- Discussion:
- there are three types of cells intimately associated with bone:
osteocytes,
osteoblasts, and
osteoclasts;
- osteocytes dwell in small lacunae within the bone matrix;
- they are oval in cross section, their longest diameter being roughly
parallel to the lamellae of mature bone;
- Woven Bone: (primary bone)
- forms embryonic skeleton, and is largely absent after age 4 yrs;
- woven bone formation is seen in fracture callus in both children and adults;
- woven bone does not contain lamellae, and rather has a relatively disorganized
array of collagen and irregular mineralization pattern;
- Lamellar Bone (secondary bone):
- layering of bone is the hallmark of mature bone;
- it is in compact cortical bone of diaphysis that
haversian systems are found;
- nutrients pass thru
haversian canals, which run in the direction of the long
axis of the bone, to bone cells that lie buried in the thick cortex;
-
osteon:
- refers to the bone surrounding the haversian system;
- the haversian canals are surrounded by concentric rings or lamellae,
each about 5 micrometers thick;
- histologically, these layers of bone will contrast with one another under polarized
light because of the different orientations of
collagen fibers in each layer;
-
cement lines surround the outer border of each osteon;
- histologically they are seen as the demarcation between the point that
bone resorption ceased, and bone formation began;
- collagen fibers and canaliculi do not cross cement lines;
- these lines do not bind adjacent lamellae but rather than separate them;
-
canaliculi:
- w/in concentric lamellae of mature bone are small lacunae containing
osteocytes;
- from lacunae extend fine canaliculi that contain cell processes of
osteocytes;
- these processes connect
osteocytes to one another and to
haversian canal,
and are the pathways of nutrition from the canal to the bone cells;
- canaliculi connect
osteocytes within one haversian system only;
- they do not cross cement lines to other systems;
- since canaliculi are much too small to permit passage of RBC's,
osteocytes
must depend on diffusion & cell pumping mechanisms for their supply of
oxygen & nutrients;
