Skeletal System Bones and Bone Tissue

Skeletal System Bones and Bone Tissue

Skeletal System • Is the framework of the body • Provides shape to the body and protection for • organs and soft tissues • Consists of cartilage, bones, tendons and ligaments • Functions • Support:Bone is hard and rigid, bears body weight • Cartilage provides firm and flexible support, such as cartilage in nose, external ear, thoracic cage and trachea • Ligaments attach bone to bone & hold them together • Protection: Bones of skull protects brain • Ribs, sternum, vertebrae protect organs of thoracic cavity

Functions of Skeletal System • Movement: Skeletal muscles attach to bones by tendons • Contraction of skeletal muscles moves the bones, produce body movement • Storage:Ca and P. Stored then released as needed. Fat stored in marrow cavities • Blood cell production:Blood cells and platelets formation takes place in bone marrow of bones

Basic Structure of Skeletal Cartilage • Consists mostly of water – accounts for resilience • contains no nerves and blood vessels • Surrounded by double layer of dense irregular connective tissue – Perichondrium • Outer layer: Contains fibroblasts • Inner layer: More delicate, has fewer fibers, contains chondroblasts and chondrocytes • Blood vessels and nerves penetrate the outer layer of pericardium but do not enter cartilage matrix • Nutrients diffuse through matrix to reach chondrocytes Skeletal Cartilages

Skeletal Cartilages • Growth of Cartilage: • Cartilage grows in two ways: • Appositional growth – Add new matrix and chondrocytes to the outside of tissue • Interstitial growth –Chondrocytes within the tissue divide and add more matrix between the cells

Skeletal Cartilages • Types of Skeletal Cartilage: • Hyaline, Elastic and Fibrocartilage • Hyaline Cartilage: Contains fine collagen fibers in matrix, Include • Articular cartilages : cover the ends of bones at movable joints • Costal cartilages: connect the rib to sternum • Respiratory cartilages: forms skeleton of larynx • Nasal cartilages: support external nose • Elastic Cartilage: Contains collagen and elastic fibers - external ears and epiglottis • Fibrocartilage: Thick bundles of collagen fibers, compressible and tough • Found in Menisci and intervertebral discs

Classification of Bones • Human skeleton consists of 206 bones • And divided into two groups: • Axial Skeleton: • Consists of skull bones, vertebral column and rib cage • Protect and support body parts • Appendicular Skeletan: • Consists of bones of upper and lower limbs and girdles ( shoulder and hip bones) • Bones of limbs – help in movement

Classification of Bones • Bones are classified by their shape as long, short, flat and irregular • Long Bones: • Are long and thin • Are found in arms, legs, hands, feet, fingers, and toes • Flat Bones • Are thin, flattened shape, usually curved • Are found in the skull, sternum, ribs, and scapula

Classification of Bones • Irregular Bones • Have complex shapes • Examples: • spinal vertebrae • pelvic bones • Short Bones • Are small and thick • Examples: • ankle • wrist bones

Bone Structure • Three levels of structure: • Chemical • Gross • Microscopy

Chemical Composition of Bone • Consists of both organic and inorganic components • Organic components include: • Cells ( Osteoprogenitor cells, osteoblasts, osteocytes and osteoclasts) and Osteoid, the organic part of the matrix • Osteoid (35%) consists mainly of collagen and proteoglycans • Inorganic components • 65% of bone tissue is calcium phosphate crystal called hydroxyapatites, CaPO4 crystals

Gross Anatomy • Bone Markings • Most bones contain features on external surface • Depressions and openings along bone surface, passage for blood vessels and nerves • Projections where tendons and ligaments attach and at articulations with other bones • Include heads, trochanters, spines etc.

Bone textures • Bone tissue is classified as woven or lamellar bone based on collagen fibers organization within bone matrix • Woven bone. Collagen fibers randomly oriented. • First formed • During fetal development • During fracture repair • Then Woven bone is remodeled into lamellar bone • Lamellar bone • Mature bone, organized in sheets called lamellae. Collagen fibers are oriented in one direction in each layer, but in different directions in different layers for strength.

Bone textures • Bones, whether woven or lamellar can be classified • according to amount of bone matrix relative to amount of space • Compact bone: • Contains dense outer layer, less space • Cancellous or spongy bone: • Has less bone matrix & more space • Consists of interconnecting rods or plates of bones called trabeculae

Structure of Long Bone • Diaphysis • Shaft, long axis of bone • Made up of Compact bone • Surrounds central medullar or marrow activity • Red marrow - blood cell formation • Yellow marrow – adipose tissue • Epiphysis • End of the bone • Cancellous bone • Joint surface of epiphysis is covered with articular (hyaline) cartilage, cushions the bone ends • Epiphyseal plate:growth plate • Growth in length occurs at E. plate • Separates epiphysis from diaphysis • When bone stops growing in length becomes Epiphyseal line

Structure of Long Bone • Membranes • External surface of bone is covered by double layer membrane called Periosteum • Fibrous layer – Outer fibrous layer is dense irregular connective tissue contains blood vessels and nerves • Cellular layer – Inner single layer of bone cells consists of osteoblasts, osteoclasts, osteochondral progenitor cells • Periosteum is attached to underlying bone by Perforating or Sharpey`s fibers, made up of collagen • Periosteum provides anchoring points for tendons and ligaments

Structure of Long Bone • Membranes • Endosteum: • Single layer of cells that lines all internal spaces, such as medullar cavity • Contains osteoblasts, osteoclasts, osteochondral progenitor cells

Structure of Flat, Short, and Irregular Bones • Flat Bones • No diaphyses, epiphyses • Sandwich of cancellous between two layers of compact bone, eg. Parietal bone of skull • Short and Irregular Bone • Similar to structure of epiphyses of long bones • Compact bone that surrounds cancellous bone center with small spaces filled with marrow • Are not elongated and no diaphyses

Location of Hematopoietic Tissue in Bones • Hematopoietic tissue, red marrow is found in trabeculae of spongy bone of long bones, diploe of flat bones (sternum) and in some irregular bones (hip bones)

Microscopic Anatomy of Bone • Four major type of Bone cells • Osteoblasts • Osteocytes • Osteoclasts • Stem cells or osteochondral progenitor cells

Bone Cells • Osteoblasts • Immature bone cells that secrete organic components of matrix known as Osteoid • Osteoblasts surrounded by bone matrix, as the material calcifies, the cell is trapped in a space called a lacuna • And becomes osteocytes (mature bone cells)

Bone Cells • Osteocytes • Mature bone cells that maintain the bone matrix • Live in lacunae • Are between layers (lamellae) of matrix • Connect by cytoplasmic extensions through canaliculi in lamellae • Do not divide • Maintains protein and mineral content of matrix • Helps repair damaged bone

Bone Cells • Osteoclasts • Giant, mutlinucleate cells • Secrete acids and protein-digesting enzymes • Breakdown bone by dissolving bone matrix

Bone Cells • Osteoprogenitor Cells • Mesenchymal stem cells that divide to produce osteoblasts • Are located in inner layer of perichondrium, inner layer of periosteum and endosteum • Assist in fracture repair

Cancellous (Spongy) Bone • Consists of interconnecting rods or plates of bone called Trabeculae • No blood vessels in trabeculae • The space between trabeculae is filled with red bone marrow: • which has blood vessels • forms red blood cells • and supplies nutrients to osteocytes

Compact Bone • The basic unit of mature compact bone is Osteon or Haversian system • Osteon is a group of hollow tubes of bone matrix, one placed outside the next • Osteon consists of single central canal and around a canal contains blood vessels • Osteocytes are arranged in concentric lamellae

Compact Bone • Perforating (Volkmann`s )Canals • Perpendicular to the central canal • Carry blood vessels into bone and marrow • Circumferential Lamellae • Present on outer surface of compact bone • Binds osteons together • Interstitial Lamellae • Present in between osteons

Bone Development • Bone formation during fetal development occurs in two patterns: • Intramembranous ossification • Takes place in connective tissue membrane • Endochondral ossification • Takes place in cartilage • Both methods of ossification • Produce woven bone that is then remodeled • After remodeling, formation cannot be distinguished as one or other

Intramembranous Ossification • Takes place in fibrous connective tissue membrane formed from embryonic mesenchyme cells around the developing brain • Starts at 8th week & completes by age 2 • Forms many skull bones, part of mandible, diaphyses of clavicles • Mesenchyme cell in the membrane become osteochondral progenitor cell • Osteochondral progenitor cell forms osteoblast

Intramembranous Ossification • Osteoblast produce bone matrix and collagen fiber • And become osteocyte and develop trabeculae • More osteoblast gather around trabeculae and produce more bone • Trabeculae join together and form cancellous bone

Intramembranous Ossification • Cells in the spongy cell produce red bone marrow • Cells surrounding the developing bone forms periosteum • Osteoblasts from the periosteum on bone matrix produce compact bone

Endochondral Ossification • Bones of the base of the skull, part of the mandible, epiphyses of the clavicles, and most of remaining bones develop through endochondral ossification • Mesenchyme cells develop into chondroblasts which secrete the matrix of hyaline cartilage & surrounded by perichondrium except where joint formation takes place

Endochondral Ossification • Chondroblasts becomes chondrocytes; Chondrocytes in the center of hyaline cartilage: • Enlarge, calcify, and die, leaving cavities in cartilage • Blood vessels grow around the edges of the cartilage and osteochondral progenitor cells in the perichondrium change to osteoblasts • Perichondrium becomes periosteum when osteoblasts begin to form bone • Osteoblast produce compact bone on the surface of cartilage and forms the Bone collar

Endochondral Ossification • Blood vessels enter the cartilage • Connective tissue surrounding the blood vessels bring osteoblasts and osteoclasts • Osteoblasts secrete bone matrix and changes the calcified cartilage of diaphysis into cancellous bone • Bone formation area – Primary ossification center • Osteoclasts remove the bone from center and forms medullar cavity • Forms red bone marrow

Endochondral Ossification • Capillaries and osteoblasts enter the epiphyses creating secondary ossification centers • Epiphyses fill with spongy bone: • All the cartilage is replaced by bones except in epihhyseal plate and an articular sufaces • In mature bone, compact and cancellous bone are fully developed and epiphyseal plate becomes epiphyseal line

Bone Growth • Bones increase in size only by Appositional growth • Formation of new bone takes place on the surface of older bone or cartilage • Growth in Bone Length • Growth in bone length occurs at the epiphyseal plate • Growth at epiphyseal plate involves the formation of new cartilage by • Interstitial cartilage growth • Followed by Appositional bone growth on the surface of the cartilage

Growth in Bone Length • Epiphyseal plate is organized into 4 zones: • Zone of Resting Cartilage: • Nearest to epiphysis, contains randomly arranged chondrocytes • Do not involve in bone growth • Zone of Proliferation: • Contains actively dividing chondrocytes • Chondrocytes produce new cartilage through interstitial cartilage growth • As the cells divide, the epiphysis moves away from the diaphysis. This in turn produces length growth in bone

Growth in Bone Length • Zone of Hypertrophy: • Contains mature and enlarge • chondrocytes • Zone of Calcification: • Matrix is calcified, chondro- • cytes die • Calcified cartilage is • replaced by bone

Growth at Articular Cartilage • Growth at articular cartilage increases the epiphyses size • Also growth at articular cartilage increases size of bones with no epiphyses: e.g., short bones • Articular cartilage persists throughout life and does not ossified as epiphyseal plate

Growth in Bone Width • Bones increases in thickness or long bones increase • in width, because appositional bone growth takes place • beneath periosteum

Growth in Bone Width

Factors Affecting Bone Growth • Size and shape of a bone determined genetically but can be modified and influenced by nutrition and hormones • Nutrition • Lack of calcium, protein and other nutrients during growth and development can cause bones to be small • Vitamin D • Necessary for absorption of calcium from intestines • Can be eaten or manufactured in the body when skin is exposed to sunlight • Rickets: lack of vitamin D during childhood • Have bowed bones • Osteomalacia: lack of vitamin D during adulthood leading to softening of bones

Factors Affecting Bone Growth • Vitamin C • Necessary for collagen synthesis by osteoblasts • Scurvy: deficiency of vitamin C, causes hemorrhage because of lack of collagen deficiency • Lack of vitamin C also causes wounds not to heal, teeth to fall out

Bone Remodeling • Bone Remodeling: Old bone is replaced with new bone • Osteoclasts remove old bone and osteoblast forms new bone • Bone remodeling converts woven bone into lamellar bone • And involve in bone growth, changes in bone shape, adjustments in bone due to stress, bone repair, and Ca ion regulation • Caused by migration of Basic Multicellular Units • Groups of osteoclasts and osteoblasts that remodel bones • Bone constantly removed by osteoclasts and new bone formed by osteoblasts

Bone Repair • Bone undergo repair after damage • Has four major steps: • Hematoma formation: • Bone fracture damages blood vessels in bone & periosteum and hematoma forms • Hematoma - Localized mass of blood released from blood vessels, Clot formation stop the bleeding • Inflammation and swelling occurs after injury

Bone Repair • Callus formation. Callus is mass of tissue that forms at a fracture site and connects the broken ends of the bone • Internal Callus– Forms between the ends of broken bones • Several days after fracture blood vessels grow into clot • Macrophages clean up debris, • osteoclasts break down dead bone • tissue, fibroblasts produce collagen and helps in put the bone together • Osteoprogenitor cells from the periosteum of healthy bone tissue produce • Chondroblasts - which secrete cartilage tissue • Ostoeblasts- which secrete bone matrix • New bone is formed

Bone Repair • External Callus – Forms collar around opposite ends of bone fragments • Periosteal osteochondral progenitor cells  osteoblasts and chondroblasts • Produce bones and cartilage

Bone Repair • Callusossification: • Callus replaced by woven, cancellous bone • Bone remodeling: Replacement of woven bone and damaged material by compact bone

Skeletal System Bones and Bone Tissue