CHAPTER 5
Cartilage and Bone
93
Bone Development and Growth
Figure 5-11A.
Intramembranous ossif
cation,
±etal
head.
H&E,
3
84; inset
3
210
Intramembranous ossif
cation
is a process of bone
formation involving the transformation of condensed
mesenchymal tissue
into bone tissue by differentiation
of mesenchymal cells into
osteoblasts
and deposition of
osteoid (prebone)
.
Osteoid
is unmineralized new bone,
which contains organic components. Soon after the new
bone is deposited, it becomes calciF
ed bone, which is
largely composed of
calcium
and
phosphate
.
Osteoblasts
often line up on the surface of the bone matrix. They are
cuboidal and low columnar in shape, and each osteoblast
contains a large round nucleus and basophilic cytoplasm
containing rich
RER
and
Golgi complexes
, indicating
their activity in producing protein and organic compo-
nents (±ig. 5-11B).
Mature
osteoblasts
are trapped inside
the bone matrix to become
osteocytes
. Osteoid appears
pink in H&E stain, in contrast to mineralized bone
matrix that appears dark red-purple in H&E stain.
Osteoid
Osteoid
(prebone)
(prebone)
Osteoid
(prebone)
Blood
vessel
Mineralized
Mineralized
bone matrix
bone matrix
Mineralized
bone matrix
Osteoblasts
Osteoblasts
Osteoblasts
Bone marrow
Bone marrow
Osteoblasts
Osteoblasts
Osteoblasts
Bone marrow
Osteoid
Osteoid
(prebone)
(prebone)
Osteoid
(prebone)
A
Euchromatic
Euchromatic
nucleus
nucleus
Mineralized
Mineralized
bone matrix
bone matrix
Osteoid (prebone)
Osteoid (prebone)
Mineralized
bone matrix
Euchromatin
Rough
endoplasmic
reticulum (RER)
Type I
collagen
fibrils
Osteoid (prebone)
B
Figure 5-11B.
Osteoblasts.
EM,
3
19,600
The three
osteoblasts
in this electron micrograph are clearly active in the synthesis and secretion of
type I collagen
and other proteins
of bone matrix. Note the high content of
euchromatin
in the nuclei and the predominance of
RER
in the cytoplasm. Minute collagen
F brils (type I collagen) are just discernible in the layer of matrix adjacent to the cells (
prebone
or
osteoid
). The deeper, mineralized
bone matrix has a homogeneous appearance that masks the presence of the collagen F
brils. The
dotted white line
indicates the inter-
face between the osteoid above and the
mineralized bone matrix
below.
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