Skeletal System: Support, Protection, and Movement

• The skeletal system provides support, protection, movement, storage of minerals and lipids, and blood cell production.
• Bones are dynamic tissues composed of bone tissue, cartilage, dense connective tissue, epithelium, adipose tissue, and nervous tissue.
• Osteology is the study of bone structure and the treatment of bone disorders.

Functions of the Skeletal System

• The skeleton offers a structural framework, aiding soft tissues and muscle tendon attachment points.
• Bones safeguard internal organs; the skull protects the brain, while the rib cage protects the heart and lungs.
• Muscles attach to bones, and their contraction results in movement.
• Bone tissue stores calcium and phosphorus, contributing to bone strength, and triglycerides in yellow bone marrow, serving as energy reserves.
• Red bone marrow is responsible for hemopoiesis, producing red blood cells, white blood cells, and platelets.

Bone Structure

• Bones are organs that incorporate bone tissue, cartilage, dense connective tissue, epithelium, adipose tissue, and nervous tissue.
• Key bone structures include the diaphysis, epiphyses, metaphyses, articular cartilage, periosteum, medullary cavity, and endosteum.
• The diaphysis is the bone's shaft.
• Epiphyses are the bone's distal and proximal ends.
• Metaphyses, located between the diaphysis and epiphyses, contain the epiphyseal plate in growing bones.
• Articular cartilage, made of hyaline cartilage, covers epiphyses at joints to reduce friction and absorb shock.
• The periosteum is a connective tissue sheath protecting the bone surface, aiding repair, nourishing tissue, and offering attachment points.
• The medullary cavity houses fatty yellow bone marrow in adults.
• The endosteum is a membrane lining the medullary cavity, containing bone-forming cells and connective tissue.

Bone Cells

• Bones contain osteogenic cells, osteoblasts, osteocytes, and osteoclasts.
• Osteogenic cells are stem cells from mesenchyme that develop into osteoblasts through cell division.
• Osteoblasts build bone, synthesizing collagen fibers and other components for its extracellular matrix.
• Osteocytes are mature bone cells derived from osteoblasts embedded in the matrix, maintaining bone tissue.
• Osteoclasts are large, multinucleated cells that break down bone matrix through resorption, using enzymes and acids.

Bone Tissue

• Bone tissue's extracellular matrix consists of 15% water, 30% collagen fibers, and 55% mineral salts.
• Calcium phosphate [Ca3(PO4)2] is the most abundant mineral salt in bone.
• Calcification is when mineral salts deposit in the collagen fiber framework, hardening the tissue.
• Bone is not completely solid, as small spaces exist between cells and matrix components.
• Compact bone has few spaces and is the strongest form of bone tissue.
• Spongy bone has larger spaces filled with red bone marrow.

Compact Bone

• Compact bone forms the external layer of all bones and a large part of long bones' diaphysis, providing protection, support, and stress resistance.
• Osteons, or Haversian systems, are repeating units in compact bone.
• Osteons feature concentric lamellae around a central canal.
• Concentric lamellae are mineralized matrix layers surrounding blood vessels and nerves in the central canal.
• Lacunae are spaces between lamellae that contain osteocytes.
• Canaliculi are small channels filled with extracellular fluid, connecting lacunae and central canals to form a canal system.
• Volkmann's canals allow blood vessels and nerves to connect the periosteum to the medullary cavity, central canals, and periosteum.

Spongy Bone

• Spongy bone lacks osteons.
• It features lamellae arranged as trabeculae.
• Spaces between trabeculae reduce bone weight.
• Trabeculae support and protect red bone marrow.
• Spongy bone is found in short, flat, irregular bones, and the epiphyses of long bones.

Bone Formation

• Ossification is the process of bone formation.
• Bone forms initially in embryos/fetuses, grows during infancy/adolescence, remodels throughout life and repairs fractures.
• Bone formation occurs via intramembranous or endochondral ossification.

Intramembranous Ossification

• Intramembranous ossification is the simpler bone formation process.
• It forms flat skull bones, facial bones, the mandible, and the clavicle's medial part.
• "Soft spots" in babies' heads indicate incomplete intramembranous ossification.
• Development of the Ossification Center: Mesenchymal cells differentiate into osteogenic cells and then into osteoblasts.
• Calcification: Osteoblasts secrete the extracellular matrix and become osteocytes; calcium and mineral salts harden the matrix.
• Formation of Trabeculae: Extracellular matrix develops into trabeculae, forming spongy bone.
• Development of the Periosteum: Mesenchyme condenses and develops into the periosteum.

Endochondral Ossification

• Endochondral ossification is the process where bone forms from hyaline cartilage.
• Most bones develop this way.
• Development of the Cartilage Model: Mesenchymal cells form chondroblasts, which secrete cartilage matrix to create a hyaline cartilage model.
• Growth of the Cartilage Model: Chondrocytes enlarge and the cartilage matrix begins to calcify.
• Development of the Primary Ossification Center: Bone tissue replaces cartilage in this region.
• Development of the Medullary Cavity: Osteoclasts break down spongy bone trabeculae, forming a cavity in the diaphysis.
• Development of the Secondary Ossification Centers: These form in the epiphyses.
• Formation of Articular Cartilage and Epiphyseal Plate: Hyaline cartilage covering the epiphyses becomes articular cartilage.

Bone Growth

• Bones grow in both length and thickness during infancy, childhood, and adolescence.
• The epiphyseal plate is responsible for lengthwise bone growth.
• Bones grow in thickness at their outer surface.

Bone Remodeling

• Bone remodeling replaces old bone tissue with new tissue.
• Bone remodeling includes bone resorption by osteoclasts and bone deposition by osteoblasts.
• Bone remodeling rates vary across the body.
• Balancing bone resorption and deposition is crucial.
• Minerals, vitamins, and hormones affect bone growth and remodeling.

Minerals

• Calcium and phosphorus in large amounts are needed for bone growth, along with smaller amounts of magnesium, fluoride, and manganese.

Vitamins

• Vitamin A stimulates osteoblast activity.
• Vitamin C is needed for collagen synthesis.
• Vitamin D increases calcium absorption.
• Vitamins K and B12 are needed for bone protein synthesis.

Hormones

• Growth hormone and insulin-like growth factors are crucial during childhood.
• Thyroid hormones and insulin stimulate osteoblasts and bone matrix synthesis.
• Estrogens accelerate epiphyseal plate closure.
• Parathyroid hormone (PTH) promotes bone resorption and calcium recovery from urine.
• Calcitonin (CT) inhibits bone resorption by osteoclasts.

Exercise and Bone Tissue

• Bone strength is altered by mechanical stress.
• Bone tissue strengthens under stress and weakens without it.
• Weight-bearing activities build and maintain bone mass.

Bone Fracture Repair

• A fracture is any break in a bone.
• Reactive Phase: Early inflammatory phase.
• Reparative Phase: Includes formation of a fibrocartilaginous callus and a bony callus.
• Bone Remodeling Phase: The bony callus is remodeled.

Classification of Bones

• Bones are classified as long, short, flat, irregular, or sesamoid.
• Long Bones: Greater length than width (e.g., femur, tibia, humerus, ulna).
• Short Bones: Cube-shaped (e.g., carpals and tarsals).
• Flat Bones: Thin, with compact bone layers enclosing spongy bone (e.g., cranial bones, sternum, ribs).
• Irregular Bones: Complex shapes (e.g., vertebrae, hip bones).
• Sesamoid Bones: Shaped like sesame seeds, developing in tendons with friction (e.g., patellae).

Bone Surface Markings

• Bone surface markings are structural features adapted for specific functions.
• Depressions/Openings: Accommodate blood vessels/nerves or form joints.
• Processes: Forming joints or serving as attachment points for ligaments/tendons.
• Examples of Depressions and Openings: fissure, foramen, fossa, meatus, or sinus.
• Examples of Processes that form Joints: condyle, facet, or head.
• Examples of Processes that form Attachment Points: crest, epicondyle, line, spinous process, trochanter, tubercle, or tuberosity.

The skeletal system provides support, protection, movement, mineral storage, and blood cell production. Bones are dynamic tissues consisting of bone tissue, cartilage, and connective tissue. Osteology studies bone structure and disorders.