OSSEOUS TISSUESKELETAL STRUCTURE
Skeletal System• 206 bones, cartilage, ligaments, and connective tissues • Functions: • support
– provides a rigid framework• storage
– calcium & phosphorus– lipids
• production of blood cells– formed in red marrow
• protection– brain is encased in skull– heart and lungs are surrounded by boney sternum and rib cage
• leverage– allows for movement due to interaction of muscular & skeletal
systems• acid-base balance
– absorbs or releases alkaline salts
Divisions of the Skeletal System• Axial skeleton
– consists of bones forming axis of the body– Skull– Hyoid– Sternum– Ribs– Vertebrae– sacrum & cocyx– auditory ossicles (not a part of either; but put here by
convention)• Appendicular skeleton
• consists of bones that anchor appendages to axial skeleton• upper & lower extremities, shoulder and pelvic girdles
Types of Bones
• Long• Flat• Short• Irregular• Sesamoid• Sutural
Long Bones• longer than wide• function as levers• act on skeletal
muscles to produce movements
• found in appendages
• fingers & toes
Short Bones• boxy & small• nearly cube-shape• found in wrist-carpals• ankle-tarsals• limited movements
Flat Bones• thin • roughly parallel surfaces• found in the roof of the
skull• sternum, ribs & scapula• enclose & protect soft
organs• provide broad surfaces for
muscle attachment
Irregular Bones• bones that do not fall into any other
category• varied, complex shapes, sizes & surface
features• vertebrae, sacrum, coccyx, temporal,
sphenoid, ethmoid, zygomatic, maxilla, mandible, palatine, inferior nasal concha, & hyoid
Sesamoid Bones• shaped like sesame
seeds develop in areas where there is a great deal of friction
• most only a few mms number in each person differs
• patella present in everyone
Sutural Bones• also called
Wormian bones• small• located in sutures • classified by
location-not by shape
Bone Composition• Osseous Tissue• supporting connective tissue• Composed of an extra cellular matrix and specialized cells
– give flexibility• two types• compact or dense bone
– dense, hard, & relatively solid– forms protective exterior of all bones
• spongy or cancellous bone– found inside most compact bone– very porous
• full of tiny holes forming open networks of struts & plates– lighter than compact bone
• reduces skeletal weight• makes it easier for muscles to move bones
Extracellular Matrix• composed of collagen fibers &
ground substance• hardened by inorganic calcium
phosphate deposits– called mineralization or
calcification• solid calcium phosphate salts
deposited around protein fibers• Calcium phosphate makes up 2/3rd
of bone weight• Calcium phosphate + calcium
hydroxide hydroxyapatite-Ca10(PO4)6(OH)2
• Calcium phosphate is hard, brittle & inflexible– can withstand compression
• Collagen fibers are stronger than steel, flexible- can be twisted & bent– not good at being compressed
• collagen makes a frame around which calcium minerals deposit
• combination makes bone flexible, strong & resistant to shattering
Bone Cell Types • Osteogenic cells– stem cells produce other
bone cells– found in cellular layer of
periosteum, endosteum & central canals
– continually divide– only bone cell that can divide
• Osteoblasts– bone-forming cells– make organic matter of
bone matrix• Osteocytes
– mature bone cells– most of bone cell population– former osteoblasts that have
become trapped in matrix they have deposited
• Osteoclasts– bone destroying cells
Osteocytes• cannot divide• function to maintain &
monitor protein & mineral content of matrix
• participate in bone repair by converting back into osteoblasts or osteogeneic cells at the site of injury
• sense strain & regulate bone remodeling
Osteoclasts• bone dissolving cells• function to remove bone by osteolysis • secrete acids & proteolytic enzymes
which degrade minerals & fibers and dissolve boney matrix
• releases matrix components into the blood restoring calcium and phosphorus concentrations in body fluids
Types of Bone Tissue• Compact Bone
– dense– covers exterior of all
bones• Spongy Bone
– cancellous– trabecular– inside compact bone– lighter
Compact Bone• basic functional unit -osteon or
Haversian system.• osteocytes are arranged in concentric
circles or layers-lamellae • around a central or Haversian canal
– runs parallel to surface– contains blood vessels
• perforating central canal are Volkmann’s canals
– run perpendicular to surface• canaliculi run through layers
– connect osteocytes to each other• interstitial lamellae fill spaces
between
Spongy Bone• matrix composition-
same• osteocytes, canalicui &
lamellae-different arrangements
• has no osteons • matrix forms plates or
struts called trabeculae (little beams)
• form a thin, branching open network filled with red bone marrow
• makes bone lighter
Bone Type & Bone Tissue Type Location
• the relationship between compact & spongy bone and the relative proportions of each varies with bone shape & with the function of the bone
Long Bone Structure• Diaphysis or shaft-long & cylindrical • Outside made of dense bone
– medullary canal or marrow cavity is filled with marrow
– Yellow bone marrow is dominated by fat cells & red marrow is responsible for forming blood cells
• Epiphysis-expanded extremities at either end of the bone– articulates with other bones-
forming joints– have broad surfaces for muscle
attachment. – filled with cancellous tissue
surrounded by thin layer of compact bone
• Metaphysis– connects diaphysis to epiphysis
Flat Bone Composition• function
– provide protection for underlying structures– broad surfaces for muscle attachment
• function can be seen by structure• resembles a spongy bone sandwich• composed of 2 thin layers of compact bone
covering a layer of spongy bone• bone marrow is present• there is no marrow cavity
Periosteum & Endosteum• Periosteum• covers all portions of compact bone
except at joint cavities• has fibrous outer layer & an inner
cellular layer• isolates bones from surrounding
tissues• provides route for blood vessels &
nerves• participates in bone growth & repair• continuous with other connective
tissues that mesh with-tendons & ligaments
• perforating or Sharpey’s fibers bond tendons & ligaments into the general structure of bone
• endosteum • consists of an incomplete cellular layer• lines marrow cavities• covers trabeculae of spongy bones • lines inner surfaces of central canals• active during bone growth, repair, and
remodeling
Blood & Nerve Supply • bone tissue is highly
vascular• Vessels pass into the
bone through the periosteum
• Periosteal arteries enter via perforating canals
• nutrient artery & vein• enter through a nutrient
foramen located in middle of the bone
Bone Growth• new bone matrix is made through
osteogenesis or ossification• process makes & releases
proteins & other organic components of matrix
• substance is osteoid–bone matrix before calcium salts
have been added• calcium salts are laid down in a
process called calcification
Bone Development & Growth• skeleton begins to form
at 6 weeks post fertilization
• does not stop until around age 25
• develops by two methods
• intramembranous ossification
• endochondral ossification
Intramembranous Ossification bone forms from
mesenchyme or fibrous connective tissue
produces flat bones of skull, most of the facial bones, mandible & medial part of the clavicle
bone develop within a fibrous sheet similar to dermis of the skin
bones are called dermal bones
Intramembranous Ossification Steps
• Step1: Development of Ossification Center
• Step 2: Calcification• Step 3: Formation of Trabeculae• Step 4: Development of
Periosteum
Step1: Development of Ossification Center
• at site where the bone is to form, chemical messages cause mesenchymal cells (embryonic connective tissue) to cluster together into a layer of soft tissue
• cells enlarge & differentiate into osteogenic cells and then into osteoblasts.
• site is the ossification center• osteoblasts begin to secrete
organic matrix• eventually become trapped &
become osteocytes
Step 2: Calcification• Calcium & other salts
deposit on organic extracellular matrix made by osteoblasts
• As trabeculae continue to grow calcium phosphate is deposited
• causes matrix to harden or calcify
Step 3: Formation of Trabeculae• osteoblasts
continue to deposit matrix
• continue to be calcified producing struts of trabeculae
• connective tissue present differentiates into red bone marrow
Step 4: Development of the Periosteum
• Mesenchyme condenses at periphery of the boneperiosteum.
• Trabeculae at surface continue to calcify until spaces between them are filled in converting spongy bone to compact bone
• process gives rise to sandwich like arrangement of flat bones
Intramembranous Ossification
Endochondral Ossification• bone forms by
replacing pre-existing hyaline cartilage model with bone
• most bones are made this way
• begins around sixth week of fetal development
• continues into the 20’s
Endochondral Ossification Steps• Step 1: Development of Hyaline Cartilage
Model• Step 2: Growth of Cartilage Model• Step 3: Development of Primary
Ossification Center• Step 4: Development of Medullary Cavity • Step 5: Development of Secondary
Ossification Centers• Step 6: Formation of Articular Cartilage &
Epiphseal Growth
Step 1: Development of Hyaline Cartilage Model
• at site when bone will form chemical messengers cause mesenchymal cells to crowed together in general shape of future bone
• cells develop into chondroblasts.
• begin to secrete cartilage extracellular matrix which develops into a hyaline cartilage bone covered with a perichondrium
Step 2: Growth of Cartilage Model
• once chondroblasts become embedded in extracellular matrix become chrondrocytes.
• cartilage model continues to grow longer from either end via interstitial or endogenous growth.
• grows in diameter or thickness via appositional or exogenous growth
– new cartilage is laid on the outside of model by chondroblasts
• as model continues to grow chondrocytes in area get larger in the mid-region area & the cartilage matrix begins to calcify
• enlarged chondrocytes are deprived of nutrients due to their size and calcification & diffusion cannot occur
• die and disintegrate• dying leaves spaces which merge into
small cavities called lacunae
Step 3: Development of Primary Ossification Center
• ossification continues inward from surface of bone to inside in the middle of model- primary ossification center
• a nutrient artery penetrates perichondrium
• stimulates osteogenic cells there to become osteoblasts
• once this occurs perichondrium is termed periosteum
• in the primary ossification center most of cartilage will be replaced with bone
• osteoblasts begin to deposit a thin collar of boney matrix around middle of cartilage model forming trabeculae of spongy bone
• primary ossification spreads from central area toward both ends of the cartilage model
Step 4: Development of Medullary Cavity
• as primary ossification center grows osteoclast cells break down some newly formed spongy bone trabeculae
• leaves a cavity• capillaries & fibroblasts migrate
to the inside of the cartilage and take over the spaces left by the dying chondrocytes
• as center is hollowed out & filled with blood and stem cells, it becomes primary marrow cavity.
• region of transition from cartilage to bone at the end of the primary marrow cavity is called the metaphysis
Step 5: Development of SecondaryOssification Centers
• when branches of the epiphyseal artery enter the epiphyses the secondary ossification centers form
• bone formation is similar to as described in the center of the bone
• here however spongy bone remains in the epiphyses
• secondary ossification proceeds outward from center of each epiphysis toward outer surface of the bone
Step 6: Formation of Articular Cartilage & Epiphseal Growth
• hyaline cartilage covering epiphyses develop into articular cartilages
• during infancy & childhood epiphyses fill with spongy bone
• cartilage is limited to articular cartilages
• prior to adulthood there is some hyaline cartilage that remains between the diaphysis and the epiphysis
• called epiphyseal or growth plate• area where bone will continue to
grow in length until it becomes adult sized
Endochondral Ossification
Endochondral Ossification
Bone Growth• bone increases in length & width• increases in length at epiphyseal
plate• interstitital growth• diameter of bone increases
through appositional growth• new tissues is deposited at
surface of the bone
Interstitital Growth• occurs at epiphyseal
plate• consists of hyaline
cartilage in middle with a transitional zone on either side
• in transitional zone cartilage is turning into bone
• epiphysis makes cartilage & ostoblasts try to overtake it by making bone
• osteoblasts cannot catch up bone gets longer
Interstitital Growth• epiphyseal plate
consists of four zones• zone of resting
cartilage• zone of proliferating
cartilage• zone of hypertrophic
cartilage• zone of calcified
cartilage
Interstitital Growth• In zone of resting cartilage small
chondrocytes present• do not participate in bone growth• cells anchor plate to the epiphysis• in zone of proliferating cartilage contains
slightly larger chondrocytes– undergo interstitial growth
• cells divide replacing those that die on diaphysis side of plate
• in zone of hypertrophy there are large, maturing chondrocytes arranged in columns
• zone of calcified cartilage contains few cells– cells are mostly dead due to extracellular matrix
around them having been calcified and no blood or nutrients can reach them
Interstitital Growth• at puberty rising
levels of sex & thyroid hormones cause osteoblasts to outpace manufacture of cartilage at epiphyseal end
• growth plate eventually fuses shut, leaving an epiphyseal line
• completes length of bone
Appositional Growth• way diameter of bone increases• new tissues is deposited at
surface of bone • at surface periosteal cells
differentiate into osteoblasts• begin to secrete organic parts of
matrix.• oteoid tissue is calcified• as osteoblasts become trapped
osteocytes• lay down matrix in layers parallel
to surface• produce circumferential lamellae
of bone
Bone Dynamics• bones constantly adapt to demands placed
on them and are continually remodeled throughout life
• part of normal growth & maintenance• 10% of skeleton tissue is replaced each year• organic and mineral components are
continuously recycled & removed through remodeling
• gives bone the ability to adapt to new stresses
Bone Dynamics• activities of both cells types are
continuous• activities must be balanced• when osteoclasts remove calcium
faster than osteoblasts can deposit itbone weakens
• when osteoblast activity predominates bones get stronger and more massive
Wolff’s law• bone’s structure is determined by mechanical stresses placed on it• one such stress is exercise• when bone is stressedmineral crystals generate small electrical
fields which attract osteoblasts• bony landmarks or bumps and ridges on surface of bone where
tendons attach may become more pronounced as muscles work to withstand increased forces
• regular exercise is needed to maintain normal bone structure• bone degeneration results from inactivity• changes in mineral content does not necessarily change shape of
bones because boney matrix contains protein fibers• bones can okay but may be soft due to no mineral deposition
– this is called osteomalacia• one form of this is rickets
– typically due to a vitamin D3 deficiency• not properly mineralized bones are flexible
– legs will bend under the weight of the body
Nutritional Needs• bone growth and maintenance requires
– calcium – phosphorous– magnesium– fluoride– manganese
• Calcitriol– from kidneys– absorption of calcium & phosphate from GI tract– synthesis of calcitriol depends on Vitamin D3
• therefore Vitamin D3 is needed for proper bone growth• Vitamin C
– needed for enzymatic reactions– needed for collagen synthesis– needed to stimulate osteoblast differentiation– without vitamin C there is a loss of bone strength and mass-scurvy
• Vitamin A– stimulates osteoblast activity– especially important for bone growth in children
• Vitamins K, and B12– needed for protein synthesis
Hormonal Needs• Growth hormone• Thyroxine• Sex hormones
–androgens in males–estrogens in females–help to close epiphyseal plates–stimulate osteoblasts to produce
bone at rate faster than epiphyseal cartilage can expand
Calcium Balance• most abundant mineral in the body• 90% is in bones• crucial to membrane functions• needed for activities of neurons & muscle
cells • for homeostatic balance three hormones
are needed• Calcitriol• Calcitonin• Parathyroid hormone
Calcitriol• active form of
vitamin D• principle
functionraise blood calcium
• increases absorption of calcium by small intestine
Calcitonin & Parathyroid Hormones
• opposite effects• Targets
–bones where calcium is stored–digestive tract where calcium is
absorbed–kidneys where calcium is
excreted
Calcitonin• made in thyroid gland• blood calcium levels rise
parafollicular or C cellsrelease calcitoninlowers blood calcium
• inhibits osteoclast activity slowing rate of calcium release from bone
• stimulates osteoblasts • encourages calcium to be
deposited into bones– more important during
childhood – also important in reducing loss
of bone mass during prolonged starvation & during late stages of pregnancy
– role in healthy adults is unknown
Parathyroid Hormone• made by parathryroid gland• calcium levels fall parathyroid
glandssecrete parathyroid hormone
• raises blood calcium levels– increases osteoclast acitivty
increases release of calcium from bones
– promotes calcium reabsorption by kidneys
– promotes final step of calcitriol synthesis in kidneys enhancing calcium uptake by intestine
– inhibits collagen synthesis by osteoblastscalcium deposition into bone decreases
Calcium Balance