A&P Chapter 6 - Lecture notes 7 PDF

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Biol 2401 with Prof. Vasefi...

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Bones: individual ones are organs within skeletal system Bone tissue continually being remodeled Bone Deposition: new tissue being formed Bone Resorption: old tissue being broken down Bone Tissue: widely scattered cells embedded in extracellular matrix Makeup of Extracellular Matrix:  25% water, 25% collagen fibers, 50% mineral salts 4 Cell Types: first 3 types are life stages of cell  osteocytes: mature bone cell that maintains bone matrix o mature osteoblast surrounded by extracellular matrix that secretes mineral salts o occupies a lacuna (pocket between layers of matrix- lamellae) o repair damaged bones o maintain protein content of matrix  osteoblasts: immature bone cell, secretes collagen fibers & starts calcification o ossification- (osteogenesis) production of new bone marrow o osteoid- organic matrix before calcium salts are deposited  osteoprogenitor (osteogenic) cells: skin cells divide & make osteoblasts o bone contains small numbers of mesenchymal cells o produces daughter cells that differentiate into osteoblasts  osteoclasts: large cells produced by fusion of many monocytes; absorb and remove bone matrix o ruffled border secretes 2 things:  digestive enzymes which degrade collagen fibers  acids which degrade mineral salts 2 Types of Bone Tissue: both can be found in a single bone  compact bone: 80%, few open spaces in ECM, o organized into osteons – osteocytes are arranged in concentric layers o perforating canals- extend perpendicular to the surface o concentric lamellae- create targetlike pattern o thickest where stresses arrive from a limited range of directions  spongy bone: 20%, many open spaces (maybe w/t red bone marrow), organized into trabeculae have lamellae, lacunae, osteocytes, & canliculi but no central canals; not arranged in osteons  blood vessels can pass through open spaces b/t trabeculae o trabeculae- matrix in spongy bone forms a meshwork of supporting bundles of fibers Calcification: deposition of mineral salts in bone tissue Structures in Osteons (Compact Bone): 4 of them  central (haversian) canal: opening for blood vessels  concentric lamellae: rings of calcified extracellular matrix  lacuna/lacunae: small spaces b/t lamellae in which osteocytes happen o

 canaliculi: small opening radiating from lacunae which connect to other lacunae/canal Bone Classification by Shape: 1. Sutural Bones  Small, flat, irregular bones between flat bones of the skull 2. Irregular Bones  Complex shape with flat, short, notched or ridged surfaces  Ex. vertebrae 3. Short Bones  Small and boxy  Ex. Carpal and tarsal bones, pollex 4. Flat Bones  Thin, parallel surfaces  Provides protection for underlying soft tissue and offer extensive surface area for the attachment of skeletal muscle  Ex. Roof of skull and scapulae, sternum 5. Long Bones  Elongated with a shaft (diaphysis) with two ends (epiphyses) that are wider than the shaft 6. Sesamoid Bones  Small, round, and flat  Develop inside tendons  Account for individual differences in the total # of bones Long Bone Structure: has compact & spongy bone tissue, 7 major structures  diaphysis: cylindrical bone shaft  epiphysis: distal & proximal ends of bone  metaphysis: where diaphysis joins epiphysis  medullary cavity: space in diaphysis that contains yellow bone marrow (adipose/fat tissue)  endosteum: epithelium that lines medullar cavity o flattened layer of osteogenic cells that cover the bone matrix  articular cartilage: thin hyaline cartilage covering epiphysis where it articulates w/t other bone  periosteum: 2 layers, outer fibrous, inner cellular layer; covers all bones except joints o outer fibrous layer: sheath of dense irregular connective tissue o inner cellular (osteogenic) layer: has cells that allow bone to grow thicker o isolates bones from surrounding tissue o provides a route for blood vessels and nerves Interstitial Growth: bone grows in length  via replacement of cartilage by bone tissue in the metaphysis Appositional Growth: bone grows in width  via addition of bone tissue at the surface of the bone  cells of inner layer of periosteum differentiate into osteoblasts

Ossification/Osteogenesis: process of bone reformation, occurs in:  bone development in embryo/fetus, bone growth in childhood  bone remodeling in adults, bone repair after fracture  Two Types of ossification o Intramembranous Ossification: bone forms directly from membranous sheets of embryonic cells ; results in dermal bones  Step 1: ossification center develops w/t osteoblasts  Step 2: osteocytes calcify, blood vessels surrounded by bone tissue  Step 3: trabeculae (spongy bone) & red bone marrow forms  bone remodeling may replace spongy bone w/t compact bone o Endochondral Ossification: bone forms in hyaline cartilage that develops from embryonic cell; replaces existing cartilage  Step 1: development of cartilage model surrounded by perichondrium  Step 2: perichondrium cells become osteoblasts & make tissue sheath  Step 3: fibroblasts in blood vessels differentiate into osteoblasts  make primary ossification center in middle of cartilage model  Step 4: ossification proceeds towards 2 ends of bones  spongy bone in diaphysis center breaks down to form medullar cavity  Step 5: osteoblasts migrate into epiphysis and produce secondary ossification center  Step 6: articular cartilage & epiphyseal cartilage (plate) develop, both are hyaline cartilage  Step 7: bone grows in length until puberty, interstitial growth  new cartilage is added to epiphyseal plate & cartilage replaces old bone 5 Major Functions of Skeletal System:  support: structural framework for soft tissue  leverage: rigid anchor: point against which skeletal muscles act o also provide attachment sites for tendons of skeletal muscle  protection: rigid cage around internal organ o cranial bones protect brain; vertebra-> spinal cord, ribs -> lungs & heart  

blood cell production: red bone marrow makes RBC, WBC & platelets via hemopoiesis lipid/mineral storage o yellow bone marrow made of adipose tissue

ground substance has mineral salts (Calcium & Phosphate) pushed into bloodstream to maintain body fluid homeostasis o deposit superficial layers of bone matrix in circumferential lamellae Bone Remodeling:  bone resorption (osteoclasts breaking mineral salts & collagen fibers down)  bone deposition (osteoblasts adding mineral salts & collagen fibers) o

Calcium Homeostasis:  many metabolic reactions need calcium  bone is calcium reservoir  constant level of calcium in blood maintained by regulating resorption & bone deposition  regulated by 2 hormones: parathyroid hormone (PTH), calcitonin (CCT) A. Negative Feedback Loop for PTH: no calcium from diet for a while o Calcium in blood decreases o Parathyroid gland cells sense change o PTH stimulates osteoclast activity o END RESULT: more calcium released in blood B. Negative Feedback Loop for CCT: calcium present from diet o Calcium in blood increases o Thyroid gland cells sense change o Thyroid glands secrete more CCT o CCT initiates osteoblast activity o END RESULT: less calcium released in blood Osteopenia- inadequate ossification; bones become thinner and weaker with age...