Bones, Bone Structure, and Joints: Study Guide for Anatomy & Physiology

Bones and Bone Structure

Bone Classifications

Bones are classified based on their shape and structure, which relates to their function in the body.

• Long Bones: Longer than they are wide (e.g., femur, humerus). Function in movement and support.

• Short Bones: Nearly equal in length and width (e.g., carpals, tarsals). Provide stability and some movement.

• Flat Bones: Thin, flattened, and often curved (e.g., sternum, ribs, skull bones). Protect internal organs and provide surfaces for muscle attachment.

• Irregular Bones: Complex shapes (e.g., vertebrae, pelvic bones).

• Sesamoid Bones: Small, round bones embedded within tendons (e.g., patella).

Example: The femur is a long bone, while the vertebrae are irregular bones.

Compact vs. Spongy Bone

Bones have two main types of tissue: compact and spongy bone, each with distinct structures and locations.

• Compact Bone: Dense and solid, forms the outer layer of bones. Contains osteons (Haversian systems), which are cylindrical structures that provide strength.

• Spongy Bone: Also called cancellous bone, found inside bones, especially at the ends of long bones and in flat bones. Contains a network of trabeculae (thin columns and plates), which help reduce bone weight and provide space for marrow.

Example: The diaphysis of long bones is mostly compact bone, while the epiphyses contain spongy bone.

Articular Cartilage

Articular cartilage is a smooth, white tissue that covers the ends of bones where they come together to form joints.

• Reduces friction and absorbs shock in joints.

• Composed of hyaline cartilage.

• Found at synovial joints (e.g., knee, elbow).

Bone Growth and Hormonal Regulation

Bone growth and remodeling are regulated by several hormones:

• Calcitonin: Lowers blood calcium by inhibiting osteoclast activity and increasing calcium excretion by kidneys.

• Parathyroid Hormone (PTH): Raises blood calcium by stimulating osteoclasts, increasing intestinal absorption, and reducing excretion by kidneys.

• Growth Hormone: Stimulates bone growth, especially during childhood.

• Sex Hormones (estrogen, testosterone): Promote bone growth and epiphyseal plate closure during puberty.

Appositional Bone Growth

Appositional growth refers to the increase in bone diameter by the addition of new bone tissue at the surface.

• Osteoblasts in the periosteum add new bone matrix to the outer surface.

• Osteoclasts remove bone from the inner surface, enlarging the medullary cavity.

Ossification: Intramembranous vs. Endochondral

Ossification is the process of bone formation. There are two main types:

• Intramembranous Ossification: Bone develops directly from mesenchymal tissue. Forms flat bones of the skull, mandible, and clavicle.

• Endochondral Ossification: Bone develops by replacing hyaline cartilage. Forms most bones of the body, including long bones.

Bone Cells: Osteoblasts vs. Osteoclasts

• Osteoblasts: Bone-forming cells that secrete bone matrix.

• Osteoclasts: Bone-resorbing cells that break down bone tissue.

Disorders of Bone Growth

• Pituitary Dwarfism: Short stature due to insufficient growth hormone.

• Gigantism: Excess growth hormone before epiphyseal plate closure, leading to abnormally large stature.

• Acromegaly: Excess growth hormone after epiphyseal plate closure, causing enlargement of hands, feet, and face.

Structure of a Long Bone

• Diaphysis: Shaft of the bone, composed mainly of compact bone.

• Epiphyses: Ends of the bone, mostly spongy bone.

• Metaphysis: Region between diaphysis and epiphysis; contains the growth plate.

• Medullary Cavity: Central cavity containing bone marrow.

• Periosteum: Outer fibrous covering of bone.

• Endosteum: Thin membrane lining the medullary cavity.

Bone Marrow: Red vs. Yellow

• Red Bone Marrow: Site of hematopoiesis (blood cell formation); found in children and in adult flat bones.

• Yellow Bone Marrow: Stores fat; increases with age as red marrow is replaced.

Example: In adults, red marrow is found in the pelvis, sternum, and proximal femur/humerus.

Hydroxyapatite

Hydroxyapatite is the mineral component of bone, providing hardness and strength.

• Chemical formula: $Ca_{10}(PO_4)_6(OH)_2$

• Composed of calcium and phosphate.

Functional Unit of Bone

The osteon (Haversian system) is the functional unit of compact bone, consisting of concentric lamellae around a central canal.

The Skeleton and Joints

Bone Identification and Structures

Students should be able to identify bones and their key features, such as processes, foramina, and articulations.

Types of Joints and Movements

Joints are classified by structure and function, allowing various types of movement.

• Fibrous Joints: Bones joined by dense connective tissue; little or no movement (e.g., sutures of the skull).

• Cartilaginous Joints: Bones joined by cartilage; limited movement (e.g., intervertebral discs).

• Synovial Joints: Freely movable joints with a synovial cavity (e.g., knee, shoulder).

Movements: Include flexion, extension, abduction, adduction, rotation, circumduction, etc.

Parts of a Synovial Joint

• Articular Cartilage: Covers bone surfaces.

• Joint (Synovial) Cavity: Space filled with synovial fluid.

• Articular Capsule: Encloses the joint cavity.

• Synovial Fluid: Lubricates and nourishes cartilage.

• Ligaments: Strengthen and support the joint.

Knee Joint

The knee is a complex synovial joint involving the femur, tibia, and patella, stabilized by ligaments and menisci.

Forms of Arthritis

• Osteoarthritis: Degenerative joint disease due to cartilage breakdown.

• Rheumatoid Arthritis: Autoimmune inflammation of joints.

• Gouty Arthritis: Inflammation due to uric acid crystal deposition.

Blood Calcium Regulation

Three systems maintain normal blood calcium:

• Skeletal System: Stores and releases calcium.

• Digestive System: Absorbs calcium from food.

• Urinary System: Excretes or conserves calcium.

Skull and Axial Skeleton

Sutures of the Skull

• Coronal Suture: Between frontal and parietal bones.

• Sagittal Suture: Between parietal bones.

• Lambdoid Suture: Between parietal and occipital bones.

• Squamous Suture: Between parietal and temporal bones.

Sinuses and Their Function

• Paranasal Sinuses: Air-filled spaces in skull bones (frontal, maxillary, ethmoid, sphenoid).

• Functions: Lighten skull, warm and moisten air, enhance voice resonance.

Fontanels

Fontanels are soft spots on a baby's skull where bones have not yet fused, allowing for brain growth and passage through the birth canal.

Vertebral Issues

• Osteoporosis: Loss of bone mass, increased fracture risk.

• Osteopenia: Reduced bone mass, less severe than osteoporosis.

• Vertebral Disorders: Include herniated discs, scoliosis, kyphosis, lordosis.

Rib Articulations

Ribs articulate with thoracic vertebrae at the head and tubercle of the rib.

Pelvic Bones

The ischial tuberosities are the parts of the pelvis you sit on.

Male vs. Female Skeleton

• Pelvis: Female pelvis is wider and shallower for childbirth.

• Skull: Male skulls are generally larger and more robust.

• Other Differences: Males have more pronounced muscle attachment sites.

Feature	Male Skeleton	Female Skeleton
Pelvic Inlet	Narrow, heart-shaped	Wide, oval
Subpubic Angle	< 90°	> 90°
Overall Robustness	More robust	Less robust
Skull Features	More pronounced	Smoother

Summary Table: Bone Cells and Functions

Cell Type	Function
Osteoblast	Builds bone matrix
Osteoclast	Breaks down bone matrix
Osteocyte	Maintains bone tissue

Additional info: Some explanations and examples have been expanded for clarity and completeness based on standard Anatomy & Physiology curriculum.