[BIO 430] Ch.6 - Skeletal System: Bone Tissue
Repair of Fractures (Steps)
(1) Formation of fracture hematoma - Swelling and inflammation; Phagocytes and osteoclasts begin to remove the dead or damaged tissue (2) Fibrocartilaginous callus formation - Fibroblasts from the periosteum invade the fracture site and produce collagen fibers; cells from the periosteum develop into chondroblasts and begin to produce fibrocartilage in this region (3) Bony callus formation - Osteogenic cells develop into osteoblasts, which begin to produce spongy bone trabeculae. The trabeculae join living and dead portions of the original bone fragments; fibrocartilage is converted to spongy bone, and the callus is then referred to as a bony (hard) callus. The bony callus lasts about 3 to 4 months. (4) Bone remodeling - Compact bone replaces spongy bone around the periphery of the fracture.
Histology of Bone Tissue (Containment)
- Bone contains an extracellular matrix that surrounds widely separated cells - The extracellular matrix is about 15% water, 30% collagen fibers, 55% crystallized mineral salts - most abundant mineral salt is calcium phosphate; combines with calcium hydroxide to form crystals of hydroxyapatite - - - As the crystals form, they combine with still other mineral salts - As these mineral salts are deposited in the framework formed by the collagen fibers of the extracellular matrix, they crystallize and the tissue hardens; this process of calcification is initiated by bone-building cells called osteoblasts - Bone is not completely solid but has many small spaces between its cells and extracellular matrix components; some spaces serve as channels for blood vessels that supply bone cells with nutrients and others act as storage areas for red bone marrow - Depending on the size and distribution of the spaces, the regions of a bone may be categorized as compact or spongy. - about 80% of the skeleton is compact bone and 20% is spongy bone
Blood Vessels and Arteries (Periosteal , Metaphyseal, Epiphyseal Arteries)
- Bone is richly supplied with blood. Blood vessels pass into bones from the periosteum. - Periosteal arteries - small arteries accompanied by nerves; enter the diaphysis through many perforating (Volkmann's) canals and supply the periosteum and outer part of the compact bone - - - ends of long bones supplied by metaphyseal and epiphyseal arteries, which arise from arteries that supply the associated joint - - - Near the center of the diaphysis, a large nutrient artery passes through a hole in compact bone called the nutrient foramen; nutrient artery divides into proximaland distal branches that course toward each end of the bone - - - Bones, like tibia, have only one nutrient artery; others, like femur, have several - Metaphyseal arteries - enter the metaphyses of a long bone and, together with the nutrient artery, supply the red bone marrow and bone tissue of the metaphyses - Epiphyseal arteries - enter the epiphyses of a long bone and supply the red bone marrow and bone tissue of the epiphyses
Bone's Role in Calcium Homeostasis
- Bone is the body's major calcium reservoir, storing 99% of total body calcium - Both nerve and muscle cells depend on a stable level of calcium ions (Ca2) in extracellular fluid to function properly - Blood clotting requires Ca2 & many enzymes require Ca2 as a cofactor; b/c of this, blood plasma level of Ca2 is very closely regulated between 9 and 11 mg/100 mL. - - - small changes in Ca2 concentration outside this range may prove fatal— the heart may stop (cardiac arrest) if the concentration goes too high, or breathing may cease (respiratory arrest) if the level falls too low. - Ca2 exchange is regulated by hormones, the most important of which is parathyroid hormone (PTH) secreted by the parathyroid glands. This hormone increases blood Ca2 level.
Remodeling of Bone
- Bone remodeling is the ongoing replacement of old bone tissue by new bone tissue - involves bone resorption (removal of minerals and collagen fibers from bone by osteoclasts) and bone deposition (addition of minerals and collagen fibers to bone by osteoblasts) - Triggers - exercise, sedentary lifestyle, changes in diet - Benefits - if newly formed bone is subjected to heavy loads, it will grow thicker and therefore be stronger than the old bone; shape of a bone can be altered for proper support based on the stress patterns experienced during the remodeling process; new bone is more resistant to fracture than old bone Factors that Affect Bone Growth: - Minerals - Calcium, phosphorus, magnesium, flouride - Vitamins - Vitamin A, C, D, K, B - Hormones - Growth hormone, thyroid hormones, sex hormones - Exercise - Aging
Bone Growth during Infancy, Childhood, and Adolescence (growth in length)
- During infancy, childhood, and adolescence, bones throughout body grow in thickness by appositional growth, and long bones lengthen by addition of bone material on the diaphyseal side of the epiphyseal plate by interstitial growth. - The growth in length of long bones involves the following two major events: (1) interstitial growth of cartilage on the epiphyseal side of the epiphyseal plate (2) replacement of cartilage on diaphyseal side of epiphyseal plate with bone by endochondral ossification.
Histology of Bone Tissue (Types of Cells in Bone Tissue)
- Osteogenic cells - unspecialized bone stem cells derived from mesenchyme; only bone cells to undergo cell division; the resulting cells develop into osteoblasts - - - Location - inner portion of periosteum, in endosteum, and in canals within bone that contain blood vessels - Osteoblasts - bone-building cells; synthesize and secrete collagen fibers and other organic components needed to build the extracellular matrix of bone tissue, and they initiate calcification ; don't undergo cell division - - - As osteoblasts surround themselves with extracellular matrix, they become trapped in their secretions and become osteocytes. - Osteocytes - mature bone cells, are the main cells in bone tissue and maintain its daily metabolism (ex. exchange of nutrients and wastes w/ blood) ; don't undergo cell division - Osteoclasts - huge cells derived from the fusion of as many as 50 monocytes (a type of white blood cell) and are concentrated in the endosteum - - - Function - cell releases powerful lysosomal enzymes and acids that digest extracellular bone matrix; this breakdown of bone extracellular matrix (called resorption) is part of the normal development, maintenance, and repair of bone. - - - Function - help regulate blood calcium level; target cells for drug therapy used to treat osteoporosis - Mnemonic to remember diff between function of osteoblasts and osteoclasts - osteoBlasts Build bone, while osteoClasts Carve out bone.
Initial Bone Formation in an Embryo and Fetus (Endochondral Ossification)
- Endochondral Ossification - bone forms within hyaline cartilage that develops from mesenchyme (1) Development of the cartilage model - mesenchymal cells develop into chondroblasts, which produces cartilage model; perichondrium develops around the cartilage model - - - appositional (exogenous) growth - process of chondroblasts that develop from the perichondrium - - - as cartilage model grows, chondrocytes increase in size and extracellular matrix hardens - - - spaces left behind by dead chondrocytes merge into lacunae (2) Development of the primary ossification center - in this region of the diaphysis, bone tissue has replaced most of the cartilage - - - Once the perichondrium starts to form bone, it is known as the periosteum - - - Near middle of the model, periosteal capillaries grow into disintegrating calcified cartilage, inducing growth of a primary ossification center, where bone tissue will replace most of cartilage - - - Osteoblasts form spongy bone trabeculae; Primary ossification spreads from this central location toward both ends of the cartilage model (3) Development of the medullary (marrow) cavity - bone breakdown by osteoclasts forms the medullary cavity; most of wall of diaphysis is replaced by compact bone (4) Development of the secondary ossification centers - usually develops around time of birth; spongy bone remains in the interior of the epiphyses (no medullary cavities formed) & proceeds outward from the center of the epiphysis toward the outer surface of the bone (5) Formation of articular cartilage and the epiphyseal (growth) plate - hyaline cartilage that covers epiphyses becomes the articular cartilage; Prior to adulthood, hyaline cartilage remains between the diaphysis and epiphysis as the epiphyseal (growth) plate, the region responsible for the lengthwise growth of long bones
Aging and Bone Tissue
- From birth through adolescence, more bone tissue is produced than is lost during bone remodeling. - In young adults the rates of bone deposition and resorption are about the same. - As the level of sex hormones diminishes during middle age, especially in women after menopause, a decrease in bone mass occurs because bone resorption by osteoclasts outpaces bone deposition by osteoblasts - In old age, loss of bone through resorption occurs more rapidly than bone gain. Because women's bones generally are smaller and less massive than men's bones to begin with, loss of bone mass in old age typically has a greater adverse effect in females. These factors contribute to the higher incidence of osteoporosis in females. Principal effects of aging on bone tissue: - Loss of bone mass - results from demineralization (the loss of calcium and other minerals from bone extracellular matrix) ; usually begins, after age 30 in females, accelerates greatly around age 45 as levels of estrogens decrease, and continues until as much as 30% of the calcium in bones is lost by age 70 - Brittleness - results from a decreased rate of protein synthesis; loss of tensile strength causes the bones to become very brittle and susceptible to fracture. (collagen fibers gives bone its tensile strength - loss of tensile strength causes bones to become brittle) - - - In some elderly people, collagen fiber synthesis slows, in part due to diminished production of human growth hormone. - - - loss of bone mass also leads to deformity, pain, loss of height, and loss of teeth
Bone Growth during Infancy, Childhood, and Adolescence - Growth in Thickness
- Growth in Thickness - bone can grow in thickness (diameter) only by appositional growth (1) At bone surface, periosteal cells differentiate into osteoblasts, which forms bone extracellular matrix and create a groove for the periosteal blood vessel; osteoblasts become surrounded by extracellular matrix and develop into osteocytes (This process forms bone ridges on either side of a periosteal blood vessel. The ridges slowly enlarge) (2) Ridges fold together and fuse, and groove becomes a tunnel that encloses blood vessel. Periosteum becomes endosteum that lines the tunnel (3) Osteoblasts in the endosteum deposit bone extracellular matrix, forming new concentric lamellae. New osteon is created (4) As an osteon is forming, osteoblasts under the periosteum deposit new circumferential lamellae, further increasing the thickness of the bone - As new bone tissue is being deposited on the outer surface of bone, bone tissue lining the medullary cavity is destroyed by osteoclasts in endosteum. The medullary cavity enlarges as the bone increases in thickness
Initial Bone Formation in an Embryo and Fetus (Intramembranous Ossification)
- Intramembranous Ossification - replacement of a preexisting connective tissue with bone; simpler bone formation - flat bones of the skull, most of the facial bones, mandible (lower jawbone), and the medial part of the clavicle (collar bone) are formed in this way - "soft spots" that help the fetal skull pass through the birth canal later harden as they undergo intramembranous ossification, which occurs as follows: (1) Development of the ossification center - Osteoblasts secrete the organic extracellular matrix of bone until they are surrounded by it (2) Calcification - secretion of extracellular matrix stops; calcium and other mineral salts are deposited and the extracellular matrix calcifies (hardens) (3) Formation of trabeculae - extracellular matrix develops into trabeculae that fuse to form spongy bonaround the network of blood vessels in the tissue (4) Development of the periosteum- mesenchyme condenses at periphery of bone and develops into periosteum; thin layer of compact bone replaces surface layers of spongy bone, but spongy bone remains in center
Types of Fractures
- Open (Compound) - broken ends of the bone protrude through the skin - Comminuted - bone is splintered,crushed, or broken into pieces at the site of impact and smaller bone fragments lie between the two main fragments - Greenstick - partial fracture; one side of the bone is broken and the other side bends - Impacted - One end of the fractured bone is forcefully driven into the interior of the other - Pott - Fracture of the distal end of the lateral leg bone (fibula), with serious injury of the distal tibial articulation - Colles - Fracture of the distal end of the lateral forearm bone (radius) in which the distal fragment is displaced posteriorly
Bone Formation
- Ossification (osteogenesis) - process by which bone forms; Occurs in 4 situations: (1) Initial Bone Formation in an Embryo and Fetus (2) Bone Growth during Infancy, Childhood, and Adolescence (3) Remodeling of Bone (4) Repair of Fractures
Skeletal System and Osteology
- Skeletal System - the entire framework of bones, cartilages, ligaments, and tendons - Osteology - study of bone structure and the treatment of bone disorders
Functions of Bone and the Skeletal System
- Support - Protection - Assistance in movement - Mineral homeostasis (storage and release) calcium and phosphorus - Blood cell production. w/in certain bones, red bone marrow produces RBCs, WBCs, and platelets, a process called hemopoiesis - Triglyceride storage. Yellow bone marrow consists mainly of adipose cells, which store triglycerides.
Blood Vessels (Veins) and Nerves
- Veins that carry blood away from long bones are evident in 3 places: - One or two nutrient veins accompany the nutrient artery and exit through the diaphysis - numerous epiphyseal veins and metaphyseal veins accompany their respective arteries and exit through epiphyses - many small periosteal veins accompany their respective arteries and exit through the Periosteum - Nerves accompany the blood vessels that supply bones; periosteum is rich in sensory nerves (some carry pain sensations); These nerves are especially sensitive to tearing or tension, which explains the severe pain resulting from a fracture or a bone tumor - - - Bone marrow needle biopsy - needle is inserted into the middle of the bone to withdraw a sample of red bone marrow to examine it for conditions; initial pain from needle penetration, but little pain afterwards
Compact Bone Tissue
- contains few spaces and is the strongest form of bone tissue - Location - beneath the periosteum of all bones; makes up the bulk of the diaphyses of long bones - Function - provides protection and support and resists the stresses produced by weight and movement - Components: osteons, central canal, concentric lamellae, lacunae, canaliculi, osteocytes, perforating canals, circumferential lamellae (outer and inner), Sharpey's fibers (connects connects outer circumferential lamella and periosteum)
Bone Growth during Infancy, Childhood, and Adolescence - epiphyseal (growth) plate
- epiphyseal (growth) plate is a layer of hyaline cartilage in the metaphysis of a growing bone that consists of four zones: (1) Zone of resting cartilage - scattered chondrocytes; cells do not function in bone growth; cells anchor the epiphyseal plate to the epiphysis of the bone (2) Zone of proliferating cartilage - chondrocytes divide to replace those that die and secrete extracellular matrix (3) Zone of hypertrophic cartilage - consists of large, maturing chondrocytes arranged in columns (4) Zone of calcified cartilage - most chondrocytes are dead b/c extracellular matrix around them has hardened - Osteoclasts replace calcified cartilage w/ bone extracellular matrix (process called endocondral ossification) - zone of calcified cartilage becomes the "new diaphysis" ; activity of epiphyseal plate is only way that the diaphysis can increase in length - As a bone grows, new chondrocytes replace older ones; thickness of the epiphyseal plate remains relatively constant, but the bone on the diaphyseal side increases in length - If a bone fracture damages the epiphyseal plate, the fractured bone may be shorter than normal once adult stature is reached; dmg to cartilage accelerates closure of epiphyseal plate due to ending of cartilage cell division, inhibiting lengthwise growth of the bone - When adolescence comes to an end (~18 in females, ~21 in males), epiphyseal plates close; epiphyseal cartilage cells stop dividing and bone replaces all remaining cartilage; epiphyseal plate fades, leaving a bony structure called the epiphyseal line & bone growth in length stops completely. - Closure of the epiphyseal plate is a gradual process and degree to which it occurs is useful in determining bone age, predicting adult height, and establishing age at death from skeletal remains, especially in infants, children, and adolescents
Spongy Bone Tissue
- spongy bone tissue (trabecular or cancellous bone tissue) does not contain osteons; always covered by a layer of compact bone for protection - Location - interior of a bone, protected by a covering of compact bone - Components: - trabeculae - irregular column consisting of concentric lamella, osteocytes in lacuna - red bone marrow exist between trabeculae spaces - Makes up most of the interior bone tissue of short, flat, sesamoid, and irregularly shaped bones. - In long bones it, forms the core of the epiphyses beneath the paper-thin layer of compact bone, and forms a variable narrow rim bordering the medullary cavity of the diaphysis Spongy Bone diffs from compact bone - spongy bone tissue is light, which reduces the overall weight of a bone. This reduction in weight allows the bone to move more readily when pulled by a skeletal muscle - trabeculae of spongy bone tissue support and protect the red bone marrow - Spongy bone in the hip bones, ribs, sternum (breastbone), vertebrae, and the proximal ends of the humerus and femur is the only site where red bone marrow is stored and, thus, the site where hemopoiesis (blood cell production) occurs in adults.
Structure of Bone
Typical long bone consists of the following parts: (1) Diaphysis - the bone's shaft or body; the long, cylindrical, main portion of the bone (2) Epiphyses - the proximal and distal ends of the bone (3) Metaphyses - regions between the diaphysis and the epiphyses - - - In a growing bone, ea. metaphysis contains an epiphyseal (growth) plate, a layer of hyaline cartilage that allows the diaphysis of the bone to grow in length - - - When a bone ceases to grow in length at about ages 14-24, the cartilage in the epiphyseal plate is replaced by bone; the resulting bony structure is known as the epiphyseal line (4) Articular Cartilage - reduces friction and absorbs shock at freely movable joints - - - b/c lacks a perichondrium and lacks blood vessels, repair of damage is limited (5) Periosteum - tough connective tissue sheath and its associated blood supply that surrounds the bone surface wherever it is not covered by articular cartilage - - - composed of an outer fibrous layer of dense irregular connective tissue and an inner osteogenic layer that consists of cells - - - Some cells enable bone to grow in thickness, but not in length - - - Functions - protects the bone, assists in fracture repair, helps nourish bone tissue, and serves as an attachment point for ligaments and tendons - - - attached to the underlying bone by perforating (Sharpey's) fibers, thick bundles of collagen that extend from the periosteum into the bone extracellular matrix (6) Medullary Cavity (Marrow Cavity) - space within the diaphysis that contains fatty yellow bone marrow and numerous blood vessels in adults (7) Endosteum - lines the medullary cavity; contains a single layer of bone-forming cells and a small amount of connective tissue
Metastases
occur most often in lungs; treatment consists of multidrug chemotherapy and removal of the malignant growth, or amputation of the limb
