Bones Flashcards
What is bone?
a tissue & organ; and parts of the skeleton
How is bone a tissue?
- bone is a CT w/ its cells embedded in a mineralized extracellular matrix (ECM)
- CTs are composed of mesenchymal cells of mesodermal origin
- the ECM of bone has an organic component (primarily collagen in the form of osteoid) & an inorganic component (primarily calcium & phosphorus in the form in hydroxyapatite)
What are the 2 families of bone cells?
those that form & maintain bone (& cartilage)
- derived from mesenchymal cells in the bone marrow
those that remove or resorb bone (& cartilage)
- derived from hematopoietic stem cells of the monocytes series that are also found in the bone marrow
stem cells are the only bone cells capable of mitosis & are, therefore, required for continuous growth & maintenance of bone
What are the 3 types of bone cells?
- osteoblasts (active or inactive)
- osteocytes
- osteoclasts
What are active osteoblasts?
- plump bone forming cells
- they line all bone-forming surfaces, produce bone matrix (osteoid), initiate mineralization of the matrix, & initiate bone resorption
What is osteoid?
- major organic component of the ECM of bone before it is mineralized
- about 90% of osteoid is collagen; 10% is amorphous ground substance that binds to hydroxyapatite
What are inactive osteoblasts?
- flat cells that are sometimes referred to as bone-lining cells
- they line bone surfaces at which neither bone formation nor bone resorption is occurring
- they can rapidly release calcium from bone if the blood calcium concentration is low, protect bone from chemical insults, & can become active osteoblasts to form new bone
What are osteocytes?
- differentiate from osteoblasts after becoming embedded in mineralized ECM
- they are located in lacunae (singular: lacuna) & communicate w/ other osteocytes & bone lining cells of the endosteum & periosteum via cell processes
- canaliculi (s. canaliculus) are the little canals w/in the mineralized ECM that contain the osteocyte processes
- osteocytes detect forces placed on bone & signal osteoblasts to either form bone or initiate the resorption of bone
- osteocytes also have a role in maintaining the ECM & plasma calcium
What are osteoclasts?
- multinucleated giant cells formed from the fusion of hematopoietic stem cells of the monocyte series that reside in the bone marrow, circulate in the blood, or both
- they are the only cells in the body capable of bone resorption
- for osteoclasts to resorb bone they must attach to the bone surface normally lined by osteoblasts
- osteoclasts create concavities in the bone surfaces called resorption lacunae or Howship’s lacunae
What is the ECM of bone?
- provides strength & hardness
- has an organic component & an inorganic component
- organic component, osteoid, is composed of fibrous stroma & amorphous ground substance
- fibers are type I collagen & the amorphous ground substance includes conjugated proteins (AAs attached to a non-protein chemical), proteoglycans, & lipids
- inorganic matrix is principally a distinctive combination of minerals called hydroxyapatite (or sometimes hydroxylapatite, HAP)
What is hydroxyapatite?
- is (Ca)10(PO4)6(OH)2; where Ca = calcium, PO4 = phosphate, & OH = hydroxide
- apatite is any grp of minerals w/ general formula 10Ca2+ : 6PO4^3- : X^-
Types of gross bone (the tissue):
- compact bone
- cortical bone
- cancellous bone
What is compact bone?
- has relatively large proportion of bone & v small proportion of interosseous space
- compact bone is composed of lamellar bone
What is cortical bone?
is the compact bone comprising the outer layer of a bone (the cortex) of the diaphysis
What is cancellous (lattice-like) bone?
- composed of anastomosing plates or trabeculae (‘little beams’; s. trabecula) of bone & relatively large proportion of interosseous space
- cancellous bone is also known as trabecular bone, spongy bone, lattice bone, & reticular bone
- cancellous bone may be composed of either woven bone or lamellar bone
KNOW
Types of microscopic bone (the tissue)?
- lamellar bone
- woven bone
What is lamellar bone?
- aka mature bone or organized bone
- collagen fibers of ECM of lamellar bone are arranged in parallel layers or concentric rings (lamellae; s. lamella)
- most (almost all) of the adult skeleton is lamellar bone
What is woven bone?
- aka immature bone, reactive bone, or new bone
- collagen fibers of the ECM of immature bone appear to be haphazardly woven together
- woven bone is temporary bone that provides short-term support during development or repair
- it is present in fetal ossification centers, at the leading edge of growing bones, around teeth, at the points of attachment of tendons and ligaments, & in response to injury (ex: fracture), inflammation, or neoplasia
What is the osteon/Haversian system or osteonal canal/Haversian canal?
- Haversian canals are a series of tubes around narrow channels formed by lamellae
- Haversian canals surround blood vessels and nerve fibers throughout the bone & communicate w/ osteocytes
- the canals + the surround lamellae are called a haversian system or an osteon
- Haversian canals are straight & long & parallel to the length of the bone
What are perforating/Volkmann’s canals?
- anatomic arrangements in cortical bones that allow blood vessels to enter the bones from the periosteum
- they interconnect the Haversian canals (running inside osteons), w/ each other, & the periosteum
- run perpendicular to the Haversian canals, & extend in random directions & at various angles
What is spongiosa?
spicules (sharp, needle-like bodies) of tissue composed partially or entirely of bone near the leading edges of ossification centers
What is primary spongiosa?
- the temporary spongiosa formed initially during ossification
- they are composed principally of calcified cartilage covered w/ woven bone but may contain some lamellar bone
What is secondary spongiosa?
- thicker than primary spongiosa & is the result of remodeling
- (replacement of calcified cartilage & woven bone w/ lamellar bone)
What is bone modeling?
- shaping
- refers to the CHANGE in size, shape, or contour of a bone in response to normal growth, altered mechanical use, or disease
- in modeling, bone surfaces may undergo mostly resorption or mostly formation, depending on the stimulus
What is Wolff’s Law?
- a bone, normal or abnormal, develops the structure most suited to resist the forces acting upon it
- in other words, bone will undergo changes in size, shape, or contour as a response to the forces applied to it
- BONE DEVELOPS BASED ON ITS ENVIRO
What is remodeling?
- the normal & continuous REPLACEMENT of existing or old bone w/ new bone to allow for the repair of microscopic injury
- in remodeling, resorption & formation are linked & occur at equal rates
What are the 4 major functions of bone as an organ?
- support
- protection
- mechanical advantage
- reservoir
- bone is also involved in acid-base homeostasis & serves as a trap for hazardous minerals (ex: lead)
How does bone function as a structural support?
bones are arranged in a skeleton that form a frame to which the soft tissues of the body are attached
How does bone function as protection for vital organs?
- bones are hard & have developed a strategic relationship w/ vital soft tissues to provide them w/ protection
- ex: brain & eyes w/in skull; spinal cord w/in the vertebral column; heart, lungs, & liver w/in the rib cage
- bones are also the location of hematopoietic tissue (bone marrow)
How does bone give us a mechanical advantage?
bones act as a series of levers that provide muscles w/ the mechanical advantage that enable pars of the body to move
How does bone function as a reservoir?
- bone is an important reservoir for certain minerals
- about 99% of the body’s calcium & 85% of the body’s phosphorus are stored in the bone
How might disease of bone affect the animal?
- not protect the animal, cant move, move in a funny way, etc.
KNOW What are the different shapes of bones?
- flat bones
- irregular bones
- tubular bones
- sesamoid bones
- short bones or cuboidal bones
what is the axial skeleton?
head, vertebral column, ribs, sternum, & pelvis
What is the appendicular skeleton?
thoracic and pelvic limbs
KNOW which bone shapes are characteristic of the axial skeleton?
flat & irregular
KNOW which bone shapes are characteristic of the appendicular skeleton?
tubular, sesamoid, and short bones
What are flat bones?
- provide maximal protection w/ minimal weight
- ex: bone of the skull, scapulae, ribs, & bones of the pelvis
What are irregular bones?
- include the vertebrae & certain bones of the skull that are not flat
- jutting processes for muscle attachment are typical of irregular bones
What are tubular (or long) bones?
- in general terms, cylindrical & conceptually, partially hollow
- humerus, radius, ulna, femur, tibia, & the 3rd metacarpal & 3rd metatarsal bones of some spp are ex of long tubular bones
- phalanges, metacarpal, & metatarsal bones of the paws of other spp are ex of short tubular bones
What are sesamoid (“seed-like”) bones?
- located near joints & serve to redirect the path of tendons & to protect tendons at areas where the greatest friction develops
- the proximal sesamoid bones of the limbs, the distal sesamoid or navicular bone of the horse’s foot, the patella, & the fabellae (s. fabella) are ex of sesamoid bones
What are short (or cuboidal) bones?
found in the carpal & tarsal regions
What are the two types of fetal ossification?
- intramembranous ossification that results in membrane or membranous bones
- endochondral ossification that results in cartilage or cartilaginous bones
What is intramembranous ossification?
- w/in a membrane
- begins w/ condensation (increased density) of mesenchymal cells
- next, some of the mesenchymal cells differentiate into OSTEOBLASTS & begin to form osteoid that is quickly mineralized to form spicules of woven bone
- spicules of woven bone increase in size & coalesce to form a continuous piece of bone
- THE ESSENCE OF INTRAMEMBRANOUS OSSIFICATION IS THE DE NOVO FORMATION OF BONE IN MESENCHYME
- some bones of the skull, including the maxillae, & portions of the mandibles are examples of membranous bones
What does de novo mean?
of new, anew, or from the beginning
What is endochondral (or enchondral: w/in cartilage) ossification?
- begins w/ the condensation of mesenchymal cells
- next, the mesenchymal cells differentiate into CHONDROBLASTS that form a cartilaginous model or anlage
- all mesenchymal cells w/in the model will differentiate into chrondroblasts & mature into chondrocytes
- therefore, the cartilaginous model is avascular & devoid of stem cells
- w/o a blood supply, the cartilaginous model degenerates & calcifies
- further development of the model depends on neovascularization, repopulation of the model w/ stem cells, & the production of woven bone via endochondral ossification
- THE ESSENCE OF ENDOCHONDRAL OSSIFICATION IS THE REPLACEMENT OF CARTILAGE W/ BONE (cartilage is not converted into bone)
- tubular bones, vertebrae, the pelvic bones, & bones of the base of the skull are examples of cartilaginous bones
Intramembranous ossification
endochondral ossification
What is articular cartilage?
the thin layer of hyaline cartilage on the articular surfaces of bones that form synovial joints
What is the cartilage btwn the articular cartilage & the outer surface of the epiphysis called?
epiphyseal cartilage
What is subchondral bone?
the bone below, or deep to, the cartilage, in this case the articular cartilage
What is the diaphysis (or shaft) ?
- (dia = btwn)
- is the elongated cylindrical portion of a tubular bone that separates, or is btwn, the 2 ends
- it is composed of a cortex surrounding the medullar cavity
- it is the site of the primary center for ossification
What is the endosteum?
- it is like the periosteum & lines the internal surfaces of bones, including the medullary cavity, trabecular bone, & the osteonal canals
What is the epiphysis?
- (epi = above or upon)
- is the wide end of a tubular bone that is covered by articular cartilage &, in growing bones, is separated from the metaphysis by the physis
- it is the site of a secondary center of ossification
- a bone might have 2 or more epiphyses
What is a medullary cavity (marrow cavity)?
the space in the diaphysis filled w/ marrow
What is the metaphysis?
- (meta = change or transition)
- the wider part(s) of a tubular bone btwn the diaphysis & the physis
- it’s composed of a think cortex surrounding cancellous bone
- in growing bones, the cancellous bone is made up of both primary & secondary spongiosa
What is the periosteum?
- (peri = around or near)
- a specialized fibrous CT that contains bone cells & covers the external surface of bones, except the articular surfaces
What is the perichondral ring?
the dense layer of fibrous CT that is continuous w/ the periosteum but overlies the periphery of a physis
what is the physis?
- (physis = growth or to grow)
- aka growth plate, metaphyseal growth plate, or epiphyseal disk
- the plate or disk of hyaline cartilage of growing endochondral bones located btwn the metaphysis & epiphysis
what is trabecular bone?
- the less dense bone found in the metaphyses & epiphyses of tubular bones
- it’s also called cancellous bone & spongy bone
What is hematopoiesis?
- blood cell production process
- cells that circulate in your blood include immune cells (WBCs), RBCs, & platelets
- your body produces an astonishing 100 billion blood cells each day
What are the two different types of bone growth?
- growth in diameter - appositional growth
- growth in length of endochondral bones - endochondral ossification
What is appositional growth?
- growth in diameter (& other dimensions)
- the diaphysis of endochondral bones & the surfaces of membranous bones grow via the apposition of bone by osteoblasts w/in the periosteum
- while the periosteum adds new bone, the endosteum removes bone via osteoclastic activity to increase the diameter of the medullary cavity & keep the cortex in relative proportion
What is apposition?
the deposition of successive layers upon those already present
MD = mature diaphysis
DD = developing diaphysis
How does growth in length of endochondral bones work?
- endochondral ossification allows endochondral bones to w/stand fxnal stresses while maintaining the ability to grow in length
- endochondral ossification at the physes is responsible for most of the growth in length while endochondral ossification w/in the EPIphyses contributes to a lesser degree
- a physis is a temporary structure composed of hyaline cartilage
- it is thickest when growth is most rapid & becomes thin as growth slows
- this event is referred to as closure of the growth plate & marks skeletal maturity
- the epiphysis contains blood vessels which provide nutrition to the physis
What are the zones of the physis?
I. reserve (or resting) zone
II. proliferative zone
III. hypertrophic zone
IV. calcifying zone
- osteogenic zone
What is the reserve (or resting) zone?
- consists of typical hyaline cartilage & serves as a source of cells for the proliferative zone
What is the proliferative zone?
- clusters of cartilage cells multiply via successive mitotic divisions & form columns of chondrocytes separated by thin bands of cartilage matrix
- this zone contributes to growth, although minimally, by adding new cells to the physis
What is the hypertrophic zone?
- chondrocytes and their lacunae become greatly enlarged & account for most of the growth in length
- then, chondrocyte cytoplasm becomes vacuolated & the nuclear & cellular membranes fragment, consistent w/ apoptosis
What happens within the calcifying zone?
- intercellular material, which is chondrocyte ECM, becomes mineralized or calcified
- mineralization of the intercellular material is necessary for phagocytic cells (chondroclasts) to attach & remove cartilage during remodeling
What is the osteogenic zone?
- capillary loops from the metaphysis invade btwn the columns of calcified cartilage
- transverse septa are removed by chondroclasts that allows the lacunae to merge & form larger spaces surrounded by calcified cartilage
- the contiguous surfaces become lined by osteoblasts which produce woven bone
