The Skeletal System: Structure, Function, and Bone Tissue

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Skeletal System Overview

Functions of the Skeletal System

The skeletal system is composed of bones, cartilage, and ligaments, all of which are types of connective tissue. It provides the framework for the body and serves several essential functions:

Support: Provides structural support for the entire body.
Storage of Minerals and Lipids: Bones store minerals, primarily calcium (Ca2+) and phosphate, as well as lipids in yellow bone marrow.
Blood Cell Production: Red bone marrow is the site of hematopoiesis, the production of blood cells.
Protection: Bones protect delicate internal organs (e.g., skull protects the brain, ribs protect the heart and lungs).
Leverage: Bones act as levers for muscles, enabling movement.

Classification of Bones by Shape

Bones are classified into several categories based on their shapes, each with specific examples and functions:

Long Bones: Longer than they are wide; e.g., femur, humerus.
Short Bones: Small and boxy; e.g., carpal (wrist) and tarsal (ankle) bones.
Flat Bones: Thin, with broad surfaces; e.g., roof of skull, sternum, scapulae. Provide large areas for muscle attachment.
Irregular Bones: Complex shapes; e.g., vertebrae.
Sesamoid Bones: Shaped like sesame seeds, develop inside tendons; e.g., patella.
Sutural Bones: Small, variable bones found between the flat bones of the skull.

Bone Tissue Structure and Composition

Bone Matrix

The bone matrix is a composite material that provides both strength and flexibility:

Collagen Fibers: Provide tensile strength, preventing shattering.
Hydroxyapatite: Crystals of calcium phosphate that make bone hard and strong. The chemical formula is:
About 99% of the body's calcium and phosphate are stored in bones.

Types of Osseous (Bone) Tissue

Compact Bone: Dense and solid, forms the outer layer of bones, provides protection and support.
Spongy Bone: Open network of trabeculae (struts and plates), found inside bones, especially at the ends. Contains red bone marrow for blood cell production.

The ratio of compact to spongy bone varies by bone type. All bones contain both types, but their distribution differs.

Structure of a Long Bone

Diaphysis: The shaft, composed mainly of compact bone.
Epiphysis: The ends, composed mainly of spongy bone with a thin outer layer of compact bone.
Metaphysis: The region where the diaphysis and epiphysis meet.
Marrow Cavity: Central space in the diaphysis containing yellow bone marrow (lipid storage).
Articular Cartilage: Hyaline cartilage covering the epiphysis at joint surfaces.

Bone Histology

Bone Cells

Bone tissue contains four main types of cells, each with specific functions:

Osteocytes: Mature bone cells located in lacunae. Maintain and monitor the mineral and protein content of the matrix; help repair damaged bone.
Osteoblasts: Bone-forming cells responsible for osteogenesis (production of new bone matrix). Once surrounded by matrix, they become osteocytes.
Osteoprogenitor Cells: Stem cells that divide to produce osteoblasts. Found in the periosteum and endosteum.
Osteoclasts: Large, multinucleated cells that break down bone matrix (bone resorption), releasing minerals into the blood.

Bone Remodeling

Bone is constantly being remodeled through the coordinated actions of osteocytes, osteoblasts, and osteoclasts.
This process is essential for mineral homeostasis and adapting bone strength to stress.

Microscopic Structure of Bone

Compact Bone

Osteon (Haversian System): The basic functional unit, consisting of concentric lamellae arranged around a central canal containing blood vessels.
Perforating (Volkmann's) Canals: Run perpendicular to the surface, connecting blood vessels of the periosteum to deeper tissues.
Lamellae: Layers of bone matrix. Types include concentric (around central canal), interstitial (between osteons), and circumferential (outer and inner surfaces).
Canaliculi: Tiny channels connecting osteocytes for nutrient and waste exchange.

Compact bone is thickest where stresses are greatest and aligned in one direction, providing strength along the axis of the bone.

Spongy Bone

Composed of trabeculae, forming an open network without osteons.
Red bone marrow fills spaces between trabeculae, supporting blood cell formation.
Spongy bone is found where bones are not heavily stressed or are stressed from multiple directions, reducing overall bone weight.

Bone Coverings

Periosteum

Outer fibrous layer (dense connective tissue) and inner cellular layer.
Functions: isolates bone, provides a route for blood vessels and nerves, supplies cells for growth and repair.
Continuous with joint capsules and tendons via perforating fibers, strengthening attachments.

Endosteum

Thin cellular layer lining the marrow cavity and covering trabeculae of spongy bone.
Contains osteoprogenitor cells, active during growth, repair, and remodeling.

Bone Development and Growth

Ossification and Calcification

Ossification: The process of bone formation.
Calcification: Deposition of calcium salts, which can occur in tissues other than bone.

Endochondral Ossification (Long Bone Development)

Hyaline cartilage model enlarges; matrix calcifies, chondrocytes die.
Blood vessels invade perichondrium; inner layer differentiates into osteoblasts, forming a bone collar.
Blood vessels and osteoblasts invade cartilage, replacing it with spongy bone (primary ossification center).
Osteoclasts create a marrow cavity by eroding trabeculae.
Secondary ossification centers form in the epiphyses.
Epiphyses fill with spongy bone; articular cartilage and epiphyseal plate remain.

Longitudinal bone growth occurs at the epiphyseal plate until it closes at puberty, leaving the epiphyseal line.

Appositional Growth

Bone increases in diameter as new layers are added to the outer surface by osteoblasts in the periosteum.
Osteoclasts remove bone from the inner surface, enlarging the marrow cavity.

Blood Supply and Innervation of Bone

Nutrient Artery and Vein: Main blood supply, entering through the nutrient foramen.
Metaphyseal Vessels: Supply the inner surface of the epiphyseal cartilage.
Periosteal Vessels: Supply superficial osteons.
Periosteum contains sensory nerves and lymphatic vessels.

Bone Remodeling and Regulation

Bone Remodeling

Continuous process involving osteocytes (maintenance), osteoblasts (formation), and osteoclasts (resorption).
Remodeling adapts bone structure to stress and maintains mineral homeostasis.
Exercise increases bone strength by stimulating bone formation.

Hormonal Regulation of Bone Remodeling

Hormone	Source	Effect on Bone
Calcitriol	Kidneys	Promotes calcium and phosphate absorption in the digestive tract
Growth Hormone	Pituitary Gland	Stimulates bone growth
Thyroxine	Thyroid Gland	Stimulates bone growth
Estrogen/Androgen	Ovaries/Testes	Stimulate bone growth at puberty; estrogen also protects bone density
Parathyroid Hormone (PTH)	Parathyroid Glands	Increases osteoclast activity when blood calcium is low
Calcitonin	Thyroid Gland	Decreases osteoclast activity when blood calcium is high

Clinical Note: Osteoporosis

Estrogen is protective of bone density; its decline at menopause increases the risk of osteoporosis (loss of bone mass).

Summary Table: Types of Bone Cells

Cell Type	Function	Location
Osteocyte	Maintains bone matrix	Lacunae within bone matrix
Osteoblast	Builds new bone matrix	Periosteum, endosteum
Osteoprogenitor Cell	Bone stem cell; produces osteoblasts	Periosteum, endosteum
Osteoclast	Resorbs bone matrix	Bone surfaces, endosteum

Example: Femur Structure

The femur is a long bone with a thick compact bone shaft (diaphysis) and spongy bone at the ends (epiphyses), demonstrating adaptation to weight-bearing and movement.

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