2Skeletal System Overview Parts of the skeletal system:Bones (skeleton)JointsCartilagesLigaments (bone to bone)Tendon* (bone to muscle)Two divisions:Axial skeletonAppendicular skeleton – limbs and girdle
3Skeletal CartilageOur skeleton is initially made up of cartilage, but most of it is replaced by bone. The cartilages that remain in adults are found mainly in region where more flexible skeletal tissues are needed.Skeletal cartilage is made of cartilage tissue that consists primarily of water. The high water content accounts for its resilience.Cartilage contains no blood vessels or nerves and is surrounded by a layer of dense, irregular connective tissue called the perichondriumThe perichondrium helps resist outward expansion during compression an is the source of blood vessels from which nutrients diffuse
4Skeletal Cartilage Locations Hyaline: support with flexibility and resilience; articular cartilage at the ends of bones at most movable joints, costal cartilages which connect ribs to the sternum, respiratory cartilage & nasal cartilagesElastic: flexible & able to withstand repeated bending; supports external ear and forms the epiglottisFibrocartilage: highly compressible, great tensile strength (intermediate); at sights of both heavy pressure and stretch such as the menisci of the knee & the discs between vertebrate
5The bones and cartilages of the human skeleton. EpiglottisLarynxThyroidcartilageCartilage inexternal earCartilages innoseTracheaCricoidcartilageLungArticularCartilageof a jointCartilage inIntervertebral discCostalcartilageRespiratory tube cartilagesin neck and thoraxPubicsymphysisBones of skeletonAxial skeletonMeniscus (padlike cartilage in knee joint)Appendicular skeletonCartilagesArticular cartilageof a jointHyaline cartilagesElastic cartilagesFibrocartilages
6Growth of CartilageCartilage consists of cells called chondrocytes encased in small cavities (lacunae) within an extracellular matrix containing jellylike ground substance and fibers.Appositional Growth: cartilage cells in the surrounding perichondrium secrete new matrix against the external face of existing cartilageInterstitial Growth: the lacunae bound chondrocytes inside the cartilage divide and secrete new matrix expanding the cartilage withinDuring normal bone growth and aging, calcium salts may be deposited in cartilage. Calcified cartilage is not bone
7Functions of Bones Support of the body Protection of soft organs Movement due to attached skeletal musclesStorage of minerals and fatsBlood cell formation
8Bones of the Human Body The adult human skeleton has 206 bones Two basic types of bone tissueCompact boneHomogeneousSpongy boneSmall needle-like pieces of boneMany open spaces
15Short Bones, Flat Bones & Irregular Bones Generally cube-shape, contain mostly spongy boneExamples: Carpals, tarsalsFlat bonesThin and flattened, usually curvedThin layers of compact bone around a layer of spongy boneExamples: Skull, ribs, sternumIrregular bonesIrregular shape, do not fit into other bone classification categoriesExample: Vertebrae and hip
16Flat bones consist of a layer of spongy bone sandwiched between two thin layers of compact bone. Trabeculae
17Changes in the Human Skeleton In embryos, the skeleton is primarily hyaline cartilageDuring development, much of this cartilage is replaced by boneCartilage remains in isolated areasBridge of the noseParts of ribsJoints
18Types of Bone CellsOsteogenic Cell: stem cell in the inner layer of the periosteum; that gives rise to osteoblastsOsteoblasts: Bone-forming cellsOsteocytes: Mature bone cellsOsteoclasts: Bone-destroying cells; Break down bone matrix for remodeling and release of calciumBone remodeling is a process executed by both osteoblasts and osteoclasts(a) Osteogenic cell(b) Osteoblast(c) Osteocyte(d) OsteoclastStem cellMatrix-synthesizingcell responsiblefor bone growthMature bone cellthat maintains thebone matrixBone-resorbing cell
19Bone DevelopmentOsteogensis & ossification both refer to the process of bone tissue formationIn embryos this leads to the formation of bony skeletonBone growth is also a form of ossification that occurs from birth into early adulthoodAlthough bones are capable of thickening throughout our lives, most ossification in adults is for remodeling and repair of bones.
20Formation of the Bony Skeleton Before week 8 – fibrous membranes & hyaline cartilageWhen bone develops from a fibrous membrane it is called intramembranous ossification (membrane bone forms)Bone formation from the replacement of hyaline cartilage is called endochondral ossification (cartilage or endochondral bone forms)
21Intramembranous Ossification I MesenchymalcellCollagenfiberOssificationcenterOsteoidOsteoblastOssification centers appear in the fibrous connective tissuemembrane.• Selected centrally located mesenchymal cells clusterand differentiate into osteoblasts, forming anossification center.1
22Intramembranous Ossification II OsteoblastOsteoidOsteocyteNewly calcifiedbone matrixBone matrix (osteoid) is secreted within the fibrous membraneand calcifies.• Osteoblasts begin to secrete osteoid, which is calcifiedwithin a few days.• Trapped osteoblasts become osteocytes.2
23Intramembranous Ossification III Mesenchymecondensing to form the periosteumTrabeculae ofwoven boneBlood vessel3Woven bone and periosteum form.• Accumulating osteoid is laid down between embryonicblood vessels in a random manner. The result is a network(instead of lamellae) of trabeculae called woven bone.• Vascularized mesenchyme condenses on the external faceof the woven bone and becomes the periosteum.
24Intramembranous Ossification IV FibrousperiosteumOsteoblastPlate ofcompact boneSpongy Bone cavities contain red marrow4Lamellar bone replaces woven bone, just deep to the periosteum.Red marrow appears.• Trabeculae just deep to the periosteum thicken, and are laterreplaced with mature lamellar bone, forming compact bone plates.• Spongy bone (diploë), consisting of distinct trabeculae, persistsinternally and its vascular tissue becomes red marrow.
25Endochondral Ossification in Long Bone Week 9Month 3BirthChildhood toadolescenceArticularcartilageSecondaryossificationcenterSpongyboneEpiphysealblood vesselArea ofdeterioratingcartilage matrixEpiphysealplatecartilageHyalinecartilageSpongyboneformationMedullarycavityBonecollarBloodvessel ofperiostealbudPrimaryossificationcenterBone collarforms aroundhyaline cartilagemodel.1Cartilage in thecenter of thediaphysis calcifiesand then developscavities.2The periostealbud invades theinternal cavitiesand spongy bonebegins to form.3The diaphysis elongatesand a medullary cavityforms as ossificationcontinues. Secondaryossification centers appearin the epiphyses inpreparation for stage 5.4The epiphysesossify. Whencompleted, hyalinecartilage remains onlyin the epiphysealplates and articularcartilages.5
27Bone Growth & Remodeling Epiphyseal plates allow for growth of long bone during childhoodEndochondral ossification occurs at the articular cartilages and epiphyseal plates as the bone grows in length.Remodeling occurs to maintain proper bone proportions.
29Importance of Ionic Calcium Necessary for numerous physiological processes:The human body contains g of calcium, 99% as bone minerals and the balance mostly within cells; only ~ 1.5g in blood.Concentration of calcium in blood ~ 9-11mg/100mLCalcium is absorbed from the intestine under the control of vitamin D metabolites
30Bone Response to Mechanical Stress I Wolff’s Law states that a bone grows or remodels in repsone to the forces or demands placed on itA bone’s anatomy reflects the stresses that are placed upon itBones are stressed when weight bears down on them or muscles pull on them.This loading is usually off center causing the bone to bendThis results in compression on one side and tension on the other
31Controls of Bone Remodeling The mechanism by which bones respond to mechanical stimulation is unclearDeforming a bone causes an electrical current, it is hypothesized that electrical signals may direct remodelingIt is also believed that the hormonal loop determines whether & when remodeling will occur in response to blood calcium levelsMechanical stress determines where remodeling occurs
34Bone Fractures A break in a bone Types of bone fractures Closed (simple) fracture – break that does not penetrate the skinOpen (compound) fracture – broken bone penetrates through the skinBone fractures are treated by reduction and immobilizationRealignment of the bone
40Osteomalacia & Rickets Osteomalacia: “soft bones”; bones are not mineralized sufficiently; often due to lack of or inability to process vitamin DOsteoid is produced, but calcium salts are not depositedBones soften & weakenPain is felt when weight is put on the affected bonesRickets: analogous disease in children; much more severe than osteomalacia because bones are in a period of rapid growthOften results in bowed legs and deformities of the pelvis, skull and ribcageEpiphyseal plates cannot be calcified so they continue to enlarge causing the ends of the bones to become visibly enlarged and abnormally long
41OsteoporosisGroup of diseases in which bone reabsorption occurs faster than bone deposit.Matrix composition remains normal, but bone mass is reduced and the bones become porous and lighter.Most common in elderly; as sex hormone production declines; insufficient exercise, diet, abnormal vitamin D receptors, smoking & hormonal conditions also contribute.Usually treated with calcium & vitamin D supplements, weight-bearing exercises & hormone replacement therapy (HRT) & other drugs.
42Paget’s DiseaseExcessive bone formation & breakdown (usually localized)Rarely occurs before age 40, mostly in elderly.The newly formed bone is made rapidly; it has a high spongy to compact bone ratio & reduced mineralization leading to spot weakeningEventually osteoclast activity wanes, but osteoblasts stay active and bones thicken irregularly or marrow fillsThere are drug therapies.
43Developmental Aspects Most bones begin ossifying by 8 weeks, by12 weeks, primary ossification centersare obviousAt birth, most long bones have ossifieddiaphysesAfter birth secondary ossification centersform and the epiphyses start to ossifyEpiphyseal plates remain and allow for long bone growth in childhood and adolescenceIn childhood and adolesence, bone formation occurs more than reabsorption, in young adults the processes are in balance and in old age, reabsorption dominates formation