Biomedical Engineering Reference
In-Depth Information
healing by secondary intention. Elongation of the distal phalangeal bone during
regeneration has only been documented for children [7], but most studies lack
radiographic data that allow for the assessment of bone regrowth. Animal models
for digit tip regeneration in adults demonstrate distal bone growth associated with
a regeneration response. There are several documented instances of regeneration
of the distal phalangeal element of the toe following traumatic injury or voluntary
resection to relieve hummer toe [8]. Thus, it would appear that the regenerative
capabilities in human limbs include the tips of both fi ngers and toes.
14.2.4
The Development of Bones: Osteogenesis
The skeleton is generated through three lineages: the somites generate the axial
skeleton, the lateral plate mesoderm generates the limb skeleton, and the cranial
neural crest gives rise to the branchial arch and craniofacial bones and cartilage.
There are two major modes of bone formation or osteogenesis, and both involve
the transformation of a preexisting mesenchymal tissue into bone tissue. The
direct conversion of mesenchymal tissue into bone is called intramembranous
ossifi cation. In other cases, the mesenchymal cells differentiate into cartilage, and
this cartilage is later replaced by bone. The process by which a cartilage intermedi-
ate is formed and replaced by bone cells is called endochondral ossifi cation. The
cranial neural crest cells form bones through intramembranous ossifi cation. In
the skull, neural crest-derived mesenchymal cells proliferate and condense into
compact nodules. As shown in Figure 14.1, some of these cells develop into capil-
laries; others change their shape to become osteoblasts, committed bone precursor
cells. The osteoblasts secrete a collagen-proteoglycan osteoid matrix that is able
to bind calcium. Upon embedding in the calcifi ed matrix, osteoblasts become
osteocytes. As calcifi cation proceeds, bony spicules radiate out from the region
Osteoid
matrix
Calcified
bone
Bone cell
(osteocyte)
Osteblasts
Loose mesenchyme
Blood vessel
Osteoblasts
Figure 14.1
Schematic diagram of intramembranous ossifi cation.
