a simplified overview:

Cartilage Model Formation: During embryonic development, a cartilage model of the long bone is formed by chondrocytes (cartilage cells). This model has the general shape of the future bone.
Formation of the Bone Collar: Blood vessels penetrate the perichondrium (the membrane surrounding the cartilage model) in the region that will become the diaphysis. This stimulates the perichondrium cells to differentiate into osteoblasts. These osteoblasts secrete bone matrix around the diaphysis, forming a bone collar.
Primary Ossification Center: As the bone collar forms, chondrocytes within the cartilage model in the diaphysis begin to hypertrophy (enlarge) and eventually die. This creates cavities within the cartilage. Blood vessels and osteoblasts invade these cavities, forming the primary ossification center.
Bone Formation: Osteoblasts in the primary ossification center begin to deposit bone matrix on the remaining cartilage, gradually replacing the cartilage with bone tissue. This process extends towards the epiphyses, lengthening the diaphysis.
Medullary Cavity Formation: As bone formation progresses, osteoclasts break down newly formed bone in the center of the diaphysis, creating the medullary cavity.
Secondary Ossification Centers: Secondary ossification centers develop in the epiphyses. Ossification in these centers follows a similar process to that in the primary ossification center, with cartilage being replaced by bone. However, in the epiphyses, cartilage remains on the articular surfaces (forming the articular cartilage) and at the epiphyseal plate (growth plate).
Epiphyseal Plate and Bone Growth: The epiphyseal plate is a layer of hyaline cartilage located between the diaphysis and epiphyses. It is responsible for longitudinal bone growth during childhood and adolescence. Chondrocytes in the epiphyseal plate proliferate and secrete cartilage matrix, which is then replaced by bone on the diaphyseal side of the plate. This process continues until the epiphyseal plate closes, typically in late adolescence or early adulthood, at which point longitudinal bone growth ceases. The epiphyseal plate becomes the epiphyseal line.

Remodeling of the Diaphysis: A Lifelong Process

Bone is a dynamic tissue that is constantly being remodeled throughout life. This process involves the continuous breakdown and formation of bone tissue, primarily by osteoclasts and osteoblasts. Bone remodeling serves several important functions:

Bone Repair: Remodeling allows the bone to repair itself after fractures or other injuries.
Calcium Homeostasis: Remodeling plays a role in regulating blood calcium levels. When blood calcium levels are low, osteoclasts break down bone to release calcium into the bloodstream. When blood calcium levels are high, osteoblasts deposit calcium in bone.
Adaptation to Stress: Remodeling allows bone to adapt to changes in mechanical stress. When bone is subjected to increased stress, it becomes stronger and denser. When bone is subjected to decreased stress, it becomes weaker and less dense.

The diaphysis, with its dense compact bone, is constantly being remodeled to maintain its strength and integrity. This remodeling process is influenced by factors such as hormones, nutrition, and mechanical stress.

Clinical Significance: When the Diaphysis is Compromised

The diaphysis, as a critical structural component of long bones, is susceptible to a variety of clinical conditions. Understanding these conditions is essential for diagnosis and treatment.

Fractures: Fractures are the most common type of injury affecting the diaphysis. These can range from hairline fractures to complete breaks. Fractures of the diaphysis can be caused by trauma, such as falls, accidents, or sports injuries. Treatment typically involves immobilization with a cast or splint, or surgical fixation with plates, screws, or rods.
Osteomyelitis: Osteomyelitis is an infection of the bone, often caused by bacteria. It can occur in the diaphysis and can lead to bone destruction, pain, and fever. Treatment typically involves antibiotics and, in some cases, surgery to remove infected tissue.
Bone Tumors: Bone tumors, both benign and malignant, can develop in the diaphysis. These tumors can cause pain, swelling, and bone destruction. Treatment depends on the type and location of the tumor and may involve surgery, radiation therapy, or chemotherapy.
Osteoporosis: Osteoporosis is a condition characterized by decreased bone density and increased risk of fractures. While it commonly affects the epiphyses and vertebrae, osteoporosis can also weaken the diaphysis, making it more susceptible to fractures. Treatment involves lifestyle modifications, such as exercise and calcium and vitamin D supplementation, as well as medications that increase bone density.
Achondroplasia: Achondroplasia is a genetic disorder that affects bone growth, resulting in dwarfism. It primarily affects the long bones, including the diaphysis, leading to shortened limbs.

The Diaphysis: A Summary of its Importance

The diaphysis is more than just the "shaft" of a long bone. It's a meticulously engineered structure that provides strength, support, and houses vital components for bone health. Here's a quick recap:

Primary Component: The long, cylindrical shaft of long bones.
Composition: Primarily composed of dense compact bone, organized into osteons.
Function: Provides strength and support, resists bending and twisting forces, and houses the medullary cavity.
Medullary Cavity: Contains bone marrow, responsible for blood cell production (in children) and fat storage (in adults).
Remodeling: Constantly remodeled throughout life to maintain strength, repair injuries, and adapt to stress.
Clinical Significance: Susceptible to fractures, infections, tumors, and conditions like osteoporosis.

Frequently Asked Questions (FAQ) about the Diaphysis

What is the difference between the diaphysis and the epiphysis?

The diaphysis is the long shaft of a long bone, while the epiphyses are the expanded ends. The diaphysis is primarily composed of compact bone and houses the medullary cavity, while the epiphyses are composed of spongy bone covered by a thin layer of compact bone and articulate with other bones.
What is the epiphyseal plate?

The epiphyseal plate (growth plate) is a layer of hyaline cartilage located between the diaphysis and epiphyses in growing bones. It is responsible for longitudinal bone growth.
What type of bone marrow is found in the diaphysis?

In adults, the medullary cavity of the diaphysis primarily contains yellow bone marrow, which is rich in fat cells. In children, it contains red bone marrow, which is responsible for blood cell production.
What is the periosteum?

The periosteum is a tough, fibrous membrane covering the outer surface of the bone (except at the articular surfaces). It is rich in blood vessels and nerves and is responsible for bone growth, repair, and nutrition.
How does the diaphysis receive blood supply?

The diaphysis is supplied with blood vessels through small openings called nutrient foramina. These foramina allow nutrient arteries to enter the bone and provide nourishment to the bone cells.

Conclusion: Appreciating the Strength Within

The diaphysis, the shaft of a long bone, is a remarkable example of biological engineering. Its structure is optimized to provide strength, support, and protection, enabling us to move, bear weight, and perform countless activities. Understanding the anatomy, development, and clinical significance of the diaphysis is essential for appreciating the complexity and resilience of the human skeletal system. From the microscopic organization of osteons to the dynamic process of bone remodeling, the diaphysis stands as a testament to the intricate design and function of our bones. By taking care of our skeletal health through proper nutrition, exercise, and preventative measures, we can ensure the diaphysis continues to provide the strength and support we need throughout our lives.