Anatomy Chapters 6&8: The Skeletal System

support, protection, movement, storage, blood cell formation

functions of the skeletal system

compact bone (8, 12, 17)

dense solid layer on the outside

spongy bone (4, 16)

porous bone with scaffolds called trabaculae, on inside of bone and end of bones

proximal epiphysis (1)

proximal end of bone

diaphysis (2)

long axis of bone

distal epiphysis (3)

distal end of bone

epiphyseal plate/line (6)

hyaline cartilage in middle of epiphysis where bone lengthening takes place, once lengthening ends leaves a line of compact bone

articular cartliage (5)

cartilage at end of bones

medullary cavity (9)

central cavity inside the diaphysis

yellow bone marrow (11)

fat storage in medullary cavity of adults

red bone marrow (9)

blood cell production in spongy bone and medullary cavity of children

periosteum (7)

double layered membrane of connective tissue covering the outside of the bone that:
- Isolates the bone from surrounding tissues
- Contains blood vessels, lymphatic vessels, and nerves; enters bone through nutrient foramen
- Participates in bone growth and repair
- Connects tendons and ligaments to bones through the weaving of collagen fibers (Sharpey’s fibers) to outer fibrous layer (dense irregular) and then to the bone matrix

endosteum (19)

connective tissue cellular layer lining trabeculae of spongy bone, central canals, Volkmann’s canals, and the medullary cavity that aids in bone growth, repair, and remodeling; contains osteoblasts, osteoclasts, and osteoprogenitor cells.

osteon (5, 19)

structural/functional unit of compact bone (cylinders or pillars tightly packed) composed of lamallae

central/Haversian canal (8, 16, 21)

tube in center of osteon that runs parallel with the surface of the bone and contains nerves and blood vessels to nourish bone cells

Volkmann's/perforating canal (9)

tubes that run perpendicular to the surface of the bone and contain blood vessels and nerves

lacuna (14)

cavities housing osteocytes that are sandwiched between lamellae, osteocytes can break down or build up extracellular matrix around them

canaliculi (14, 20)

passageways between lacunae which permit the exchange of nutrients, gases, hormones, and wastes to pass between osteocytes (gap junctions) and ultimately to a central canal

lamellae (cocentric) (7, 12)

hollow tubes made of inorganic hydroxyapatites around parallel collagen fibers. Collagen fibers of adjacent run in different directions to resist twisting

interstitial lamellae (22)

incomplete lamellae between osteons

circumferential lamellae (6)

lamellae surrounding the entire outside of compact bone; under periosteum

osteocytes (13) (yellow arrow)

mature bone cells within lacunae, maintain the protein and mineral content of surrounding matrix, when bone is damaged they are released from lacunae and convert to osteoblasts

osteoblasts (blue arrow)

bone forming cells, produce osteoid and convert osteoid to bone by adding calcium salts to it; when trapped become osteocytes. Produce hormone Osteocalcin that plays a role in activating the fight-or-flight response by switching off the parasympathetic nervous system

osteoprogenitor cells

stem cells which produce osteoblasts, located in periosteum and endosteum; become dormant in adult bone that is not growing or remodeling but provide nutrients to osteocytes via gap junctions

osteoclasts (green arrow)

giant multinucleated cells that break down and recycle bone matrix, located in periosteum and endosteum. Secretes lysosomal enzymes to digest organic matrix and dead osteocytes and hydrochloric acid to digest calcium salts

trabeculae (10)

plates that run in all directions forming branches with spaces between them. Made of lamellae that lack blood vessels, so nutrients diffuse between canaliculi that open onto the surface of trabeculae and blood vessels of the endosteum

Sharpey's fibers (14)

matrix of connective tissue consisting of bundles of strong collagenous fibers connecting periosteum to bone

long bone (a)

longer than wide (ex: humerous)

short bone (b)

more square shaped (ex: wrist or ankle bones)

flat/sutural bone (c)

flatter bones (ex: ilium, skull)

irregular bone (d)

weirdly shaped, doesn't fit into other classifications (ex: vertebrae, isonium)

sesamoid bone (e)

upside down pyramid (ex: patella)

shape, location, formation

bones are classified by...

axial skeleton (blue)

longitudinal axis of body; skull, vertebrae, thoracic cage

appendicular skeleton (pink)

bones of limbs and the girdles that connect them to the trunk

ossification

process of bone formation (osteogenesis)

calcification

deposition of calcium salts in cartilage, during ossification or in other tissues

intramembranous ossification

- forms flat bones: bones of skull, clavicles and the maxilla and mandible
- begins as sheet of fibrous connective tissue

fontanels

tough fibrous membrane where skull bones have not ossified (canvas). Fuse at 18 mo-2 yrs

endochondral ossification

- forms long, short, and irregular bones of the body below the base of the skull
- begins as pieces of hyaline cartilage

lengthening of bone at epiphyseal plate

- on the epiphysis side, chondroblasts divide adding new cartilage to lengthen bone
- on the diaphysis side, osteoblasts replace cartilage with spongy bone toward the epiphyseal side (both happen in the same direction which lengthens bone)
- osteoclasts digest spongy bone in the same direction as lengthening, forming a growing medullary cavity
- at puberty --> epiphyseal closure

epiphyseal closure

Bone lengthening ends because at puberty, the sex, growth, and thyroid hormones stimulate osteoblasts to produce bone faster than the chondroblasts can make cartilage. Therefore, the epiphyseal plate gets thinner and thinner until it completely ossifies. The remnants of the epiphyseal plate can be seen as an ossified line called the epiphyseal line.

appositional growth

causes bones to grow larger in diameter (bones increase in diameter from birth through adulthood and can continue into adulthood when stressed by weight-bearing activities and muscle activity)

low levels of blood calcium

parathyroid gland secretes parathyroid hormone (PTH) which stimulates osteoclasts to break down bone, increasing blood calcium levels

high levels of blood calcium

thyroid gland secretes calcitonin which stimulates osteoblasts to take up calcium from blood and build up bone, lowering blood calcium levels

spongy bone

replaced every 3-4 years

compact bone

replaced every 10 years

after age 30, bone resorption outpaces bone deposition and bone density declines. A person could maximize peak bone density by regular weight-bearing activities and maintaining blood calcium levels (1000-1030 mg/day in adults)

at what age does bone density decline and what are things to do that maintain bone density?

joints

sites where two or more bones meet. Their two functions are securing bones together and allowing the skeleton to be mobile (consist of tendons, ligaments, and cartilage)

tendons

connect muscle to the bone

ligaments

connect bone to the bone

skeletal cartilage

consists of a perichondrium (membrane covering outside of cartilage made of dense irregular connective tissue; vascularized and cellular) and cartilage tissue (gel-like extracellular matrix with fibers and high water content, chondrocytes inside of lacunae, no nerves or blood vessels). The three types are hyaline, elastic, and fibrocartilage

hyaline cartilage

Location: at ends of long bones, connect ribs to sternum, nose, larynx and trachea
Structure: circular chondrocytes in doublets and further apart with matrix filled with fine collagen fibers
Function: surface of bones glide with no friction, absorb, cushion

elastic cartilage

Location: external ear, epiglottis, eustachian tubes
Structure: chondrocytes appear closer together and more elastic fibers than hyaline cartilage.
Function: make structure flexible

fibrocartilage

Location: Intervertebral discs, symphysis pubis, menisci of the knee
Structure: parallel rows of chondrocytes appear farther apart and in between is extracellular matrix filled with thick collagen fibers.
Function: withstand heavy pressure, compression, shock absorption

appositional growth

growing from the outside, chondroblasts in perichondrium secrete new matrix onto existing cartilage

interstitial growth

growing from the middle, chondrocytes in lacunae divide and secrete new matrix to expand cartilage from within

nose and ears

two locations cartilage continues to grow throughout lifetime

synarthroses or immovable joints

No active movement between bones
Bones secured together with fibrous connective tissue or cartilage
Ex. Sutures, Vertebrosternal ribs and sternum

amphiarthroses or slightly movable joints

Limited amount of movement, designed for strength and flexibility
Bones joined by fibrocartilage discs or by ligaments
Ex. Intervertebral joints, Pubic symphysis, Sacroiliac joint, Interosseous membrane along radius and ulna

diarthroses or freely movable joints

Varying ranges of movement possible
Bones joined by fibrous articular capsule and ligaments with joint capsule in between
Ex: predominate in limbs, ends of long bones

synarthrotic joint

amphiarthrotic joint

diarthrotic joint

fibrous joints

bones joined by fibrous tissue (dense fibrous connective or ligaments), no joint cavity, immovable to slightly movable

sutures

fibrous joint: between skull bones; short dense fibrous connective tissue (later synostoses)

syndesmoses

fibrous joint: between tibia and fibula, radius & ulna; connected by long ligaments

gomphoses

fibrous joint: connect teeth to sockets in jaw bones; connected by periodontal ligament

cartiliginous joints

bones joined by cartilage (hyaline and fibrocartilage), no joint cavity, immovable to slightly movable

synchondroses

cartiliginous joint: epiphyseal plates, between first rib and sternum; plate of hyaline cartilage between bones (immovable)

symphyses

cartiliginous joint: pubic symphysis, intervertebral discs; hyaline cartilage on ends of bones connected to fibrocartilage in the middle. (slightly movable)

synovial joints

bones separated by a joint cavity and are freely movable; joints of the limbs

articular cartilage (1)

smooth hyaline cartilage covering ends of opposing bone surfaces of synovial joint

joint cavity (5)

space between bones filled with synovial fluid

joint/articular capsule (2)

double layered capsule enclosing and connecting bones, composed of fibrous capsule and synovial membrane

fibrous capsule (4)

outer tough articular capsule (dense irregular connective tissue) that connects to periostea of bones, strengthens joint

synovial membrane (3)

inner layer of articular capsule (loose areolar connective tissue) that lines the fibrous capsule internally and covers all internal joint surfaces that are not hyaline cartilage; secretes synovial fluid

synovial fluid (5)

viscous fluid with consistency of egg white that fills the joint cavity and inside of articular cartilage, less viscous when joint warms. Functions: lubrication, nutrient distribution, shock absorption, and contains macrophages to help digest microbes or cellular debris. Inside articular cartilage to aid in bearing pressure and nourish cartilage.

reinforcing ligaments (6)

(dense regular) strengthens joints, are either thickened parts of fibrous capsule or are independent structures; many nerves here and in the articular capsule sense position (proprioceptors)

menisci or articular discs

accessory structure found in some synovial joints: disc or wedge of fibrocartilage that subdivides the synovial cavity and is used for shock absorption and making the joint more stable (knee, jaw, sternoclavicular joints)

bursae

accessory structure found in some synovial joints: fluid filled sacs located between skin and bones that aid in movement of tendons, ligaments, muscle, or skin passing over bones; reduces friction and acts as shock absorbers.

tendon sheaths

accessory structure found in some synovial joints: bursae wrapped around a tendon that is subjected to friction when it crosses bony surfaces (shoulder joint)

fat pads

accessory structure found in some synovial joints: cushioning adipose pads located superficial to the joint cavity or between the fibrous capsule and the synovial membrane; which protect and act as packing material (hips and knees)

first factor that influences joint stability

Shape of articular surfaces
- Joints are more stable when two bone surfaces fit snugly into one another (hip)
- Joints are not stable when sockets are shallow or two bones do not fit together.

Number and positioning of ligaments
- The more ligaments the stronger the joint.
- Ligaments can only stretch 6% of their length before they break
- A joint is not stable when the main method of reinforcement is ligaments

third factor that influences joint stability

Muscle tone
- Joints are stable when there is good muscle tone that allows the muscle tendons crossing the joint to be taught; most important factor in most joints (shoulder, knee, arches of foot)
- There are many sensory nerves around the articular capsule and ligaments, which quickly send signals to the muscles to regulate muscle tone (proprioceptors).

Shoulder joint because the bones fit more snugly together

is a shoulder joint or knee joint more stable?

anterior cruciate ligament

1

lateral meniscus

2

fibular collateral ligament

3

patellar ligament

4

posterior cruciate ligament

5

medial/tibial collateral ligament

6

medial meniscus

7

hyperextension, extension, flexion

1, 2, 3

dorisflexion, plantar flexion

4, 5

adduction, abduction

6, 7

extension, flexion

8, 9

rotation, circumduction

12, 13

supination, pronation

10, 11