Bone

Overview of blood supply

Overview of Terms

Anatomical Zones of a Long Bone

To understand the vascular layout, a long bone is divided into three distinct structural regions:

1. The Epiphysis: The rounded, expanded tip found at both ends of a long bone. It is primarily composed of spongy (trabecular) bone and contains red bone marrow. Its outer surface is typically covered by articular cartilage to form joints.
2. The Metaphysis: The transitional zone sandwiched between the epiphysis and the diaphysis. In growing children, this area contains the epiphyseal (growth) plate, a layer of hyaline cartilage responsible for longitudinal bone growth. Once growth ceases after puberty, this cartilage ossifies into an epiphyseal line, and the metaphysis functions primarily to transfer mechanical loads from the joint to the shaft.
3. The Diaphysis: The main elongated cylindrical shaft of the bone. It features a thick outer wall of compact bone surrounding a central medullary (marrow) cavity, which houses yellow bone marrow in adults.

Bone Development

1. Intramembranous Ossification

Bone develops directly from sheets of mesenchymal (embryonic) connective tissue without any pre-existing cartilage.

1. Formation of Ossification Center: Mesenchymal cells gather at a specific site and differentiate directly into bone-forming cells (osteoblasts).
2. Matrix Secretion: Osteoblasts secrete osteoid (the unmineralized, collagen-rich organic bone matrix).
3. Calcification: Calcium and phosphate minerals deposit onto the osteoid, hardening the matrix. Osteoblasts trapped inside their own matrix become mature osteocytes.
4. Formation of Trabeculae (Spongy Bone): The mineralizing matrix forms a network of trabeculae around embryonic blood vessels, creating spongy (trabecular) bone.
5. Periosteum & Compact Bone Formation: Mesenchyme on the outside condenses to form the periosteum. Just beneath the periosteum, a thin layer of compact bone replaces the outer edges of the spongy bone.

Key Characteristics & Locations

• Types of Bones: Exclusively forms flat bones.
• Anatomical Examples: * Flat bones of the skull vault (e.g., frontal, parietal, occipital, and temporal bones).
  • Most of the facial bones (e.g., maxilla, zygomatic).
  • The clavicle (collarbone) — notably the first bone in the body to begin ossification.

2. Endochondral Ossification

Bone develops by replacing a pre-existing, temporary model made of hyaline cartilage.

1. Cartilage Model Blueprint: Mesenchymal cells differentiate into chondrocytes to create a miniature hyaline cartilage model of the future bone, covered by a perichondrium.
2. The Bone Collar: Chondrocytes in the center of the shaft hypertrophy (enlarge) and signal the surrounding matrix to calcify. Cut off from nutrients, these chondrocytes die, leaving empty spaces. Concurrently, blood vessels invade the outer membrane, converting the perichondrium into a periosteum, and osteoblasts lay down a ring of compact bone called a periosteal bone collar around the shaft.
3. Primary Ossification Center: A "periosteal bud" (consisting of blood vessels, osteoblasts, and osteoclasts) invades the internal cavities. Osteoblasts use the remaining calcified cartilage framework to lay down spongy bone tissue. This central shaft area is the primary ossification center.
4. Medullary Cavity Development: As the primary center spreads toward the ends, osteoclasts break down the newly formed internal spongy bone to open up the central medullary (marrow) cavity.
5. Secondary Ossification Centers: Around the time of birth, blood vessels enter the ends of the bone, creating secondary ossification centers within the epiphyses.
6. Retention of Cartilage: Hyaline cartilage is preserved at two crucial sites:
   • On the joint surfaces as articular cartilage.
   • Between the diaphysis and epiphysis as the epiphyseal (growth) plate, allowing the long bone to grow in length throughout childhood.

Key Characteristics & Locations

• Types of Bones: Forms long, short, and irregular bones.
• Anatomical Examples: * Long bones of the limbs (e.g., femur, tibia, humerus, radius).
  • Vertebrae of the spine.
  • Bones of the pelvic girdle.
  • Bones at the base of the skull (e.g., sphenoid, ethmoid).

Summary of Differences

Classification of Epiphyses

To understand why traction epiphysis is correct, let's look at how epiphyses are classified based on their function and development:

• Traction Epiphysis: This type is strictly non-articular and non-weight bearing. It develops under the tensile pulling forces (traction) exerted by powerful attached tendons or muscles.
  • Examples: The greater and lesser trochanters of the femur, and the greater and lesser tubercles of the humerus.
• Pressure Epiphysis: This type is articular and weight-bearing. It is found at the ends of long bones that transmit body weight and form joints.
  • Examples: The head of the femur (hip joint) and the head of the humerus (shoulder joint).