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Lecture 6

Week 6 Bones (1).docx

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Emily Agard

Week 6: Bones & Skeletal Tissues Skeletal Cartilages Basic Structures, Types, and Locations • Skeletal Cartilage – Made up of cartilage tissue (primarily watter) o resilience o No nerves or blood vessels o Surrounded by dense irregular connective tissue • Hyaline Cartilage—provides support with flexibility & resilience; Most abundant & spherical o Only fiber in their matrix is fine collagen  Includes: articular cartilages  costal cartilages  respiratory cartilages  nasal cartilage • Elastic Cartilage – look like hyaline, but contain more elastic fibers (bending) • Fibrocartilages – highly compressible, great tensile strength(Intermediate b/w Hyaline & Elastic) Growth of Cartilage • Cartilage has a flexible matrix that accommodates mitosis (unlike bone) o Appositional Growth (growth from outside) o Interstitial Growth (inside) • Cartilage growth ends during adolescence when skeleton stops growing • Under certain conditions, calcium salts may be deposited in matrix causing it to harden Classification of Bones • Axial skeleton – • Appendicular • Unique shape/size of each bone fulfills a particular need o Long Bones – Has a shaft plus two ends; All limb bones except patella, wrist, and ankle o Short Bones – form in a tendon (patella); Vary in size & number in individuals o Flat Bones – thin, flattened and a bit curved (sternum, scapulae, ribs, skull bones) o Irregular Bones – complicated shapes that fit none of preceding classes (vertebrae/hip) Functions of Bones 1. Support 2. Protection 3. Movement 4. Mineral & Growth Factor Storage 5. Blood Cell Formation – hematopoiesis 6. Triglyceride (fat) storage Bone Structure Gross Anatomy Bone Markings - External surface of bones have projections, depressions, openings that serve as sites of muscle, ligament and tendon attachment – bone markings - Table 6.1 Bone Textures: Compact & Spongy Bone - External layer is compact bone, and internal layer is spongy bone (honeycomb of trabeculae) - In living bones, open spaces b/w trabeculae are filled with red/yellow bone marrow Structure of a Typical Long Bone • Diaphysis - Tubular diaphysis/shaft forms long axis of bone - Constructed of thick collar of compact bone that surrounds a central medullary cavity - In adults, medullary cavity contains fat & is called yellow marrow cavity • Epiphysis - Bone ends; Exterior = Compact & Interior = Spongy - Joint surface covered with a thin layer of articular cartilage which cushions opposing bone ends - Between diaphysis & epiphysis is an epiphyseal line, a remnant of the epiphysial plate, a disc of hyaline cartilage that grows during childhood to lengthen bone - Region where D & E meet is the metaphysic • Membranes - External surface of entire bone except joint surfaces covered by periosteum (double membrane) - Outer fibrous layer = dense irregular connective tissue - Inner osteogenic layer = consists of osteoblasts which secrete bone matrix elements, and osteoclasts, as well as primitive stem cells called osteogenic cells - Richly supplied with nerve fibers, lymphatic/blood vessels which enter D via nutrient foramina - Secured to underlying bone for perforating fibers – tufts of collagen fibers that extend from its fibrous layer into bone matrix - Also provides anchoring points for tendons/ligaments - Internal surface covered with delicate connective tissue membrane – endosteum - Covers trabeculae of spongy bone and lines canals that pass through compact bone - Contains both bone forming and destroying cells Structure of Short, Irregular, and Flat Bones • Simple design o Have no shaft/epiphysis b/c they are not cylindrical o Contain bone marrow, but no significant cavity is present • In flat bones, spongy bone is called the diploe (resembles stiffened sandwich) Location of Hematopoietic Tissue in Bones • Hematopoietic tissue, red marrow- typically found within trabecular cavities of spongy bone of long bones and in diploe of flat bones • Often referred to as red marrow cavities MicroscopicAnatomy of Bone Compact Bone • Looks dense & solid, but has passageways that serve as conduits for nerves and B/LV • Structural unit is osteon/Haversian system o Each osteon is an elongated cylinder parallel to long axis of bone o Tiny weight bearing pillars • Osteon = group of hollow tubes of bone matrix, placed outside the next like rings of a tree trunk • Each matrix tube is a lamella (compact bone often called lamellar bone) • Although all collagen fibers in a lamella run in a single direction, the collagen fibers in adjacent lamellae always run in different directions (withstand torsion stresses) • Bone salt crystals align with collagen fibers & also alternate their direction in adjacent lamellae • Core of each osteon is the central canal/Haversian canal – containing small BV/Nerve fibers • Perforating Canals/Volkmann’s canals – lie @ right angles to the long axis of the bone and connect the blood and nerve supply of the periosteum to those in the central canals and the medullary cavity (Also lined with endosteum) • Osteocytes occupy lacunae @ junctions of lamellae • Hairlike canals called canaliculi connect lacunae to eachother and the central canal • Cannluci form when bone matrix hardens and the osteocytes become trapped within it • Tiny canals, formerly tentacle-like extensions containing gap junctions of the osteocytes are thusly formed. The canaliculi tie all the osteocytes in an osteo together, permitting nutrient and wastes to be relayed from 1 osteocyte to the next. It also permits cell to cell relays through its gap junctions to allow bones to be well nourished • Osteocytes maintain bone matrix and act as stress/strain sensors in case of bone deformation/damage. They also communicate with osteoblasts & osteoclasts. • Interstitial Lamellae – incomplete lamellae lying between intact osteons; They either fill gaps between forming osteons or are remnants of osteons cut through by bone remodeling • Circumferential Lamellae – extend around entire surface of diaphysis and resist twisting of the long bone Spongy Bone • Trabeculae in spongy bone align precisely along lines of stress & help bone resist stress as much as possible • Tiny bone struts are carefully positioned as the flying buttresses that help to support a Gothic cathedral • Trabeculae contain irregularly arranged lamellae & osteocytes interconnected by canaliculi; No osteons are present • Nutrients reach osteocytes of spongy bone by diffusing through canaliculi from capillaries into endosteum surround trabeculae Bone Development Ossification & osteogenesis – process of bone formulation Formation of Bony Skeleton • Before week 8, skeleton of a human embryo made from fibrous membranes/hyaline cartilage • Bone tissue develops and eventually replaces most of fibrous/cartilage structures • Membrane Bone- develops from fibrous membrane; process is intramembranous ossification • Cartilage/Endochondral Bone – replaces hyaline cartilage; endochondral ossification • Benefit of using flexible structures to fashion embryonic skeleton = mitosis Intramembranous Ossification • Formation of cranial bones (frontal, parietal, occipital and temporal) & clavicles • Figure 6.8 Endochondral Ossification • All bones below base of skull except clavicles • 2 month of development • More complex because hyaline cartilage must be broken down • Formation of long bone: Begins in center of hyaline cartilage shaft @ primary ossification center o Perichondrium covering cartilage is infiltrated with BV, vascularizing it o Underlying mesenchymal cells specialized into osteoblasts 1. Bone collar laid down around diaphysis of hyaline cartilage a. Osteoblasts secret osteoid against hyaline encasing it; peristeal bone collar 2. Cartilage in center of diaphysis calcifies & develops cavities a. Chondrocytes within shaft enlarge and signal cartilage matrix to calcify. Because calcified matrix is imperable to nutrients, chondrocytes die and matrix deteriates. (Opens up cavities) 3. Periosteal bud invades internal cavities & spongy bone forms a. Month 3: cavities invaded by periosteal bud which contains a nutrient artery, vein, LV, Nerve fibers, red marrow elements, osteoblasts & clasts b. Entering osteoclasts erode calcified matrix and osteoblasts secret osteoid around remaining fragments of hyaline cartilage, forming bone covered trabeculae 4. Diaphysis elongates & medullary cavity forms a. As ossification center enlarges,
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