Skeletal System: Structure of Long Bones and Comparison of Compact vs. Spongy Bone

Skeletal System

Overview

The skeletal system provides structural support, protection, and facilitates movement in the human body. This section focuses on the anatomy of long bones and the differences between compact and spongy bone tissue.

Long Bones

Definition and General Structure

• Long bones are bones that are longer than they are wide and primarily function as levers for movement.

• Examples include the femur, humerus, and tibia.

• They have a characteristic structure with distinct regions: diaphysis, epiphyses, and metaphysis.

Gross Anatomy of a Long Bone

• Diaphysis: The tubular shaft of the bone, composed mainly of compact bone that surrounds the medullary (marrow) cavity.

• Epiphyses: The proximal and distal ends of the bone, consisting of a thin layer of compact bone surrounding spongy bone. Covered with articular cartilage (hyaline cartilage) at joint surfaces.

• Metaphysis: The region between the diaphysis and epiphysis, containing the epiphyseal plate (growth plate) in children or the epiphyseal line in adults.

• Periosteum: A double-layered membrane covering the outer surface of the bone except at joint surfaces. The outer fibrous layer is dense irregular connective tissue; the inner cellular layer contains osteogenic cells.

• Endosteum: A thin membrane lining the medullary cavity and covering trabeculae of spongy bone.

• Medullary cavity: The central cavity within the diaphysis, filled with bone marrow (red in children, yellow in adults).

Key Features and Functions

• Nutrient foramen: Openings in the bone for the entry of blood vessels and nerves.

• Sharpey's (perforating) fibers: Collagen fibers that anchor the periosteum to the underlying bone, providing structural reinforcement.

• Articular cartilage: Hyaline cartilage covering the epiphyses at joint surfaces, reducing friction and absorbing shock.

Bone Marrow

• Red bone marrow: Site of hematopoiesis (blood cell formation); found in the medullary cavity of children and in spongy bone of certain adult bones (e.g., hip bones, ribs, sternum, vertebrae, proximal femur and humerus).

• Yellow bone marrow: Consists mainly of adipose tissue; replaces red marrow in the medullary cavity of long bones in adults.

Microscopic Anatomy of Bone Tissue

Compact Bone

Compact bone, also known as cortical bone, forms the dense outer layer of bone and provides strength and protection.

• Osteon (Haversian system): The structural unit of compact bone, consisting of concentric rings (lamellae) of bone matrix surrounding a central (Haversian) canal.

• Central (Haversian) canal: Contains blood vessels and nerve fibers.

• Perforating (Volkmann's) canals: Transverse or diagonal channels that connect blood and nerve supply of the periosteum to those in the central canals and medullary cavity.

• Lamellae: Layers of bone matrix; can be concentric (within osteons), interstitial (between osteons), or circumferential (encircling the bone).

• Lacunae: Small spaces between lamellae that house osteocytes (mature bone cells).

• Canaliculi: Tiny canals connecting lacunae, allowing for nutrient and waste exchange between osteocytes and blood vessels.

• Collagen fibers in lamellae run in alternating directions, providing resistance to twisting and stretching forces.

Spongy Bone

Spongy bone, or cancellous bone, is found primarily at the ends of long bones and inside flat bones. It is less dense than compact bone and contains spaces filled with bone marrow.

• Trabeculae: Irregular latticework of thin bony plates; align along lines of stress to provide structural support.

• Osteocytes are located in lacunae within the trabeculae, connected by canaliculi.

• No true osteons are present in spongy bone.

• Spaces between trabeculae are filled with red or yellow bone marrow, depending on age and bone location.

Comparison of Compact and Spongy Bone

Feature	Compact Bone	Spongy Bone
Location	Diaphysis of long bones, outer layer of all bones	Epiphyses of long bones, interior of flat/irregular bones
Structure	Osteons (Haversian systems), dense matrix	Trabeculae, irregular lamellae, no osteons
Function	Strength, protection, support	Lightweight support, site of hematopoiesis
Vascularization	Central and perforating canals	Blood vessels in marrow spaces

Key Terms and Definitions

• Osteocyte: Mature bone cell that maintains bone tissue.

• Periosteum: Membrane covering the outer surface of bone.

• Endosteum: Membrane lining the internal surfaces of bone.

• Epiphyseal plate: Hyaline cartilage plate in growing bones; site of lengthwise growth.

• Epiphyseal line: Remnant of the epiphyseal plate in adult bones.

• Hematopoiesis: Formation of blood cells, occurring in red bone marrow.

Sample Questions and Applications

• Perforating fibers are composed of collagen protein.

• Osteocytes are located within lacunae.

• Canaliculi are necessary to keep deeper bone cells alive by allowing nutrient and waste exchange.

• Endosteum is located lining the medullary cavity and covering trabeculae of spongy bone.

• Articular cartilage is composed of hyaline cartilage.

• Epiphyseal plate is present in a 6-year-old (indicating growth), while an epiphyseal line is found in a 30-year-old (growth has ceased).

• Bone marrow in the epiphysis: In an adult tibia, yellow marrow is present; in a child's tibia, red marrow is present.

• Bone marrow in the ilium: In both adult and child, the ilium contains red marrow, but the amount decreases with age.

• Osteons are found only in compact bone, not in spongy bone. (True/False question: False)

Additional info:

• Short bones, such as those in the wrist and ankle, have a different structure compared to long bones, lacking a diaphysis and often containing more spongy bone relative to compact bone.

• Bone tissue is highly vascularized, allowing for efficient nutrient delivery and waste removal.