One of the most obvious features of cancellous or spongey bone is the absence of Haversian Systems or Osteons.  Remember, these are only found in compact bone.

During bone formation, the first bone to form is always cancellous.  Where compact bone is needed, cancellous bone forms first and later is remodeled into the denser, compact type.

As bone forms, a special cell(derived from mesenchyme) called the osteoprogenitor cell forms fibroblasts and osteoblasts.  The fibroblasts form the collagen fibers of the matrix.  Thereafter, osteoblasts influence local conditions to initiate deposition of the hydroxyapatite ground substance around the collagen fibers.  The first crystals of bone that form are pointed and needle-like so histologists call these spicules.  Here is a view of a forming spicule .  A single osteocyte is visible within a lacuna.  Also, on the edge of the bone is a single cell that could be an osteoblast.  Remember, when osteoblasts are surrounded by the hydroxyapatite, they become osteocytes.  Osteocytes occupy lacunae!

As the spicules of cancellous bone grow in various directions, they meet and fuse to form the interconnecting "struts" of cancellous bone called trabeculae.  A close examination of a single trabeculum in areas of active bone formation enables one to see successive layers or lamellae of bone.  There appear to be three  lamellae here.  This is an area of active bone formation, evidenced by numerous osteoblasts lined up along the edge of the newly forming bone.

Open spaces between trabeculae of cancellous bone are typically occupied by myeloid tissues or bone marrow.  Myeloid tissues are the source for all "formed elements" of the blood.  The development and mobilization of these cells and cellular products requires numerous blood vessels.  So, if you think you are looking at cancellous bone, look for myeloid tissue!  A very characteristic cell found here is the megakaryocyte, the cell that forms blood platelets!

As indicated previously, cancellous bone is always the first type of bone to form.  Therefore, in bone repair, this type precedes compact.  Repair of bone can involve both intramembranous and endochondral bone formation processes as mesenchyme cells of periosteal and endosteal membranes mobilize to form the new supportive tissues needed.

Now, lets take a look at cancellous bone forming:

• within mesenchyme via intramembranous bone formation!  Can you see the osteoblasts along the edges and the osteocytes in lacunae?

• within cartilage via endochondral bone formation!  Note the large lacunae containing chondrocytes on the left.  Smaller lacunae containing osteocytes can be seen to the right.

• within cartilage at higher magnification.  You can see new osteocytes in lacunae, osteoblasts along the edge of bone, remnant cartilage, myeloid cells, and a blood vessel!  See if you can locate these.  Here is a labelled view!

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