CH 6 Bones PDF

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CH 6 Bones and skeletal system...

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CH 6 BONES AND SKELETAL SYSTEM Skeletal Cartilage Basic Structure, Types, and Locations • • •

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The human skeleton initially consists of just cartilage, which is replaced by bone, except in areas requiring flexibility. - Composed of cartilage and fibrous connective tissue Made of some variety of cartilage tissues (which mainly consist of water, which lends resilience) The cartilage is surrounded by a layer of DRCT, the perichondrium - contains blood vessels for nutrients that diffuses to the cartilage and act like a girdle to resist outward expansion when the cartilage is compressed All contain chondrocytes in lacunae and extracellular matrix No blood vessels or nerves Three types of cartilage tissues in the body: 1. Hyaline cartilage: - Location: articular surfaces - Most common/abundant type of cartilage - Collagen fibers only - Function: reduces friction, provide support with flexibility and resilience - Skeletal hyaline cartilage includes: o Articular cartilage: covers the ends of most bones at movable joints o Costal cartilage: connects the ribs to the sternum o Respiratory cartilage: form the skeleton of the larynx (voice box) and reinforce other respiratory passageways o Nasal cartilage: Support the external nose 2. Elastic cartilage: - Location: nose, ear, epiglottis - Function: stretched and rebounds back into original shape; can stand repeated bending - Look a lot like hyaline cartilage (collagen fibers) but they contain more stretch (elastic fibers) 3. Fibrocartilage: - Location: intervertebral discs, menisci and pubic symphysis - Strongest type of cartilage - Function: carry and resist weight, highly compressible and have a great tensile strength - Consist of roughly parallel rows of chondrocytes alternating with thick collagen fibers - Occurs in sites that are subject to both heavy pressure and stretch (such as the menisci of the knee and discs between the vertebrae)

Growth of Cartilage •

Chondrocytes within lacunae surrounded by variation of matrix 1. Appositional growth- Cells secrete matrix against external face of existing cartilage 2. Interstitial growth- Chondrocytes divide and secrete new matrix, expanding cartilage from within 3. Calcification of cartilage: occurs during normal bone growth, youth and old age; hardens but calcified cartilage is not bone.

Classification of Bones •

206 named bones in the skeleton

→ Axial Skeleton: long axis of body; bones of the skull, vertebral column and rib cage → Appendicular skeleton: bones of the upper and lower limbs and the girdles attach limbs to axial skeleton Classification of bone by shape: • Long bones - Elongated shape with a shaft plus two ends  Longer than they are wide - All limbs bones except the wrist and ankle bones are long bones (the phalanges are considered long bones) - Interstitial growth- site of longitudinal bone growth, growth plate - Appositional growth- long axis or shaft of long bones • Medullary cavity→ yellow “fat” marrow • Short bones - Roughly cube shaped - E.g. Bones of the wrist and ankle  Sesamoid bones: a special type of short bone that forms within a tendon  E.g. patella (vary in size and number in different individuals; may be placed with short bone) • Flat bones - Thin, flattened, usually a bit curved - E.g. Sternum, scapulae, ribs, most skull bones, clavicles  Why clavicle: because they ossify by intramembranous ossificaiton • Irregular bones - Have complicated shapes that fit none of the above categories - E.g. Vertebrae, hip bones, facial Function of Bones •

Besides contributing to body shape and form, our bones perform several important functions 1. Support: For body and soft organs 2. Protection: For brain, spinal cord, and vital organs 3. Movement: Levers for muscle action 4. Mineral and growth factor storage: Calcium and phosphorus, and growth factors reservoir 5. Blood cell formation (hematopoiesis): in red marrow cavities of certain bones 6. Triglyceride (fat) storage: in bone cavities (Energy source) 7. Hormone production: Osteocalcin secreted by osteoblasts a. Protects against obesity, regulates insulin secretion and glucose homeostasis (diabetes mellitus)

Bone Structure •

Bones are organs (contain different types of tissue) • Bone (osseous) tissue, nervous, cartilage, fibrous CT, muscle and epithelial cells in its blood vessels

GROSS ANATOMY OF BONE Bone Markings:  Site for muscle, ligament, and tendon attachment on external surfaces  Projections: most indicate stresses created by muscle pull or joint modification  Depressions or openings: usually allow nerves and blood vessels to pass

Bone Textures →Compact (or lamellar): the dense outer layer of a bone; smooth and solid →Spongy (cancellous or trabeculae): internal layer; a honeycomb of small needle-like or flat piece called trabeculae. The space between trabeculae are filled with red or yellow bone marrow Structure of short, irregular and flat bones All these bones share a simple design: • Thin plates of spongy bone (aka diploe) cover by compact bone - Thin plates sandwiched between CT membrane  Periosteum-covered bone on the outside (outer layer)  Endosteum-covered spongy bone within (inner layer) - No diaphysis or epiphyses • Contain bone marrow throughout spongy bone but no bone marrow cavity - Trabeculae filled with marrow • Hyaline cartilage covers auricular surfaces Structure of Typical Long Bones • Diaphysis: a tubular diaphysis, or shaft, forms the long axis of the bone. Constructed of a thick collar of compact bone that surrounds a central medullary cavity (marrow cavitycontains fat, or yellow marrow) • Epiphysis: the bone ends. The joint surface of each epiphysis is covered with a thin layer of articular (hyaline) cartilage. • Compact bone forms the exterior of the epiphysis and spongy bone forms the interior. • The joint surface of each epiphysis is covered with a thin layer of articular (hyaline) cartilage. • Epiphyseal line: the region where the diaphysis and epiphysis meet - a remnant of childhood bone growth at the the epiphyseal, or growth plate Membranes → Periosteum: a glistening white, double-layered membrane that cover the external surface of the entire bone, except the joint surfaces - The outer fibrous layer is dense irregular connective tissue (DICT). - Sharpey’s fibers secure it to bone matrix - The inner osteogenic layer, abutting the bone surface, consist primarily of bone-forming stem cells, called osteoblast or osteogenic cells - Manu nerve fibers and blood vessels - Process: anchoring points for tendons and ligaments → Endosteum: delicate connective tissue membrane covering internal bone surfaces - Covers trabeculae of spongy bone - Lines canals that pass though compact bone - Contains osteogenic cells that can differentiate into other bone cells  Contain both bone-forming (osteoblast) and bone-destroying cells (osteoclast)

Location of bone marrow In children: • Medullary cavity and all areas of the spongy bone contain red bone marrow In adults: • Red marrow (Hematopoietic tissue) is found within the trabecular cavities of spongy bone of long bones and in the dipole of the flat bones (e.g. sternum) • Medullary cavity extends well into the epiphysis and produces yellow bone marrow (fat). Very little red marrow is present in the spongy bone cavities. • For this reason, blood cell production in adult long bones occurs only in the heads of the femur and humerus (yellow marrow in the medullary cavity can convert to red marrow if a person become anemic and needs enhanced blood cell production) Microscopic Anatomy of Bone Four major cell type populates bone tissue: • Osteogenic cells (or osteoprogenitors cells) - mitotically active stem cells found in the periosteum and endosteum - When stimulated differential into osteoblast or bone lining cells - Some persist as osteogenic cells • Osteoblast- bone-forming cells - Secrete unmineralized bone matrix or osteoid  Includes collagen and calcium-binding proteins (collagen 90% of bone protein)  Sill mitotically active • Osteocytes- Monitor and maintain bone matrix - Mature bone cells in lacunae - Act as stress or strains sensors - Respond to and communicate mechanical stimuli to osteoblast and osteoclast so bone remodeling can occur • Osteoclast- bone-destroying cells - Derived from hematopoietic stems cell that become macrophages - Giant, multinucleated cells for bone reabsorption - When active, rest in resorption bay and have ruffled board - Ruffled board increases surface area for enzymes degradation of bone and seals off area from surrounding matrix. • Bone Lining cells: flat cells on bone surfaces believed to help maintain matrix. - Periosteal cells: External bone surface - Endosteal cells: Lining internal surfaces

Compact bone (lamellar bone) • Osteon (haversian system)- the structural unit of compact bone. It is an elongated cylinder parallel to the long axis of the bone - Lamellae: hollow tubes of bone matrix - Collagen fibers in adjacent lamella run in different directions to withstand torsions stresses • Central (haversian) canal runs through core of osteon - Contains blood vessels and nerve fibers • Perforating (Volkmann's) canals: - Canals lined with endosteum at right angles to central canal - Connect blood vessels and nerves of periosteum, medullary cavity, and central canal • Lacunae: small cavities that contain osteocytes • Canaliculi: a system of tiny hair-like canals that connect lacunae to each other and central canal Canaliculi formation • Osteoblasts secreting bone matrix maintain contact with each other and osteocytes via cell projections with gap junctions • When matrix hardens and cells are trapped the canaliculi form - Allow communication - Permit nutrients and wastes to be relayed from one osteocyte to another throughout osteon Lamellae • Interstitial lamellae: - Incomplete lamellae not part of complete osteon - Fill gaps between forming osteons - Remnants of osteons cut by bone remodeling • Circumferential lamellae - Just deep to periosteum - Superficial to endosteum - Extend around entire surface of diaphysis - Resist twisting of long bone Spongy bone • Appears poorly organized - The trabeculae in spongy align precisely along lines of stress and help the bone resist stress as much as possibly • Trabeculae contain irregularly arranged lamellae and osteocytes interconnected by canaliculi • No osteon is present • Capillaries in endosteum supply nutrients

CHEMICAL COMPOSITION OF BONE Bone has both organic and inorganic components • Cells, osteoid and inorganic compounds →Organic components • The cells (osteogenic cells, osteoblast, osteocytes, osteoclast and bone lining cells) • Osteoid- the organic part of the matrix secreted by osteoblast • ~1/3 of matrix • Includes: o Ground substance  Proteoglycans Both made and secreted by osteoblast  Glycoproteins o Collagen fibers  Contribute to bone’s structure, flexibility and tensile strength • Resilience of bone due to Sacrificial bonds- in or between collagen fiber. These bonds stretch and break easily on impact, dissipating energy to prevent fractures. If no additional trauma, bonds reform →Inorganic components (*65% of bone by mass) • Calcium Hydroxyapatites (mineral salts)- largely calcium phosphates present around the collagen fibers in the extracellular matrix o Responsible for hardness and resistance to compression The proper combination of organic and inorganic matrix elements allows bones to be exceeding durable and strong without being brittle. Because of the salts they contain, bones last long after death. Nature of Bone • Half as strong as steel in resisting compression • As strong as steel in resisting tension • Last long after death because of mineral composition • Reveal information about ancient people • Can display growth arrest lines - Horizontal lines on bones - Proof of illness: when bones stop growing so nutrients can help fight disease

Bone Development Ossification (or Osteo/genesis): process of bone formation (Os= bone; Genesis=beginning) • Formation of bony skeleton o Begins in 2ndmonth of development • Postnatal bone growth o Until early adulthood • Bone remodeling and repair o Lifelong Formation of the Bony Skeleton Intramembranous ossification • The process when bone is developed from fibrous membrane • Bones called membrane bones • Most bone formed by this process are flat bones o Formation of cranial bone of the skull and the clavicle  Frontal, parietal, occipital, temporal bones  Process • Begins within fibrous connective tissue membranes formed by mesenchymal cells • Ossification centers appear • Osteoid is secreted • Woven bone and periosteum form • Lamellar bone replaces woven bone & red marrow appears

Endochondral ossification (IMPORTANT) • Begins in the 2nd month of development • Bone forms by replacing hyaline cartilage • Bones called cartilage (endochondral) bones • Forms most of skeleton (apart from the skull bones and the clavicle) • Uses hyaline cartilage models • Requires breakdown of hyaline cartilage prior to ossification  Process • Begins at primary ossification center in center of shaft of hyaline cartilage o Blood vessel infiltration of perichondrium (DICT) converts it to periosteum →underlying mesenchymal cells change to osteoblasts • Bone collar forms around diaphysis of cartilage model • Central cartilage in diaphysis calcifies, then develops cavities • Periosteal bud invades cavities → formation of spongy bone • Diaphysis elongates & medullary cavity forms • Epiphyses ossify

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Osteoblasts secrete osteoid against the hyaline cartilage diaphysis, encasing it in bone (this layer of bone is called the periosteal bone collar) As the bone collar forms, chondrocytes (the only cell found in cartilage) within the shaft enlarge and signal cartilage matrix to calcify. Because calcified cartilage matrix is permeable to nutrients the chondrocytes die and the matrix starts to deteriorate. This opens up the cavities, but the bone collar keeps the model in shape. Cartilage elsewhere continues to grow, causing the model to elongate. In month 3, the forming cavities are invaded by a collection of elements called the periosteal bud, which contains a nutrients artery and vein, lymphatic vessels, nerve fibers, red marrow elements, osteoblasts, and osteoclast. The osteoblast secretes osteoid around the remaining hyaline cartilage, forming trabeculae (spongy bone is developing). Osteoclast break down the newly formed spongy bone and open up a medullary cavity in the centre of the diaphysis. At birth, most of our long bones have a bony diaphysis surrounding remnants of spongy bone, a widening medullary cavity, and two cartilaginous epiphyses. Shortly before birth, secondary ossification centre appear on one or bot epiphyses and the epiphyses gain bony tissue. Secondary ossification is exactly the same as primary ossification except that the spongy bone in the interior is retained and no medullary cavity is formed. When secondary ossification is complete, hyaline cartilage remains only at two places: • On the epiphyseal surfaces, as the auricular cartilage • At the junction of the diaphysis and epiphyses, where it forms the epiphyseal plates

Postnatal Bone Growth • • •

During infancy and youth, long bones length entirely by interstitial growth (length) of the epiphyseal plate cartilage and its replacement by bone, and all grow in (thickness) by appositional growth Most bones stop growing during adolescence- but some facial bones, such as those of the nose and lower jaw, continue to grow almost invisibly through life Epiphyseal plate closure- when the bone of the epiphysis and diaphysis fuses (this happens at 18 in females and 21 in males)

Interstitial growth • Requires presence of epiphyseal cartilage • Epiphyseal plate maintains constant thickness o Rate of cartilage growth on one side balanced by bone replacement on other • Concurrent remodeling of epiphyseal ends to maintain proportion • Result of five zones within cartilage o Resting (quiescent) zone  Cartilage on epiphyseal side of epiphyseal plate  Relatively inactive o Proliferation (growth) zone  Cartilage on diaphysis side of epiphyseal plate  Rapidly divide pushing epiphysis away from diaphysis → lengthening o Hypertrophic zone  Older chondrocytes closer to diaphysis and their lacunae enlarge and erode → interconnecting spaces o Calcification zone  Surrounding cartilage matrix calcifies, chondrocytes die and deteriorate o Ossification (osteogenic) zone  Chondrocyte deterioration leaves long spicules of calcified cartilage at epiphysis diaphysis junction  Spicules eroded by osteoclasts  Covered with new bone by osteoblasts  Ultimately replaced with spongy bone • Near end of adolescence chondroblasts divide less often • Epiphyseal plate thins then is replaced by bone • Epiphyseal plate closure o Bone lengthening ceases  Requires presence of cartilage o Bone of epiphysis and diaphysis fuses o Females about 18 years o Males about 21 years Appositional growth • Allows lengthening bone to widen • Occurs throughout life • Osteoblasts beneath periosteum secrete bone matrix on external bone • Osteoclasts remove bone on endosteal surface • Usually more building up than breaking down o → Thicker, stronger bone but not too heavy

Hormonal regulation of bone growth: • Growth hormone: most IMPORTANT for stimulating epiphyseal plate activity in infancy and childhood • Thyroid hormone modulates activity of growth hormone, ensures proper proportions • Sex hormones (testosterone and estrogen) also effect bone growth at puberty o Promote adolescent growth spurts o End growth by inducing epiphyseal plate closure  Excess or deficient of any cause abnormal skeletal growth Bone Homeostasis: Remodeling and Repair Bone Remodeling Bone remodeling • In healthy bone, rate of bone deposit and removal are equal • Bone deposit and bone removal occur both at the surface of the periosteum and at the surface of the endosteum. • Recycle 5-7% of bone mass each week o Spongy bone replaced ~ every 3-4 years •

o Compact bone replaced ~ every 10 years Older bone becomes more brittle o Calcium salts crystallize

o Fractures more easily Consists of bone remodeling and bone repair: o Consists of both bone deposit and bone resorption o Occurs at surfaces of both periosteum and endosteum • Remodeling units: o Adjacent osteoblasts and osteoclasts • Bone deposit occur wherever bone is injured or added bone strength is required o Evidence of new matrix deposit by osteoblasts  Osteoid seam • Unmineralized band of bone matrix  Calcification front • Abrupt transition zone between osteoid seam and older mineralized bone o Trigger not confirmed  Mechanical signals involved  Endosteal cavity concentrations of calcium and phosphate ions for hydroxyapatite formation  Matrix proteins bind and concentrate calcium  Enzyme alkaline phosphatase for mineralization Bone Resorption • Is function of osteoclasts o Dig depressions or grooves as break down matrix o Secrete lysosomal enzymes that digest matrix and protons (H+) o Acidity converts calcium salts to soluble forms • Osteoclasts also o Phagocytize demineralized matrix and dead osteocytes  Transcytosis allow release into interstitial fluid and then into blood o Once resorption complete, osteoclasts undergo apoptosis • Osteoclast activation involves PTH and T cell-secreted proteins •

Control of remodeling • The remodeling that occurs continuously in the skeleton is regulated by genetic factors and two control loops: o A negative feedback hormonal loop that maintains Ca2+ homeostasis in the blood  Controls blood Ca2+ levels; not bone integrity o responses to mechanical and gravitational forces acting on the skeleton • Calcium play a huge role of importance for the body o Necessary for transmission of nerve Impulse, muscle contraction, blood coagulation, secretion by glands and nerve cells and cell division o 1200-1400 grams of calcium in body  99% as bone minerals  Amount in blood tightly regulated (9-11 mg/dl)  Intestinal absorption requires Vitamin D metabolites  Dietary intake required Hormonal...