Bones and Skeletal Tissues: Structure, Function, and Development

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Bones and Skeletal Tissues

Introduction

The skeletal system is composed of bones and cartilages that provide structure, protection, and support for the human body. Understanding bone anatomy and physiology is essential for diagnosing and treating bone-related diseases such as osteoporosis.

Overview of the Human Skeleton

Bones and Cartilages of the Human Skeleton

Bones form the rigid framework of the body, supporting and protecting organs.
Cartilage is a flexible connective tissue found in joints, rib cage, ear, nose, bronchial tubes, and intervertebral discs.
The skeleton is divided into the axial skeleton (skull, vertebral column, rib cage) and the appendicular skeleton (limbs and girdles).

Functions of Bones

Seven Major Functions

Support: Provides a framework for the body and supports soft organs.
Protection: Protects vital organs such as the brain, spinal cord, and thoracic organs.
Movement: Acts as levers for muscles to produce movement.
Mineral and Growth Factor Storage: Reservoir for minerals (especially calcium and phosphate) and growth factors.
Blood Cell Formation: Hematopoiesis occurs in red marrow cavities of certain bones.
Triglyceride (Fat) Storage: Fat is stored in bone cavities and serves as an energy source.
Hormone Production: Bones produce osteocalcin, a hormone involved in bone formation and energy metabolism. Additional info: This function is often included in modern textbooks.

Classification of Bones

Based on Shape

Long Bones: Longer than they are wide (e.g., femur, humerus).
Short Bones: Cube-shaped (e.g., wrist and ankle bones); sesamoid bones (e.g., patella) form within tendons.
Flat Bones: Thin, flattened, and usually curved (e.g., sternum, ribs, scapulae).
Irregular Bones: Complicated shapes (e.g., vertebrae, hip bones).

Bone Structure

Gross Anatomy

Compact Bone: Dense outer layer; smooth and solid.
Spongy Bone (Cancellous Bone): Honeycomb of trabeculae filled with red or yellow marrow.

Structure of a Flat Bone

Consists of a layer of spongy bone sandwiched between two thin layers of compact bone.

Structure of a Long Bone

Diaphysis: Shaft; compact bone surrounding medullary cavity.
Epiphyses: Ends of the bone; spongy bone interior.
Membranes: Periosteum (outer) and endosteum (inner).

Microscopic Anatomy of Bone

Bone Cells

Osteogenic (Osteoprogenitor) Cells: Stem cells that differentiate into osteoblasts.
Osteoblasts: Bone-forming cells responsible for bone growth.
Osteocytes: Mature bone cells that maintain the bone matrix.
Bone-Lining Cells: Flat cells on bone surfaces believed to help maintain matrix.
Osteoclasts: Large cells that resorb or break down bone matrix.

Compact Bone Structure

Osteon (Haversian System): Structural unit of compact bone; consists of concentric lamellae around a central canal.
Canals and Canaliculi: Pathways for blood vessels and communication between osteocytes.
Interstitial and Circumferential Lamellae: Fill spaces between osteons and encircle the bone, respectively.

Bone Marrow

Types and Locations

Red Marrow: Site of hematopoiesis; found in trabecular cavities of spongy bone and diploë of flat bones (e.g., sternum, hip bone).
Yellow Marrow: Stores fat; can convert to red marrow if necessary (e.g., in anemia).

Bone Markings

Purpose and Types

Sites of muscle, ligament, and tendon attachment.
Areas involved in joint formation or conduits for blood vessels and nerves.

Type	Description
Projection	Bone prominence (e.g., tuberosity, crest)
Depression	Bowl- or groove-like cut (e.g., fossa, sulcus)
Opening	Hole or canal (e.g., foramen, canal)

Bone Development (Ossification)

Overview

Ossification (Osteogenesis): Process of bone tissue formation.
Begins in the second month of embryonic development.
Postnatal bone growth continues until early adulthood.
Bone remodeling and repair are lifelong processes.

Types of Ossification

Endochondral Ossification: Bone forms by replacing hyaline cartilage; forms most of the skeleton.
Intramembranous Ossification: Bone develops from fibrous membrane; forms flat bones (e.g., skull, clavicles).

Bone Growth

Growth in Length

Occurs at the epiphyseal plate via interstitial growth of cartilage, which is then replaced by bone.
Epiphyseal plate closure occurs at the end of adolescence, leaving the epiphyseal line.

Growth in Thickness

Appositional growth increases bone thickness throughout life.

Bone Remodeling and Repair

Remodeling

Bone is continuously deposited and resorbed in response to mechanical stress and hormonal signals.
Wolff's Law: Bones grow or remodel in response to the demands placed on them (e.g., weight-bearing exercise increases bone density).

Calcium Homeostasis

Regulated primarily by parathyroid hormone (PTH), which increases blood calcium by stimulating osteoclast activity.
Calcitonin (from the thyroid) can lower blood calcium levels, though its effect is less significant in adults.

Equation for Calcium Homeostasis:

Homeostatic Imbalances

Hypocalcemia: Low blood calcium; causes hyperexcitability of nerves and muscles.
Hypercalcemia: High blood calcium; causes nonresponsiveness and can lead to kidney stones.

Bone Fractures and Repair

Common Types of Fractures

Type	Description
Comminuted	Bone fragments into three or more pieces
Compression	Bone is crushed
Spiral	Ragged break due to twisting forces
Epiphyseal	Epiphysis separates from diaphysis
Depressed	Broken bone portion pressed inward
Greenstick	Bone breaks incompletely

Stages of Bone Healing

Hematoma Formation: Blood clot forms at the fracture site.
Fibrocartilaginous Callus Formation: Soft callus of collagen and cartilage forms.
Bony Callus Formation: New bone trabeculae appear in the callus.
Bone Remodeling: Compact bone replaces spongy bone; bone regains original shape.

Bone Disorders

Major Diseases

Osteomalacia and Rickets: Bones are poorly mineralized; soft and weak due to vitamin D or calcium deficiency.
Osteoporosis: Bone resorption exceeds deposit; bones become fragile and porous, increasing fracture risk.
Paget's Disease: Excessive and disorganized bone remodeling; bones are enlarged, deformed, and weak.

Developmental and Age-Related Aspects

Embryonic and Fetal Development

Skeleton ossifies predictably; fetal age can be estimated by bone development.
Primary ossification centers appear by week 8; most bones are well-ossified by week 12.

Postnatal and Age-Related Changes

Bone formation exceeds resorption in children and adolescents.
In young adults, formation and resorption are balanced.
In older adults, resorption exceeds formation, leading to bone loss.
Bone mass, mineralization, and healing ability decrease with age, especially in females and individuals of European descent.