UNIT 7: The Skeletal System

how many bones are in an adult human?

206

what makes up the axial skeleton?

bones of…

* skull
* Hyoid
* vertebral column
* thorax

what makes up the appendicular skeleton?

bones of…

* upper & lower limbs
* pectoral & pelvic girdles

long bones

long longitudinal axes & expanded ends

* e.g. forearm, femur

short bones

lengths and widths the same

* wrists, ankles
* sesamoid/round bones

sesamoid/round bones

small, nodular, develops within tendon or adjacent to joint

* e.g. patella

flat bones

platelike, broad surfaces

* e.g. ribs, scapulae, some skull

irregular bone

variety shapes, most connected to other bones

* vertebrae, some face bones

think back to the relationship between the anatomy of body parts and their functions.

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complete the statement:

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"bone’s _____ makes possible its ________.”

bone’s __shape__ makes possible its __function.__

elevations or bone projections (processes) are useful for what?

tendon & ligament attachment

depressions, grooves, & tunnels are useful for what?

blood vessels or nerve passages

A

diaphysis

* shaft of long bone, between epiphysis

B

top B = proximal epiphysis

* nearest attachment to trunk of body

lower B = distal epiphysis

* farthest attachment to trunk of body

C

epiphyseal lines

* region where epiphysis & diaphysis fuse together during growth

D

articular cartilage

* outer surface of articulating portion of epiphysis coated in layer of hyaline cartilage

E

spongy bone

bracket below E

spaces containing red marrow

F

compact bone

line below F

medullary cavity

* cavity containing red or yellow marrow within diaphysis of long bone

G

yellow marrow

H

periosteum

* dense connective tissue covering bone surface

point to the endosteum

tissue lining medullary cavity

where can marrow be found

1. medullary cavities of long bones
2. irregular spaces of spongy bone
3. larger central canals of compact tissue

compact bone (cortical bone)

**what?** dense bone tissue in which cells are organized in osteons without apparent spaces

**where?** wall of diaphysis

spongy bone (cancellous bone)

**what?** bone that consists of bars & plates separated by irregular spaces

* trabeculae

**where?** epiphysis

trabeculae

branching bony plate that separates irregular spaces within spongy bone

why is trabeculae important?

help reduce weight of bone

describe the microscopic structure of compact bone

1. compact bone consists of densely packed, cylinders called *osteons*. each osteon contains osteocytes & extracellular matrix deposited in rings (*lamellae*) around a central canal.
2. blood vessels & nerves occupy the central canal & provide a blood supply & neural control to the bone tissue.
3. central canals extend longitudinally through bone tissue & transverse perforating canals (Volkmann’s canals) connect them.
4. osteocytes lie within tiny chambers called lacunae; they communicate & share nutrients with other osteocytes through canaliculi, tiny passages through which their extensions (processes) pass.

describe the microscopic structure of spongy bone

1. composed of osteocytes and extracellular matrix, but cells do not aggregate around central canals
2. rather, cells lie in trabeculae & get nutrients from substances diffusing into canaliculi that lead to surface of these thin, bony plates

bone functions

1. support, protection, movement
2. blood cell production (hematopoiesis)
3. lipid storage
4. storage of inorganic salts & minerals

support

supports WEIGHT

protect

body parts & internal organs

movement

light enough to work with muscles → leverage

blood cell production (hematopoiesis)

red marrow: functions in formation of red blood cells (erythrocytes), white blood cells (leukocytes), & blood platelets

where is red marrow found

spongy bone of skull, ribs, sternum, clavicles, vertebrae, & hip bones

why is red marrow red?

hemoglobin (oxygen-carrying pigment in red blood cells)

lipid storage

yellow marrow: fat storage in medullary cavities of certain bones

what is the relationship between red and yellow marrow?

* yellow marrow replaces much of red marrow into adulthood
* yellow marrow temporarily turns into red marrow if supply of blood cells is deficient

storage of inorganic salts & minerals

store calcium phosphate

* releasing calcium when low blood calcium level **(Parathyroid hormone)** & storing calcium when high blood calcium level

why is maintaining sufficient blood calcium levels important

* muscle contraction
* nerve cell contraction
* blood clotting

ossification

formation of bone

* begins in embryo & continues into adulthood

bones composed of:

1. collagen fibers
2. calcified matrix (calcium phosphate)

collagen fibers

* 1/3 of bone weight
* strong & flexible
* bends if compressed

calcified matrix (calcium phosphate)

* 2/3 of bone weight
* found in crystals
* hard, inflexible, brittle

why is a combo of collagen & calcified matrix benefit bones

make bone strong, somewhat flexible, & highly resistant to shattering

bone growth

1. interstitial growth
2. appositional growth

interstitial growth

* grows in length
* occurs at epiphyseal plate
* happens within bone

appositional growth

* growth in diameter & thickness
* bone gets wider
* happens on bone’s outer surface
* \

intramembranous bones

originate between sheetlike layers of connective tissue

* CT vascular
* major skull bones

cartilage cells

chondrocytes: mature cell, live in lacunae

chondroblasts: young, active cells that produce matrix

endochondral bones

bones that begin as hyaline cartilage that is subsequently replaced by bone tissue

describe the development of endochondral bones

1. start w hyaline cartilage model. cartilage breakdown in middle of future diaphysis
2. bone collar develops to form periosteum, which surrounds future diaphysis
3. primary ossification center forms as osteoblasts & blood vessels from periosteum invade area of decomposing cartilage; here spongy bone tissue is deposited, & later a layer of compact bone is deposited over spongy bone; medullary/marrow cavity starts remodeling
4. secondary ossification centers form at the epiphysis, where spongy bone is produced; medullary/marrow cavity continues remodeling
5. image shows developed bone; interstitial growth occurs at epiphyseal plates between primary & secondary ossification centers
6. adult bone is represented by joined primary & secondary ossification centers, separated by epiphyseal lines (what remains after epiphyseal plate growth & closure)

epiphyseal plate

cartilaginous later between epiphyses & diaphysis of a long bone that grows

why is the epiphyseal plate important?

lengthens the bone, GROWTH!

what factors remodel bone

bones remodeled, or reshaped, throughout life, when a


1. person’s activity level changes


2. bone is recovering from fracture
3. level of blood calcium changes

explain how osteoclasts & osteoblasts remodel bone

1. osteoblasts deposit matrix
2. osteoclasts resorb, or remove matrix

what % of the skeleton is replaced every year due to bone tissue remodeling?

3-5%

both compact & spongy bone contain 3 types of cells:

osteocytes: ECM surrounds osteoblasts

osteoblasts: bone-forming cell

osteoclasts: break down matrix

what factors affect bone development, growth, & repair

1. nutrition
2. exercise
3. hormones

Vitamin A

activates osteoblasts

Vitamin C

promotes collagen production

Vitamin D

stimulates absorption & transport of calcium phosphate ions

why is Vitamin D important?

without this, bones lack normal mineral content & are soft & deformed

exercise

* builds muscles
* stress increases thickness & strength of bone

growth hormone

causes division of cartilage cells in epiphyseal plates

sex hormones

* stimulate osteoblasts
* promote epiphyseal plate growth & eventual closure

calcitonin

released when there is a high blood calcium level; stimulates osteoblasts to build bone and store excess calcium; inhibits osteoclasts

parathyroid hormone (PTH)

released when there is a low blood calcium level; stimulates osteoclasts to resorb bone and release stored calcium into blood

fracture

break in a bone

greenstick fracture

* incomplete


* break occurs on convex surface of bend in bone

fissured fracture

incomplete longitudinal break

comminuted fracture

complete & bone fragments

transverse fracture

* complete
* break occurs at right angle to the axis of the bone

oblique fracture

at an angle other than a right angle to the axis of the bone

spiral fracture

caused by excessive twisting of bone

step 1 of bone repair

blood escapes from ruptured blood vessels & forms hematoma

\+ periosteum likely to tear

\+ surrounding tissue vessels dilate, swell, & inflame the tissue

step 2 of bone repair

spongy bone forms (osteoblasts job) in regions close to developing blood vessels & fibrocartilage forms (fibroblasts job) in more distant regions

\+ phagocytic cells remove blood clot & dead or damaged cells in affected area

\+ osteoclasts resorb bone fragments - “clean up”

step 3 of bone repair

a hard bony callus replaces fibrocartilage (cartilaginous soft callus)

\+ blood vessels & osteoblasts invade area

step 4 of bone repair

osteoclasts remove excess bony tissue, restoring new bone structure much like the original

joint (articulations)

union of 2 or more bones

joint functions

1. make possible bone growth
2. permit parts of skeleton to change shape during childbirth
3. enable body to move in response to skeletal muscle contractions

diarthrotic

free movement

synovial joints

* surrounded by joint capsule of ligaments & synovial membranes; ends of bones covered by articular cartilage & separated by synovial fluid
* diarthrotic

why is synovial fluid important?

lubricates joints, prevents friction

types of synovial joints

1. **ball-and-socket joint (focus for this class)**
2. condylar joint
3. plane joints
4. **hinge joint (focus for this class)**
5. pivot joint
6. saddle joint

ball-and-socket joint

* ball-shaped head of one bone articulates w cup-shaped cavity of another
* movements in all planes, including rotation
* shoulder, hip

hinge joint

* convex surface of one bone articulates w concave surface of another
* flexion & extension
* elbow, joints of phalanges

rheumatoid arthrisis (RA)

* autoimmune disorder where the immune system attack the body’s healthy tissues - painful & debilitating
* synovial membrane inflamed/thickened, articular cartilage damaged, fibrous tissue infiltrates

osteoarthritis (OA)

* degenerative disorder that results from aging or poorly healed injury
* articular cartilage softens & disintegrates

bone mineral density test

* X-ray measures how much calcium & minerals are in bone segment
* when? older age = people heightened risk for osteoporosis
* why? osteoporosis diagnosis & helps understand risk of breaking a bone in the future

osteoporosis

bone diseases that causes bones to lose strength, making them susceptible to fractures

physiology behind osteoporosis

normally, osteoblasts deposit new bone and osteoclasts resorb bone to maintain its shape in remodeling process, which works to keep bone young;

HOWEVER, w osteoporosis, osteoclasts resorb bone at a faster rate than osteoblasts can deposit new bone; thus, bone is lost.

uncontrollable risk-factors

age, sex, body frame/size, family history, genetics, race

controllable risk factors

level of exercise, diet insufficient in calcium & Vitamin D, smoking tobacco, excessive drinking of alcohol

symptoms

1. height loss
2. stooped posture
3. back pain due to weakened vertebrae
4. easily fractured bones

why does osteoporosis happen?

with natural aging, a person’s bones have weakened regenerative abilities

achondroplasia

* slower conversion of epiphyseal plate cartilage to bone (less interstitial growth)
* short, stocky limbs result, but trunk normal size


* genetic

gigantism

* overproduction of growth hormone before puberty (increased interstitial growth)
* puberty often delayed = more time for **interstitial growth**
* seen in childhood

acromegaly

* overproduction of growth hormone after epiphyseal cartilages close → **appositional growth**
* bones get thicker, not longer, esp face, jaw, & hands
* seen in adulthood

1. zone of resting cartilage

normal hyaline cartilage; chondrocytes in lacuna surrounded by ECM