Biomedical Engineering Reference
In-Depth Information
outermost 'fibrous' layer is composed of fibroblasts, collagen and elastin
fibers,24
24
along with a distinctive nerve and microvascular network.
25
The
inner 'cambium' or 'cellular' layer is positioned in direct contact with
the bone surface. It contains mesenchymal stem cells (Mscs) which have
the potential to differentiate into osteoblasts and chondrocytes,
26-28
and
differentiated osteogenic progenitor cells, fibroblasts, microvessels and
sympathetic nerves.
29
The localization of Mscs and osteoprogenitor cells
within the cambium layer has made the periosteum a target for drug therapies
and cell harvesting for tissue engineering purposes.
The endocortical or inner surface of a bone faces the medullary canal
and is lined with a membranous sheath called the endosteum (Fig. 2.3G).
The endosteum is lined with a single thin layer of bone lining cells (mature
osteoblasts) and osteoblasts which form a membrane over endocortical and
trabecular bone surfaces to enclose the bone marrow.
30
Osteoclasts can also be
present in the endosteum in regions of active bone resorption. The endosteum
contains osteoprogenitor cells, but does not appear to contain either Mscs
or hematopoietic stem cells (hscs). however, a portion of hscs can be
found next to the endosteum, suggesting reciprocal communication between
cells within the endosteum and multipotent hscs.
31
This forms a so-called
'stem cell niche' whereby the cells of the endosteum physically support and
influence stem cell activity.
32, 33
2.3.2 Microscopic bone structure
cortical and trabecular bone are both composed microscopically of bone
structural units (Bsus) or osteons. In cortical bone, the osteons are commonly
referred to as haversian systems (Fig. 2.3a-c). haversian systems are
cylindrical in shape and form an anastomosing network.
34
They contain a
central haversian canal housing blood vessels and nerves which are enveloped
in concentric layers or lamellae of bone tissue. This arrangement gives each
osteon an appearance of a tree trunk in cross-section with a circumferential
cement line representing the tree's bark. The cement line represents the
outermost boundary of an osteon and consists of a 5 µm ring of highly or
similarly mineralized bone as contained within the osteon.
35
uniformly spaced
throughout the lamellae are lenticular cavities called lacunae. Radiating in
all directions from each lacuna are branching canaliculae. These penetrate
the lamellae of the interstitial substance and anastomose with canaliculae of
neighboring lacunae to form a continuous network of interconnecting cavities.
Interstitial bone fills the region between adjacent osteons and appears as
short arched layers which are remnants of older partially resorbed osteons.
In trabecular bone, osteons are referred to as a bone packets which are saucer
shaped and consist of stacks or layers of lamellae (Fig. 2.3d,e). adjacent
packets in trabecular bone are separated by a cement line.
