Page 267 - Atlas of Histology with Functional Correlations

differentiate into osteoprogenitor cells. Angiogenic factors produced by

chondrocytes induce the formation of new blood vessels from the vascular
periosteum. Osteoprogenitor cells and blood-forming cells invade the
degenerating cartilage model with the blood vessels and differentiate into
osteoblasts that proliferate, attach to the calcified cartilage remnants, and begin
to deposit the bone matrix. Initially, the bone matrix is a soft collagenous osteoid

that lacks minerals but is quickly mineralized into bone. The osteoblasts
eventually become surrounded by bone in the lacunae and become osteocytes
with one osteocyte per lacuna. Osteocytes establish a complex cell-to-cell

connection through the canaliculi, which eventually open into central canals with
blood vessels. In addition, osteoprogenitor cells also arise in endosteum, a single
layer of cells that lines all internal cavities in the bone.

Mesenchymal tissue, osteoblasts, blood-forming cells, and blood vessels
form the primary ossification center in the developing bone, which first

appears in the diaphysis or the shaft of the long bone. This is followed by a
secondary ossification center in the epiphysis or the articular surface of the
expanded end of the bone. In all developing long bones, cartilage in the
diaphysis and epiphysis is gradually replaced by bone, except in the epiphyseal

plate region, located between the diaphysis and epiphysis. Interstitial cartilage
growth in this region continues, lengthening the bone until bone growth stops.
Expansion of the two ossification centers eventually replaces all cartilage with
bone, including the epiphyseal plate, ending the bone growth in length. Hyaline

cartilage is not replaced by bone on the free or articulating ends of long bones,
where a layer of permanent hyaline cartilage covers the bone; this is the
articular cartilage.

Intramembranous Ossification

In intramembranous ossification, bone development is not preceded by a
hyaline cartilage model. Instead, bone develops directly from condensation of
mesenchyme cells that produce ossification centers; flat bones develop by this

method. The mesenchymal cells differentiate directly into osteoblasts that
produce the surrounding osteoid matrix, which quickly calcifies. Numerous
ossification centers are formed, anastomose, and produce a network of spongy
bone that consists of thin rods, plates, and spines called trabeculae; this bone

formation is by appositional growth. Located between the trabeculae is the
hematopoietic tissue where the mesenchyme cells transform into blood-forming
cells. In the bony matrix, osteoblasts also become surrounded by bone in the

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