chapter 6 in depth 
Human Anatomy and Physiology - Chapter 06: Bones and Bone Tissue
The Skeletal System
Components of the Skeletal System:
Consists of bones, joints, and other supporting tissues.
Bones:
Main organs of the skeletal system; adults typically have 206 bones.
Each bone includes:
Bone (Osseous) Tissue
Dense Regular Collagenous Tissue
Dense Irregular Connective Tissue
Bone Marrow
Functions of the Skeletal System
### Protection
Bones such as the skull, sternum, and ribs protect underlying organs.
### Mineral Storage and Acid-Base Homeostasis
Bones store minerals including calcium, phosphorus, and magnesium salts.
These minerals act as electrolytes, acids, and bases in the blood, essential for maintaining electrolyte and acid-base balance.
### Blood Cell Formation
Red Bone Marrow is the site of Hematopoiesis (the formation of blood cells).
### Fat Storage
Yellow Bone Marrow contains adipocytes with stored triglycerides.
### Movement
Bones serve as attachment points for most skeletal muscles; muscle contraction pulls on bones, generating movement around joints.
### Support
The skeleton supports body weight and provides structural framework.
Bone Structure
### Classification of Bone by Shape
Long Bones: Longer than wide. Examples include limb bones.
Short Bones: Approximately cube-shaped, e.g., wrist and ankle bones.
Flat Bones: Thin and broad, e.g., most skull bones, bones of the pelvis.
Irregular Bones: Have irregular shapes, e.g., vertebrae.
Sesamoid Bones: Small, flat, oval-shaped bones in tendons, e.g., kneecap.
Structure of a Long Bone
Periosteum:
Outer dense irregular connective tissue membrane containing blood vessels and nerves.
Perforating Fibers:
Collagen anchors that penetrate into the bone matrix to attach the periosteum.
Diaphysis:
Shaft of the bone with a medullary (marrow) cavity lined by the endosteum and filled with marrow.
Epiphyses:
Ends of long bones filled with red marrow, covered by articular cartilage (hyaline cartilage).
Compact Bone:
Dense outer bone resists linear compression and twisting forces.
Spongy (Cancellous) Bone:
Inner honeycomb-like framework resists forces from multiple directions and houses bone marrow.
Epiphyseal Lines:
Remnants of the epiphyseal (growth) plate, a line of hyaline cartilage that is actively growing in children and adolescents.
Structure of Short, Flat, Irregular, and Sesamoid Bones
Share similarities with long bones but consist of fewer structures.
In flat bones, the spongy bone is called Diploë; some flat and irregular skull bones have air-filled spaces called sinuses to reduce weight.
Blood and Nerve Supply to Bone
Bones are enriched with blood vessels and sensory fibers.
Short, Flat, Irregular, and Sesamoid Bones: Blood supply is chiefly through the periosteum.
Long Bones: Blood supply is from the periosteum and Nutrient Arteries via the Nutrient Foramen in the diaphysis.
Red and Yellow Marrow
Yellow Bone Marrow:
Composed mainly of blood vessels and adipocytes.
Red Bone Marrow:
Comprising a network of reticular fibers supporting hematopoietic cells.
Infants predominantly have red marrow, transitioning to yellow marrow by age 5. Adult patterns include red marrow only in certain areas such as pelvic bones, proximal thigh and arm bones, vertebrae, ribs, and sternum.
Bone Marrow Transplantation
Indications: Patients with leukemia, sickle-cell anemia, and aplastic anemia may benefit from bone marrow transplants.
Procedure:
A needle is inserted into the pelvic bone of a matching donor; Bone Marrow Harvest can retrieve up to 2 quarts of red marrow.
The recipient's marrow is destroyed via chemotherapy/radiation before donor marrow is administered, allowing new blood cell production in 2-4 weeks, with risks of infection and rejection.
Alternative: Peripheral Blood Stem Cell (PBSC) Donation
Donor receives the injection to encourage hematopoietic cell release into the bloodstream. Blood is filtered to extract these cells.
The Extracellular Matrix of Bone (Bone Matrix)
### Inorganic Matrix
Accounts for about 65% of bone weight, primarily calcium and phosphorus forming Hydroxyapatite Crystals, which confer strength and compressive resistance.
Includes bicarbonate, potassium, magnesium, and sodium salts.
### Organic Matrix (Osteoid)
Approximately 35% of bone weight, includes:
Protein fibers (mainly collagen)
Proteoglycans
Glycosaminoglycans
Glycoproteins
Bone-specific proteins such as Osteocalcin.
Functions of components:
Collagen provides toughness and resistance to torsion and tensile forces.
Osteocalcin binds calcium and hydroxyapatite, organizing bone’s inorganic matrix.
Glycosaminoglycans and proteoglycans promote water retention and resist compression.
Glycoproteins facilitate bindings between hydroxyapatite and bone cells.
Bone Cells
Bone is a dynamic tissue; new bone continually forms while old bone is resorbed, facilitated by:
Osteoblasts: Build bone and mature into osteocytes.
Osteocytes: Mature osteoblasts that maintain bone integrity.
Osteoclasts: Multinucleated cells responsible for bone resorption, breaking down bone matrix.
Comparison of Osteoblasts, Osteocytes, and Osteoclasts
Osteoblasts:
Derived from osteogenic cells; Responsible for bone deposition by secreting organic matrix and assisting inorganic matrix formation.
Osteocytes:
Trap within a lacuna, maintain the ECM; stimulate osteoblasts under stress/strain.
Osteoclasts:
Large cells with ruffled borders; break down bone ECM through hydrogen ion secretion, affecting mineral and nutrient levels.
Osteopetrosis
Condition known as Marble Bone Disease, characterized by defective osteoclasts that fail to degrade bone properly.
Results in increased bone mass that is weak and brittle.
Infantile Osteopetrosis: Most severe inherited form, leads to nerve trapping causing blindness/deafness, and crucial reduction in red marrow can be fatal.
Adult Osteopetrosis: Milder inherited form, symptoms include bone pain, fractures, and joint pain.
Histology of Bone
Compact Bone: Hard, dense outer layer, stress-resistant, organized into Osteons (Haversian Systems).
Spongy Bone: Composed of Trabeculae, forming a framework for bone marrow but is not weight-bearing.
Bone Formation: Ossification
Ossification (Osteogenesis): Process of bone formation, typically completes by age 7.
Primary (Woven) Bone: Immature form with irregular collagen arrangements.
Secondary (Lamellar) Bone: Mature, strong form with organized lamellae and higher inorganic matrix percentage.
### Two Forms of Ossification:
Intramembranous Ossification: Begins with embryonic connective tissue (mesenchymal membrane), resulting in many flat bones (e.g., skull, clavicles).
Endochondral Ossification: Completes with hyaline cartilage models for bone formation under most conditions (all bones below the head except clavicles).
Steps of Intramembranous Ossification
Osteoblast formation from mesenchymal cells in primary ossification center.
Osteoblasts secrete organic matrix which calcifies, leading to osteocyte development.
Osteoblasts lay down trabeculae of spongy bone, surrounding mesenchyme differentiates into periosteum.
Osteoblasts in periosteum lay down compact bone; larger bones may require fusion of primary ossification centers.
Steps of Endochondral Ossification
Chondroblasts in the perichondrium differentiate into osteoblasts.
The bone initiates ossification from the outside with:
2a. Osteoblasts forming a bone collar.
2b. Internal cartilage begins calcifying as chondrocytes die.
In the primary ossification center, osteoblasts replace calcified cartilage with early spongy bone.
Medullary cavity expands, replacing cartilage remaining with bone while epiphyses begin ossification, maintaining cartilage for growth.
Bone Growth in Length
Longitudinal Growth: Involves chondrocyte division at the epiphyseal plate, which has 5 different zones:
Zone of Reserve Cartilage: Non-dividing cells.
Zone of Proliferation: Active chondrocyte division occurs here.
Zone of Hypertrophy and Maturation: Contains mature chondrocytes.
Zone of Calcification: Cells and matrix die; further away from blood supply.
Zone of Ossification: Calcified chondrocytes; osteoblasts build bone.
Bone Growth in Width
Appositional Growth: Growth of bone width continues post-length growth; osteoblasts between periosteum and bone surface deposit new bone.
Involves new circumferential lamellae formation while deeper ones are incorporated into osteons and medullary cavity expands via osteoclasts.
Hormones and Bone Growth
Growth Hormone: Secreted by the anterior pituitary, stimulates chondrocytes in epiphyseal plates and osteoblasts in periosteum.
Testosterone: Promotes thicker bones and greater calcium deposits, accelerates plate closure in males.
Estrogen: Similar effects to testosterone but less pronounced, tends to close plates earlier in females.
Osteoporosis
Disease from inadequate inorganic matrix, leading to brittle bones and higher fracture risk.
Causes include calcium and vitamin D deficiency, female sex, older age, lack of exercise, hormonal changes (especially in postmenopausal women), genetic predisposition, and diseases affecting digestive or urinary systems.
Preventative Measures: Adequate calcium and vitamin D intake, weight-bearing exercises, potential hormone replacement therapy for women post-menopause, medications that inhibit osteoclasts or stimulate osteoblasts.
Achondroplasia
Most common cause of dwarfism caused by mutations affecting growth factor receptors influencing endochondral ossification.
Typical height around 4 feet; effects include shortened limbs, long trunk, characteristic features, and potential spinal and chest wall deformities.
Bone Repair: Process of Fracture Healing
Hematoma Formation: Fills gaps due to ruptured blood vessels, leading to cell death in the region of injury.
Soft Callus Formation: Fibroblasts and chondroblasts infiltrate creating dense tissue and hyaline cartilage.
Bone Callus Formation: Osteoblasts from periosteum create a primary bone collar.
Remodeling of the Bone Callus: Primary bone models are replaced with secondary bone.
Types of Fractures
Simple Fracture (Closed): Skin remains intact.
Compound Fracture (Open): Skin or surrounding tissues damaged around the fracture.
Spiral Fracture: Caused by twisting forces.
Compression Fracture: Bone crushes under weight; common in older individuals.
Comminuted Fracture: Bone shattered into multiple parts, difficult to repair.
Avulsion Fracture: Pieces of bone pulled off by ligaments or tendons, common in ankle injuries.
Greenstick Fracture: Bone breaks on one side but bends on the opposite side; prevalent in children.
Epiphyseal Plate Fracture: Occurs in children and young adults, may affect growth.