Untitled Quiz

Functions of Bones

• Support: Provides structural support for the body and protects internal organs.
• Protection: Shields the central nervous system; skull protects the brain, and vertebrae protect the spinal cord, while the rib cage encases thoracic and upper abdominal organs.
• Attachment Points: Serves as attachment points for skeletal muscles via tendons.
• Storage: Stores essential minerals and fat; yellow marrow located in adult bones.
• Blood Cell Formation: Hematopoiesis occurs in red bone marrow, responsible for producing blood cells.
• Hormone Production: Osteocalcin plays a role in regulating insulin release, glucose homeostasis, and energy expenditure.

Cartilage and Skeletal Formation

• Initial Skeleton: Composed of cartilage and connective tissue fibers before ossification.
• Characteristics of Cartilage: Strong, resilient, flexible, and mainly composed of water; lacks nerve supply and blood vessels, encased by perichondrium containing blood vessels.

Types of Cartilage

• Hyaline Cartilage: Most prevalent type, containing spherical chondrocytes and collagen; examples include articular, costal, respiratory, and nasal cartilages.
• Elastic Cartilage: Similar to hyaline but with more elastic fibers, making it more flexible; found in the external ear and epiglottis.
• Fibrocartilage: Contains rows of chondrocytes interspersed with thick collagen bands; offers great tensile strength and is found in areas bearing weight, such as vertebral discs and knee joints.

Cartilage Growth

• Appositional Growth: New cartilage forms on the surface of existing cartilage, increasing thickness.
• Interstitial Growth: Growth occurs within the cartilage as cells divide and secrete new matrix, increasing length.

Bone Tissue

• Skeletal Structure: Divided into axial (skull, vertebral column, ribs) for protection and appendicular (limbs and girdles) for movement.
• Bone Shapes:
  • Long bones: Longer than wide (e.g., limb bones).
  • Short bones: Cube-shaped (e.g., wrist and ankle bones).
  • Flat bones: Thin, flat, and curved (e.g., sternum, ribs).
  • Irregular bones: Unclassified shapes (e.g., vertebrae).

Gross Anatomy of Bone

• Bone Composition: All contain outer compact bone and inner spongy bone.
• Compact Bone: Appears smooth and solid; spongy bone has open spaces filled with marrow.
• Membranes: Surrounded by periosteum (external covering) and endosteum (internal covering).
• Vascularization: Involves nutrient arteries and veins for blood supply to the bone structure.

Microscopic Anatomy

• Osteon: Fundamental structural unit of compact bone; composed of lamellae organized to withstand stress.
• Central Canals: Host nerves and blood vessels, while perforating canals connect with neighboring osteons.

Hematopoietic Tissue

• Blood Formation: Found in long and flat/irregular bones; red marrow is present in infants and adults, where it converts to yellow marrow with age.
• Red Marrow Functionality: Converts to yellow marrow over time; can revert during instances of anemia or blood loss.

Cellular Composition of Bone

• Osteoprogenitor Cells: Stem cells that divide to produce osteoblasts or remain undifferentiated.
• Osteoblasts: Immature bone-forming cells that secrete unmineralized matrix (osteoid).
• Osteocytes: Mature bone cells responsible for monitoring bone matrix and responding to mechanical stress.
• Osteoclasts: Bone-reabsorbing cells that help remodel bones and maintain calcium homeostasis.

Chemical Composition of Bone

• Organic Components: Include cells and osteoid; sacrificial bonds in collagen offer resilience.
• Inorganic Components: Primarily mineral salts like calcium phosphate that provide strength.

Bone Ossification

• Types of Ossification:
  • Endochondral Ossification: Most common, replaces cartilage with bone tissue; initiated by the formation of a bone collar at about nine weeks of fetal development.
• Stages of Endochondral Ossification:
  • Formation of bone collar and primary ossification center in the diaphysis.
  • Development of a cavity within the diaphysis as cartilage calcifies and cells die off.### Bone Development and Ossification
• Matrix within the primary ossification center (POC) deteriorates, creating a cavity in the diaphysis.
• Cartilage outside the bone continues to grow, supported by the bone collar which prevents collapse under weight.
• Initial spongy bone formation occurs in the diaphysis around the third month of gestation when the periosteal bud invades the cavity, containing nutrients and various cell types.
• Osteoblasts create matrix around the calcified cavity, leading to initial trabecular bone formation.
• Formation of the medullary cavity begins at birth, where initial spongy bone is resorbed by osteoclasts and replaced by medullary cavity.
• Secondary ossification begins in epiphyses, with spongy bone retained while actively growing cartilage persists.

Intramembranous Ossification

• Involves bone formation from a fibrous membrane with no cartilage involved.
• Begins with mesenchymal cells differentiating into osteoblasts, forming an ossification center.
• Osteoblasts lay down osteoids which calcify, trapping them to form osteocytes.
• Blood vessels invade calcified matrix, leading to the creation of trabeculae known as woven bone.
• Periosteum formation occurs as mesenchyme condenses, creating a fibrous layer; compact bone replaces some spongy bone with red marrow development.

Bone Growth

• Length growth occurs through interstitial growth at the epiphyseal plate which has three regions: active, resting, and hypertrophy.
• Active regions divide and lay down new matrix, pushing epiphyses away from diaphysis.
• Cartilage is calcified and replaced, with growth continuing into adolescence until epiphysis and diaphysis fuse at the epiphyseal plate closure.
• Width growth is appositional; osteoblasts secrete new matrix under periosteum while osteoclasts break down tissue internally to maintain thickness.

Hormonal Regulation of Bone Growth

• Growth hormone, released by the anterior pituitary gland, regulates epiphyseal plate activity.
• Excess growth hormone leads to gigantism, while insufficient levels cause dwarfism.
• Sex hormones (estrogen and testosterone) influence growth spurts during puberty; estrogen leads to earlier closure of epiphyseal plates in females, while testosterone influences male skeletal features.

Bone Remodeling

• Continuous process of bone deposition (new formation) and resorption (breakdown), vital for calcium homeostasis.
• Parathyroid hormone (PTH) increases osteoclast activity when blood calcium levels drop, enhancing bone resorption to release calcium into the bloodstream.
• Wolff’s Law asserts that bones strengthen in response to mechanical stress, leading to denser and thicker areas where pressure is higher.
• Bone density can decrease after menopause due to lower estrogen levels.

Bone Repair Process

• Requires immobilization of the fracture site to initiate repair.
• Hematoma formation occurs at fracture site, with new blood vessels bringing in macrophages and chondroblasts to form fibrocartilaginous callus.
• Bony callus formation replaces cartilage with bone; remodeling is necessary for normal stress adaptation.
• Hyaline cartilage and epiphyseal plates persist into adulthood, crucial for ongoing skeletal function.