Chapter 6 - Bones and Skeletal Tissues
Resorption
breakdown of bone extracellular matrix
Growth of Cartilage
Unlike bone, which has a hard matrix - cartilage has a flexible matrix which can accommodate mitosis. Ideal tissue to use to rapidly lay down the embryonic skeleton and to provide for new skeletal growth. Ends during adolescence when the skeleton stops growing. Grows in 2 ways: 1. appositional growth. 2. interstitial growth.
In what stage of endochondral ossification does the secondary ossification center form?
during diaphysis elongation and the development of medullary cavitity
In what stage of endochondral ossification does the epiphyseal plate form?
during epiphyseal ossification
When do nutrient arteries/veins begin to form during endochondral ossification?
during the stage in which the periosteal bud invades the internal cavities and spongy bone forms
Skeletal System
entire framework of bones and their cartilages, ligaments, and tendons
Epiphyseal Line
location of growth plate that turns to cartilage once growing has stopped
In short bones, only the (blank) center is formed.
primary ossification
Fotanels
soft spots on fetal skull that later harden through intramembranous ossification
When we eat, serotonin is secreted and circulated
via the blood to the bones where it interferes with osteoblast activity. - reduction of bone turnover after eating may lock calcium in bone when new calcium is flooding into the bloodstream.
Fracture
any break in a bone
Osteomalacia
"soft bones" - includes a number of disorders in which the bones are poorly mineralized. - osteoid is produced, but calcium salts are not adequately deposited, so bones are soft and weak. - main symptom is pain when weight is put on the affected bones.
Endosteum
"within the bone". - delicate connective tissue membrane. - covers internal bone surfaces. - covers trabeculae of spongy bone, lines canals that pass through compact bone. - contains osteogenic cells that can differentiate into other bone cells.
Interstitial Growth
- "growth from inside" - chondrocytes divide and secrete new matrix, expanding the cartilage from within.
Appositional Growth
- "growth from outside". - cartilage forming cells in perichondrium secrete new matrix against the external face of existing cartilage.
Compact Bone
- 80% of skeleton. - contains few spaces. - strongest form. - makes up the bulk of diaphysis of long bones. - provides protection, support, and resists stressed produced by weight and movement.
Structure of Compact Bone
- Osteon (Haversian System). - Canals and Canaliculi. - Interstitial and Circumferential Lamellae.
When blood levels of ionic calcium decline
- PTH is released. - increased PTH stimulates osteoclasts to resorb bone, releasing calcium into blood. - when activated, osteoclasts break down both old and new matrix.
Structure of Osteon - Compact Bone
- a group of hollow tubes of bone matrix, one place outside the next. - each matrix tube is a lamella and for this reason compact bone is called lamellar bone. - the collagen fiber in adjacent lamellae always run in different directions. - this alternating patterns is designed to withstand torsion stresses. - the adjacent lamellae reinforce one another to resist twisting.
Leptin
- a hormone released by adipose tissue. - plays a role in regulating bone density. - best known for its effects on weight and energy balance. - appears to inhibit osteoblasts through an additional pathway mediated by the hypothalamus, which activates sympathetic nerves serving bones.
Osteogenic Cells
- a.k.a osteoprogenitor cells. - mitotically active stem cells found in the periosteum and endosteum, - in growing bones, they are flattened or squamous cells. - when stimulated, they differentiate into osteoblasts or bone lining cells, while others persist as osteogenic cells.
Bone Resorption
- accomplished by giant osteoclasts. - osteoclasts move along a bone surface, digging depressions or grooves as they break down the bone matrix. - ruffled border of the osteoclast clings tightly to the bone, sealing off the area of bone destruction and secreting lysosomal enzymes that digest the organic matrix and protons (H+). - resulting acidic brew in the resorption bay converts the calcium salts into soluble forms that pass easily into solution. - osteoclasts may also phagocytize the demineralized matrix and dead osteocytes. - the digested matrix end products, growth factors, and dissolved minerals are then endocytosed, transported across the osteoclast (by transcytosis), and released at the opposite side. - there they enter the interstitial fluid and then the blood.
3. Intramembranous Ossification. Woven bone and periosteum form.
- accumulating osteoid is laid down between embryonic blood vessels in a manner that results in a network of trabeculae called woven bone. - vascularized mesenchyme condenses on the external face of the woven bone and becomes the periosteum.
Function of Osteocytes
- act as stress or strain sensors and respond to mechanical stimuli, which are bone loading, bone deformation or weightlessness. - they communicate this information to the cells responsible for bone remodeling, osteoblasts and osteoclasts, so that bone matrix can be made or degraded as necessary to preserve calcium homeostasis.
What are risk factors of osteoporosis?
- aged, postmenopausal women. - men develop it to a lesser degree. - androgens (male) and estrogens (females) help maintain the health and the normal density of the skeleton by restraining osteoclasts and promoting deposit of new bone. - after menopause, estrogen secretion lessens.
Longitudinal growth is accompanied by
- almost continuous remodeling of the epiphyseal ends to maintain the proportion between the diaphysis and epiphyses. Bone remodeling involves both new bone formation and bone resorption.
Hypercalcemia
- an abnormally high level of calcium in the blood. - can lead to undesirable deposits of calcium salts in the blood vessels, kidneys and other soft organs.
Bone Deposit
- an osteoid seam marks areas of new matrix deposits by osteoblasts. - between the osteoid seam and the older mineralized bone, there is an abrupt transition called the calcification front.
Rickets
- analogous disease in children. - because young bones are still growing rapidly, rickets is more severe that osteomalacia. - bowed legs, deformities of the pelvis, skull and rib cage. - because the epiphyseal plates cannot calcify, they continue to widen and the ends of long bone become visibly enlarged and abnormally long.
4. Endochondral ossification. The diaphysis elongates and a medullary cavity forms.
- as primary ossification center enlarges, osteoclasts break down the newly formed spongy bone and open up a medullary cavity in the center of diaphysis. - throughout the fetal period (week 9 until birth), the epiphysis consist only of cartilage, and the hyaline cartilage models continue to elongate by division of viable cartilage cells at the epiphyses. - ossification chases cartilage formation along the length of the shaft as cartilage calcifies, erodes and then is replaced by bony spicules on the epiphyseal surfaces facing the medullary cavity.
2. Endochondral ossification. Cartilage in the center of the diaphysis calcifies and then develops cavities
- as the bone collar forms, chondrocytes within the shaft hypertrophy enlarge and signal the surrounding cartilage matrix to calcify. - then, because calcified cartilage matrix is impermeable to diffusing nutrients, the chondrocytes dies and the matrix begins to deteriorate. - this deterioration opens up cavities, but the bone collar stabilizes the hyaline cartilage model. - elsewhere, the cartilage remains healthy and continues to grow briskly, causing the cartilage model to elongate.
Development of Bone - Birth to Young Adulthood
- at birth, most long bones of the skeleton are well ossified except for their epiphyses. - after birth, secondary ossification centers develop in a predictable sequence. - epiphyseal plates persist and provide for long bone growth through childhood and the sex hormone mediated growth spurt at adolescence. - by age 25, nearly all bones are completely ossified and skeletal growth ceases.
5. Endochondral ossification. The epiphyses ossify
- at birth: long bones have a bony diaphysis surrounding remnants of spongy bone, a widening medullary cavity, and two cartilaginous epiphyses. - shortly before or after birth: secondary ossification centers appear in one or both epiphyses, and the epiphyses gain bony tissue. - cartilage in the center of epiphyses: calcifies and deteriorates, opening up cavities that allow a periosteal bud to enter. - the bone trabeculae appear.
Bone Growth in Length - calcified cartilage
- at the epiphysis-diaphysis junction and they are long, slender spicules. - calcified spicules become part of the ossification or osteogenic zone and are invaded by marrow elements from the medullary cavity. - osteoclasts partly erode the cartilage spicules, osteoblasts cover them with new bone, and spongy bone replaces them. - as osteoclasts digest the spicule tips, the medullary cavity also lengthens.
Formation of the Bony Skeleton
- before week 8, skeleton of embryo is constructed entirely from fibrous membranes and hyaline cartilage. - bone tissue begins to develop and replaces most of existing fibrous or cartilage structures. - using flexible structures (membranes and cartilages) to make the embryonic skeleton accommodates mitosis.
4. Bone remodeling occurs. Repair of Bone Fracture.
- beginning during bony callus formation and continuing for several months, the bony callus is remodeled. - excess material on the diaphysis exterior and within the medullary cavity is removed and compact bone is laid down to reconstruct shaft walls. - final structure of the remodeled are resembles the original unbroken bony region because it responds to the same set of mechanical stressors.
Greenstick Fracture
- bone breaks incompletely, much in the way a green twig breaks. - only one side of the shaft breaks; the other side bends. - common in children, whose bones have more organic matrix and are more flexible than those of adults.
Osteoblasts
- bone forming cells. - synthesize and secrete un-mineralized bone matrix that includes collagen (90% of bone protein) and calcium binding proteins that make up the initial un-mineralized bone or osteoid. - initiate matrix calcification.
Comminuted Fracture
- bone fragments into 3 or more pieces. - common in the aged, whose bones are more brittle.
Hormonal Regulation of Bone Growth
- bone growth that occurs until young adulthood is controlled by hormones. - during infancy and childhood: stimulus of epiphyseal plate activity is growth hormone released by the anterior pituitary glad. - thyroid hormones: modulate the activity of growth hormone, ensuring that the skeleton has proper proportions as it grows.
Compression Fracture
- bone is crushed. - common in porous bone (osteoporotic bone) subjected to extreme trauma, as in a fall.
Between the Cells
- bone is not completely solid, there are many small spaces (channels for blood vessels, red bone marrow for storage) - depending on size and distribution of the spaces, bone is classified as compact or spongy
Projections (bone markings)
- bone markings that bulge outward from the surface. - include heads, trochanters, spines and others. - each has distinguishing features and functions. - indicate stresses created by muscles attached to and pulling on them or are modified surfaces where bones meet and form joints.
Endochondral Ossification
- bone replaces hyaline cartilage skeleton. - most bones below the base of the skull formed this way. - late in the 2nd month of development, this process uses hyaline cartilage for bone construction. - more complex than intramembranous ossification because the hyaline cartilage must be broken down as ossification proceeds.
Diaphysis
- bone shaft. - forms the long axis of the bone. - constructed of a thick collar of compact bone that surrounds a central medullary cavity or marrow cavity. - in adults, the medullary cavity contains fat (yellow marrow) and is called yellow marrow cavity.
Response to Mechanical Stress
- bone's response to mechanical stress and gravity keep the bones strong where stressors are acting.
Serotonin
- brain, intestines and skeleton help regulate the balance between bone formation and destruction using serotonin. - it is a neurotransmitter that regulates mood and sleep. - most is made in the gut and the blood-brain barrier bars it from entering the brain.
Depressed Fracture
- broken bone portion is pressed inward; - typical of skull fracture
Treating Osteoporosis
- calcium and Vitamin D supplements, weight bearing exercise and hormone (estrogen) replacement therapy (HRT). - HRT slows loss of bone but does not reverse it. - bisphosphonates: decrease osteoclast activity and number and partially reverse osteoporosis in the spine. - Selective estrogen receptor modulators (SERMs), such as raloxifene, dubbed "estrogen light," mimic estrogen's bone-sparing properties without targeting the uterus or breast. - statins, drugs used to lower cholesterol levels, have a side effect of increasing bone mineral density up to 8% over 4 years. - monoclonal antibody drug denosumab reduces fractures in men fighting prostate cancer and improves bone density in the elderly.
The hormonal feedback becomes much more meaningful when you understand
- calcium's importance in the body. Ionic calcium is necessary for: - transmission of nerve impulses. - muscle contraction. - blood coagulation. - secretion by glands and nerve cells. - cell division.
Spongy Bone
- looks like an unorganized, haphazard tissue. - the trabeculae align precisely along lines of stress and help the bone resist stress. - tiny bone struts are carefully positioned.
Paget's Disease
- characterized by excessive and haphazard bone deposition and resorption. - newly formed bone, Pagetic bone, has a high ratio of spongy bone to compact bone. - along with reduced mineralization, causes a spotty weakening of bones., - late in the disease, osteoclast activity wanes, but osteoblasts continues to work, forming irregular bone thickenings or filling the marrow cavity with Pagetic bone.
Development of Bone - Age-Related Changes in Bone
- children and adolescents: bone formation exceeds bone resorption. - young adults: these processes are in balance. - old age: resorption predominates. - genetics plays a role in determining how much a person's bone density will change. - at 40, bone mass decreases with age; only exception is the bones of the skull. - skeletal mass is greater in males than in females. - bone loss is faster in whites than in blacks.
Bones
- come in many size and shapes. - the unique shape of each bone fulfills a particular need. - continuously growing, remodeling, and repairing itself - movement puts stress on bones forcing them to remodel - skeletal system
Irregular Bones
- complex shapes that cannot be grouped into any of the previous categories. examples: vertebrae, hip bones, some facial bones, calcaneus.
Inorganic Components of Bones
- consist of inorganic hydroxyapatites or mineral salts. - largely calcium phosphates present as tiny, tightly package, needle-like crystals in and around collage fibers in the extracellular matrix. - the crystals account for the exceptional hardness of bones, which allows it to resist compression. - because of the mineral salts they contain, bone last long after death.
Appendicular Skeleton
- consists of the bones of the upper and lower limbs and the girdles - shoulder and hip bones - that attach the limbs to the axial skeleton. *Bones of the limbs helps us move from place to place - locomotion - and manipulate our environment.
Trabeculae - Spongy Bone
- contain irregularly arranged lamellae and osteocytes interconnected by canaliculi. - no osteons present. - nutrients reach the osteocytes by diffusing through the canaliculi from capillaries in the endosteum surrounding the trabeculae.
Chemical Composition of Bone
- contain organic and inorganic substances. - organic components: bone cells and osteoid. - inorganic components: mineral salts.
Short Bones
- cube-shaped. - nearly equal in length and width. examples: carpals, tarsals
How do mechanical forces communicate with the cells responsible for remodeling?
- deforming a bone produces an electrical current. - compressed and stretched regions are oppositely charged, so electrical signals direct remodeling. - this principle underlies some of the devices used to speed bone repair and heal fractures. - fluid flows within the canaliculi appear to provide stimuli that direct the remodeling process.
Osteoclasts
- derived from hematopoietic stem cells that differentiate into macrophages. - giant multi-nucleate cells located at sites of bone resorption.
As adolescence ends, the chondroblasts of the epiphyseal plates
- divide less often and the plates become thinner and thinner until they are entirely replaced by bone tissue.
Epiphyses
- ends of the bone, proximal and distal. - broader than diaphysis. - an outer shell of compact bone forms the exterior and their interior contains spongy bone. - a thin layer of articular (hyaline) cartilage covers the joint surface of each epiphysis, cushioning the opposing bone ends during movement and absorbing stress.
Between the diaphysis and each epiphysis of an adult long bone is an
- epiphyseal line, a remnant of the epiphyseal plate, a disc of hyaline cartilage that grows during childhood to lengthen the bone.
Epiphyseal Fracture
- epiphysis separates from the diaphysis along the epiphyseal plate; - tends to occur where cartilage cells are dying and calcification of the matrix is occurring.
Bone Homeostasis - Remodeling and Repair
- every week, we recycle 5-7% of our bone mass. - spongy bone is replaced every 3-4 years. - compact bone is replaced every 10 years. - when bones are broken, self-repair occurs.
Bone Lining Cells
- flat cells. - help maintain matrix. - found on bone surfaces where bone remodeling is not occurring. - bone lining cells on the external surface: periosteal cells; internal surface: endosteal cells.
Intramembranous Ossification
- forms the clavicles and cranial bones of the skull; frontal, parietal, occipital, and temporal bones. - most bones formed are flat bones. - week 8: ossification begins within fibrous connective tissue membranes formed by mesenchymal cells.
Axial Skeleton
- forms the long axis of the body. - includes the bones of the skull, vertebral column and ribcage. - protect, support or carry other body parts.
Red marrow in flat bones and irregular bones
- found in diploe of flat bones (e.g. sternum) and some irregular bones (e.g. hip bones) is much more active in hematopoiesis. - clinicians obtain red marrow samples from these sites if they suspect problems with blood-forming tissue. - yellow marrow in the medullary cavity can revert to red marrow if a person becomes very anemic and needs more red blood cells.
Preventing Osteoporosis
- get enough calcium while your bones are still increasing in density, bones reach their peak density during early adulthood. - excessive intake of carbonated beverages and alcohol leaches minerals from bone and decreases bone density. - get plenty of weight-bearing exercise throughout life, which will increase bone mass above normal values and provide a greater buffer against age-related bone loss.
Periosteum
- glistening white, double layered membrane that covers the external surface of the entire bone, expect the joint surfaces. - surrounds and protects bone surface. - assists in fracture repair. - nourishes bone tissue. - attachment point for ligaments/tendons.
Bone Growth in Thickness
- growing bones widen as they lengthen. - increase in size by appositional growth. - osteoblasts beneath the periosteum secrete bone matrix on the external bone surface as osteoclasts on the endosteal surface of the diaphysis remove bone. - there is more building up than breaking down which produces a thicker, stronger bone but prevents it from becoming too heavy.
Canaliculi - Compact Bone
- hair-like canals. - connect the lacunae to each other and to the central canal. - tie all osteocytes in a mature osteon together allowing them to communicate and permitting nutrients and wastes to be relayed from one osteocyte to the next throughout the osteon. - the canaliculi and cell-to-cell relays allow bone cells to be well nourished.
Fibrocartilages
- highly compressible with great tensile strength. - consist of roughly parallel rows of chondrocytes alternating with thick collage fibers. - occur in sites that are subjected to both pressure and stretch. - found in pad-like cartilages (menisci) of the knee and the discs between vertebrae.
Wolff's Law
- holds that a bone grows or remodels in response to the demands placed on it. - a bone's anatomy reflects the common stresses it encounters. - a bone is loaded (stressed) whenever weight bears down on it or muscles pull on it. - this loading is usually off center and tends to bend the bone. - bending compresses the bone on one side and subjects it to tension (stretching) on the other. - both compression and tension are minimal toward the center of the bone, they cancel each other out, so a bone can "hollow out" for lightness, using spongy bone instead of compact, without jeopardy.
Growth arrest lines
- horizontal line on long bones that provide visible proof of illness when the body uses nutrients to fight disease and the bones stop growing.
Bone Remodeling
- in adult skeleton, bone deposit and resorption occur at the surfaces of both the periosteum and the endosteum. - packets of adjacent osteoblasts and osteoclasts called remodeling units coordinate bone remodeling with the help from stress-sensing osteocytes. - in healthy young adults, total bone mass remains constant, an indication that the rates of bone deposit and resorption are essentially equal. - remodeling does not occur uniformly.
3. Endochondral ossification. The periosteal bud invades the internal cavities and spongy bone forms.
- in month 3, the forming cavities are invaded by a collection of elements called the periosteal bud, which contains a nutrient artery and vein, nerve fibers, red marrow elements, osteogenic cells and osteoclasts. - osteoclasts: partially erode the calcified cartilage matrix. - osteogenic cells: become osteoblasts and secrete osteoid around the remaining calcified fragments of hyaline cartilage, forming bone-covered cartilage trabeculae. - where the earliest version of a spongy bone forms in a developing long bone.
Depressions and openings (bone markings)
- include fossae, sinuses, foramina and grooves. - allow nerves and blood vessels to pass.
Organic Components of Bone
- includes osteogenic cells, osteoblasts, osteocytes, bone-lining cells, osteoclasts, and osteoid.
Postnatal Bone Growth
- infancy and youth: long bones lengthen entirely by interstitial growth of the epiphyseal plate cartilage and its replacement by bone, and all bones grow in thickness by appositional growth. - most bones stop growing during adolescence. - some facial bones, nose and lower jaw, continue to grow throughout life.
Osteomalacia and rickets are caused by
- insufficient calcium in the diet or by a vitamin D deficiency. - can be cured by drinking vitamin D fortified milk and exposing skin to sunlight, which spurs the body to form Vitamin D.
Perforating (Volkmann's) Canals - Compact Bone
- lie at right angle to the long axis of the bone. - connect the blood and nerve supply of the periosteum to those in the central canals and the medullary cavity. - not surrounded by concentric lamellae, but lines with endosteum.
Circumferential lamellae - Compact Bone
- located just deep to the periosteum and just superficial to the endosteum. - extend around the entire circumference of the diaphysis. - effectively resist twisting of the long bone.
Long Bones
- longer than they are wide. - has a shaft plus 2 ends which are often expanded. - all limb bones expect the patella (kneecap) and the wrist and ankle bones are long bones. - named for their elongated shape, not their overall size. examples: femur, tibia, fibula, humerus, ulna, radius, phalanges.
Hyaline Cartilage
- looks like frosted glass when freshly exposed. - provide support with flexibility and resilience. - most abundant skeletal cartilage. - chondrocytes are spherical. - only fiber type in their matrix is fine collagen fibers.
Skeletal Cartilages
- made up of cartilage tissue molded to fit its body location and function. - consist primarily of water. - surrounded by perichondrium because cartilage contains no nerves or blood vessels.
Where is Paget's disease usually localized?
- may affect any part of the skeleton, but is usually localized. - spine, pelvis, femur and skull are most involved and become deformed and painful. - rarely occurs before 40 and affect 3% of N.A. elderly people. - cause is unknown, but a virus may trigger it. - drug therapies include calcitonin (nasal inhaler) and bisphosphonates, which prevent bone breakage.
Bone Growth in Length - epiphysis and diaphysis
- mimics endochondral ossification and depends on presence of epiphyseal cartilage. - cartilage is inactive on the side of the epiphyseal plate facing the epiphysis, a region called the resting or quiescent zone. - epiphyseal plate cartilage abutting the diaphysis organizes into a patter that allows fast, efficient growth. - the cartilage cells form tall columns. - cell at the top, epiphysis-facing side of the stack abutting the resting zone comprise the proliferation or growth zone. - these cells divide quickly, pushing the epiphysis away from the diaphysis and lengthen the entire long bone.
Qualitative changes in Age-Related Changes in Bone
- more osteons remain incompletely formed. - mineralization is less complete. - the amount of nonviable bone increases, reflecting a diminished blood supply to the bones in old age. - electrical stimulation of fracture sites and daily ultrasound treatments hasten repair and healing. - electrical fields inhibit PTH stimulation of osteoclasts and induce formation of growth factors that stimulate osteoblasts.
Interstitial Lamellae - Compact Bone
- not part of complete osteons. - lying between intact osteons and are incomplete lamellae. - fill gaps between forming osteons or are remnants of osteons that have been cut through by bone remodeling.
Lacunae - Compact Bone
- occupied by spider-shaped osteocytes at the junctions of the lamellae.
Bone Growth in Length - chondrocytes
- older chondrocytes in the stack, which are closer to the diaphysis (hypertonic zone), hypertrophy and their lacunae erode and enlarge, leaving large interconnecting spaces. - the surrounding cartilage matrix calcifies and these chondrocytes die and deteriorate, producing the calcification zone.
Osteoid
- organic part of the matrix. - makes up approximately 1/3 of the matrix. - includes ground substance (proteoglycans and glycoproteins) and collagen fibers, both of which are made and secreted by osteoblasts. - collagen contribute both to a bone's structure and to the flexibility and tensile strength that allow it to resist stretch and twisting.
2. Intramembranous Ossification. Osteoid is secreted within the fibrous membrane and calcifies.
- osteoblasts is secreted within the fibrous membrane and calcifies. - trapped osteoblasts become osteocytes.
Hormonal Controls of Bone
- primarily involves parathyroid hormone (PTH) produced by the parathyroid gland. - calcitonin, produced by parafollicular cells of the thyroid gland, may also be involved. - hormonal controls act to preserve blood calcium levels.
The formation of a long bone typically begins in the center of the hyaline cartilage shaft at a region called the
- primary ossification center. - 1st, blood vessels infiltrate the perichondrium covering the hyaline cartilage, converting it to a vascularized periosteum. - as a result of this change in nutrition, the underlying mesenchymal cells specialize into osteoblasts. The stage is now set for ossification to begin.
Bone Markings
- projections, depressions, and openings that serves as sites of muscle, ligament, and tendon attachment, as joint surfaces, or as conduits for blood vessels and verves.
Spiral Fracture
- ragged break occurs when excessive twisting forces are applied to a bone; - common sports fractures
Hematopoietic Tissue
- red marrow. - found within the trabecular cavities of spongy bone of long bones and in the diploe of flat bones. Therefore, both these cavities are called red marrow cavities. - newborn: medullary cavity of the diaphysis and all areas of spongy bones contain red bone marrow.
Osteoporosis
- refers to a group of diseases in which bone resorption outpaces bone deposit. - the bone becomes very fragile. - the composition of the matrix remains normal but bone mass declines, and the bones become porous and light. - affects entire skeleton, but spongy bone of the spine is most vulnerable and compression fractures of the vertebrae are common. - the neck of the femur is very susceptible to fracture, a broken hip.
Control of Remodeling of Bone
- remodeling goes on continuously, regulated by genetic factors and 2 control loops. - one is a negative feedback hormonal loop that maintains Ca2+ homeostasis in the blood. - the other involves responses to mechanical and gravitational forces acting on the skeleton.
Benefits of Bone Remodeling
- removes injured bone. - if bone is subject to heavy loads, it will grow thicker. - shape of a bone can be altered for proper support based on the stress patterns. - new bone is more resistant to fracture than old bone.
Secondary ossification
- reproduces almost exactly the events of primary ossification, except that the spongy bone in the interior is retained and no medullary cavity forms in the epiphyses.. - when it is complete, hyaline cartilage remains only at 2 places: 1. On epiphyseal surfaces, as the articular cartilages. 2. At the junction of the diaphysis and epiphysis, where it forms the epiphyseal plates.
Elastic Cartilage
- resembles hyaline cartilage but have more stretchy elastic fibers and so are better able to stand up to repeated bending. - only found in two skeletal locations: 1. External ear. 2. Epiglottis - the flap that bends to cover the opening of the larynx each time we swallow.
When actively resorbing (breaking down) bone, the osteoclasts
- rest in a shallow depression called a resorption bay and exhibit a distinctive ruffled border which directly contacts the bone. - the deep plasma membrane of the ruffled border tremendously increase the surface area for enzymatically degrading the bones and seal off that area from the surrounding matrix.
Central (Haversian) Canals - Compact Bone
- running through the core of each osteon. - contain small blood vessels and nerve fibers that serves the osteon's cells. - surrounded by concentric lamellae.
Where does bone's resilience come from?
- sacrificial bonds in or between collagen molecules. - these bonds stretch and break easily on impact, dissipating energy to prevent the force from rising to a fracture value. - in the absence of continued or additional trauma, most to of the sacrificial bonds re-form.
2. Fibrocartilaginous callus forms. Repair of Bone Fracture.
- soft granulation tissue or soft callus forms. - capillaries grow into the hematoma and phagocytic cells invade the area and clean up debris. - fibroblasts, cartilage and osteogenic cells invade the fracture site from the periosteum and endosteum and begin reconstructing the bone. - fibroblast: produce collagen fibers that span the break and connect the broken bone ends. - precursor cells differentiate into chondroblasts that secrete cartilage matrix. - osteoblasts begin forming spongy bone. - cartilage cells farthest from the capillaries secrete an externally bulging cartilaginous matrix that later calcifies. - entire mass repair tissue, called fibrocartilaginous callus, splints the broken bone.
Sesamoid Bones
- special type of short bone that form in a tendon. - vary in size and number in different individuals. - some act to alter the direction of pull of a tendon. example: patella
Osteocytes
- spidery mature bone cells. - occupy spaces (lucunae) that conform to their shape. - monitors and maintains bone matrix. - if they die: surrounding matrix is resorbed.
Osteon (Haversian System) - Compact Bone
- structural unit of compact bone. - an elongated cylinder oriented parallel to the long axis of the bone. - functionally are tiny weight-bearing pillars.
Male and Female sex hormones in growth
- testosterone and estrogens are released in increasing amounts in puberty. - initially, these hormones promote growth spurt, as well as masculinization and feminization of specific parts of the skeleton. - later, the hormones induce epiphyseal closure, ending longitudinal bone growth. - excess or deficits of these hormones can result in abnormal skeletal growth.
Red marrow in adult long bones
- the fat containing medullary cavity extends well into the epiphysis. - little red marrow is present in the spongy bone cavities. - for this reason, blood cell production in adult long bones routinely occurs only in the heads of the femur and humerus.
Metaphysis
- the flared portion of a long bone, where the diaphysis and epiphysis meet, whether it is the epiphyseal plate or line.
Ossification and Osteogenesis
- the process of bone formation. - in embryos: leads to the formation of the bony skeleton. - bone growth: form of ossification that goes on until early adulthood as the body increases in size. - bones are capable of growing thicker throughout life. - in adults: mainly for bone remodeling and repair.
As blood concentrations of calcium rise
- the stimulus of PTH release ends. - the decline of PTH reverse its effects and causes blood Ca2+ levels to fall. - calcitonin lower blood calcium levels temporarily.
Structure of Short, Irregular, and Flat Bones
- they all consist of thin plates of spongy bone covered by compact bone. - these plates are covered outside - periosteum - and inside - endosteum - by connective tissue membranes. - they are not cylindrical so they have no shaft or epiphyses. - they contain bone marrow between their trabeculae but no well defined marrow cavity. - where they form movable joints with their neighbors, hyaline cartilage covers their surface.
Flat Bones
- thin, flattened and usually a bit curved. examples: sternum (breastbone), ribs, scapulae (shoulder blade), most skull bones.
When the Ca2+·Pi product reaches a certain level
- tiny crystals of hydroxyapatite form spontaneously and catalyze further crystallization of calcium salts in the area.
4. Intramembranous Ossification. Lamellar bone replaces woven bone, just deep to the periosteum. Red marrow appears.
- trabeculae just deep to the periosteum thicken. Mature lamellar bone replaces them, forming compact bone plates. - spongy bone (diploe) consisting of distinct trabeculae, persists internally and its vascular tissue becomes red marrow.
Fracture Treatment and Repair
- treatment involves reduction: realignment of broken bone ends. - closed (external) reduction: physician's hands coax the bone ends into position. - open (internal) reduction: the bone ends are secured together surgically with pins or wires. - immobilization of bone by cast or traction is needed for healing. - time needed for repair depends on break severity, bone broken, and age of patient.
1. A hematoma forms. Repair of Bone Fracture
- when a bone breaks, blood vessels in the bone, periosteum and surrounding tissues are torn and hemorrhage. - as a result, a hematoma - a mass of clotted blood - forms at the fracture site. - bone cells deprived of nutrition die, and the tissues at the site becomes swollen, painful and inflammed.
Role of Matrix on Osteoblasts
- when actively depositing matrix: cube shaped. - when inactive: resemble flattened osteogenic cells or may differentiate into bone lining cells. - when become completely surrounded by matrix being secreted: become osteocytes.
How are canaliculi formed?
- when bone matrix is forming: osteoblasts secreting bone matrix surround blood vessels and maintain contact with one another and local osteocytes by tentacle-like projections containing gap junctions. - then, as newly secreted matrix hardens and the maturing cells become trapped within it, the canaliculi filled with tissue fluid and containing the osteocyte extensions is formed.
3. Bony callus forms. Repair of Bone Fracture.
- within a week, new bone trabeculae appear in the fibrocartilaginous callus and gradually convert it to bony (hard) callus of spongy bone. - bony callus formation continues until a firm union forms about 2 months later. - this process generally repeats the events of endochondral ossification.
Perichondrium
-dense irregular connective tissue that contains blood vessels and surrounds skeletal cartilage. -acts like a girdle to resist outward expansion when the cartilage is compressed.
Steps in Endochondral Ossification
1. A bone collar forms around the diaphysis of the hyaline cartilage model. 2. Cartilage in the center of the diaphysis calcifies and then develops cavities. 3. The periosteal bud invades the internal cavities and spongy bone forms. 4. The diaphysis elongates and a medullary cavity forms. 5. The epiphyses ossify.
Hyaline cartilage includes:
1. Articular cartilages - cover the ends of most bones at movable joints. 2. Costal cartilages - connects the rib to the sternum (breastbone). 3. Respiratory cartilages - form the skeleton of the larynx (voicebox) and reinforce other respiratory passageways. 4. Nasal cartilages - support the external nose.
3 types of Cartilage
1. Hyaline. 2. Elastic. 3. Fibrocartilage.
Types of Bone
1. Long, 2. Short, 3. Sesamoid, 4. Flat, 5. Irregular.
Steps in Intramembranous Ossification
1. Ossification centers appear in the fibrous connective tissue membrane. 2. Osteoid is secreted within the fibrous membrane and calcifies. 3. Woven bone and periosteum form. 4. Lamellar bone replaces woven bone, just deep to the periosteum. Red marrow appears
Types of Bone Cells
1. Osteogenic cells. 2. Osteoblasts. 3. Osteocytes. 4. Bone Lining cells. 5. Osteoclasts. - all of these, except the osteoclasts, originate from mesenchymal cells. - each is a specialized form of the same basic cell type that transforms to a mature or functional form that serves bone growth in some specific way. - surrounded by extracellular matrix of their making.
Bone Repair - Fracture Classification
1. Position of the bone ends after fracture: In nondisplaced fractures, the bone ends retain their normal position. In displaced fractures, the bone ends are out of normal alignment. 2. Completeness of the break: If the bone is broken through, the fracture is a complete fracture. If not, it is an incomplete fracture. 3. Whether the bone ends penetrate the skin: If so, the fracture is an open (compound) fracture. If not, it is a closed (simple) fracture.
Types of Bone Formation
1. endochondral ossification - a bone develops by replacing hyaline cartilage. The resulting bone is called a cartilage, or endochondral bone. 2. intramembranous ossification - bone develops from a fibrous membrane and the bone is called a membrane bone.
Repair of Bone Fracture
1. formation of fracture hematoma. 2. fibrocartilaginous callus formation. 3. bony callus formation. 4. bone remodeling.
Functions of Bones
1. support. 2. protection of organs. 3. movement, in conjunction with muscular system. 4. mineral homeostasis. 5. blood cell production. 6. triglyceride storage. 7. hormone production (osteocalcin).
The human body contains (blank) g of calcium, more than (blank) present as bone minerals.
1200-1400, 99%. - less than 1.5 g is present in blood, and the hormonal control loop normally maintains blood Ca2+ within the range of 9-11 mg per dl (100 ml) of blood. - calcium is absorbed from intestines under the control of vitamin D metabolites. - the daily dietary calcium requirement is 400-800 mg from birth until age 10, and 1200-1500 mg from ages 11 to 24.
The (blank) named bones of the human skeleton are divided into two groups:
206. Groups: 1. Axial. 2. Appendicular.
Osteoid Seam
An un-mineralized band of gauzy-looking bone matrix 10-12 micrometers wide -- marks areas of new matrix deposits by osteoblasts.
Check Your Understanding - What is the status of bone structure at birth?
At birth, most bones are well formed and ossified except for their epiphyses.
Check Your Understanding - How do bone growth and bone remodeling differ?
Bone growth increases bone mass, as during childhood or when exceptional stress is placed on the bones. Bone remodeling follows bone growth to maintain the proper proportions of the bone considering stresses placed upon it.
Check Your Understanding - Describe two functions of a bone's marrow cavities.
Bone marrow cavities serve as sites for blood cell formation and fat storage.
Check Your Understanding - Which component of bone—organic or inorganic—makes it hard?
Bone's inorganic component (bone salts) makes it hard.
Check Your Understanding - Bones don't begin with bone tissue. What do they begin with?
Bones begin as fibrous membranes or hyaline cartilages.
Functions of Bones: Mineral and Growth Factor Storage
Bones is a reservoir for minerals - calcium and phosphate. Stored minerals are released in their ionic form as needed for distribution to all parts of the body. Deposits and withdrawals of minerals to and from the bones go on almost continuously. Mineralized bone matrix stores important growth factors.
Function of Bones: Hormone Production
Bones produce osteocalcin - hormone that helps regulate bone formation and protects against obesity, glucose intolerance and diabetes mellitus.
Check Your Understanding - Are crests, tubercles, and spines bony projections or depressions?
Bony projections.
(blank) is the basic component of all cartilage
Chondrocytes encased in small cavities, lucunae, within an extracellular matrix containing a jelly-like ground substance and fibers.
Check Your Understanding - How does the structure of compact bone differ from that of spongy bone when viewed with the naked eye?
Compact bone looks fairly solid and homogeneous whereas spongy bone has an open network of bone spicules.
In flat bones, the spongy bone is called the
Diploe and the whole arrangement resembles a sandwich.
Check Your Understanding - Which membrane lines the internal canals and covers the trabeculae of a bone?
Endosteum.
Check Your Understanding - What two body structures contain flexible elastic cartilage?
Epiglottis and external ear.
Bone Textures: Compact and Spongy Bone
Every bone has a dense external layer that looks smooth and solid - compact bone. Spongy bone (trabecular bone) - a honeycomb of small needle-like or flat pieces called trabeculae. Open spaces between trabeculae are filled with red or yellow bone marrow.
Function of Bones: Triglyceride (fat) storage
Fat, a source of energy for the body, is stored in bone cavities.
Check Your Understanding - What bone class do the ribs and skull bones fall into?
Flat bones.
Function of Bones: Support
Form the framework that supports the body and cradles soft organs. Bones of lower limbs act as pillars to support the body trunk when we stand and the rib cage supports the thoracic wall.
Function of Bones: Protection
Fused bone of the skull - protect brain. Vertebrae - surrounds the spinal cord. Rib cage - protects vital organs of the thorax.
Check Your Understanding - Cartilage grows by interstitial growth. What does this mean?
Growth from within.
Check Your Understanding - Which type of cartilage is most plentiful in the adult body?
Hyaline.
Check Your Understanding - If osteoclasts in a long bone are more active than osteoblasts, how will bone mass change?
If bone-destroying cells, osteoclasts, are more active than bone-forming cells, osteoblasts, bone mass will decrease.
Check Your Understanding - How does an open fracture differ from a closed fracture?
In an open fracture, the bone ends are exposed to the external environment. In a closed fracture, the bone ends do not penetrate the external boundary of the skin.
Check Your Understanding - What two types of substances are stored in bone matrix?
Minerals and growth factors.
Function of Bones: Blood Cell Formation
Most blood cell formation, or hematopoiesis, occurs in the red marrow cavities of certain bones.
1. Endochondral Ossification. A bone collar forms around the diaphysis of the hyaline cartilage model.
Osteoblasts of the newly converted periosteum secrete osteoid against the hyaline cartilage diaphysis, encasing it in a cuff or collar of bone called the periosteal bone collar.
Check Your Understanding - Which cell has a ruffled border and acts to break down bone matrix?
Osteoclasts.
The inner layer of periosteum
Osteogenic layer that gives rise to all bone cells except bone destroying cells.
Check Your Understanding - What name is given to "adult rickets"?
Osteomalacia.
Check Your Understanding - Which bone disorder is characterized by excessive deposit of weak, poorly mineralized bone?
Paget's disease.
Role of water in cartilage
Resilience - its ability to spring back to its original shape after being compressed.
1. Intramembranous Ossification. Ossification centers appear in the fibrous connective tissue membrane.
Selected centrally located mesenchymal cells cluster and differentiate into osteoblasts, forming an ossification center that produces the first trabeculae of spongy bone.
Structure of a Typical Long Bone
Shaft - diaphysis, Bone ends - epiphyses, and Membranes - periosteum
Check Your Understanding - What is the functional relationship between skeletal muscles and bones?
Skeletal muscles use bones as levers to cause movement of the body and its parts.
Function of Bones: Movement
Skeletal muscles, which attach to bones by tendons, use bones as levers to move the body and its parts. As a result, we can walk, grasp objects and breather. The design of joints determines the type of movement possible.
Check Your Understanding - What are three measures that may help to maintain healthy bone density?
Sufficient vitamin D, calcium, and weight-bearing exercise all help to maintain healthy bone density.
Check Your Understanding - When describing endochondral ossification, some say "bone chases cartilage." What does that mean?
The cartilage model grows, then breaks down and is replaced by bone.
Check Your Understanding - As a long bone grows in length, what is happening in the hypertrophic zone of the epiphyseal plate?
The chondrocytes are enlarging and their lacunae are breaking down and leaving holes in the cartilage matrix.
Check Your Understanding - Which stimulus—PTH (a hormone) or mechanical forces acting on the skeleton—is more important in maintaining homeostatic blood calcium levels?
The hormonal stimulus maintains homeostatic blood calcium levels.
Check Your Understanding - Contrast the general function of the axial skeleton to that of the appendicular skeleton.
The major function of the axial skeleton is to establish the long axis of the body and to protect structures that it encloses. The general function of the appendicular skeleton is to allow us mobility for propulsion and manipulation of our environment.
Because the osteoid seam is always of constant width and the change from un-mineralized to mineralized matrix is sudden
The osteoid must mature for about a week before it can calcify. - mechanical signals are involved in the calcification. - once critical factor is the product of the local concentrations of calcium and phosphate ions in the endosteal cavity. Other factors: - matrix proteins that bind and concentrate calcium. - the enzyme alkaline phosphate, shed in matrix vesicles by the osteoblasts, which is essential for mineralization.
What limits cartilage thickness?
The perichondrium that contains blood vessels from which nutrients diffuse through the matrix to reach the cartilage cells internally. This mode of nutrition limits cartilage thickness.
Nutrient foramina of periosteum
The periosteum is richly supplied with nerve fibers and blood vessels, which pass through the shaft to enter the marrow cavity.
Check Your Understanding - Where is the primary ossification center located in a long bone? Where is (are) the secondary ossification center(s) located?
The primary ossification center in a long bone is in the center of the shaft. The secondary ossification centers are in the epiphyses (bone ends).
Check Your Understanding - The decrease in bone mass that begins in the fourth decade of life affects nearly all bones. What are the exceptions?
The skull bones do not appear to lose mass with aging.
Check Your Understanding - What are the components of the axial skeleton?
The skull, the vertebral column, and the thoracic cage.
Why are bones organs?
They contain different type of tissues. Bone (osseous) tissue dominates bones, but they also contain nervous tissues in their nerves, cartilage in their articular cartilages, fibrous connective tissue lining their cavities and muscle and epithelia tissues in their blood vessels.
Femur
Withstands great pressure and its hollow cylinder design provides maximum strength with minimum weight to accommodate our upright position.
Hyperexcitability
blood calcium levels too low.
Open (Compound) Fracture
broken ends of the bone protrude through the skin
Under certain conditions, during normal bone growth in youth and during old age, cartilage can become _____________.
calcified - hardened due to deposit of calcium salts.
The outer fibrous layer of the periosteum is
dense irregular connective tissue
Most irregular bones develop from several (blank)
distinct ossification centers.
Closed (Simple) Fracture
does not break the skin
Articular Cartilage
hyaline, at joint
Medullary Cavity
marrow cavity
Sutural Bones
small bones located in sutures of cranial bones
Why is the bone collar important?
the bone collar holds up and stabilizes the cartilage model as the osteoblasts secrete osteoid against hyaline cartilage diaphysis
Longitudinal bone growth ends when
the bone of the epiphysis and diaphysis fuses. This process, called epiphyseal plate closure, happens at about 18 years of age in females and 21 years of age in males. Once this has occurred, only the articular cartilage remains in bones. an adult bone can still widen by appositional growth if stressed by excessive muscle activity or body weight.
When resorption of a given area of bone is completed
the osteoclasts undergo apoptosis.
During growth, the epiphyseal plate maintains a constant thickness because
the rate of cartilage growth on its epiphysis-facing side is balanced by its replacement with bony tissue on its diaphysis-facing side.
What are perforating (Sharpey's) fibers of the periosteum?
tufts of collagen fibers that extend from its fibrous layer into the bone matrix and secure the periosteum to the underlying bone. Extremely dense.
Wolff's law also explains several other observations:
■ Handedness, being right or left handed, results in the bones of one upper limb being thicker than those of the less-used limb. Vigorous exercise of the most-used limb leads to large increases in bone strength. ■ Curved bones are thickest where they are most likely to buckle. ■ The trabeculae of spongy bone form trusses, or struts, along lines of compression. ■ Large, bony projections occur where heavy, active muscles attach. The bones of weight lifters have enormous thickenings at the attachment sites of the most-used muscles. ■ Featureless bones of the fetus and the atrophied bones of bedridden people—situations in which bones are not stressed.
Hormonal and Mechanical control of the skeleton
■ Hormonal controls determine whether and when remodeling occurs in response to changing blood calcium levels. ■ Mechanical stress determines where remodeling occurs. - when bone must be broken down to increase blood calcium levels, PTH is released and targets the osteoclasts. - mechanical forces determine which osteoclasts are most sensitive to PTH stimulation, so that bone in the least stressed areas (which is temporarily dispensable) is broken down.
Several other factors can contribute to osteoporosis:
■ Petite body form. ■ Insufficient exercise to stress the bones. ■ A diet poor in calcium and protein. ■ Abnormal vitamin D receptors. ■ Smoking (which reduces estrogen levels). ■ Hormone-related conditions such as hyperthyroidism, low blood levels of thyroid-stimulating hormone, and diabetes mellitus.
