Unit 6: Bones & Skeletal Tissue PDF

Unit 6: Bones & Skeletal Tissue Tags function support holds up body cradles organs protection central nervous system s...

Unit 6: Bones & Skeletal Tissue Tags function support holds up body cradles organs protection central nervous system skull protects brain vertebrae wrap around spinal cord visceral organs rib cage wraps around organs in thorax and upper abdominal cavity attachment point skeletal muscle attaches to bone via tendons storage minerals fat yellow marrow in adult bones blood cell formation hematopoiesis: formation of blood cells in red bone marrow hormone production osteocalcin: regulates insulin release, glucose homeostasis, and energy expenditure if it is increased, it increases insulin release Unit 6: Bones & Skeletal Tissue 1  cartilage & skeleton formation before osseous skeleton forms in embryo, skeleton is composed of cartilage and connective tissue fibers cartilage cells develop faster characteristics strength and resilience can be compressed and return to original shape cartilage is mostly water so it is flexible no nerve supply and is avascular surrounded externally by fibrous connective tissue called perichondrium it contains blood vessels and resists outward expansion of cartilage all cartilage has two basic components, chief cell type is chondrocyte types of cartilage hyaline cartilage most abundant type chondrocytes are spherical contains collagen fibers ex: articular cartilage (surrounding ends of bones), costal cartilage (where ribs join to sternum), respiratory cartilage, nasal cartilage (helps keep nasal passageways open) also exist within the lungs to keep them from collapsing elastic cartilage similar to hyaline but contains more elastic fibers more flexible ex: external ear, epiglottis Unit 6: Bones & Skeletal Tissue 2  fibrocartilage contains rows of chondrocytes alternation with thick collagen bands most compressible, great tensile strength can push and pull and will likely not be damaged found where a lot of weight is bore ex: vertebral discs, knee, pubic symphysis knee joint has this cartilage because knees are basically bearing weight of entire body types of cartilage growth appositional laying down new cartilage on old cartilage like laying down bricks cells just under perichondrium deposit new matrix on top of old cartilage occurs at surface of cartilage tissue growth in thickness/width interstitial growth from within cells divide and secrete matrix in pre-existing cartilage occurs throughout cartilage tissue grows in length does not occur at surface bone tissue location of bone axial skeleton: makes up long axis of body Unit 6: Bones & Skeletal Tissue 3  skull, vertebral column, ribs mostly protective in function appendicular skeleton: makes up limbs and girdles pectoral and pelvic girdle, arms, legs important for movement and mobility shape of bone long bones longer than wide ex: almost all limb bones short bones: cube shaped ex: bones in wrists (carpal) and ankles (tarsal) sesamoid bones: bone that forms IN a tendon (produces tension in a tendon) flat bones: thin, flat, curved ex: sternum, scapulae, ribs, most cranial bones irregular bones: do not fit in the above categories ex: vertebrae, ox coxae, ethmoid, sphenoid gross anatomy of bone all bones contain outer compact bone and inner spongy bone compact bone: looks smooth and solid spongy (trabecular) bone: has open spaces with needle like pieces of bone called trabeculae open space is filled with red or yellow marrow found mostly along lines of stress flat, irregular, short bone thin plate of spongy bone covered by compact bone Unit 6: Bones & Skeletal Tissue 4  no well defined large cavities for bone marrow covered externally by bone membrane called periosteum long bone diaphysis: bone shaft, composed of compact bone “collar” with internal medullary cavity cavity filled with yellow marrow as adults barely any spongy bone in diaphysis epiphysis: rounded ends of bone composed of compact bone externally, spongy bone internally forms joint surfaces ends are covered with hyaline cartilage don’t need periosteum here membranes periosteum: covers external bone surface, except at joints double layered outermost layer is fibrous, inner layer composed of osteoprogenitor cells (are stem cells) very well vascularized and innervated endosteum: covers internal bone covers trabeculae in spongy bone, cavities in compact bone like medullary cavity also has osteoprogenitor cells vascularization and innervation nutrient artery and nutrient vein → diaphysis epiphyseal artery and vein → epiphyses nerves travel with blood vessels Unit 6: Bones & Skeletal Tissue 5  microscopic anatomy osteon: structural unit of compact bone function: help bone withstand pressure/stress especially torsion/twisting single osteon composed of several layers (LAMELLA) packed closely together for a single lamella, collagen fibers run in one direction fibers in adjacent lamella run opposite so that it can be twisted in different directions but not too much twisting so they bones wont break central canals run through center of each osteon contains nerves and blood vessels (VERTICAL) perforating canals connect nerve/blood supply of marrow cavity to central canal and allow passageway to neighboring osteons (HORIZONTAL) interstitial lamellae: incomplete lamellae found in between complete osteons function: fill gaps between osteons, makes bones more solid and sturdy circumferential lamellae: found just deep to periosteum extend completely around circumference of diaphysis function: resists twisting of long bone hematopoietic tissue blood forming tissue found in both long and flat/irregular bones flat/irregular bones: marrow found between trabeculae long bones: marrow found in medullary cavity of diaphysis red bone marrow (hematopoietic tissue) in infants: found in all spongy bone and marrow cavities of all diaphyses because they are growing and need to increase body size Unit 6: Bones & Skeletal Tissue 6  in adults: found around trabeculae of bones of skull, ribs, hips, sternum, clavicles, scapula, vertebrae, heads of femur and humerus converts to yellow at end of growth yellow marrow in adults: found in diaphysis of long bones contains more fat and less blood supply than red marrow can be converted back to red marrow has less vasculature if someone is anemic enough, yellow will convert back to red to get more oxygen, last ditch effort, once resolved it will revert back to yellow losing blood in large amounts, yellow will convert to red as well cellular composition of bone osteoprogenitor cells stem cell: undifferentiated cell type mitotically active, will divide into daughter cells that either remain as these cells or are differentiated into osteoblasts osteoblasts -blast = immature cell type, lay down ground substance and fibers bone forming cells secrete unmineralized matrix (OSTEOID) that forms bone tissue activity of cell depends on shape cube shaped → secreting matrix flattened/squamous → inactive only secrete matrix until it has surrounded itself then transforms into an osteocyte Unit 6: Bones & Skeletal Tissue 7  osteocytes mature bone cells monitor and maintain bone matrix respond to mechanical stress on bone ex: weightlessness, bone loading chemical signals ex: calcium levels in bone often have several projections of membrane surface if bones become too hard they can break easier often have several projections of membrane surface that they use to touch osteocytes and osteoclasts osteoclasts bone degrading cells portion of cell contacts bone directly produces degrading enzyme collagenase that breaks down the bone/collagen fibers functions: maintains, repairs, and remodels bones, important function in blood calcium homeostasis every 7-8 years you get a completely new skeleton chemical composition of bone organic cells and osteoid sacrificial bonds in or between collagen molecules stretch and break easily inorganic Unit 6: Bones & Skeletal Tissue 8  mineral salts → mostly calcium phosphate packed around collagen fibers osteomalacia (adults) or Ricket’s (children) less mineral salts deposited in bone bone becomes weak and soft, bend and break more easily caused by insufficient calcium in diet or vitamin D deficiency adult skeleton does not have that much damage because it is fully formed children have growing skeletons that is bent abnormally so it can lead to bone deformities bone ossification two types: endochondral formation of ossified bone by replacement of cartilage with bone tissue more frequent form occurs in most bones below the skull hyaline cartilage used as a blueprint to form ossified bone and is eventually broken down and replaced with bone step 1: formation of bone collar starts around week 9 of fetal development cells of perichondrium specialize into osteoblasts they lay down bone matrix against cartilage surface formation of primary ossification center (POC) occurs after bone collar formation, found in center of diaphysis POC is rigid and tough on outside step 2: cavity forms in diaphysis center cartilage inside POC is calcified Unit 6: Bones & Skeletal Tissue 9  cells inside POC dies off, no diffusion! matrix inside POC deteriorates and a cavity is created inside diaphysis cartilage outside the bone continues to grow cartilage is heavy, so bone collar prevents it from collapsing because the epiphysis are heavy and it holds diaphysis in place step 3: formation of initial spongy bone in diaphysis month 3 periosteal bud invades cavity contains nutrient artery/vein, nerve fibers, red marrow, osteoprogenitor (stem cells) cells and osteoclasts osteoblasts secrete matrix around calcified inside cavity initial formation of spongy bone is bone covered CARTILAGE trabeculae step 4: formation of medullary (marrow) cavity and elongation of diaphysis birth initial spongy bone broken down by osteoclasts and medullary cavity is formed cartilage is calcified, broken down, replaced spreads throughout the diaphysis born with cartilaginous epiphysis step 5: secondary ossification begins in epiphyses similar to primary ossification cartilage deteriorates and bone forms however, spongy bone is retained there will still be actively growing cartilage intramembranous no cartilage, formation of bone from fibrous membrane Unit 6: Bones & Skeletal Tissue 10  forms cranial bones of skull, clavicles (flat bones) step 1: mesenchymal cells in membrane differentiate into osteoblasts to form ossification center osteoblasts lay down osteoids forms a clump of osteoblasts, which acts as the center step 2: matrix secreted by osteoblasts matrix is calcified after it is secreted osteoblasts trapped in calcified matrix form osteocytes step 3: formation of periosteum and immature spongy bone blood vessels invade area of calcified matrix osteoids have to be laid around the blood vessels creates trabeculae called “woven bone” mesenchyme at external surface of initial bone forms periosteum the cells will condense, forming a fibrous layer → periosteum step 4: compact bone replaces some spongy bone, red marrow develops trabeculae just deep to periosteum is replaced with compact bone at center, immature spongy bone remodeled to mature spongy bone mature spongy bone fills with red marrow bone growth growth in length: accomplished by interstitial growth (cartilage will be the same) interstitial: growing from within, increasing volume occurs at epiphyseal plate: cartilage plate found between epiphysis and diaphysis of bone three regions of epiphyseal plate Unit 6: Bones & Skeletal Tissue 11  active region: actively dividing and laying down matrix/new cartilage, found closer to epiphyses than diaphysis it is physically pushing epiphysis farther from diaphysis resting region: towards center of plate, cells are not dividing, they hypertrophy and calcify, so they do not survive and die off closest to plate, bone cells, osteoblasts cells lay down matrix, osteoclasts break down the cartilage bone growth continues through adolescents until about 18-21 at end of adolescence, chondrocytes divide less often new cartilage at epiphyseal plate is no longer produced epiphysis and diaphysis fuse together with bone, called the epiphyseal plate closure growth in width: appositional growth have to occur at same time as lengthening to ensure bone is at the thickness it should be, otherwise the bones would snap on outside: osteoblasts secrete new matrix just deep to periosteum on inside: osteoclasts break down bone tissue at the endosteum osteoblasts occur at a faster rate to ensure growth bone growth and hormones growth hormone controls activity at epiphyseal plate released by anterior pituitary gland in brain hyper-secretion of growth → gigantism, epiphyseal plate will be overstimulated hypo-secretion of growth → dwarfism helps to remain epiphyseal plate remains active Unit 6: Bones & Skeletal Tissue 12  does not have that much of an effect on changing shape of bones sex hormones influence bone shape and growth estrogen causes growth spurt at puberty in high levels → induce epiphyseal plate closure females will be taller than males because women have more estrogen growth spurt earlier but will stop growing before males do epiphyseal plate usually closes around 18 female hip bones are set wider and pubic angle is greater than 90 deg testosterone causes masculinization of certain parts of skeleton less wide pelvic girdles, pubic angle less than 90 deg mandible is usually thicker in males epiphyseal usually closes around 21 bone remodeling bone deposition and resorption involved in bone remodeling new bone continuously laid down, old bone tissue resorbed by body maintenance of Ca2+ homeostasis have to maintain calcium levels which is important for nervous system function → allowing neurons to communicate also important for muscle tissue contraction puts mechanical stress on skeleton, body can remodel to withstand it importance of bone remodeling control of deposition and resorption Unit 6: Bones & Skeletal Tissue 13  1. parathyroid hormone (PTH) released in response to decreasing blood Ca2+ levels effect: number of osteoclasts at bone increases, begin to resorb more bone tissue this is so that the calcium salts will be released to the blood stream once level is normal, PTH decreases, only care about calcium numbers 2. mechanical stress wolff’s law: bones are strongest where they are subject to higher pressure or greater stress (bone loading) more trabecular bone and thicker compact bone in those areas big muscle = big tendons = big bones bone density decreases after menopause due to decrease in estrogen for WOMEN astronauts skeletons become weaker because no gravity bone repair must be reduced for repair to begin hairline fracture: not broken, just a crack doctor will line the bones up and then immobilized (casts, boots) 1. hematoma forms: the blood vessels also break when the bone breaks 2. fibrocartilaginous callus forms: new blood vessels form and bring in macrophages (phagocytosis, clean out the blood) and cartilage cells (chondroblasts lay down cartilage tissue). the callus is the body’s splint, physically holding bones in place 3. bony callus forms: same as bone growth, the cartilage dies and is replaced with bone, here bone is no longer broken 4. bone remodeling occurs: must be remodeled to withstand normal stressors, for ex need to reform medullary cavity and compact bone Unit 6: Bones & Skeletal Tissue 14  in xray, there is a swollen nodule which is shaved down to be flat, but can still tell it was broken before if they do not do anything to strengthen the bone after healed, the bone itself will be weaker 💡 hyaline cartilage and epiphyseal plate are maintained into adulthood Unit 6: Bones & Skeletal Tissue 15