Bones PDF

BONES Macroscopic Structure of Bone There are two primary forms of bone tissue Bone is a type of connective tissue that visible macroscopically: consists of cells, fibers, and ground substance, with...

BONES Macroscopic Structure of Bone There are two primary forms of bone tissue Bone is a type of connective tissue that visible macroscopically: consists of cells, fibers, and ground substance, with its extracellular matrix 1. Compact Bone (Substantia calcified, distinguishing it from other Compacta): This is dense, solid connective tissues. The calcified matrix bone tissue that forms the outer layer makes bone an ideal tissue for its primary of bones. It appears as a continuous functions: mass, with only microscopic spaces visible. Support and Protection: Bone 2. Spongy Bone (Substantia provides the internal support for the Spongiosa): This is lighter and more body and forms the skeleton. It serves as porous, consisting of a lattice of a framework for the attachment of branching bone spicules called muscles and tendons, which are trabeculae. The spaces between essential for movement and locomotion. trabeculae are filled with bone Bones also protect vital organs, such as marrow. the brain and heart. Reservoir for Calcium: Bone acts as a In long bones such as the femur or humerus, dynamic storage depot for calcium. It the structure is organized into: plays an important metabolic role by storing calcium, which can be mobilized 1. Diaphysis (Shaft): The central part of as needed to maintain the calcium ion a long bone, made up of a thick- concentration in the blood and other walled cylinder of compact bone bodily fluids. surrounding the medullary cavity, Hematopoiesis: The bone encloses which contains bone marrow. bone marrow, which is the site of blood 2. Epiphysis (Ends): The ends of long cell production (hematopoiesis). bones consist mostly of spongy bone covered by a thin layer of compact Physical Properties of Bone bone. The spongy bone here is continuous with the marrow cavity in Bones exhibit a remarkable combination of adults. physical properties: 3. Epiphyseal Plate: In growing bones, the epiphysis is separated from the Tensile Strength: Resistance to stretching. diaphysis by a cartilaginous plate, Compressive Strength: Ability to which allows for bone growth in withstand forces that compress the bone. length. Bone growth occurs here, with Elasticity: Some flexibility allows bones cartilage continuously being replaced to absorb shock. by bone tissue. 4. Metaphysis: The area between the Lightweight: Despite its strength, bone is relatively light, thanks to its efficient epiphysis and diaphysis, containing internal architecture. the growth zone where the transition from cartilage to bone occurs. Dynamic Tissue: Bone is continually The surface of bones, except where tendons, remodeled in response to mechanical ligaments, or articular cartilage cover it, is stresses, metabolic needs, and nutritional invested by the periosteum, a fibrous layer influences. Disuse leads to atrophy, and with osteogenic potency (ability to produce increased use results in hypertrophy. bone tissue). The periosteum is absent in areas covered by articular cartilage and where tendons and ligaments insert into the bone. The inner surfaces, including the medullary cavity and the cavities within spongy bone,  are lined by the endosteum, a thin cellular of osteons, containing blood vessels layer with similar osteogenic potential as the and connective tissue. periosteum. TOV Volkmann’s Canals: Transverse or oblique channels that connect In flat bones (like those of the skull), Haversian canals to the bone's compact bone forms two layers, the outer surface and marrow cavity. These and inner tables, which enclose a thin layer channels are not surrounded by of spongy bone called the diploe. The concentric lamellae and allow blood pericranium covers the outer surface of vessels from the periosteum to skull bones, while the dura mater covers penetrate the bone. the inner surface. Microscopic Structure of Bone At the microscopic level, bone is mostly composed of bone matrix (mineralized extracellular substance) arranged in layers called lamellae: Osteocytes are the mature bone cells, housed in small cavities called lacunae, spaced uniformly throughout the bone matrix. Canaliculi are slender channels that ***SHARPEY’S FIBERS radiate from each lacuna, allowing the osteocytes to communicate and - are coarse bundles of collagen fibers that exchange nutrients with neighboring extend from the outer layer of the cells. periosteum into the bone matrix, anchoring the periosteum to the underlying bone. The lamellae of compact bone are arranged in three common patterns: - occupy irregular channels, while calcified fibers appear as faint streaks in the outer 1. Haversian Systems (Osteons): portions of compact bone. Cylindrical structures consisting of Osteoprogenitor cells concentric lamellae surrounding a Bone Cells - Osteoblasts Osteoclasts central Haversian canal, which Osteocytes contains blood vessels and nerves. Four types of cells are involved in bone Each osteon typically contains 4-20 development, maintenance, and remodeling: lamellae. 2. Interstitial Systems: Irregular pieces Osteoprogenitor Cells of lamellar bone found between Haversian systems, remnants of old Origin: Derived from embryonic osteons that have been partially mesenchymal cells, which have broad resorbed. developmental potential. 3. Circumferential Lamellae: Located Location: Found on or near all free on the external and internal surfaces surfaces of bones, including the of compact bone, these lamellae run endosteum, inner layer of the around the circumference of the periosteum, and trabeculae of bone, just beneath the periosteum or calcifying cartilage in growing bones. endosteum. Morphology: o Pale-staining, elongated or oval Two types of vascular channels are found in nuclei. compact bone: o Scant cytoplasm that is acidophilic or faintly basophilic. HCL Haversian Canals: Longitudinal Function: channels that run through the centers  o They have limited differentiation o Produce growth factors that potential and can only become regulate bone growth and osteoblasts or chondroblasts. respond to hormonal signals o Most active during bone growth (e.g., parathyroid hormone). but can be reactivated for fracture repair. Osteocytes o They proliferate and differentiate to replenish osteoblasts during the Origin: Mature osteoblasts that have continual internal remodeling of become embedded in the bone bone. matrix. o Unlike osteoblasts, they are capable Location: Reside in lacunae within of cell division. the mineralized matrix. o Bone-lining cells: Morphology: OSTEOBLASTS that have o Flattened cells with long, slender become inactive after completing processes that extend through their role in bone formation. canaliculi, connecting them with Although they are inactive, they neighboring osteocytes and are not morphologically surface cells. distinguishable from o These processes are osteoprogenitor cells. These cells interconnected through gap can become reactivated and turn junctions, allowing for nutrient back into osteoblasts when needed, and signal exchange. such as during bone repair after Function: injury. o Maintain the bone matrix and regulate mineral content. Osteoblasts o Play a role in detecting mechanical stress on bone, Origin: Formed from which influences bone osteoprogenitor cells. remodeling. Location: Found on bone surfaces o Controversial role in osteocytic where active bone formation is osteolysis (once thought to occurring. mobilize calcium directly from Morphology: the matrix, now largely attributed o Cuboidal or columnar cells to osteoblast and osteoclast arranged in an epithelioid layer. interaction). o Intense basophilic cytoplasm o Some osteocytes revert to with a prominent Golgi complex. quiescent bone-lining cells upon o The nucleus is typically release from their lacunae during positioned away from the bone bone resorption. surface. Function: Osteoclasts o Responsible for synthesizing bone matrix (osteoid), which Origin: Formed from mononuclear includes type-I collagen, precursors in the bone marrow, proteoglycans, and non-collagen specifically from granulocyte- proteins (osteocalcin, macrophage progenitors. osteopontin, etc.). Location: Found on bone surfaces o When embedded in the matrix undergoing resorption, occupying they secrete, they differentiate shallow depressions called into osteocytes. Howship’s lacunae. o Release enzymes involved in bone resorption, including "Osteoclasts sail on Howship's procollagenase and plasminogen Lacunae, where they clash and dig activator, helping to expose their bone pits!" mineralized bone for osteoclast action. "Osteo-CLASH with the ship!"  Morphology: o Large, multinucleated cells with diameters of up to 150 µm, containing 10-50 nuclei. o Their bone-facing surface forms Bone Matrix a ruffled border, an area with deep membrane infoldings The bone matrix is composed of two main responsible for bone degradation. components: o Surrounded by a sealing zone, which isolates the resorption area Organic Matrix (35%): Composed from the rest of the bone. mostly of type-I collagen fibers embedded in a ground substance rich in proteoglycans. These fibers give bone its tensile strength and flexibility. Inorganic Matrix (65%): Comprised primarily of calcium phosphate in the form of hydroxyapatite crystals. These crystals are deposited along the collagen fibers and give bone its hardness and ability to resist compression. Periosteum and Endosteum The periosteum consists of two layers: Function: 1. Outer Fibrous Layer: Dense o Responsible for bone resorption connective tissue with numerous by creating an acidic environment blood vessels. in the subosteoclastic 2. Inner Osteogenic Layer: Contains compartment, which dissolves the osteoprogenitor cells that can mineral matrix. differentiate into osteoblasts when o Secrete lysosomal enzymes such bone growth or repair is needed. as acid hydrolases that degrade Sharpey’s Fibers are collagen organic components of bone. bundles from the periosteum that o Regulated by several hormones: penetrate the outer layers of bone, ▪ Parathyroid hormone anchoring it securely. (PTH) indirectly stimulates osteoclasts by triggering The endosteum lines the internal surfaces of osteoblasts to release an bones and is composed of a thin layer of osteoclast-activating factor. squamous cells with osteogenic potential. ▪ Calcitonin inhibits osteoclast activity, reducing bone resorption. dormancy Types of Ossification period of o Osteoclasts remain quiescent until activated by metabolic or There are two primary processes by which hormonal signals and are long- bone forms: lived, modulating between active and resting phases. 1. Intramembranous Ossification: o Osteoclast differentiation is highly dependent on signals from Definition: Bone formation occurs osteoblasts, which are thought to directly in mesenchymal tissue release factors that guide their without a cartilage precursor. formation and activation.  Location: Occurs in flat bones of Osteoblasts lay down bone the skull (e.g., frontal, parietal, matrix on the remaining occipital, and temporal bones). cartilage framework, forming trabeculae. The center of the cartilage model (diaphysis) becomes the Process: primary ossification center. Later, similar processes occur Mesenchymal cells in at the ends of the bone connective tissue condense (epiphyses), forming and differentiate into secondary ossification osteoblasts. centers. Osteoblasts begin secreting The remaining cartilage bone matrix, forming between the diaphysis and trabeculae (thin strands of epiphysis becomes the bone matrix). epiphyseal plate (growth The trabeculae grow and plate), allowing bones to grow interconnect, forming a in length. network of bone. Osteoblasts trapped in the End result: Formation of long matrix become osteocytes. bones with distinct diaphyses and The bone matrix mineralizes, epiphyses. forming woven bone (with randomly oriented collagen Growth of Long Bones fibers). Over time, woven bone is Primary Ossification Center: replaced by lamellar bone Long bone development begins with (highly-ordered in layers). a primary ossification center in the diaphysis (shaft). End result: Formation of flat Chondrocytes in the center enlarge, bones, which provide protection and and cartilage matrix calcifies. Blood structure (e.g., skull, clavicle). vessels invade, bringing osteoprogenitor cells which form 2. Endochondral Ossification: bone around the calcified cartilage. Definition: Bone formation that Periosteal Bone Formation: occurs by replacing a pre-existing Concurrently, bone forms beneath the cartilage model with bone. periosteum, forming a periosteal Location: Occurs in most long collar around the diaphysis. bones (e.g., femur, tibia) and bones of This appositional growth continues the vertebral column. to thicken the diaphysis. Process: Secondary Ossification Centers: The cartilage model (hyaline Secondary ossification centers form cartilage) grows in length and later in the epiphyses (ends of long width. bones), typically after birth. Chondrocytes in the center of These centers differ from the primary the cartilage model enlarge center, as they lack periosteal bone (hypertrophy), and the formation. surrounding matrix begins to calcify. Growth in Length: The calcified cartilage is Bone lengthens due to continuous invaded by blood vessels and interstitial growth at the epiphyseal osteoprogenitor cells, which plates located between the epiphyses differentiate into osteoblasts. and diaphysis.  Chondrocytes in the epiphyseal osteogenic potential and plates proliferate and align in begins depositing new bone, columns. As they move toward the forming a bony callus diaphysis, they mature and consisting of woven bone hypertrophy, leading to calcification trabeculae, which starts to and eventual replacement by bone. bridge the bone fragments. 3. Remodeling: o The bone is remodeled over time, where excess bone is Long bone growth occurs at the resorbed, re-establishing the epiphyseal plate through the marrow cavity and restoring continuous replacement of cartilage by the bone to its normal shape bone. The process is divided into four and function. distinct zones: 1. Zone of Proliferation: Joints And Synovial Membranes Chondrocytes divide, contributing to elongation. Types of Joints: Bones are connected at 2. Zone of Maturation: Chondrocytes joints (articulations) which allow varying enlarge. degrees of movement: 3. Zone of Hypertrophy: Chondrocytes become vacuolated Synarthroses: Joints with little or no and VERY large. movement. 4. Zone of Provisional Calcification: o Synostoses: Bone connects Matrix calcifies around hypertrophic directly to bone (e.g., skull chondrocytes. sutures). 5. Zone of Degeneration: o Synchondroses: Bones are Chondrocytes die, osteoprogenitor connected by cartilage. cells invade the calcified cartilage o Syndesmoses: Bones are and deposit bone matrix, replacing connected by dense the cartilage with bone. connective tissue (e.g., between long bones). After puberty, the epiphyseal plate Diarthroses (Synovial Joints): closes, and no further growth in length is These joints allow free movement. possible. The articular surfaces of bones are Bone Repair covered with hyaline cartilage. These joints are enclosed in a joint 1. Formation of Blood Clot capsule made of two layers: (Hematoma): 1. Outer fibrous layer: Dense o After a bone fracture, the first connective tissue, continuous step in the repair process is with the periosteum. the formation of a blood clot 2. Inner synovial layer (hematoma) at the site of (synovium): A cellular layer injury. This provides the that secretes synovial fluid. foundation for healing. 2. Granulation Tissue and Callus Synovium: Formation: The synovium lacks a continuous o The hematoma is replaced by epithelial lining; instead, connective granulation tissue, which tissue is exposed to the synovial condenses into connective fluid. tissue. This leads to the It contains two main types of cells: formation of a 1. Fibroblast-like cells: Secrete fibrocartilaginous callus collagen, proteoglycans, and that temporarily stabilizes the lubricating molecules like fracture site. hyaluronate and lubricin. o The periosteum (bone's outer layer) reactivates its  2. Macrophages: Remove 3. What covers most of the outer surface of debris from joint wear-and- bones, providing an osteogenic layer? tear. a) Endosteum Lymphocytes are found deeper b) Marrow within the synovium. c) Periosteum In some areas, synovium may rest d) Synovial membrane directly on the fibrous layer or be separated by loose connective or Answer: c) Periosteum adipose tissue. Rationale: The periosteum is a specialized connective tissue covering bone surfaces, Synovial Fluid: except where articular cartilage is present Synovial fluid is a transudate from 4. Which part of the bone is responsible for blood capillaries, similar in its longitudinal growth during development? composition to interstitial fluid. It a) Compact bone contains hyaluronate and lubricin b) Epiphyseal plate for lubrication. c) Bone marrow Fluid exchange occurs via d) Perichondrium transudation from blood capillaries into the joint cavity, and movement Answer: b) Epiphyseal plate through the synovium, facilitated by Rationale: The epiphyseal plate (growth joint flexion. plate) allows for the elongation of bones during growth Aging: As individuals age, villi in the synovium increase in size and number, and 5. Where is the articular cartilage of a long small areas of cartilage may form within bone found? them. a) On the diaphysis b) At the epiphysis c) In the medullary cavity QUESTIONS d) Surrounding the periosteum 1. Which type of bone consists of a three- Answer: b) At the epiphysis Rationale: Articular cartilage covers the dimensional lattice of branching bony joint surfaces at the ends of long bones to spicules? facilitate smooth movement a) Compact bone 6. Which component in bone matrix is b) Cancellous (spongy) bone responsible for hardness and strength? c) Dense bone d) Periosteum a) Collagen fibers b) Glycoproteins Answer: b) Cancellous (spongy) bone c) Hydroxyapatite crystals Rationale: Spongy bone consists of d) Elastic fibers trabeculae that create a labyrinthine structure filled with bone marrow Answer: c) Hydroxyapatite crystals Rationale: Bone hardness comes from 2. The primary centers of ossification in long hydroxyapatite, a crystalline form of bones appear in which part of the bone? calcium phosphate a) Epiphyses b) Diaphysis 7. Which hormone increases bone resorption? c) Metaphysis d) Periosteum a) Calcitonin b) Growth hormone Answer: b) Diaphysis c) Parathyroid hormone Rationale: Primary ossification centers form d) Insulin in the diaphysis during fetal development Answer: c) Parathyroid hormone Rationale: Parathyroid hormone stimulates osteoclast activity, leading to increased bone resorption 8. Which of the following is absent where tendons insert into bone? a) Endosteum b) Periosteum c) Bone marrow d) Compact bone Answer: b) Periosteum Rationale: The periosteum is absent where tendons and ligaments insert directly into the bone 9. What is the name of the structure formed when periosteum becomes incarcerated in bone matrix? a) Sharpey’s fibers b) Osteons c) Canaliculi d) Trabeculae Answer: a) Sharpey’s fibers Rationale: Sharpey's fibers anchor the periosteum to the bone by penetrating the bone matrix 10. In which area of long bones does appositional growth contribute to bone diameter? a) Epiphyseal plate b) Diaphysis c) Marrow cavity d) Periosteum Answer: d) Periosteum Rationale: Appositional growth increases bone diameter by adding layers to the bone surface from the periosteum