Skeletal Overview: The Axial & Appendicular Skeletal System

Skeletal Overview: The Axial & Appendicular Skeletal System

The human skeleton is divided into two main parts: the axial skeleton and the appendicular skeleton. Together, these structures provide support, protection, and movement for the body.

Axial vs. Appendicular Skeleton

• Axial Skeleton: Consists of the skull, vertebral column, and thoracic cage (ribs and sternum). It forms the central axis of the body and protects vital organs such as the brain, spinal cord, and heart.

• Appendicular Skeleton: Includes the bones of the limbs and girdles (shoulder and pelvic girdles) that attach the limbs to the axial skeleton. It is primarily responsible for facilitating movement.

Bone Structure

Bones are complex organs composed of several types of tissues. Their structure varies depending on their function and location in the body.

Long Bone Structure

• Epiphysis: The rounded end of a long bone, typically covered with articular cartilage for joint movement.

• Diaphysis: The shaft or central part of a long bone, providing strong support.

• Periosteum: A dense, fibrous membrane covering the outer surface of bones except at the joints. It contains nerves and blood vessels that nourish the bone.

• Bone Substance: Refers to the hard, mineralized matrix that gives bone its strength.

• Articular Cartilage: Smooth, white tissue that covers the ends of bones where they come together to form joints, reducing friction and absorbing shock.

• Yellow Marrow (in medullary cavity): Stores fat and is found in the central cavities of long bones.

• Compact Bone: Dense bone tissue that forms the outer layer of bone, providing strength and protection.

• Spongy Bone: Lighter, less dense bone found at the ends of long bones and inside flat bones; contains red marrow for blood cell production.

• Epiphyseal Line (Plate): The remnant of the growth plate, where bone growth occurs in children and adolescents.

Long Bone (Transverse Section)

• Periosteum: As above, the outer covering of bone.

• Outer Circumferential Lamellae: Layers of bony matrix that encircle the bone just beneath the periosteum.

• Inner Circumferential Lamellae: Layers of bony matrix that encircle the marrow cavity.

• Lacunae: Small spaces within the bone matrix that house osteocytes (bone cells).

• Osteocytes: Mature bone cells responsible for maintaining bone tissue.

Compact Bone Histology

• Osteon (Haversian System): The fundamental functional unit of compact bone, consisting of concentric lamellae arranged around a central canal.

• Central (Haversian) Canal: Contains blood vessels and nerves.

• Lamellae: Concentric rings of bone matrix.

• Lacunae: Spaces containing osteocytes.

• Canaliculi: Tiny channels that connect lacunae and allow for nutrient and waste exchange between osteocytes and blood vessels.

Types of Bones

• Long Bones: Longer than they are wide (e.g., femur, humerus).

• Short Bones: Approximately equal in length and width (e.g., carpals, tarsals).

• Flat Bones: Thin and often curved (e.g., skull, ribs, sternum).

• Irregular Bones: Complex shapes (e.g., vertebrae, pelvis).

• Sesamoid Bones: Small, round bones embedded within tendons (e.g., patella).

Example: Femur Structure

• The femur is a long bone with a proximal epiphysis (head), diaphysis (shaft), and distal epiphysis.

• It contains compact bone along the shaft and spongy bone at the ends, with yellow marrow in the medullary cavity.

Key Terms and Definitions

• Epiphysis: End part of a long bone, initially growing separately from the shaft.

• Diaphysis: The shaft or central part of a long bone.

• Periosteum: Membrane that covers the outer surface of all bones.

• Articular Cartilage: Cartilage covering the surfaces of bones where they come together to form joints.

• Compact Bone: Dense bone tissue that forms the outer layer of bone.

• Spongy Bone: Porous bone tissue found inside bones.

• Marrow: Soft tissue inside bones; yellow marrow stores fat, red marrow produces blood cells.

Additional info:

• Bone markings, such as foramina, processes, and tubercles, are important anatomical features for muscle attachment and passage of nerves and blood vessels (not fully detailed in the provided images but commonly included in this topic).