Bone Physiology PDF

Functions of skeleton - Support of the body (bears weight) - Mechanical protection (protects brain) - Leverage for movement (acts like stiff rods, moving fingers needs the contraction of muscles in forearm) - Storage for minerals and lipids (Ca+, phosphorus as minerals and ye...

Functions of skeleton - Support of the body (bears weight) - Mechanical protection (protects brain) - Leverage for movement (acts like stiff rods, moving fingers needs the contraction of muscles in forearm) - Storage for minerals and lipids (Ca+, phosphorus as minerals and yellow marrow for lipid) - Hematopoesis (red bone marrow makes white/red blood cells) Bone shape - Sutural bones (sutures on skull) ex. sagittal suture - Sesamoid bone (grows between ligaments) ex. Patella ^^most important - Flat bone (parietal bone) - Long bone (femur) - Short bone (carpal bones) - Irregular bone (vertebra) Bone [osseous] tissue Compact (cortical): dense and organized, made up of collagen fibers - Lamellae: are found in the rings of bone ie. circumferential, interstitial, concentric. - Osteons (collection of rings): rings of lamellae make up osteons - Haversian canal (central): are veins and arteries that bring nutrients to lacunae, are vertical - Volkmann canal (perforating): go through bone horizontally and have veins and arteries - Lacunae: house osteocytes - Canaliculi: microscopic canals that provide nutrients to lacunae Spongy (cancellous): honeycombed, trabeculae, many open spaces - Made up of trabeculae, empty space between trabecular is red/yellow marrow, has tiny holes in trabecula for canaliculi, endosteum lines outside of trabecula - Cross-section of trabecula will have osteoblasts lining outside, osteoclasts on outside, lacuna with osteocytes in it, each ring has lots of lamellae and canaliculi running though Structure - Diaphysis: shaft of bone, long center - Metaphysis: in between diaphysis and epiphysis, where bone grows in length - Epiphysis: ends of bone, proximal/distal - Medullary cavity: middle of bone, where marrow sits - Articular cartilage: ends of bone, acts as lubricant between bones Periosteum - Lines only outer surface - Made of dense irregular tissue - Has a fibrous layer, then a cellular layer - Sharpey’s (perforating) fibers are made of collagen fibers and hold periosteum to bone Endosteum - Lines inner surfaces of bone (medullary cavity, trabeculae, canals) - Doesn’t have any fibers, just cells (same cells as cellular layer of periosteum) Flat bone structure - Is a “sandwich of spongy bone”, spongy bone between two layers of compact bone - Called a diploe with marrow** - Only has red marrow, no yellow marrow - Doesn’t have a medulary cavity since it’s not thick enough Bone marrow Red: hematopoesis - In medullary cavity when young - Between trabeculae even as an adult Yellow: adipose CT, energy storage - In medullary cavity when an adult Osseous tissue composition - Hydroxyapatites are a specific combo of minerals to make bone (calcium salts, Fl, Na, Mg ions) - Brings harness to bones, without minerals bone would be soft - Compression, job of minerals is to resist compression / hold your weight - Collagen fibers - Flexibility, tensile strength - Stretching and twisting Osseous cells - Osteocytes maintain matrix, sense when bone needs to be made / removed - Osteoblasts maintain bone 1. Secrete osteoid = glycoprotein, protoglycan, collagen fibers 2. Mineralize osteoid by secreting alkaline phosphatase, which raises pH locally and causes osteoid to crystallize (hydroxyapatites) 3. Osteoblasts get trapped in calcified matrix, turn into osteocytes in lacuna - Osteoclasts break down bone, etc 1. Secrete HCl acid, which lowers pH in localized area and causes minerals to dissolve 2. Secrete enzymes (collagenase), which break down the matrix further. In turn will release osteocytes from lacuna and turns back into osteoblasts - Osteoprogenitor cells whose division makes osteoblasts Bone formation [ossification] - Endochondral (inside bone) - Replaces hyaline cartilage with spongy bone then compact bone - Most bones use this formation - Intramembranous (between membranes) - From fibrous membranes ex. flat bones of skull, clavicle Endochondral ossification Starts from the diaphysis center and moves out Step 1: - Chondrocytes in hyaline cartilage - Enlarge / hypertrophy - Calcify matrix: secrete alkaline phosphate, which increases pH locally and will crystallize in matrix (hydroxyapatite) - Cells dye once calcified because there are no canaliculi Step 2: - Cartilage has perichondrium. Perichondrium is DICT, DICT has fibroblasts which are stell cells. Blood vessels pick up fibroblasts and deliver it to the dying matrix of hyaline cartilage, where fibroblasts turn into osteoblasts. - Bone collar forms on surface of dying cartilage, which is spongy bone - Perichondrium becomes periosteum Step 3: - Blood vessels move from just being in the perichondrium, to the center of the bone - Brings in fibroblasts to turn into osteoblasts (secretes osteoid, mineralizes with alkaline phosphatase, makes spongy bone) - The calcified cartilage disintegrates - Osteoblasts secrete HCl to dissolve minerals, enzymes (collagenase) to break down matrix) - The primary ossification center is spongy bone (builds bone from center to outside) Step 4: - Remodeling - Marrow cavity: had spongy bone, but will remove it with osteoclasts to make a marrow cavity - Thicker shaft: turns spongy bone to compact bone, needs osteoblasts and osteoclasts to do it - Bone replaces cartilage at metaphysis (slowly moving down ends of bone to remodel) Step 5: - Secondary ossification centers in epiphysis - Same process again - Chondrocytes are enlarged and die once matrix starts to calcify. Blood vessels will bring in fibroblasts from periosteum and will turn into osteoblasts. Osteoblasts and osteoclasts will start to remove dying cartilage and build spongy bone. Will remodel spongy bone into compact bone with osteoblasts and osteoclasts. Step 6: - Grow length of bone at epiphyseal plate - Hyaline cartilage grows at resting zone, bone grows at ossification zone - Remaining cartilage is articular and epiphyseal cartilage Growth of long bone Epihysis / epiphyseal side / hylaine cartilage growth this direction - Resting zone: matrix production (chondrocytes in lacunae and cartilage) - Proliferation zone: mitosis with interstitial growth (looks like stacks of chondrocytes) - Hypertrophic zone: chondrocytes enlarge - Calcification zone: cell death (osteoclasts help clear out disintegrating cartilage) - Ossification zone: osteoblasts swarm up from diaphysis and secrete osteoid onto cartilage remnants, makes spongy bone from that Metaphysis / metaphyseal side / bone growth this direction - Epihyseal plate = still growing (looks darker on x-rays) - Epiphyseal line = done growing (white thin line on x-ray) Intramembranous ossification - Similar to endochondral but.. - Has no cartilage, so it will start within membranes (fibrous membranes of embryonic [mesenchymal] CT) - Mesenchymal cells turn into osteoblasts - Mesenchymal are high level stem cells, and has no cartilage so you don’t need to kill it off - Ossifies: makes spongy bone, remodels into compact bone - Blood vessels invade deeper - CT turns into periosteum - CT becomes more and more condensed until it eventually becomes periosteum Appositional growth 1. Osteoblasts beneath the periosteum secrete bone matrix, forming ridges that follow the course of periosteal blood vessels 2. As the bony ridges enlarge and meet, the groove containing the blood vessel becomes a tunnel 3. The periosteum lining the tunnel is transformed into an endosteum and the osteoblasts just deep to the tunnel endosteum secrete bone matrix, narrowing the canal 4. As the osteoblasts beneath the endosteum form new lamellae, a new osteon is created. Meanwhile, new circumferential lamellae are elaborated beneath the periosteum and the process is repeated, continuing to enlarge bone diameter. Bone remodeling - Weight and compression will stimulate bone growth - If you put weight on one shoulder more than the other, bone will grow thicker on the side with weight - Breaking down bone will increase Ca in blood by releasing it from bone - Osteoclasts secrete HCl to dissolve calcium salts, collagenase to break matrix - Building up bone will decrease Ca in blood, since it uses Ca to build bone - Osteoblasts secrete osteoid (glycoproteins, protoglycans, collagen fibers, mineralize with alkaline phosphatase which increases pH, osteoblasts turn into osteocytes once solution becomes mineralized  - Every time you build / remove bone, it will reflect in your Ca levels Ca homeostasis High blood calcium level: hypercalcemia - Stimulus: raising blood Ca+ levels - Thyroid gland releases calcitonin - Calcitonin will stimulate Ca+ deposition in bones (build more bone) and reduce Ca+ uptake in kidneys (release all extra Ca+ into urine) - Results in lowered Ca+ level and back to normal Lower blood calcium level: hypocalcemia - Stimulus: falling blood Ca+ level - Parathyroid glands release parathyroid hormone (PTH) - PTH will stimulate Ca+ release from bones (break down bone), increase Ca+ uptake in kidneys (doesn’t let Ca+ go into urine), and increased Ca+ uptake in intestines (absorbs from food given that you have vit. D) - Results in raised Ca+ level and back to normal Importance of Ca - Transmission of nerve impulses - Muscle contraction - Blood coagulation: clotting blood, to not bleed to death - Secretion by glands, nerve - Cell division Diseases - Osteoporosis: condition of pores / holes in bone - Removes spongy bone first and then moves to compact bone - Estrogen in women is very important in keeping bone density - Causes bone to fracture easily, happens earlier in women - Reduce your risk by eating calcium, vit. D, and exercise Fracture repair - Healed bone will be thicker and stronger 1. Hematoma formation: clotting blood 2. Fibrocartilage callus formation: fibrocartilage is made fast and softer, holds bone together as its healing 3. Bony callus formation: adds spongy bone where fibrocartilage was 4. Bone remodeling: bone will be thicker. Converts spongy bone to compact bone at edges. Medullary cavity with spongy bone will clear out Bone conditions - Rickets: lack of Ca+ and / or vit.D - Bow-legged and has wide uncalcified epiphyseal plates even as adults - Osteomalacia: in adults - Pain when bearing weight - Pituitary gigantism - Happens with children while they still have the epiphyseal plate. The excess growth hormone (GH) secreted makes bones keep growing. With epiphyseal lines, they will continue to grow in hands, feet and face because of the GH - Acromegaly - People who have pituitary gigantism will usually have actomegaly as well - Enlargement of face, hands, and feet - Achondroplastic dwarfism - Problem with epiphyseal plate, cartilage doesn’t grow so long bones don’t grow in length