Bones and Skeletal Tissues

Introduction

The skeletal system is fundamental to human anatomy and physiology, providing structure, protection, and facilitating movement. Understanding bone anatomy and the process of bone remodeling is essential for diagnosing and treating bone diseases such as osteoporosis.

Skeletal Cartilages

Overview of Cartilage in the Human Skeleton

In early development, the human skeleton consists primarily of cartilage, which is gradually replaced by bone. Cartilage remains in adult areas requiring flexibility.

• Cartilage: A resilient, molded tissue composed mainly of water, providing flexibility and cushioning.

• Perichondrium: A dense connective tissue layer surrounding cartilage, aiding in resistance to outward expansion and delivering nutrients via blood vessels.

• Chondrocytes: Specialized cells encased in lacunae within the extracellular matrix of cartilage.

Types of Cartilage

• Hyaline cartilage: Most abundant; provides support, flexibility, and resilience. Contains only collagen fibers. Found in articular (joint) surfaces, costal (ribs), respiratory (larynx), and nasal regions.

• Elastic cartilage: Similar to hyaline but contains elastic fibers. Located in the external ear and epiglottis.

• Fibrocartilage: Contains thick collagen fibers, providing great tensile strength. Found in menisci of the knee and vertebral discs.

Growth of Cartilage

Mechanisms of Cartilage Growth

Cartilage grows by two primary mechanisms, contributing to skeletal development and repair.

• Appositional growth: Cartilage-forming cells in the perichondrium secrete new matrix on the surface of existing cartilage.

• Interstitial growth: Chondrocytes within lacunae divide and secrete new matrix, expanding cartilage from within.

Calcification of cartilage occurs during normal bone growth in youth and can also occur in old age. Hardened cartilage is not equivalent to bone.

Functions of Bones

Major Functions of Bones

Bones perform several vital functions necessary for the maintenance and protection of the body.

• Support: Provides structural framework for the body and soft organs.

• Protection: Shields the brain, spinal cord, and vital organs from injury.

• Movement: Acts as levers for muscle action, enabling locomotion and manipulation of the environment.

• Mineral and growth factor storage: Reservoir for calcium, phosphorus, and growth factors essential for various physiological processes.

• Blood cell formation: Hematopoiesis occurs in red marrow cavities of certain bones.

• Triglyceride (fat) storage: Fat stored in bone cavities serves as an energy source.

• Hormone production: Bones secrete osteocalcin, which helps regulate insulin secretion, glucose levels, and metabolism.

Classification of Bones

Axial and Appendicular Skeleton

The human skeleton is divided into two main groups based on location:

• Axial skeleton: Forms the long axis of the body, including the skull, vertebral column, and rib cage.

• Appendicular skeleton: Comprises the bones of the upper and lower limbs and the girdles attaching limbs to the axial skeleton.

Classification by Shape

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

• Long bones: Longer than wide; found in limbs.

• Short bones: Cube-shaped; found in wrist and ankle. Includes sesamoid bones (e.g., patella) formed within tendons.

• Flat bones: Thin, flat, and slightly curved; includes sternum, scapulae, ribs, and most skull bones.

• Irregular bones: Complicated shapes; includes vertebrae and hip bones.

Bone Structure

Bone as an Organ

Bones are organs composed of various tissues, including bone (osseous) tissue, nervous tissue, cartilage, fibrous connective tissue, muscle cells, and epithelial cells in blood vessels.

• Levels of structure: Gross (macroscopic), microscopic, and chemical.

Gross Anatomy: Compact and Spongy Bone

Bones consist of two types of tissue:

• Compact bone: Dense outer layer that appears smooth and solid.

• Spongy bone: Honeycomb-like structure of trabeculae, with spaces filled by red or yellow bone marrow.

Structure of Short, Irregular, and Flat Bones

• Thin plates of spongy bone (diploë) covered by compact bone.

• Compact bone is sandwiched between connective tissue membranes: periosteum (outer) and endosteum (inner).

• Bone marrow is scattered throughout spongy bone; no defined marrow cavity.

• Hyaline cartilage covers areas of bone involved in movable joints.

Structure of Long Bones

• Diaphysis: Tubular shaft forming the long axis, consisting of compact bone surrounding the medullary cavity (yellow marrow in adults).

• Epiphyses: Bone ends, with compact bone externally and spongy bone internally; articular cartilage covers joint surfaces.

• Epiphyseal line: Remnant of the childhood epiphyseal plate, where bone growth occurs.

Bone Membranes

• Periosteum: White, double-layered membrane covering external surfaces except joints. Contains fibrous (outer) and osteogenic (inner) layers, nerve fibers, blood vessels, and serves as anchoring points for tendons and ligaments.

• Endosteum: Delicate connective tissue membrane covering internal bone surfaces, including trabeculae of spongy bone and canals in compact bone. Contains osteogenic cells.

Hematopoietic Tissue in Bones

• Red marrow: Found within trabecular cavities of spongy bone and diploë of flat bones. In newborns, present in medullary cavities and all spongy bone; in adults, mainly in heads of femur and humerus, and active in flat and irregular bones.

• Yellow marrow: Can convert to red marrow if the person becomes anemic.

Bone Markings

Bone markings are features on bones that serve as sites for muscle, ligament, and tendon attachment, joint formation, or passageways for blood vessels and nerves.

• Projection: Outward bulge due to muscle pull or joint modification.

• Depression: Bowl or groove-like cutout serving as passageways or joint surfaces.

• Opening: Hole or canal serving as passageways for blood vessels and nerves.

Microscopic Anatomy of Bone

Types of Bone Cells

Five major types of bone cells, all derived from the same basic cell type:

• Osteogenic cells (osteoprogenitor cells): Mitotically active stem cells in periosteum and endosteum; differentiate into osteoblasts or bone-lining cells.

• Osteoblasts: Bone-forming cells that secrete unmineralized bone matrix (osteoid), composed of collagen and calcium-binding proteins.

• Osteocytes: Mature bone cells in lacunae; maintain bone matrix and act as stress sensors, communicating with osteoblasts and osteoclasts for bone remodeling.

• Bone-lining cells: Flat cells on bone surfaces, helping maintain matrix; called periosteal cells (external) and endosteal cells (internal).

• Osteoclasts: Derived from hematopoietic stem cells; giant, multinucleate cells responsible for bone resorption, located in resorption bays with ruffled borders to increase surface area for enzyme degradation.

Microscopic Structure of Compact Bone

• Osteon (Haversian system): Structural unit of compact bone; elongated cylinders running parallel to the bone's long axis, composed of concentric rings (lamellae) with alternating collagen fiber orientation for strength and resistance to twisting.

• Canals and Canaliculi: Central (Haversian) canal contains blood vessels and nerves; perforating (Volkmann's) canals connect periosteum, medullary cavity, and central canal. Canaliculi connect lacunae and allow communication and nutrient/waste exchange between osteocytes.

• Interstitial and Circumferential Lamellae: Interstitial lamellae fill gaps between osteons or are remnants of remodeled osteons; circumferential lamellae encircle the diaphysis, providing resistance to twisting.

Microscopic Structure of Spongy Bone

• Composed of trabeculae arranged along lines of stress for strength.

• No osteons present, but trabeculae contain lamellae and osteocytes interconnected by canaliculi.

• Capillaries in endosteum supply nutrients.

Chemical Composition of Bone

Organic Components

• Includes osteogenic cells, osteoblasts, osteocytes, bone-lining cells, osteoclasts, and osteoid (unmineralized bone matrix).

• Osteoid consists of ground substance and collagen fibers, providing tensile strength and flexibility.

• Sacrificial bonds between collagen molecules dissipate energy and prevent fractures; these bonds reform if no additional trauma occurs.

Inorganic Components

• Hydroxyapatites (mineral salts): Make up 65% of bone by mass; primarily tiny calcium phosphate crystals in and around collagen fibers, responsible for bone hardness and resistance to compression.

• Bones are half as strong as steel in resisting compression and equally strong in resisting tension.

• Mineral composition allows bones to last long after death and provides information about ancient populations.

Example Table: Classification of Bones by Shape

Key Formula: Bone Matrix Composition

The bone matrix is composed of organic and inorganic components:

• Organic: Collagen fibers, ground substance

• Inorganic: Hydroxyapatite crystals (calcium phosphate)

Percentage of inorganic component:

Additional info: The notes have been expanded to include definitions, examples, and academic context for clarity and completeness.