Skeletal System

Overview and Objectives

The skeletal system provides the framework for the human body, supporting movement, protecting organs, and serving as a site for blood cell formation. This section covers the classification of bones by shape and structure, the identification of bone markings, and the comparison of compact and spongy bone.

• Classify bones according to shape and structure, with examples and functional significance.

• Identify cell types in bone and their major functions.

• Compare the structures and functions of compact and spongy bone.

Classification of Bones

Axial and Appendicular Skeleton

• Axial skeleton: 80 bones (skull, vertebral column, rib cage, sternum)

• Appendicular skeleton: 126 bones (pectoral girdle, upper limbs, pelvic girdle, lower limbs)

• Total bones in adult human: 206

Bone Shapes and Examples

Bones are classified by their shape and structure, which relate to their function.

• Sutural bones: Small, flat, irregularly shaped bones between the flat bones of the skull.

• Irregular bones: Complex shapes (e.g., vertebrae, mandible, sacrum, pubis, ischium, facial bones).

• Short bones: Boxy, nearly equal in length and width (e.g., carpal bones, tarsals).

• Flat bones: Thin, parallel surfaces (e.g., parietal bone, sternum, scapulae, ribs). Protect vital organs and provide large surfaces for muscle attachment.

• Long bones: Longer than they are wide, with a shaft and two ends (e.g., humerus, femur, radius, ulna, tibia, fibula, metacarpals, metatarsals, phalanges). Important for movement and weight bearing.

• Sesamoid bones: Small, round, and flat; develop inside tendons (e.g., patella, base of big toe). Improve mechanical efficiency of muscles.

Summary Table: Bone Types and Examples

Bone Markings

Types and Functions

Bone markings are surface features that serve as sites for muscle, tendon, and ligament attachment, or as passages for nerves and blood vessels. They are essential for joint formation and movement.

• Projections: Raised areas for muscle/tendon/ligament attachment or joint formation (e.g., condyle, process, ramus, epicondyle, crest, line, trochanter, tuberosity, tubercle, spine).

• Openings: Allow passage of blood vessels or nerves (e.g., foramen, canal, fissure, sinus, meatus).

• Depressions: Indentations or grooves for passage of soft tissues or to form joints (e.g., fossa, sulcus/groove).

Summary Table: Major Bone Markings

Bone Structure

Long Bone Anatomy

• Epiphysis: End of long bone, made of spongy bone; site of red bone marrow.

• Metaphysis: Region between diaphysis and epiphysis; contains epiphyseal plate (growth area).

• Diaphysis: Shaft; composed of compact bone surrounding the medullary cavity.

• Medullary cavity: Central cavity containing red (hematopoietic) or yellow (fat storage) bone marrow.

Flat Bone Anatomy

• Composed of two layers of compact bone with a middle layer of spongy bone (diploë).

• Red bone marrow is present in the diploë, even without a medullary cavity.

Bone Tissue Composition

• Matrix: Extracellular protein fibers and ground substance.

• Cell types:

  • Osteoprogenitor (osteogenic) cells: Stem cells that develop into osteoblasts.

  • Osteoblasts: Bone-building cells; secrete bone matrix.

  • Osteocytes: Mature bone cells; maintain bone tissue.

  • Osteoclasts: Bone-resorbing cells; break down bone matrix.

Compact vs. Spongy Bone

Compact Bone

• Dense, strong bone tissue forming the outer layer of bones.

• Composed of osteons (Haversian systems) with concentric lamellae around a central canal.

• Provides strength for weight bearing, especially along the long axis of bones.

Spongy Bone

• Porous, lightweight bone tissue found at the ends of long bones and inside flat bones.

• Composed of trabeculae (lattice-like network) with spaces containing red bone marrow.

• Reduces bone weight and provides space for marrow.

Key Terms and Definitions

• Osteon: The structural unit of compact bone.

• Epiphyseal plate: Growth plate in long bones where lengthening occurs.

• Diploë: Spongy bone layer in flat bones of the skull.

• Foramen: Opening in bone for passage of nerves or blood vessels.

• Fossa: Shallow depression in bone.

Example: Functional Significance of Bone Markings

• Epicondyles on the humerus serve as attachment points for forearm muscles, enabling movement of the wrist and fingers.

• Foramen magnum in the occipital bone allows the spinal cord to connect with the brain.

• Trochanters on the femur provide leverage for large thigh muscles, aiding in walking and running.

Additional info: Detailed examination of bone markings is crucial in fields such as anthropology and forensic science, where skeletal remains can reveal information about an individual's age, sex, and general health.