Bone Structure and Function

• The skeletal system includes bones, joints, and supporting tissues.
• Bones are organs composed of osseous tissue, dense connective tissue, and bone marrow.

Skeletal System Functions

• Protection: The skeleton protects vital organs like the brain.
• Mineral Storage and Acid-Base Homeostasis: Bones store minerals such as calcium and phosphate, which help with electrolyte and acid-base balance.
• Blood Cell Formation: Red bone marrow is the site of blood cell formation.
• Fat Storage: Yellow bone marrow stores triglycerides.
• Movement: Muscles produce body movement via their attachment to bones.
• Support: The skeleton supports the body weight.

Bone Classification by Shape

• Long Bones: Longer than they are wide, mostly in the arms and legs.
• Short Bones: Cube-shaped or as long as they are wide, like wrist and ankle bones.
• Flat Bones: Thin and broad, including ribs, pelvis, sternum, and most skull bones.
• Irregular Bones: Vertebrae and certain skull bones that don't fit into other categories.
• Sesamoid Bones: Specialized bones within tendons, small, flat, and oval-shaped, like the patella.

Long Bone Structure

• Periosteum: Membrane of dense irregular collagenous connective tissue with blood vessels and nerves; surrounds the outer surface.
• Perforating Fibers (Sharpey's Fibers): Collagen that anchors the periosteum.
• Diaphysis: The shaft of the long bone; each end is an epiphysis, covered with hyaline cartilage.
• Medullary Cavity: Contains red or yellow bone marrow, depending on bone and age.
• Compact Bone: Hard, dense outer region that resists linear compression and stress.
• Spongy Bone: Inner bone tissue with bony struts to resist forces from multiple directions; provides a cavity for bone marrow.
• Endosteum: Membrane covering bony struts of spongy bone that is involved in bone maintenance.
• Epiphyseal Lines: Remnants of epiphyseal plates (growth plates) in developing bones of children.

Short, Flat, Irregular, and Sesamoid Bone Structure

• These bones do not have diaphyses, epiphyses, medullary cavities, epiphyseal lines, or epiphyseal plates.
• They are surrounded by periosteum with perforating fibers, blood vessels, and nerves
• They feature two outer layers of thin compact bone with a middle layer of spongy bone (diploë).
• Sinuses: Some flat and irregular skull bones contain air-filled spaces, which reduce bone weight.

Blood Supply to Bone

• Bones are well supplied with blood vessels and sensory nerve fibers.
• Short, flat, irregular, and sesamoid bones receive blood supply mostly from vessels in the periosteum.
• Long bones receive ⅓ of their blood supply from the periosteum, and ⅔ from one or two nutrient arteries.
• Nutrient arteries enter the diaphysis through a small hole (nutrient foramen).
• Epiphyses receive blood from nutrient arteries and vessels through small holes in compact bone.

Bone Marrow

• Red Bone Marrow: Loose connective tissue that supports blood-forming hematopoietic cells.
• The amount of red marrow decreases with age.
• In adults, red marrow is in the pelvis, proximal femur and humerus, vertebrae, ribs, sternum, clavicles, scapulae, and skull bones.
• Children have more red marrow to assist in growth and development.
• Yellow Bone Marrow: Contains triglycerides, blood vessels, and adipocytes.

Bone Marrow Transplantation

• Bone marrow transplantation can help with blood diseases like leukemia, aplastic anemia and sickle-cell anemia.
• Red marrow is withdrawn through a needle from the donor's pelvic bone.
• The recipient's marrow is destroyed and the donor marrow is given intravenously.
• Cells from the donor marrow travel to the recipient's marrow cavities and help to produce new blood cells in 2-4 weeks.
• Complications of transplant are similar to flu-like symptoms (first 2-4 weeks), infection, or transplant rejection.

Bone Tissue Composition

• Osseous Tissue: Primarily composed of extracellular matrix; includes a small population of cells.
• Extracellular Matrix: Unique in bone tissue.
  • Inorganic Matrix: Composed of minerals, makes up about 65% of bone's weight.
  • Organic Matrix: Makes up remaining 35%; it's largely made of collagen fibers.

Extracellular Matrix Components

• Inorganic Matrix: Predominantly calcium salts -Bone stores around 85% of total calcium ions -Contains calcium and phosphorus salts in hydroxyapatite crystals -[Ca₁₀(PO₄)₆(OH)₂] -Crystalline structure makes bone hard, strong, and resistant to compression
• Allows bone to be both protective and supportive
• Bicarbonate, potassium, magnesium, and sodium are also in inorganic matrix
• Organic Matrix: known as osteoid
• Composed of collagen protein fibers
• Collagen fibers resists twisting (torsion) and tensile forces.
• Fibers align with hydroxyapatite crystals to enhance hardness
• Creates an osmotic gradient, draws water into osteoid, and helps resist compression

Bone Cells

• Bone is dynamic tissue that’s constantly renewed by breaking down and building new bone.
• Three types of bone cells maintain bone's dynamic nature.
  • Osteoblasts
  • Osteocytes
  • Osteoclasts

Specific Bone Cell Function

• Osteoblasts: Active in periosteum and endosteum; perform bone deposition and form matrix.
  • Osteogenic cells: Differentiate into osteoblasts when chemically stimulated.
• Osteocytes: Derived from osteoblasts, they maintain bone extracellular matrix.
• Matrix surrounds matrix in small cavities (lacunae)
• Osteoclasts: Responsible for bone resorption; secrete hydrogen ions and enzymes to break down bone matrix.
• Eventually reside in depressions on bone surfaces.

Compact Bone Composition

• Osteon (Haversian System): Structural unit made of rings of thin bone layers (4 to 20). -Greatly enhance compact bone's strength
• Lamellae connect with central canal via blood vessels
• Canaliculi are vessels for sharing resources
• Cytoplasmic extensions of osteocytes extend through networks

Osteon Structures

• Interstitial Lamellae: outside the osteon -Lamellae outside of the osteon -Remnants of resorbed osteons -Fill gaps between forming osteons
• Circumferential Lamellae:
  • Extend around the diaphysis; are
  • Just deep to the periosteum
  • Resisting twisting of the entire bone
• Perforating canals connects with blood vessels in the periosteum, linking to central canals of neighboring osteons

Spongy Bone

• Spongy bones are usually not weight-bearing like compact one and not as densely packed
• Network reinforces bone, resisting forces from many directions
• Protects bone marrow tissue
• Trabeculae: Composed of struts of bone and covered with endosteum
  • Not arranged into osteons -Composed of concentric lamellae and osteocytes in lacunae
  • Communicate through canaliculi

Bone Formation

• Ossification (Osteogenesis): Bone formation occurs in the embryonic period and continues through childhood to age 7.
  • Immature primary (woven) bone occurs first
  • Collagen bundles, osteocytes, and sparse inorganic matrix are irregularly arranged
  • broken down by osteoclasts
  • Mature secondary bone follows primary bone formation
  • Contains more inorganic matrix with increased strength

Ossification Types

• Intramembranous Ossification: Built on a membrane model of embryonic connective tissue -Forms flat bones of the skull and clavicles during fetal development -Forms primary bone, which will become secondary bone
• Endochondral Ossification: Built on a model of hyaline cartilage -Forms most of the primary bones except skull and clavicles -Hyaline cartilage model includes chondrocytes, collegn, and ECM -These structures are surrounded by a membrane of connective tissue (perichondrium) and chondroblasts

Endochondral Ossification

• Cartilage remains in epiphyseal and articular surfaces where bones interact
• Articular cartilage persists into adulthood
• Epiphyseal plates replaced with bone/ossified, ceasing elongation

Bone Health and Disease

• Osteoporosis occurs when bones become brittle do to inadequate inorganic matrix
• Bones become weak due to dietary factors (calcium and/or vitamin D deficiency), gender, age, lack of exercise, estrogen, genetic factors, and other diseases
• Preventative measures, like balanced diet, weight-bearing exercise, estrogen or supplementation may mitigate effects
• Treatment may require drugs that inhibit osteoclasts or stimulate blasts

Lengthening Bones

• Chondrocyte division occurs in epiphyseal plate, and bone growth takes place at epiphysis closest to diaphysis
• Epiphyseal plate consists of 5 zones that compose ossification, and composed of hyaline cartilage
• The 5 zones are: -Ossification -Calcification -Hypertrophy & Maturation -Proliferation -Reserve cartilage

Bone Widening

• Appositional growth occurs through osteroblasts within the periosteum, and bone surface layering -Starts with circumferential lamellae
• Deeper portions are removed and mixed

Bone Remodeling

• Hormones affect bone growth
  • Growth hormone from anterior pituitary gland, and enhances protein cells including bones -Testosterone increases cells including growth and increases rate of mitosis -Estrogen: enhances bone growth, inhibits activity and causes rapid cell division
• Remodeling repairs, grows, and maintains bone - Repairs calcium levels and tension and homeostasis -Replaces primary bones by primary ones
• This process maintains skeleton homeostasis

Bone Remodeling Cells

• Periosteum and endosteum build organic matrix/formation of material/minerals, binding calcium ions
• Vesicles and enzymes bind to collagen fibers
• Hydrogen and calcium ions (pH sensitivity) break down and relocate
• Enzymes degrade minerals

Homeostasis Factors

• Increased exercise, bone tension, hormones, diet, Calcium increase
• Rest, poor diet, lack of water decrease ion intake and absorption

Maintaining Balance

• Calcium ions must remain balanced via absorption and release -PTH aids intestine and kidneys to maintain calcium
• PTH regulates bone activity which requires nerves & cells

Fractures

• Two main types: -Simple where tissue around fracture remains -Compound in tissues damaged
• Comminuted: shattering
• Disclaimer:* These notes are intended as a study aid and should not replace comprehensive textbook study or professional medical advice.

Test your knowledge of bone composition, including the organic and inorganic matrix. This quiz covers the role of collagen, hydroxyapatite, and minerals in bone strength and function. Questions address bone classification, calcium storage, and skeletal system functions.