NS 3410 1st Edition Lecture 17Outline of Last LectureI. LeukocytesII. Platelets III. HemostasisIV. PlasmaV. Blood TypingOutline of Current LectureI. Structure of Long BonesII. Bone LamellaeIII. Microscopic Structure of Bone: Compact BoneIV. Chemical Composition of BoneV. Skeletal OssificationCurrent Lecture- Diaphysis-Tubular shaft that forms the axis of long bones-Composed of compact bone that surrounds the medullary cavity-Bone marrow is contained in the medullary cavity- Metaphysis-Narrow zone that connects diaphysis to epiphysis- Epiphyses-Expanded ends of long bones-Exterior is compact bone and interior is trabecular bone-Joint surface is covered with hyaline cartilageThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.-Epiphyseal line is a remnant of the epiphyseal plate- Growth plate-Expanded ends of long bones- Periosteum- double-layered protective membrane-Outer fibrous layer of connective tissue-Inner osteogenic layer is composed of mesenchymal progenitor cells-Richly supplied with nerve fibers, blood, lymphatic vessels which enter bone via nutrient foramina-Secured to underlying bone by Sharpey’s fibers- Endosteum- delicate membrane covering internal surfaces of bone- Osteogenic cells are found in the inner periosteum and in the endosteum and in the canals within bones- Osteocyte acts as stress or strain sensorsII. Bone Lamellae- Concentric lamellae: within osteon- Interstitial lamellae: fill the gaps between forming osteons or are remnants of osteons that have been cut from remodeling- Circumferential lamellae: within the periosteum and superficial to endosteumIII. Microscopic Structure of Bone: Compact Bone- Haversian system or osteon: structural unit of compact bone- Lamella- weight-bearing, column-like matrix tubes composed mainly of collagen- Haversian or central canal- central channel containing blood vessels and nerves- Volkmann’s canals- channels lying at right angles to central canal, connecting blood and nerve supply of periosteum to that of Haversian canal- Osteocytes- mature bone cells found in lacunae- small cavities in bone with canaliculi (hair like canals that connect lacunae to each other and the central canal)IV. Chemical Composition of Bone- Inorganic components: sixty five percent of bone by mass, mainly calcium phosphate, responsible for bone hardness and its resistance to compression- Organic components:-Osteoblasts: uninucleated cell that forms bone osteoid, bone-forming cells-Osteocytes: osteoblasts that have become surrounded by mineralized bone-Lining cells: quiescent osteoblasts lining surface of periosteum-Osteoclasts: large cells that resorb or break down bone matrix-Osteoid: unmineralized bone matrix composed of proteoglycans, glycoproteins and collagen- Osteoblasts are cuboidal in shape with a central nucleus found on the bone surface. They have gap junctions with neighboring osteoblasts, allowing cells to communicate with each other. They come from bone marrow precursor cells. These precursors are capable of turning into either osteoblasts or fat cells and various factors determine which kind of cells will be made. The job of osteoblasts is to make the proteins that will form the organic matrix of bone and to control mineralization of bone.- Osteoblasts have 3 fates; they have receptors for hormones such as vitamin D, estrogen, and parathyroid hormone-When the team of osteoblasts has finished making new bone, some become surrounded with matrix and differentiate into osteocytes-Others will remain on the surface of the new bone and differentiate into lining cells-The rest undergo apoptosis and disintegrate; regulated by proteins from other cells- Lining cells are former osteoblasts which have become flat and pancake-shaped. They line the entire surface of the bone. They are responsible for immediate release of calcium from the bone if the blood calcium is too low. They protect the bone from chemicals in the blood which dissolve crystals. They have receptors for hormones and factors that initiate bone remodeling. - Osteoclasts have two fates; they are large cells with many nuclei and share lineage with blood cells. Osteoclasts resorb the bone. They form sealed compartments next to the bone surface and secrete acids and enzymes which degrade the bone. The edge next to the bone is called the ruffled border. After they finish resorbing bone, they undergo apoptosis. This process is regulated by proteins from other cellsV. Skeletal Ossification- Fetal skeleton initially made up of fibrous membranes and hyaline cartilage; begins to ossify at approximately 7-9 weeks of gestation - Bones from via 2 processes: intramembranous ossification and endochondral ossification- Intramembranous ossification-Begins at week 8 of embryo development and is completed by about 2 years of age-Many skull bones and other bones such as part of the mandible develop in this way. All bones formed in this process are flat bones-Centers of ossification are where the ossification of the membrane begins-Non-ossified larger membrane covered spaces between the developing skull bones are called fontanels-Process:1) Mesenchymal cells aggregate, differentiate into osteoblasts, and begin the ossification process. The bone expands as a series of spicules that spread into surrounding tissue2) As the spicules connect, they trap blood vessels within the bone3) Over time, the bone assumes the structure of spongy bone. Areas of spongy bone may later be removed, creating medullary cavities. Through remodeling, spongy bone formed in this way can be converted to compact bone- Fontanelles are non-ossified larger membrane covered spaces between the developing skull bones- Endochondral Ossification: begins in the second month of development-Uses hyaline cartilage bones as models for bone construction-Bone forms by replacing hyaline