Bone growth, remodeling, calcium homeostasis
endochondral ossification
responsible for growth and development of long bones (ex. femur, humerus)
hyaline cartilage is replaced w/ bone tissue after it dies
intramembranous ossification
occurs in flat bones of the skull, mandible, and clavicle
starts w/ a sheet of fibrous connective tissue (mesenchyme) → fibroblasts/mesenchymal cells are transformed into osteoblasts → secrete matrix that ultimately becomes spongy bone
trabeculae
small branches/spicules found in spongy bone
provide structural support and help distribute forces within the bone
located at the ends of long bones and in the pelvis, ribs, skull, and vertebrae
functions of the skeleton
Support
Protection
Movement
Electrolyte balance: stores calcium and phosphate ions and releases them into tissue fluid/blood according to physiological needs
Acid-Base balance: bone tissue buffers blood against excessive pH changes by absorbing/releasing alkaline phosphate and carbonate salts
Blood formation: red bone marrow
Hormone secretion: influences secretion and action of insulin and moderates the stress response
marrow (medullary) cavity
contains bone marrow in long bones
diaphysis
shaft of long bones — provides leverage
epiphysis
expanded head at each end of long bones — strengthens joints and provides added surface area for the attachment of tendons and ligaments
epiphyseal line
slightly denser spongy bone b/t the epiphysis and diaphysis in mature bones — remnant of the epiphyseal plate (childhood growth zone)
articular cartilage
a layer of hyaline cartilage on the joint surface where one bone meets another
periosteum
a tough surface membrane of bones
outer fibrous layer of collagen, inner osteogenic layer of bone-forming cells
provides strong attachment and continuity from muscle to tendon to bone
no periosteum over articular cartilage
endosteum
a thin layer of reticular connective tissue that lines the internal marrow cavity, covers all the honeycombed surfaces of spongy bone, and lines a canal system found throughout compact bone
flat bone
shieldlike plates that protect delicate organs such as the brain and heart and form broad surfaces for muscle attachment
two layers of compact bone (inner and outer tables) enclosing a middle layer of spongy bone (diploe)
osteogenic cells
stem cells that develop from embryonic mesenchyme → most other bone cell types
occur in endosteum and inner layer of periosteum
multiply continually — only cells capable of dividing and producing more bone cells
osteoblasts
bone-forming cells that synthesize the organic matter of the bone and then promote its mineralization (osteogenesis)
form rows in endosteum and inner layer of periosteum
have abundant mitochondria and ER to support their role in osteogenesis
osteocytes
former osteoblasts that have become embedded in the matrix they deposited
90-95% of bone cells
reside in cavities called lacunae (interconnected by canaliculi)
have dendrites that contact neighboring osteocytes (gap junctions), blood vessels, and osteoblasts on the bone surface
some resorb bone matrix and others deposit it
strain sensors
osteoclasts
bone-dissolving cells on the bone surfaces (osteolysis)
develop from the same bone marrow stem cells as blood cells (not osteogenic cells)
large (up to 150 µm) w/ 3-4 nuclei (up to 50)
reside in pits called resorption bays
ruffled border faces the bone surface
compact bone structure
osteon: functional unit of compact bone, concentric layers of bone tissue (lamellae — torsional strength) surrounding the Haversian canal (where oxygen/nutrients, blood vessels, nerves, and connective tissue are)
lacunae: small spaces in each lamella where osteocytes are located
canaliculi: tiny channels that connect lacunae and allow nutrient and waste product exchange b/t osteocytes
bone remodeling occurs…
in response to hormonal changes
in response to mechanical stress
bone turnover is constant
osteoporosis
too much bone breakdown (more osteoclasts than osteoblasts)
Paget’s disease
inappropriate buildup of bone (more osteoblasts than osteoclasts)
pituitary giantism
excess growth hormone during development (before adulthood)
pituitary dwarfism
deficiency of growth hormone during development (proportional)
seckel syndrome
slow growth before birth
acromegaly
excess of growth hormone after epiphyseal plate closes → acts on cartilage (changes in nose and ears, square jaw)
growth hormone (source & effects)
source: pituitary gland
effects: increased muscle and bone growth through intermediate growth factors (insulin-like growth factors)
thyroxine (source & effects)
source: thyroid gland
effects: stimulates osteoblasts & promotes protein synthesis → bone growth
calcitriol/vitamin D3 (source & effects)
source: skin (sun) — produced by the sequential action of the skin, liver, and kidneys
effects: increases calcium in plasma — promotes reabsorption of calcium (filtered from the blood) from kidneys, promotes absorption of calcium from foodstuffs that’re in the digestive system
without calcitriol = calcium and phosphate levels in the blood are too low for normal deposition → bone softness (rickets/osteomalacia)
calcitonin (source & effects)
source: parafollicular cells of the thyroid gland
effects: secreted when blood/plasma [Ca2+] is too high → lowers [Ca2+] through osteoclast inhibition & osteoblast stimulation
more important in children b/c osteoclasts are more highly active
parathyroid hormone (source & effects)
source: parathyroid glands (on posterior surface of the thyroid gland)
effects: secreted when blood calcium is low → raises [Ca2+] by
binding to osteoblast receptors → stimulate osteoclast population & promote bone resorption
promote Ca2+ reabsorption by the kidneys → less calcium is lost in the urine
promotes final step of calcitriol synthesis in kidneys → enhancing the calcium-raising effect of calcitriol
inhibits collagen synthesis by osteoblasts → inhibits bone deposition
estrogen (source & effects)
source: ovaries of females; smaller amounts by adrenal glands and testes in males
effects: stimulates osteoblasts & adolescent growth; prevents osteoporosis
testosterone (source & effects)
source: testes in males; smaller amounts in adrenal glands and ovaries in females
effects: stimulates osteoblasts and promotes protein synthesis, thus promoting adolescent growth and epiphyseal closure
osteoporotic bone loss
weakening of spongy bone tissue (osteoclast > osteoblast)
common in women post-menopause in head of femur & vertebrae
During intramembranous ossification, what type of tissue is replaced with bone?
fibrous connective tissue/mesenchyme
During endochondral ossification, what type of tissue is replaced w/ bone?
hyaline cartilage
Which bones would be formed by intramembranous ossification?
flat bones: mandible, frontal bone, occipital bone, etc.
Which bones would be formed by endochondral ossification?
long bones: femur, humerus, rib, vertebrae, etc.
The growth in bones in width most resembles _________
intramembranous ossification
The growth in bones in length most resembles __________.
endochondral ossification