Skeletal System Anatomy and Physiology

• Bones and bone tissues explained
• The functions of the skeletal system are outlined
• Classification of bones by shape is covered
• Bone structure is explained
• Includes discussion and definition of ossification

Skeletal System Functions

• Protection of vital organs
• Mineral storage
• Acid-base homeostasis
• Blood cell formation in red bone marrow
• Fat storage via triglycerides in yellow bone marrow
• Movement via muscle attachment to bones
• Support: the weight of the body

Bone Classification by Shape

• Long bones are longer than they are wide, such as arms and legs
• Short bones are cube-shaped, such as wrists (carpals) and ankles (tarsals)
• Flat bones are thin and broad, such as ribs, pelvis, sternum (breastbone), and most skull bones
• Irregular bones include vertebrae and some skull bones, not fitting other classes
• Sesamoid bones are specialized, located within tendons, and give tendon a mechanical advantage, the patella (kneecap)

Long Bone Structure

• Periosteum forms a covering with blood vessels on the outer surface
• Perforating fibers anchor the periosteum to the bone matrix
• Diaphysis is the shaft of a long bone
• Epiphyses are each end of a long bone
• Ends are covered with hyaline cartilage (articular cartilage) between bones
• Marrow cavity is within the diaphysis and contains red or yellow marrow
• Compact bone is the dense outer region, resists linear compression and twisting
• Spongy bone is located inside compact bone, providing a honeycomb-like framework
• Endosteum covers inner surfaces of bone
• Epiphyseal lines separate epiphyses from the diaphysis
• Epiphyseal plates (growth plates) are lines of hyaline cartilage in developing bones

Short, Flat, Irregular, and Sesamoid Bones

• Lack diaphysis, epiphyses, medullary cavities, epiphyseal lines, or epiphyseal plates
• Covered by periosteum with associated perforating fibers, blood vessels, and nerves
• Internal structure: two outer layers of compact bone and a middle layer of spongy bone (diploe)
• Some flat and irregular skull bones have air-filled spaces called sinuses, which reduce bone weight
• Blood supply comes mainly from nutrient arteries, which enter through the nutrient foramen in the diaphysis
• Epiphyses receive blood from nutrient arteries and small vessels through the compact bone

Red and Yellow Bone Marrow Differences

• Red marrow is hematopoietic (produces blood cells)
• Red marrow decreases as a person ages
• Red marrow in adults is only in the pelvis
• Children need more red marrow for growth
• Yellow marrow contains triglycerides, blood vessels, and adipocytes

Extracellular Matrix Composition

• Inorganic matrix makes up about 65% of bone by weight
• Primarily calcium salts, and about 85% of calcium ions in the body, plus phosphorus, and exists as hydroxyapatite crystals
• The crystalline structure gives bone its hardness and compressive strength
• Bicarbonate, potassium, magnesium, and sodium are also in inorganic matrix
• Organic matrix makes up 35% of bone weight, containing collagen fibers and ECM components
• Organic matrix: also known as osteoid, consists of protein fibers, proteoglycans, glycosaminoglycans, glycoproteins, and bone-specific proteins
• Collagen: predominantly resists torsion (twisting) and tensile (pulling or stretching) forces
• Collagen fibers align with hydroxyapatite crystals, enhancing hardness
• Glycosaminoglycans and proteoglycans create an osmotic gradient, drawing water to resist compression
• Glycoproteins bind all components of the osteoid and inorganic matrix together

Bone Cell Types and Functions

• Osteoblasts: metabolically active cells on the periosteum and endosteum
• Osteoblasts carry out bone deposition, secreting organic matrix materials and forming inorganic matrix
• Osteocytes: relatively inactive cells surrounded by bone matrix in lacunae, derived from osteoblasts
• Osteoclasts: responsible for bone resorption
• Secrete hydrogen ions and enzymes to break down bone matrix, derived from fused cells
• Hydrogen ions dissolve inorganic matrix, while enzymes break down organic matrix

Ossification (Osteogenesis)

• The is the process of bone formation
• Begins in the embryonic period and continues through childhood (bones complete by age 7)
• Intramembranous ossification forms flat bones like skull bones and clavicles during fetal development
• Endochondral ossification is bone development for all bones below the head (except clavicle), beginning in the fetal stage
• Wrist and ankles ossify much later
• Hyaline cartilage serves to scaffold developing bone

Long Bone Growth

• Long bones grow in length via longitudinal growth
• The dividing chondrocytes (not osteocytes or osteoblasts) are closest to the diaphysis

Longitudinal vs. Appositional Bone Growth

• Longitudinal bone growth is long bones growing in arms and legs
• Appositional growth is bones growing in width

Bone Deposition and Resorption

• Bone deposition is bone formation carried out by osteoblasts
• Bone resorption is the process of old bone removal and is carried out by osteoclasts

Factors Influencing Bone Remodeling

• Hormones: Testosterone promotes bone deposition, estrogen inhibits osteoclast activity
• Age: Growth hormones and sex hormones decline, affecting bone dynamics
• Calcium ion intake: Supports bone deposition
• Vitamin D: Promotes calcium absorption from the gut and prevents calcium ion loss
• Vitamin C: Needed for collagen synthesis
• Vitamin K: Synthesis of calcium ion-binding glycoproteins secreted by osteoblasts
• Protein Intake: Collagen fibers, found in an organic matrix

Calcitonin, Parathyroid Hormone, and Vitamin D Roles

• Calcitonin is secreted by the thyroid gland, active during bone growth
• Parathyroid hormone (PTH) increases blood calcium ion levels
• Vitamin D aids in absorption of calcium ions from the gut

Test your knowledge of bone cells, growth, and hormones. Explore bone resorption, ossification, and the roles of key vitamins. Understand skeletal system functions.