Overview of the Skeleton: Structure and Bone Types

Overview of the Skeleton

Major Structures of Bone

The human skeleton is composed of various types of bones, each with distinct structural features. Understanding these features is essential for studying anatomy and physiology, as they relate to bone function, growth, and health.

• Periosteum: A dense, fibrous membrane covering the outer surface of bones except at joints. It contains blood vessels, nerves, and cells important for bone growth and repair.

• Endosteum: A thin membrane lining the inner surface of the medullary cavity and spaces within spongy bone. It is involved in bone growth, repair, and remodeling.

• Circumferential Lamellae: Layers of bony matrix that encircle the entire bone, contributing to its strength and structure.

• Interstitial Lamellae: Remnants of older osteons found between newer osteons in compact bone, filling spaces and providing additional support.

• Compact Bone: Dense bone tissue forming the outer layer of bones, providing strength for weight-bearing.

• Spongy Bone (Cancellous/Trabecular Bone): Porous bone tissue found at the ends of long bones and inside other bones, containing red bone marrow and providing structural support with less weight.

• Epiphysis: The expanded ends of long bones, typically containing spongy bone and red bone marrow.

• Diaphysis: The shaft or central part of a long bone, composed mainly of compact bone and enclosing the medullary cavity.

• Medullary Cavity: The central cavity within the diaphysis, containing yellow bone marrow in adults.

• Epiphyseal Line: A remnant of the epiphyseal plate (growth plate) in adults, marking the site where bone growth occurred in children.

Sites of Key Features in Living Bone

• Articular Cartilage: Smooth, white tissue covering the ends of bones at joints, reducing friction and absorbing shock.

• Red Bone Marrow: Found within spongy bone, especially in the epiphyses of long bones; responsible for hematopoiesis (blood cell formation).

• Yellow Bone Marrow: Located in the medullary cavity of long bones; primarily composed of adipose tissue and serves as an energy reserve.

• Periosteum: Covers the outer surface of bones except at joint surfaces.

• Endosteum: Lines the inner surfaces of the medullary cavity and trabeculae of spongy bone.

Classification of Bone Shapes

Bones are classified according to their shapes, which relate to their functions and locations in the body.

Bone Shape	Description	Examples
Long Bones	Longer than they are wide; primarily composed of compact bone with spongy bone at the ends.	Femur, humerus, tibia
Short Bones	Approximately equal in length, width, and thickness; mostly spongy bone covered by a thin layer of compact bone.	Carpals (wrist bones), tarsals (ankle bones)
Sesamoid Bones	Small, round bones embedded within tendons; protect tendons from stress and wear.	Patella (kneecap)
Flat Bones	Thin, flattened, and usually curved; provide protection and broad surfaces for muscle attachment.	Sternum, ribs, scapulae, cranial bones
Irregular Bones	Complex shapes that do not fit into other categories; often have specialized functions.	Vertebrae, pelvic bones

Key Terms and Definitions

• Osteon: The structural unit of compact bone, consisting of concentric lamellae arranged around a central canal.

• Lamellae: Layers of bone matrix in compact bone.

• Hematopoiesis: The process of blood cell formation, occurring in red bone marrow.

Example: Structure of a Long Bone

• The femur is a classic example of a long bone, featuring a diaphysis (shaft), two epiphyses (ends), a medullary cavity filled with yellow marrow, and articular cartilage covering the joint surfaces.

Additional info: The periosteum is essential for bone growth in diameter and for healing fractures, while the endosteum plays a role in bone remodeling by housing osteoblasts and osteoclasts.