Bone Objectives Flashcards
Ectoderm
On the external surface of the embryo
- skin, adnexa, neural tissue
Mesoderm
Middle layer
- bone, muscle, and all connective tissues
Endoderm
Inner layer, is an outpocket of the pharynx
- epithelial lining of GIT, associated ducts/glands, viscera
- also lines pharynx, respiratory tract, urethra, bladder
Mesenchyme originates from which tissue layer?
Mesoderm
How does undifferentiated mesenchyme become organized and develop into a limb?
Mesoderm of left and right lateral body wall forms Wolff’s crest (cylindrical thickening) –> focal ectodermal thickenings occur over Wolff’s crest at sites of future limb placements = apical ectodermal ridge
- AER initiates and organizes limb bud formation and remains throughout limb development
_______ induces mesenchymal cells to condense to form a cartilage model for each bone of the developing limb
Apical ectodermal ridge
What is the difference in how bone and cartilage grow?
Bones only undergo appositional growth (new chondrocytes applied to existing structure), cartilage tissue grows by appositional growth and by interstitial growth (cartilage tissue expands within)
Long bone development
Condensation of mesenchyme to form cartilage model –> formation of perichondrium and invasion of nutrient artery –> degeneration of chondrocytes to form medullary cavity –> perichondrium becomes periosteum –> periosteum forms concentric layers of cortical bone in diaphysis –> vessels at joint capsule insertion invade epiphyseal area –> epiphyseal and metaphyseal physes form
Epiphyseal physis
3D growth of epiphysis by proliferation of chondrocytes followed by enchondral ossification to form epiphyseal cancellous and subchondral bone
Epiphyses
Ends of long bone
Diaphysis
Long compact shaft of the bone
Epiphyseal plate
Separates the epiphysis and diaphysis
- growth plate
Metaphysis
Transitional spongy bone that joins the diaphysis to the epiphyseal plate
Medullary space
Area of spongy bone in the middle of the diaphysis
- is not hollow, consists of bony spicules to provide space for bone marrow (cancellous bone)
Periosteum
Outside of bone covered by cells
Endosteum
Internal surfaces of bone trabeculae within the marrow cavity
- also lined with osteoprogenitor cells
Osteoblasts
- line periosteal and endosteal surface
- produce collagen-rich osteoid matrix that later mineralizes
- main cells involved in ossification process
Osteoclasts
- derived from mononuclear phagocyte cell line
- fusion of cells to form multinucleated cell, responsible for dissolving bone matrix and collagen to allow reshaping of bone to accommodate stress
How does growth in length occur?
Osteoblasts differentiate –> layer of loosely organized collagen matrix is deposited –> new bone matrix calcifies –> osteoclasts move into the metaphysis to resorb the woven bone and calcified cartilage –> new osteoblasts form organized sheets of type 1 collagen
*cells of the zone of proliferation are advancing the growth plate away from the diaphysis, while the ostoblasts/clasts are converting primary woven bone to lamellar bone at the diaphyseal side of the growth plate
How does growth in diameter occur?
Deposition of new bone on the outside by cells within the periosteum
- bone resorption is key
- deposition of new bone is accompanied by resorption of older bone at the interior edge of the shaft of the diaphysis
- allows marrow cavity within the bone to expand
Anatomy of a growth plate
- zone of proliferation: chondrocytes furthest from the diaphysis
- zone of maturation: chondrocytes are not dividing and are enlarged
- zone of hypertrophy: hypertrophy and vacuolation of the chondrocytes
- zone of calcified cartilage: cartilaginous matrix surrounding cells begins to calcify
Process of endochondral ossification
Proliferating chondrocytes from inner mitotic layer mature to produce collagen and proteoglycan matrix –> mature chondrocytes hypertrophy –> hypertrophic chondrocytes form matrix vessicles and die –> hydroxyappetite crystals form at matrix vesicle sites calcifying the cartilage –> capillary loops with osteogenic mesenchyme invade tunnels formed by dying hypertrophic chondrocytes after cartilage calcifies –> osteoblasts layer woven bone on spicules of calcified cartilage = bony trabeculae –> osteoclasts remodel calcified cartilage spicules and bone trabeculae to produce cancellous bone
What allows bone to bend and absorb concussion?
Trabecular plates (cancellous bone)
What portion of bone is dense and strong?
Cortical bone of the diaphysis
- compact bone
What strengthens the bone?
Osteons
What 3 things generally result from decreased calcium?
- limb deformity during growth
- osteomalacia
- Bran’s disease: big head, hyperparathyroidism (weak bones, lameness)
What occurs as a result of inadequate Ca during growth?
Faulty endochondral ossification
- poor calcification of cartilage in physis
- delayed bone formation
- wide unstable cartilage zone in physis
- limb deviation
What occurs as a result of inadequate Ca in diet of mature horses?
Bran disease
- osteoclasts reabsorb bone to maintain serum Ca level –> osteoblasts produce too much osteoid to compensate for lost bone –> osteoid fails to mineralize in low Ca conditions –> bone is fibrous and thickened
Ca effect on the nervous system
Essential for nerve conduction by stabilizing voltage gated channels
- decreased Ca leads to nerve cell hyperexcitablity with tetany (eclampsia), or paralysis in cows
- too much calcium allows channels to become resistant to opening = nerve transmission stops
Ca effect on the blood
Essential for blood clotting
- decreased Ca leads to inadequate clot formation
- ex: EDTA chelates Ca to stop its effect on coagulation
Hypercalcemia
Depresses nervous system and muscle activity
- apparent at Ca levels > 12 mg/dl
- decreased appetite
- constipation
How are Ca levels in plasma tightly regulated?
- increase/decrease GI absorption
- increase/decrease renal losses
- move Ca into or out of bone
What systems are involved in controlling Ca levels?
- vitamin D
- parathyroid hormone via chief cells of parathyroid gland
- calcitonin via C cells of the thyroid
Vitamin D
D3 must be activated to have an effect on Ca levels
- active vit D increases intestinal absorption of Ca
- is necessary for parathyroid hormone to work on bone
PTH
- Most powerful mechanism of controlling Ca and phosphate
- increases Ca by increasing GI absorption, and increasing reabsorption of Ca and P from bone
- decreases renal losses of Ca, increases excretion of P
Calcitonin
Stimulated by increased plasma Ca
- decreases Ca (opposite of PTH)
- decreases absorptive action of osteoclasts
- decreases formation of new osteoclasts
- minor effect on kidneys and GIT
- limited effect compared to PTH (no lasting effect after thyroids are removed)
Milk fever
Need for adequate Ca at motor end plate is more pronounced in cows than other species
- decreased Ca causes decreased Ach release = nerve excitability in cows
- paralysis and death
Where is Ca stored/located in the body?
- bone
- kidneys
- GIT
What are the 3 forms of Ca found in plasma?
- ionized: diffusible thru membranes (50%)
- protein bound: nondiffusible (41%)
- complexed to negative ions and nonionized (9%)
How much total Ca is in plasma?
9.4 mg/dl = 2.4 mmol
Hypoparathyroidism
Inadequate PTH secretion
- decreases osteocyte/clast bone resorption
- Ca level in ECF drops
- bone stays strong
Hyperparathyroidism - primary
Excess PTH secretion (due to tumor)
- extreme osteoclastic reabsorption of bone
- weak bone with fractures
- high Ca levels and low P levels in ECF
- crystals deposit in tissues
- mild elevations lead to kidney stones
- high Ca = nerve depression, constipation, death
Hyperparathyroidism - secondary
Poor nutrition and decreased Ca intake
- causes low serum Ca
- seen also when kidney damage causes inadequate activation of vitamin D = inadequate intestinal absorption of Ca, low serum Ca
- stimulates PTH to maintain blood levels –> increased PTH leads to Ca reabsorption from bone
- nutritional form common in horses, renal form common in dogs/cats with chronic renal failure
Rickets
Inadequate vitamin D
- usually seen in spring after staying indoors during winter
- Ca absorption from bone prevents clinical signs for a few months, liver stores of D3 are depleted by spring
- mildly low Ca in ECF, pronounced decrease in P
- weak bones due to increased PTH
- late stage: tetany as bone stores are depleted and ECF Ca drops
Excessively rapid administration of Ca containing solutions
Used to treat milk fever in cows
- monitor heart rate
- too rapid administration slows heart = death
- stop administration if arrhythmia noted
Excessively rapid administration of Ca depleting solutions
Rapid IV tetracycline antibiotic administration
- chelates Ca and takes it out of solution
- acute death due to hypocalcemia and cardiac arrhythmia
