anatomy units

skeletal system

-includes bones of skeleton, cartilage, ligaments, and other connective tissues that stabilize or connect the bones

-bones of skeleton are primary organs of the skeletal system; they form rigid framework of the body

most common bone connective tissue in bones of the body

-compact bone and spongy bone

compact bone (dense or cortical bone)

- relatively rigid connective bone tissue that appears white, smooth, and solid

-80% of total bone mass

-dense outer layer

spongy bone (cancellous or trabecular bone)

-located internal (deep) to compact bone, appears porous, and makes up about 20% of total bone mass

-covered by layer called endosteum

-outer layer - periosteum

cartilage

-semirigid connective tissue that is more flexible than bone

-mature cartilage is avascular (lacking a blood supply)

-three subtypes of cartilage (two subtypes associated with skeletal system)

hyaline cartilage

-attaches ribs to sternum

-covers ends of some bones

-is the cartilage w/in growth plates

-provides a model during development for the formation of the fetal skeleton

fibrocartilage

-withstands compression

-forms the intervertebral discs, the pubic symphysis, and the cartilage pads of the knee joints

tissues that make up bones

-bone connective tissue (compact bone and spongy bone)

-dense irregular ct

-reticular ct

-muscle tissue

-epithelial tissue in blood vessels

-nervous tissue

functions of bones

-Support and protection

-levers for movement

-hematopoiesis

-storage of mineral and energy reserves

support and protection

bones provide structural support; protect delicate tissues and organs from injury and trauma. ex: rib cage which shields the heart and lungs

levers for movement

-bones serve as attachment sites for skeletal muscles, other tissues

-muscles attached to bones of skeleton contract and exert a pull on skeleton, which functions as system of levers (bones by themselves dont move)

-movements range from powerful contractions for running and jumping to even removing a splinter from finger

***hematopoiesis

-the process of blood cell production (RBC, WBC, platelets)

-occurs in red bone marrow of spongy bone ct, which contains stem cells that form blood cells and platelets

storage of mineral and energy reserves

-most of the body’s reserves of the minerals calcium and phosphate are stored within and then released from bone connective tissue; when calcium or phosphate is needed by body, some bone ct is broken down and minerals are released into blood

-chemical energy is stored in adipocytes (fat cells)

calcium

essential mineral for body functions like muscle contraction, release of neurotransmitter from nerve cells, and blood clotting

phosphate

structural component of ATP, other nucleotides, and phospholipids; is an important component of the plasma membrane

four classes of bone

long bones, short bones, flat bones, and irregular bones

long bones

-greater in length than width

-most common bone shape

-elongated, cylindrical shaft

-found in the upper limbs (the arm, forearm, palm, and fingers) and lower limbs (thighs, leg, sole of foot, toes)

short bones

-have a length nearly equal to their width

-includes wrist bones and bones in the foot, sesamoid bones (like the patella/kneecap)

sesamoid bones

-example of short bones

-small, sesame seed-shaped bones along the tendons of some muscles

flat bones

-flat, thin surfaces that may be slightly curved

-provide extensive surface areas for muscle attachment and protect underlying soft tissue

-form the roof of the skull, shoulder blades, sternum (breastbone), and ribs

irregular bones

-have elaborate, sometimes complex shapes

-vertebrae, hip bones, and several bones (ethmoid, sphenoid) in the skull are examples

hormone production (osteocalcin)

regulates bone formation, protects against obesity, glucose intolerance, diabetes etc

structure of typical long bone

-diaphysis (bone shaft): forms the long axis, provides for the leverage and major weight support of a long bone, compact bone surrounding medullary cavity

-epiphyses: bone ends, external compact bone and internal spongy bone, articular cartilage covers articular surfaces, between is epiphyseal line

medullary (marrow) cavity

cylindrical space (hollow) within the diaphysis; in children, this cavity contains red bone marrow which is later replaced by yellow bone marrow in adults

articular cartilage

meant for smoother movement (reduces friction and absorbs shock in movable joints), covers the joint surface of the epiphysis, thin layer of hyaline cartilage

epiphysis

an expanded, knobby region that is at the end of a long bone

proximal epiphysis is the end of the bone closest to the body trunk, while distal epiphysis is the end of the long bone farthest from the trunk

metaphysis

region of long bone where bone widens and transfers forces between the diaphysis and the epiphysis

epiphyseal line

marks the border between proximal epiphysis to diaphysis or between diaphysis and distal epiphysis; used to be epiphyseal plate (growth plate) - it is a remnant of childhood bone growth

epiphyseal plate

layer of hyaline cartilage located between long bone diaphysis and epiphysis; allows longitudinal growth of the bone

periosteum

-covers the outer surface of bone expect for areas covered by articular cartilage

-consists of two layers; outer layer = fibrous layer of dense irregular ct (sharpens fibers secure to bone matrix); inner layer composed of bone stem cells (osteogenic cells) and osteoblasts (both provide circumferential layers of bone matrix)

-serves as point of attachment for tendons and ligaments

-periosteum is responsible for growth in bone width

-anchored to the bone by numerous collage fibers = perforating fibers or sharpeys fibers

endosteum

-located w/in (inside) bone in medullary cavity; delicate connective tissue membrane covering internal bone surface

-covers trabeculae of spongy bone

-contains osteogenic cells that can differentiate into other bone cells; cells are active during bone growth, repair, and remodeling

-lines canals that pass through compact bones

sharpeys fibers

connect periosteum to bone

red (bone) marrow

-hematopoiesis (hemat- blood, poiesis- to create) = process of creation of blood cells (RBC, WBC, platelets) which happens in the red bone marrow

-spongy bone contains red bone marrow, usually in the proximal and distal epiphysis

-found within trabecular cavities of spongy bone and flat bones (sternum)

-in medullary cavities and spongy bone of newborns

-adult long bones have little red marrow (heads of femur and humorous only)

-red marrow in flat and some irregular bones is most active

-contains reticular ct, developing blood cells, and adipocytes

yellow bone marrow

-contains fat inside the medullary cavity (in adults)

-decrease in developing blood cells and increase in adipocytes

-may convert into red bone marrow when needed (like in cases of anemia)

bone markings

sites of muscle, ligament, and tendon attachment on external surfaces; joint surfaces; conduits for blood vessels and nerves

-projections: most indicate stresses caused by muscle pull or joint modifications

-depressions and openings: usually allow nerves and blood vessels to pass

blood supply and innervation of bone

blood is highly vascularized (has many blood vessels); blood vessels enter bones from the periosteum

bone connective tissue (osseous connective tissue)

-primary component of bone

-composed of both cells and extracellular matrix, like all ct

four types of cells in bone connective tissue

osteoprogenitor cells, osteoblasts, osteocytes, osteoclasts

osteogenic cells (osteoprogenitor cells)

-cell of bone tissue

-stem cells in periosteum and endosteum

-when cells divide, some of the stem cells will become osteoblasts while others stay as stem cells

-actively mitotic (divide)

osteoblasts

• responsible for making fibers (collagen = 90% of bone protein)

• responsible for making ground substance (secrete unmineralized bone matrix or osteoid)

• cell of bone tissue

• formed from osteogenic (osteoprogenitor stem cells) cells

• produce new bones

• once osteoblasts become entrapped in the matrix they produce and secrete, they differentiate into osteocytes

• actively mitotic - divide

• MAKING BONE

osteocytes

-come from osteoblasts; lost their bone-forming ability after osteocytes became embedded with calcified osteoid

-mature bone cells in lacunae

-monitor and maintain bone matrix; detect mechanic stress on bone

-regulate activity of osteoblasts and osteoclasts

-act as stress or strain sensors; if stress is detected, osteoblasts are signaled, resulting in possibly the deposition of new bone matrix at the surface

bone lining cells (NOT AS IMPORTANT)

-flat cells on bone surfaces to help maintain matrix

-on external bone surface called periostea cells, lining internal surfaces called endosteal cells

osteoclasts

-macrophages (WBC specialized in removing debris or engulfing bacteria; cleaning crew to remove waste)

-derived from hematopoietic stem cells (they are wbc; so the site of formation is red bone marrow)

-large, multinucleate cells for bone resorption (break down bone through this process)

-when active rest in resorption bay and have ruffled border; ruffled border increases surface area for enzyme degradation of bone and seals off area from surrounding matrix

-BREAKING BONE

Bone growth

when osteoblasts more active than osteoclasts; when we get older, we stop growing b/c equal amount of activity - same amount bone breaking and same amount bone formation. osteoclasts starts increasing their activity

the osteon (Haversian system)

-structural unit of compact bone

-elongated cylinder parallel to long axis of bone

-hollow tubes of bone matrix called lamellae

-collage fibers in adjacent rings run in different directions (withstands stress, resist twisting)

lamellae

• made up of ground substance and calcium, also collagen fibers (which are tough/strong fibers and they run in different directions; this arrangement prevents the bone from getting twisted)

• in the osteon

spongy bone structure

-appears poorly organized

-trabeculae: align along lines of stress to help resist it, no osteons, contain irregularly arranged lamellae and osteocytes interconnected by canaliculi, capillaries in endosteum supply nutrients

osteogenesis imperfecta

(osteo=bone, genesis=creation, imperfecta= imperfect)

genetic disorder with collagen fibers

9 types

type 1 osteogenesis imperfecta

make the collagen fiber but not an enough amount

type 2 osteogenesis imperfecta

problem with quality and quantity of collagen fiber

central (Haversian) canal

-runs through the core of osteon (center of each osteon)

-contains blood vessels and nerve fibers (also a vein there)

-provide all the oxygen and nutrients and nerve signals to all the cells (osteocytes, osteoblasts, osteoclasts) within the osteon

perforating (Volkmann’s canals)

-canals lined with endosteum at right angles to central canal

-connect blood vessels and nerves of periosteum, medullary cavity, and central canal

-blood vessel/artery running through this canal; they run through the entire width of the compact bone— end in the central canal of the osteon system as the branches are sent upwards

lacunae

small cavities that contain osteocytes (look like tiny spiders surrounding in the diagram)

canaliculi

-hairlike canals that connect lacunae to each other and central canal

-small canals

-small processes of osteocytes run through canals

canaliculi formation

-osteblasts secreting bone matrix maintain contact with each other and osteocytes via cell projections with gap junctions

-when matrix hardens and cells are trapped the canaliculi form—-allow communication, permit nutrition and wastes to be relayed from one osteocyte to another throughout osteon

interstitial lamellae

-incomplete lamella not part of complete osteon

-fill gaps between forming osteons

circumferential lamellae

-actually create the osteon

why do we have spongy bone

-lighter in weight than compact bone so easier for muscle to contract and pull on bones b/c makes bones lighter (reduces weight of skeleton)

-locations not heavily stressed, locations w/stresses arriving from many different directions

-example: epiphyses of long bones where stresses transferred across joints

-can help form red bone marrow

chemical composition of bone matrix

-include cells (osteogenic cells, osteoblasts, osteocytes, bone-lining cells, and osteoclasts) and osteoid

osteoid

-the ground substance that glue all the cells and create the lamella

-collagen fibers embedded in it

-1/3 of organic bone matrix secreted by osteoblasts

-made by the osteoblasts, which will create the collagen fibers and

bone is made up of

-organic components (40%): collagen (90%) type 1, proteoglycan + glycoprotein, matrix proteins-proteins that are non-collagen, growth factors

-inorganic components (60%): calcium hydroxyopetite

sacrificial bonds

-between the collagen bonds

-resilience of bone due to sacrificial bonds in or between collagen molecules

-stretch and break easily on impact to dissipate energy and prevent fracture

-if no additional trauma, bonds re-form

hydroxyapatites (mineral salts)

-65% of bone by mass

-mainly of tiny calcium phosphate crystals in a and around collagen fibers

-responsible for hardness and resistance to compression

osteogenic or ossification

-process of bone tissue formation

-formation of bony skeleton (begins in 2nd month of development)

-postnatal bone growth (until early adulthood)

-bone remodeling and repair (lifelong)

two types of ossification based on location

endochondral ossification and intramembranous ossification

endochondral ossification

-(endo=inside, chondral = cartilage) - you take cartilage and convert it into bone (do it from inside the cartilage)

-bone forms by replacing hyaline cartilage

-bone called cartilage (endochondral) bones

-forms most of skeleton inferior to base of skull except clavicles

intramembranous ossification

-bone start from some fibrous membrane

-bones called membrane bones

-forms flat bones like clavicles and cranial bones

endochondral ossification, detailed steps

-all starting as a hyaline cartilage (from the inside) and eventually we going to get to the bone

-primary ossification center where the center of shaft is where starts turning into bone

-blood vessels infiltrate perichondrium, converting it to periosteum

-cells convert to osteoblasts (cells to form bone)

-secondary ossification center in the epiphyses area giving rise to spongy bone

-articular cartilage on the surface

-epiphyseal plate/growth plate (some cartilage left) which will get ossified (deposit calcium, make it tough, shut it down) and then becomes epiphyseal line once growth is done

primary ossification center

-primary ossification center: center of the shaft at whats going to become a bone; center gives rise to the bone

intramembranous ossification

-forms frontal, parietal, occipital, temporal bones, and clavicle; specifically for flat bones

-form inside membrane, fibrous connective tissue (not cartilage)

-within membrane there are stem cells, which will become osteoblasts—osteoblasts becomes osteocytes, forming a bone from within that membrane

-production of ground substance and fibers. calcium will toughen it. part of it will die and will start forming trabeculae (gives rise to spongy bone which is covered on both sides by compact bone)

interstitial growth

-responsible for making bone taller, increasing its length, until early adulthood

-requires presence of epiphyseal cartilage, epiphyseal plate maintains constant thickness

-five zones within cartilage: resting zone, proliferation (growth zone), hypertrophic zone, calcification zone, ossification (osteogenic) zone

appositional growth

growth is going towards the side, bone is made wider, lifelong

bone growth

two types: interstitial and appositional growth

growth plate zones for interstitial growth

1) resting/reserve zone - closer to the epiphyseal end; chondrocytes help keep cartilage alive, prevent the spongy bone from taking over

2) proliferation zone - cell replication/ mitosis

3) hypertrophy zone- increase in size of the cells. lipids, glycogen, etc. will cause to get bigger and bigger

4) calcification zone- cells are starting to die and all the content is getting calcified (calcium deposits)

5) ossification zone - final product; add into bone matrix, added more into length of bone

eventually growth will cease

appositional growth

-growth in width, allows lengthening bone to widen, happening throughout life

-osteoblasts deposit bone and bone matrix under periosteum, create bone from outer surface

-osteoclasts from inner surface of bone start to break bone apart

-usually more building up than breaking down

-thicker, stronger bone but not too heavy

-growth appositional b/c happening in opposite directions

osteocytes in relation to osteoclasts and osteoblasts

tell the osteoclasts and osteoblasts when and how and how much to work (in regards to building and breaking bone)

growth hormone; one hormone important for bone growth

-stimulates growth

-stimulates epiphyseal plate (growth plate) activity in infancy and childhood

-mostly secreted during the sleep

-lack of growth hormone can cause dwarfism

-too much growth hormone can cause gigantism (bones are not strong enough lacking calcium despite being tall/long)

ONCE EPIPHYSEAL PLATE IS CLOSED AND BECOMES EPIPHYSEAL LINE NO GROWTH CAN OCCUR

thyroid hormone

-responsible for all metabolism (all chemical rxn taking place in body) in our body

-help modulate activity of growth hormone

-ensure proper proportions of bone

testosterone (males) and estrogens (females) at puberty

-during puberty; promote adolescent growth spurts

-end growth by inducing epiphyseal plate closure

excess or deficits in growth, thyroid, testosterone/estrogen hormones

will cause abnormal skeletal growth

bone homeostasis

-older bone become more brittle as calcium salts crystalize, so fractures more easily take place

-recycle 5-7% of bone mass each week (spongy bone replaced every 3-4 years while compact bone replaced every 10 years)

-consists of bone remodeling and bone repair

bone remodeling

-happens throughout life

-activity of osteoblasts and osteoclasts

-osteoblasts responsible for bone deposit, build ground substance with collagen fibers. next there will be lacunae and osteocytes

-osteoclasts responsible for breaking down the bone (bone resorption)

bone repair

-more active process involving fracture

-repair part of bone because its broken

bone resorption

-function of osteoclasts

-secrete enzymes which contain acid; acidity converts calcium salts to soluble forms, calcium can go back to the blood

-osteoclasts also phagocytize dead osteocytes, making room for new osteocytes

-once resorption complete, osteoclasts undergo apoptosis —- they will die

-osteoclast activation involves parathyroid hormone and T cell secreted proteins

bone remodeling regulation

-occurs continuously but regulated by genetic factors and two control loops

-negative feedback hormonal loop for Ca2+ homeostasis ; must maintain level of calcium in blood in right amount. if need more calcium, we go to bones to give to the blood. when osteoclasts break bone, this is when they release the calcium

-when bone is responding to mechanical and gravitational forces- Wolff’s law (if there is pressure on bones, there is more calcium on bones compared to not enough pressure, which doesn’t need as much calcium like when going to space + astronauts)

importance of calcium

-functions in:

nerve impulse transmission, muscle contraction, blood coagulation, secretion by glands and nerve cells, cell division.

-99% as bone minerals

-intestinal absorption requires vitamin d metabolites

-dietary intake required

-level of calcium in blood drops, then these functions are jeopardized

parathyroid hormone (PTH)

-produced by parathyroid glands

-removes calcium from bone regardless of bone integrity

-level of calcium in blood must be maintained - 9/11 mg/dl but in this case it has dropped and we have less calcium. blood travels everywhere in body and it will pass by parathyroid glands, sense low level of calcium in blood and parathyroid glands secrete hormone called parathyroid hormone

-parathyroid hormone will travel into bones and stimulate activity of osteoclasts. calcium can go back into blood, which will raise level of calcium in blood

calcitonin

-produced by parafollicular cells of thyroid gland

-high level of calcium in blood, and blood goes everywhere, passing by thyroid glands and we have cells called parafollicular cells. sense high level of calcium, so they secrete hormone called calcitonin into blood stream

-caarry calcitonin which will be delivered into bone. lowering activity of osteoclasts so it won’t release too much calcium

-works with kidney to urinate more calcium. more calcium in urine can lead to kidney stones

fracture in bone causes…

change in structure and function of bone

bone repair

-fractures (breaks)

-for youth: most result from trauma

-old age: most result of weakness from bone thinning

three fracture classifications

1. position of bone ends after fracture

2. completeness of break

3. whether skin is penetrated

also described by location of fracture, external appearance, and nature of break

position of bone ends after fracture

non displaced: ends retain normal position vs displaced: ends out of normal alignment

completeness of break (pertaining to bone fractures)

complete: broken all the way through vs. incomplete: not broken all the way through

whether skin is penetrated (in terms of bone fracture)

open compound: skin is penetrated, closed (simple): skin is not penetrated

fracture treatment and repair

reduction: realignment of broken bone ends

-closed reduction: physician manipulates to correct position (not surgery)

-open reduction: surgical pins or wires secure ends

-immobilization by cast or traction for healing

-depends on break severity, bone broken, and age of patient

bone repair in fracture

1)hematoma forms (gap between bones is filled with blood - internal bleeding, clot Is formed)

2) fibrocartilage callus forms. fibrous tissue and cartilage is mixed. creation of external callus and internal callus

3) convert fibrocartilage callus into bony callus - osteoblasts will change cartilage into bone

4) bone remodeling occurs - osteoblasts (remove unnecessary debris left from fracture) and osteoclasts (continue to make new bone). bone can sometimes be restored to original shape, but sometimes we can tell if person had structure when seeing calluses in bone

osteopenia

-bones become thinner and weaker as normal part of aging

-ossification process starts to decrease

-osteoblast activity slows, osteoclast activity remains constant,

-begins between age 30-40, women lose about 8% of skeletal mass every decade while male about 3%

osteoporosis

-loss of bone mass that impairs normal function and can lead to more fractures

-more common in women because after menopause there is a decline in estrogen, this will lead to decrease in bone mass

-condition less common in men due to levels of androgens (male sex hormones)

-causes increased bone fractures and decreased ability to repair

skeletal system

-composed of bones, cartilage, joints, ligaments

-20% of body mass index

-2 major parts: axial and appendicular

axial skeleton includes

-skull, vertebral column, and thoracic cage