- Discussion:
- haversian systems are of two types, primary and secondary;
- primary systems are formed on the growing surface of bone;
periosteal vessels running in long axis of bone become surrounded
by bone, first in a trough, then completely buried in canal;
- this canal is at first much larger than vessels;
-
osteoblasts line canal, &, by producing new concentric rings of
bone at its periphery, gradually narrow the lumen until vessels,
now in a much smaller canal, come to lie within the concentric
lamellae of bone that characterize an Haversian system;
- it should be noted that these systems are formed from periphery of
canal toward the center;
- secondary haversian systems are formed in newly eroded channels within
depths of the cortex;
- constant internal
remodeling of cortex of bone takes place
throughout life;
-
osteoclasts from surface or from adjacent canals resorb bone to
form new channels within the cortex;
- these channels are referred to as
resorption canals;
- advancing front of the
resorption canal is called the cutting cone;
- vessels follow
osteoclasts into these
resorption canals; later, canal
walls become lined w/
osteoblasts, & new haversian
system is formed;
- this continuous alteration of internal architecture of bone enables it
to adapt to changes in stress & metabolic demands throughout life;
