Bones and Skeletal Tissues: Structure, Function, and Classification

Bones and Skeletal Tissues

Introduction to the Skeletal System

The skeletal system is composed of bones, cartilages, and joints, providing structural support, protection, and facilitating movement. This chapter focuses on the types, structure, and functions of bones and skeletal tissues.

• Bones are rigid organs that form the skeleton and serve as sites for muscle attachment, mineral storage, and blood cell formation.

• Cartilage is a flexible connective tissue found in various locations, including joints, respiratory structures, and the ear.

• Joints are connections between bones that allow for movement and flexibility.

Cartilages as Organs of the Skeletal System

Types and Locations of Cartilage

Cartilage is a specialized connective tissue with a firm, yet flexible matrix. It is classified into three main types based on fiber composition and function.

• Hyaline cartilage: Most abundant type; provides support with flexibility and resilience. Found in the nose, articular surfaces of joints, costal cartilages, and respiratory tubes.

• Elastic cartilage: Contains more elastic fibers, allowing for greater flexibility. Found in the epiglottis and external ear.

• Fibrocartilage: Contains abundant collagen fibers, making it very strong and able to withstand compression. Found in intervertebral discs and menisci of the knee.

Perichondrium is a dense connective tissue membrane that surrounds most cartilages, providing nutrients and serving as a protective girdle.

Microscopic Structure of Cartilage

• Cartilage cells, called chondrocytes, are located in small spaces called lacunae within the matrix.

• The matrix is rich in water, collagen, and sometimes elastic fibers, depending on the cartilage type.

• Cartilage is avascular (lacks blood vessels); nutrients diffuse through the matrix.

Classification of Bones

Types of Bones by Shape

Bones are classified according to their shapes, which relate to their functions.

• Long bones: Longer than they are wide; consist of a shaft and two ends (e.g., femur, humerus).

• Short bones: Cube-shaped; found in the wrist and ankle (e.g., carpals, tarsals).

• Flat bones: Thin, flattened, and usually curved (e.g., sternum, ribs, skull bones).

• Irregular bones: Complicated shapes that do not fit other categories (e.g., vertebrae, hip bones).

Bone Markings

Surface Features of Bones

Bones display various markings that serve as attachment sites for muscles, ligaments, and tendons, or as conduits for nerves and blood vessels.

• Projections: Sites of muscle and ligament attachment (e.g., tuberosity, crest, trochanter).

• Surfaces: Form joints (e.g., head, facet, condyle).

• Depressions and openings: Allow passage of blood vessels and nerves (e.g., foramen, groove, notch).

Gross Anatomy of Bone

Structure of a Long Bone

Long bones have a complex structure that supports their function in movement and weight-bearing.

• Diaphysis: The shaft of the bone, composed mainly of compact bone surrounding a medullary cavity.

• Epiphyses: The ends of the bone, consisting of spongy bone covered by a thin layer of compact bone.

• Epiphyseal line: Remnant of the epiphyseal plate (growth plate), marking where bone growth occurred in youth.

• Periosteum: A dense irregular connective tissue membrane covering the external surface of bones, except at joint surfaces.

• Endosteum: A thin membrane lining the internal surfaces of the bone, including the medullary cavity and trabeculae of spongy bone.

Structure of Short, Irregular, and Flat Bones

• These bones consist of thin plates of spongy bone (diploë) covered by compact bone.

• No shaft or epiphyses; bone marrow is present between trabeculae but no defined medullary cavity.

Microscopic Anatomy of Bone Tissue

Compact Bone

Compact bone is dense and forms the outer layer of all bones, providing strength and protection.

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

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

• Lacunae: Small spaces housing osteocytes (bone cells).

• Canaliculi: Tiny canals connecting lacunae, allowing for nutrient and waste exchange.

• Interstitial lamellae: Fill spaces between osteons.

• Circumferential lamellae: Extend around the entire circumference of the diaphysis.

Spongy Bone

Spongy bone (cancellous bone) is found inside bones, particularly at the ends of long bones and in flat bones.

• Composed of a network of trabeculae ("little beams") that align along lines of stress.

• Spaces between trabeculae are filled with bone marrow.

• Trabeculae contain lamellae and osteocytes but lack osteons.

• Nutrients reach osteocytes by diffusing through canaliculi from capillaries in the endosteum.

Bone Growth and Remodeling

How Bones Grow in Length

Longitudinal bone growth occurs at the epiphyseal plate through a process involving cartilage proliferation and ossification.

• Cartilage grows on the epiphysis side of the plate.

• Bone replaces cartilage on the diaphysis side.

• When growth ceases, the epiphyseal plate is replaced by the epiphyseal line.

Bone Remodeling

Bone is a dynamic tissue that undergoes continuous remodeling throughout life.

• Osteoclasts: Cells that resorb bone by secreting hydrochloric acid and lysosomal enzymes, releasing calcium into the blood.

• Osteoblasts: Cells that form new bone by secreting bone matrix; become osteocytes when surrounded by matrix.

• Remodeling helps maintain bone strength and mineral homeostasis.

Key Terms and Structures

• Periosteum: Outer membrane of bone; contains nerves and blood vessels.

• Endosteum: Inner membrane lining bone cavities.

• Yellow marrow: Fat storage tissue found in the medullary cavity of long bones.

• Red marrow: Hematopoietic tissue found in spongy bone; site of blood cell formation.

• Central canal: Channel in the center of each osteon containing blood vessels and nerves.

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

Summary Table: Types of Cartilage

Type	Main Locations	Main Features
Hyaline	Nose, articular surfaces, costal cartilages, respiratory tubes	Most abundant, provides support and flexibility
Elastic	Epiglottis, external ear	Contains elastic fibers, very flexible
Fibrocartilage	Intervertebral discs, menisci	Abundant collagen, resists compression

Additional info:

• Bone matrix is composed of organic (collagen fibers) and inorganic (hydroxyapatite crystals) components, providing both flexibility and strength.

• Bone markings are critical for identifying muscle attachment sites and understanding joint mechanics.

• Bone remodeling is regulated by hormones (e.g., parathyroid hormone, calcitonin) and mechanical stress.