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
Formation of the Bony Skeleton
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
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