Chapter 6 & 7: The Skeletal System

Skeletal Cartilages: structures, types & locations

Skeletal cartilage – Made from cartilage

Consists primarily of water Allows for resilience No nerves or blood vessels

Surrounded by a layer of dense irregular connective tissue – perichondrium

Resists outward expansion when compressed Source of blood vessels – feeds the matrix & chondrocytes

Hyaline cartilage – Hyaline cartilage is the most abundant skeletal cartilage,

and includes the articular (cover bone ends @ movable joints), costal (connects ribs to sternum), respiratory (larynx & reinforce passageways), and nasal (external nose) cartilages.

Provide support & flexibility (due to collagen fibers)

Skeletal cartilages cont.

Elastic cartilage – More flexible than hyaline

Contains more elastic fibers Located in the external ear &

epiglottis Fibrocartilage –

Located in areas that need to withstand a great deal of pressure & stretch

Chondrocytes & collagen fibers Knee & intervertebral discs

Growth of cartilage

Appositional – “growth from the outside” Outward expansion due to production of

cartilage matrix on the outside of tissue Secrete new matrix against the external

surface of the existing cartilage Occurs in the shafts of long bones

Interstitial – “growth from the inside” Expansion within the cartilage matrix due to

divisions of lacunae-bound chondrocytes & secretions of the matrix

Occurs in the ends of bone

Classifications of bones

206 bones in the body 2 divisions –

Axial – Consists of:

The skull, vertebral column, & rib cage Involved in protection, support, or carrying

other body parts Appendicular –

Consists of: The bones of the upper & lower limbs & the

girdles (shoulder & hip bones) that attach them to the axial skeleton

Shape

Long bones – Longer than they are wide Have a definite shaft & two ends Consist of all limb bones except:

Patellas, carpals, & tarsals Named for their shape not size (fingers

are long bones even though they are small)

Short bones – Somewhat cube-shaped Include –

the carpals & tarsals Sesamoid – bones that form with in tendons

(patella)

Shape cont.

Flat bones – Thin, flattened, and often curved

bones Include –

Skull bones, sternum, scapulae, and ribs

Irregular bones – Complicated shapes Don’t fit into any other class Include –

Vertebrae Hip bones & coxae

Functions

5 main functions – Support –

Support body Cradle soft organs

Protection – Protect vital organs

Movement – Allow movement

Muscles attach to bones acting as levers for movement

Mineral storage – Store calcium & phosphate Released into the blood stream as ions for

distribution to the body Blood cell formation –

House hematopoietic tissue

Bone structure: gross anatomy

Bone markings – Projections;

Muscle attach to and pull Modified for where bones meet (joints) E.g. heads, trochanters, spines

Depressions and openings; Allows passages of nerves and blood

vessels E.g. fossae, sinuses, foramina, grooves

Table 6.1 pg. 179

Bone Textures

External layer = compact bone Internal layer = spongy bone

Made of trabeculae

Long bones cont.

Diaphysis – The bone shaft Contains cavity with yellow marrow

Epiphysis – Ends of long bones Typically wider than diaphysis (shaft) Consist of internal spongy bone & outer layer

of compact bone Ends are covered with hyaline (protects bone

ends where they meet at the joint) Epiphyseal line/plate –

Between epiphysis & diaphysis Line = remnant of plate (hyaline cartilage

disc in young adults that lengthens bone)

Long bones cont.

The external surface of bone is covered by the periosteum Double layered membrane Covers all bones except joint surfaces Contains osteoblasts & osteoclasts Richly supplied w/ blood, nerve fibers, & lymphatic

vessels – enter bone shaft via nutrient foramen Secured to bone shaft by – Sharpey’s fibers – tufts of

collagen fibers Provides insertion points for tendons and ligaments

The internal surface of bone is lined by a connective tissue membrane called the endosteum Covers trabeculae of spongy bone Lines canals that run through compact bone Also contains osteoblasts & osteoclasts

Short, flat, & irregular bones

Short, flat, & irregular bones consist of thin plates of periosteum-covering compact bone on the outside, and endosteum-covered spongy bone inside, which houses bone marrow between the trabeculae

No shaft or epiphyses Flat bones – internal layer of

spongy bone = diploë

Hematopoietic tissue

Hematopoietic tissue = red bone marrow

Located within trabecular cavities of the spongy bone, in diploë of flat bones & epiphysis of long bones

Infants – all areas of spongy bone contain red marrow

Adults – epiphysis of long bones – diaphysis – yellow marrow

Gross anat. cont.

2 types of bone texture – Compact –

Appears dense, smooth & solid Contains passageways for blood vessels & nerves Osteon –

structural unit of bones tiny weight bearing pillars arranged like tree rings Each matrix tube = lamella

Collagen fibers run in same direction – in opposing lamella they run in opposing directions – allow extra strength

Haversian canal – Center of osteon Contain blood vessels & nerves

Lacunae – Contain osteocytes – mature bone cells Canaliculi – connect lacunae to each other and the central canals

Interstitial lamellae – Incomplete lamellae between osteons

Circumferential lamellae – Deep to the periosteum Superficial to endosteum Resist twisting of long bones

Microscopic anatomy

Compact bone – dense and solid Structural unit = osteon Contains lamellae, Haversian canal, & blood

vessels and nerves Volkmann’s canals –

Lie at right angles to the long bone axis Connect blood & nerve supplies of the periosteum to

the central canals & medullary cavity Osteocytes –

Occupy lacunae & lamella junctions Connected by canaliculi

Lamellae – Circumferential –

Beneath periosteum Interstitial –

Between osteons

Compact Bone

Gross anat. cont.

Spongy – Internal to compact bone Honeycomb, needle-like, flat pieces

= trabeculae Align along the lines of stress Help the bone to resist stress Contain irregularly arranged lamellae &

osteocytes connected by canaliculi No osteons present

Nutrients – diffused from canaliculi from capillaries in the endosteum

Chemical Composition of Bone

Organic components Cells (osteogenic cells, osteoblasts,

osteocytes, and osteoclasts) Osteoid – ground substance and collagen

fibers Contribute to bone’s structure and

flexibility

Inorganic components 65% mineral salts (calcium phosphates) Tightly packed crystals around collagen

fibers Contribute to bone’s hardness – resists

compression

Ossification or osteogenesis = process of bone formation

Before week 8, skeleton made up of fibrous membranes and hyaline cartilage Flexible and resilient, can accommodate

mitosis Intramembranous ossification –

Formation of cranial bones of the skull and clavicles

Endochondral ossification – All bones below base of the skull (except

clavicles) Hyaline cartilage broken down as ossification

proceeds

Formation of the Bony Skeleton

Intramembranous Ossification

Endochondral Ossification

Postnatal bone growth

During youth bones lengthen entirely by interstitial growth from the epiphyseal plates

Growth in length – The cartilage cells at the top of the

epiphyseal plate (closest to the epiphysis) push the epiphysis away from the diaphysis causing the bone to grow

Old chondrocytes (closer to the diaphysis) calcify & replace the cartilage with bone tissue

Growth in width – Occurs through appositional growth

Bone growth due to deposition of bone matrix by osteoblasts beneath the periosteum

Growth in Length of Long Bones

Postnatal Bone Growth

Hormonal regulation Infancy and childhood – growth hormone

stimulates epiphyseal plate activity Released by anterior pituitary gland

Thyroid hormones regulate the activity of growth hormone ensuring proper bone proportions

Testosterone and estrogens are released in increasing amounts at puberty

Initially – growth spurt, later induce epiphyseal plate closure

Bone homeostasis

Bone remodeling – Weekly recycle 5-7% of bone mass Spongy bone replaced every 3-4 yrs Compact bone replaced every 10 yrs Adults –

Balanced due to deposit & removal Bone deposit occurs at a greater rate when bone is injured Bone resorption allows minerals to be absorbed into the

blood Vit C (collagen synthesis), Vit D (absorption of dietary

calcium), Vit A (needed for balance between deposit & removal of bone)

Bone Modeling & Remodeling Control of bone remodeling –

Hormones – maintain blood calcium homeostasis Mechanical stress & gravity – affect bone growth & allow

bone to withstand stresses

Bone Remodeling

Response to mechanical stress Wolff’s Law: a bone grows or

remodels in response to the demands placed on it

Long bones are thickest midway along the diaphysis (where bending stresses are greatest)

Curved bones are thickest where they are most likely to buckle

Trabeculae of spongy bone form trusses or struts along lines of compression

Large, bony projections occur where heavy, active muscles attach

Bone repair

Classification of fractures – Position of bone ends after fracture

Nondisplaced fractures = bone ends retain normal position

Displaced fracture = bone ends out of normal alignment

Completeness of break Complete fracture = bone broken through Incomplete = not broken all the way through

Orientation of break relative to the long axis of bone

Linear = parallels the long axis Transverse = perpendicular to the bones long axis

Whether the bone ends penetrate the skin Open/compound = bone ends penetrate the skin Closed/simple = bone ends don’t penetrate the skin

Bone repair

Fractures are treated by reduction – Closed (external) reduction

Bone ends coaxed into position by physician’s hands

Open (internal) reduction Bone ends secured together surgically with pins or

wires Check out Fig 6.2 on pg. 192 for other types

Comminuted – common in elderly (brittle bones) Compression – common in porous bones Spiral – common sports fracture Epiphyseal Depressed – typical skull fracture Greenstick – common in children (more organic

matter)

Spiral fractures

Comminuted

Compression

Dislocations & open fractures

Bone repair cont.

4 stages of fracture repair – 1. Hematoma formation –

Blood vessels are torn during the break, blood clot forms Nearby bone cells deprived of nutrition and die

2. Fibroncartilaginous callus formation – Vessels begin to form Phagocytic cells clean up debris Fibroblasts (produce collagen fibers that reconnect the bone)

& osteoblasts (begin forming spongy bone) begin to reform the bone

3. Bony callus formation – Bone trabeculae convert callus into bone Begins 3-4 weeks after injury Continues for about 2-3 months

4. Remodeling of bony callus – Excess material removed Compact bone is laid down

Steps of Bone Repair

Homeostatic Imbalances

Osteomalacia & Rickets (in children) Bones inadequately mineralized Caused by insufficient calcium or vitamin D deficiency

Osteoporosis Bone resorption outpaces bone deposit, bone mass reduced

Spongy bone of spine most vulnerable and neck of femur (“broken hip”)

Caucasian women most susceptible group Estrogen helps restrain osteoclast activity

GET ENOUGH CALCIUM WHILE BONES STILL INCREASING IN DENSITY! (Also, drink fluoridated water)

Paget’s Disease Excessive and haphazard bone deposit and resorption High ratio of spongy to compact -> spotty weakening