Page 232 - Fundamentals of anatomy physiology

Figure 6–12 Intramembranous Ossification.
Parietal bone
Frontal
bone
Occipital bone
Mandible
Intramembranous ossification
starts about the eighth week of
embryonic development. This
type of ossification occurs in the
deeper layers of the dermis,
forming dermal bones.
Areas of spongy bone are
remodeled forming the
diploë and a thin covering
of compact (cortical) bone.
Chapter 6 Osseous Tissue and Bone Structure 219
1 Mesenchymal cells cluster Bone matrix
Osteoid
together, differentiate into Mesenchymal cell
osteoblasts, and start to Ossification center
secrete the organic Blood vessel
components of the matrix. Osteoblast
The resulting osteoid then
becomes mineralized with 6
calcium salts forming
bone matrix. Spicules
2 As ossification proceeds, Osteocyte
some osteoblasts are Blood vessel
trapped inside bony trapped within
pockets where they bone matrix
differentiate into osteo-
cytes. The developing bone
grows outward from the
ossification center in small
struts called spicules.
3 Blood vessels begin to
branch within the region
and grow between the
spicules. The rate of bone
growth accelerates with
oxygen and a reliable
supply of nutrients. As
spicules interconnect, they
trap blood vessels within
the bone.
4 Continued deposition of
bone by osteoblasts
located close to blood
vessels results in a plate
of spongy bone with
blood vessels weaving
throughout.
5 Subsequent remodeling Fibrous periosteum
Blood vessels
around blood vessels trapped within
produces osteons typical bone matrix
of compact bone.
Osteoblasts on the bone Cellular periosteum
surface along with
connective tissue around
the bone become the
periosteum.