Human Anatomy: The Skeletal System and Bone Tissue – Study Notes

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Introduction to the Skeletal System

Overview of Skeletal System Functions

The skeletal system is composed of several different tissues working together, including bone, cartilage, dense connective tissue, blood-forming tissues, adipose tissue, and nervous tissue. Bones, along with their cartilages, form the skeletal system, which serves multiple essential functions in the human body.

Support: Provides a structural framework for the body by supporting soft tissues and providing attachment points for skeletal muscles.
Protection: Protects internal organs from injury; for example, cranial bones protect the brain, and the vertebrae shield the heart and lungs.
Movement: Skeletal muscles attach to bones, and their contraction pulls on bones to produce movement.
Mineral Homeostasis: Acts as a reservoir for minerals, especially calcium and phosphorus.
Blood Cell Production (Hemopoiesis): Red bone marrow produces blood cells.
Triglyceride Storage: Yellow bone marrow stores adipose tissue.

Osteology is the study of bone structure and treatment of bone disorders. Orthopedics is the medical specialty concerned with correcting disorders of the musculoskeletal system.

Bone Tissue and Bone Cells

Types of Bone Cells

Bone tissue is dynamic and consists of several types of cells, each with specific functions in bone formation, maintenance, and resorption.

Osteoprogenitor Cells: Undifferentiated cells capable of cell division. They are found in the inner layer of the periosteum and endosteum and can develop into osteoblasts.
Osteoblasts: Bone-forming cells that synthesize the bone matrix and collagen fibers. They cannot divide.
Osteocytes: Mature bone cells derived from osteoblasts. They maintain bone tissue but do not secrete matrix.
Osteoclasts: Large cells formed from fused monocytes (a type of white blood cell). They are responsible for bone resorption, breaking down bone matrix at surfaces such as the endosteum.

Additional info: Bone tissue also contains other supporting tissues such as epithelium, nerve tissue, and blood-forming tissue.

Anatomy of a Long Bone

Structural Features of Long Bones

Long bones have distinct regions, each with specific functions and characteristics.

Diaphysis: The shaft or central part of a long bone.
Epiphysis: The ends of a long bone.
Metaphysis: The areas between the diaphysis and epiphysis, which include the epiphyseal plate in growing bones. The epiphyseal plate is responsible for bone growth in length.
Articular Cartilage: Covers joint surfaces, acting as a friction reducer and shock absorber.
Medullary Cavity: The central cavity of bone shafts where marrow is stored.
Endosteum: The lining of the medullary cavity.
Periosteum: A tough membrane covering the bone except at joint surfaces. It contains cells that enable bone growth in thickness, not length.

Additional info: The periosteum consists of a fibrous layer (dense irregular connective tissue) and an osteogenic layer (bone-forming cells).

Histology of Bone Tissue

Bone Matrix Composition

Bone tissue consists of widely separated cells surrounded by a matrix composed of inorganic salts and collagen fibers.

Inorganic Mineral Salts: Provide bone hardness. The main minerals are hydroxyapatite (calcium phosphate) and calcium carbonate.
Collagen Fibers: Provide bone flexibility and tensile strength, allowing resistance to stretching and tearing.

Calcification or mineralization of bone occurs only in the presence of collagen fibers. The matrix is not completely solid, as it contains small spaces for cells and red bone marrow.

Types of Bone Tissue

Compact Bone vs. Spongy Bone

Bone tissue is classified into two main types based on structure and function.

Compact Bone: Arranged in units called osteons (Haversian systems). It forms the dense outer layer of bones and the shaft of long bones, providing strength to resist stresses from weight and movement.
Spongy Bone (Cancellous Bone): Does not contain osteons. It consists of trabeculae (plates of bone) oriented along lines of stress and contains many spaces filled with red marrow. It forms most of the structure of short, flat, and irregular bones, and the epiphyses of long bones.

Additional info: The trabeculae in spongy bone provide strength, flexibility, and help transfer forces from joints toward the cortical bone.

Blood and Nerve Supply of Bone

Vascularization of Bone Tissue

Bones are highly vascularized, receiving blood supply from several sources:

Periosteal Arteries: Supply the periosteum and outer compact bone.
Nutrient Arteries: Enter through nutrient foramina and supply the diaphysis and red marrow.
Epiphyseal and Metaphyseal Arteries: Supply the red marrow and bone tissue of the epiphyses and metaphyses.

Bone Growth and Development

Appositional Growth (Thickness)

After bones stop growing in length, they can continue to grow in thickness or diameter throughout life in response to mechanical stress or muscle activity. This process is called appositional growth.

Occurs at the endosteum or periosteum.
Osteoclasts resorb old bone lining the medullary cavity, while osteoblasts produce new bone tissue beneath the periosteum.

Growth in Length

Long bones grow in length by the activity of the epiphyseal plate, which consists of four zones:

Zone of Resting Cartilage: Anchors the growth plate to bone.
Zone of Proliferating Cartilage: Rapid cell division; chondrocytes are stacked like coins.
Zone of Hypertrophic Cartilage: Chondrocytes enlarge and accumulate collagen.
Zone of Calcified Cartilage: Matrix becomes calcified; cells die, and osteoclasts remove the matrix. Osteoblasts and capillaries move in to create bone over the calcified cartilage.

Growth in length stops when the epiphyseal plate closes and is replaced by bone, forming the epiphyseal line (usually by age 25).

Factors Affecting Bone Growth

Nutrition and Hormones

Bone growth and maintenance depend on adequate nutrition and hormonal regulation.

Minerals: Calcium and phosphorus are essential for bone growth.
Vitamins: Vitamin C is needed for collagen formation; vitamins K and B12 are required for protein synthesis.
Hormones: Insulin-like growth factor (IGF), human growth hormone (hGH), thyroid hormones (T3 & T4), and insulin are important during childhood. Sex steroids (estrogen and testosterone) stimulate growth and skeletal modifications at puberty.

Hormonal Abnormalities: Excess hGH during childhood causes giantism; deficiency leads to dwarfism. Lack of estrogen receptors can result in tall stature due to delayed closure of the growth plate.

Fractures and Bone Repair

Types of Fractures

Fractures are classified by the shape or position of the fracture line and the extent of damage.

Greenstick: Partial fracture, common in children.
Impacted: One side of the fracture is driven into the interior of the other side.
Closed (Simple): No break in the skin.
Open (Compound): Skin is broken.
Comminuted: Broken ends of bones are fragmented.
Specific Fractures: Potts fracture (ankle), Colles fracture (wrist), metacarpal fractures (hand).

Repair of Bone Fractures

Bone repair is a multi-step process:

Formation of Fracture Hematoma: Damaged blood vessels produce a clot within 6-8 hours; inflammation brings in phagocytic cells for clean-up.
Formation of Fibrocartilaginous (Soft) Callus: Fibroblasts lay down collagen fibers; chondroblasts produce fibrocartilage to span the broken ends.
Formation of Bony (Hard) Callus: Osteoblasts secrete spongy bone that joins the broken ends; lasts 3-4 months.
Bone Remodeling: Compact bone replaces spongy bone in the callus; the surface is remodeled back to normal shape.

Reduction: The procedure to realign fractured bone ends, either by manipulation (closed reduction) or surgery (open reduction).

Aging and Bone Tissue

Effects of Aging

Aging affects bone tissue primarily through loss of calcified matrix (demineralization) and decreased collagen production, leading to brittle bones and increased susceptibility to fractures.

Demineralization begins in women around age 40-45 due to decreased estrogen levels; in men, it starts after age 60.
Decreased growth hormone also contributes to bone brittleness.

Osteoporosis

Osteoporosis is characterized by decreased bone mass and porous bones, increasing fracture risk.

Risk factors: Menopausal women, low calcium intake, smoking, excessive alcohol, eating disorders, and certain athletes.
Prevention: Adequate diet, weight-bearing exercise, and estrogen replacement therapy (for menopausal women).

Disorders of Bone Ossification

Common Disorders

Rickets/Osteomalacia: Inadequate mineralization leads to soft bones, deformities, and increased fracture risk. Rickets occurs in children due to vitamin D deficiency; osteomalacia affects adults.
Osteosarcoma: Bone cancer affecting osteoblasts, common in teenagers during growth spurts. Most often affects the femur, tibia, or humerus.
Osteomyelitis: Bone infection, often caused by Staphylococcus aureus, characterized by fever, swelling, pus formation, and edema.

Bone Scan

Diagnostic Imaging

A bone scan is a diagnostic procedure in which a radioactive tracer is injected intravenously. Uptake of the tracer is related to blood flow and metabolic activity in bone tissue.

Normal Tissue: Appears as a consistent gray color.
Hot Spots: Areas of increased metabolic activity, indicating cancer, abnormal healing, or growth.
Cold Spots: Areas of decreased metabolism, indicating decalcified bone, infection, or degenerative bone disease.

Bone scans can detect abnormalities earlier than X-rays and expose patients to less radiation.

Summary Table: Types of Bone Cells

Cell Type	Function	Location
Osteoprogenitor Cell	Stem cell; divides to produce osteoblasts	Inner layer of periosteum and endosteum
Osteoblast	Forms bone matrix and collagen fibers	Bone surface
Osteocyte	Maintains bone tissue	Within bone matrix (lacunae)
Osteoclast	Resorbs bone matrix	Bone surfaces (endosteum)

Key Equations

Tensile Strength: The amount of load or stress a material can withstand before stretching and breaking.
Mineralization Equation: