Skeletal System Flashcards
Functions of the skeletal system
• Support and protection
• Mineral storage
– Calcium and phosphorus
• Triglyceride storage
– Yellow bone marrow
• Blood cell production (hemopoiesis)
– Red bone marrow
• Movement
– With skeletal muscles
Typical long bone
• Diaphysis: long shaft
– Around medullary cavity with red or
yellow bone marrow (age dependent)
• Epiphysis: proximal and distal
• Metaphysis (also epiphysis in some texts)
– Epiphyseal (growth) plate: child -teen
• Hyaline cartilage
• Allows diaphysis growth in length
• Replaced by:
– Epiphyseal line: adult “scar”
• Bone replaces cartilage
• Bone stops growing
• Articular cartilage that remains:
– Hyaline cartilage at end of long bone
– Joints
Membranes of long bones
• Periosteum:
– Covers outside of bone except where
have articular cartilage
– Functions:
• Attach ligaments, tendons
• Anchors blood vessels/nerves
• Contain bone cells (osteo-blasts, -
clasts, -genic)
• Endosteum:
– Lines surfaces inside bone: canals,
medullary cavity, spongy bone
trabeculae
– Contains bone cells (osteo-blasts, -
clasts, -genic)
Microscopic anatomy of bone
• Cells: osteocytes, osteoblasts,
osteoclasts, osteoprogenitor cells
• Extracellular matrix: osteoid +
calcium salts
– Osteoid (organic): osteoblasts
make
• Collagen fibers: strength
(but alone = flexible)
• Ground substance
– Mineral (Ca2+) salt crystals
(inorganic): Hardness
• Hydroxyapatite crystals
– Ca2+ phosphate
– Ca2+ hydroxide
• Needed for calcification
Types of bone textures
• Compact bone with
osteons
• Spongy bone with
trabeculae
Compact bone
• Hard, dense, resists
bending
• Osteons: Structural units
– Parallel to diaphysis
– Central canal in middle
• Connected by
perforating canals
– Concentric lamellae
– Osteocytes in lacuna,
canaliculi
– Align parallel with lines
of stress, remodel
• Lamellae
– Concentric
– Circumferential: internal and external
• Periosteum
Compact bone
Spongy bone
• AKA trabecular bone
• Always covered with compact bone!
• Locations:
– Where bone not heavily stressed OR
stress arrives from many directions
• Short, flat, irregular bones
• Long bone:
– Around medullary cavity
– Epiphyses
• Benefits:
– Light, easily moved
– Helps bone resist stress
– Protects red bone marrow
• Trabeculae
• No osteons
– Contains lamella,
osteocytes in lacuna
– Endosteum covering
– Spaces between
trabeculae
• Red bone marrow:
blood cell formation
• Yellow bone marrow:
fat storage
– Canaliculi open onto
bone surface
Bone cells (the “O” cells) and origins
• Osteoprogenitor (osteogenic) cells: stem cells
• Osteoblasts: bone-building cells, secrete ECM
• Osteocytes: after osteoblasts become trapped, maintain bone
• Osteoclasts: bone-chewing cells, break down ECM
• Origins:
– Mesenchyme:
• Osteoprogenitor cells
• Osteoblasts
• Osteocytes
– Red bone marrow
• Osteoclast
Osteoprogenitor cells
• Originate from mesenchyme (embryo connective tissue)
– Mitotic stem cells
– Maintain osteoblast population
• Location:
– In endosteum
– Inner cellular periosteum
Osteoblasts
• Formed from osteoprogenitor
cells
• Bone-building cells:
responsible for bone
formation (ossification)
• Secrete matrix (osteoid): Bone
matrix w/out Ca2+ salts
– Make collagen, proteins
• Role in calcification: osteoid to
bone
– Hydroxyapatite formation
• Become trapped as osteocytes
Osteocytes
• Mature bone cells,
once osteoblasts
• Majority of cells,
connected
• Functions:
– Maintains bone
matrix
– Detect mechanical
stress on bone-
signal osteoblasts
Osteoclasts (pac-man)
• Large, multinucleated, phagocytic cells
– Fusion of many bone marrow cells =
many nuclei!
• Job: bone chewing resorption (osteolysis)
• Has ↑ contact with bone
– Releases HCl (acid) and enzymes
– Matrix products (including Ca2+) released
to blood
• Function:
– Regulate Ca2+ and phosphate in blood
– Normal bone remodeling: balance with
osteoblasts
The importance of balance
• Osteoblast > osteoclast activity: bone growth, stronger/more
massive
• Osteoclast > osteoblast activity: bone break down, weaken…(age,
osteoporosis)
Bone formation
• Process by which bone forms:
ossification or osteogenesis
– Replace other tissue with bone
– Center of ossification: location in
tissue where bone formation
begins
– Requires Calcification: deposit of
Ca2+ salt
• Why needed?
– Formation of bony skeleton in
embryo/fetus
– Growth of bone until adult
– Remodeling of bone
– Repair of bone (fractures)
Formation of bony skeleton:
embryonic/fetal ossification
- Endochondral ossification
• Cartilage made from
mesenchyme tissue
• Mesenchyme replaced by
cartilage
• Cartilage replaced by bone
• Most bones of body (i.e. long
bones) - Intramembranous ossification
• Bone made directly from
mesenchyme tissue membrane
• No cartilage stage
• Flatbones of skull, mandible,
some facial bones, baby “soft
spots”
Intramembranous ossification
• Start with mesenchyme connective tissue membrane
• Steps:
1. Formation of ossification center
• Mesenchymal cells become osteoprogenitor, which become
osteoblasts
• Ossification center established
Intramembranous ossification
2. Osteoid production and calcification:
– Osteoblasts make osteoid
– Calcium and mineral salts deposited = calcification
– Early bone forms and traps osteoblasts
– Trapped osteoblasts become osteocytes
3. Early spongy bone trabeculae with periosteum forms
– More bone produced and fuses to form trabeculae of immature
spongy bone
– Mesenchyme covering bone forms periosteum
- Compact bone
formation/red bone
marrow appears
• Compact bone forms
under periosteum
• Spongy bone remains
inside
• Red bone marrow
forms within spongy
bone cavities
• Continual remodeling
Endochondral ossification
• Start with mesenchyme →replaced by
hyaline cartilage → replaced by bone
• Early embryo to adulthood
• Cartilage model developed
– Mesenchymal cells become cartilage
cells
– Cartilage cells form hyaline cartilage
model with perichondrium cover
• Note: chondroblasts, chondrocytes =
cartilage cells of different ages
- Periosteum and bone collar
forms
– Cells under perichondrium
become osteoblasts
– Make bone matrix and form
bone collar (early compact
bone)
– Internal cartilage - Inside diaphysis
– Cartilage cells divide to
make more
– Make cartilage, cells grow
bigger (hypertrophy),
– Cartilage begins to calcify
– Cartilage cells die leaving
big holes - Primary ossification center forms
in diaphysis (~3 month fetus)
– 1st center of bone formation
– Blood vessels and osteoblasts
invade calcified cartilage
– Osteoblasts replace cartilage
with spongy bone
– Bone development spreads in
both directions from primary
ossification center towards
epiphyses
– Bone collar continues to grow - Medullary cavity formed
– Osteoclasts enter and form
medullary cavity in diaphysis
center
– Medullary cavity fills with red
bone marrow, surrounded by
spongy bone and compact
bone
– Cartilage continues to form at
epiphyseal plates, bone
continues to replace cartilage =
lengthening of bone called
(interstitial growth - Secondary ossification
centers form in epiphyses
– Cartilage replaced by
spongy bone in epiphyses
– Ossification moves out
from center in all
directions
– BUT! No medullary cavity
formed in epiphyses,
spongy bone remains - Replacement of all cartilage
by bone continues until:
Hyaline cartilage only present in
2 places:
– Articular cartilage:
epiphyseal surface, joint
area
– Epiphyseal plate:
• Allows for bone
lengthening (interstitial
growth) through
adolescence
Bone lengthening: interstitial growth
• Infants – adolescence bone lengthening: interstitial growth
• Epiphyseal growth plates: like endochondral ossification event!
– Chondrocytes divide (proliferate), produce cartilage
– Osteoblast replace cartilage with bone
– With continued cartilage production and ossification – bone grows
Epiphyseal (growth) plate
• Children, adolescents bone formation > bone resorption
• Growth plate closes post-puberty leaving epiphyseal line
– Cartilage cells stop dividing and make less cartilage,
osteoblasts bone production completely replaces cartilage
Bone widening: Appositional growth
• Osteoblasts under periosteum produce bone and widen bone
Bone remodeling
• Bone remodeling: old bone
tissue replaced by new
– Bone resorption:
osteoclasts
– Bone deposition:
osteoblasts
– Continues even after
epiphyseal plate closure
Healthy adult (<40yo): total bone mass remains constant
– Bone formation = Bone resorption
• ~10% of bone mass removed and replaced/year
• Needed for fracture repair!
• Influenced by Ca 2+ homeostasis, mechanical stress, exercise, dietary
changes, sedentary lifestyle
Factors affecting bone health:
growth and remodeling
- Nutrition: vitamins and minerals
- Hormones
– Affects calcium and phosphate levels - Weight-bearing exercises
Nutrition and bone health
• Protein: collagen fibers
• Calcium: calcification to harden bone
• Vitamin C:
– Important in collagen synthesis, stimulates
osteoblasts
– Deficiency: Scurvy
• Vitamin D: activation to calcitriol
– Absorption of calcium in GI tract
– Deficiency: Rickets, osteomalacia
Results of poor mineralization
• Rickets (children)/osteomalacia (adult)
– Can be result of lack of vitamin D, calcium or problem absorbing fats
– Collagen fibers present but ECM not hardened normally
– Growth plates affected in children: rickets
– Osteomalacia – “Adult rickets”, bones soft due to Ca2+ depletion
