Biomedical Engineering Reference
In-Depth Information
the size of ordinary capillaries. Despite the thin walls of these vessels, Trueta
and Harrison [21] were not able to demonstrate open fenestrations between the
endothelial cells. However, it should be noted that Zamboni and Pease [22], using
electron microscopy, considered the vessels in red marrow to consist of flattened
reticulum cells with many fenestrations and no basement membrane. This would
mean that there is minimal hindrance at the sinusoid wall for molecular exchange.
In the fatty marrow, the capillaries are closed and continuous like those of other
tissues such as muscle [21]. This is supported by
in vivo
observations in the rabbit
[23] that the vessels varied according to the functional state of the marrow. It was
estimated that the sinusoid was up to seven times the size of the marrow capillaries,
which have a diameter of 8
µ
m.
9.4
Microvascular Network of Cortical Bone
Throughout the cortex of long bones, there is a capillary network housed in small
passages (Figures 9.4 and 9.5). In immature bones, these are arranged rather
haphazardly, but as the bone remodels and matures, a more distinct pattern
emerges. In the mature dog and the human, there are two basic systems, the
Haversian canals, which run longitudinally, and the Volkmann canals, which run
radially (Figures 9.2 and 9.5). The two systems are intimately anastomosed to
each other. The vessels within the Haversian canals of the human tibia have been
examined by microscopy of decalcified sections [24]. The majority of the vessels
were observed to be a single layer of endothelial cells. Occasionally, near the
endosteal surface of the cortex, small arterioles with a muscular coat were seen,
usually accompanied by a larger vein.
A comprehensive examination of the cortical bone of mature and immature
dogs by electron microscopy has been reported by Cooper
et al
. [25]. This revealed
considerable detail of the capillaries in bones. The Haversian canals ranged in size
from 5 to 70 µm and contained either one or two vessels that had the ultrastructure
of capillaries. On the transverse section, they were lined by one or more endothelial
cells, which were surrounded by a continuous basement membrane 400-600
˚
A
thick. The junctions of the endothelial cells varied from simple juxtapositioning to
a complex interlocking. These investigators found no smooth muscle cells in the
walls of the vessels in the Haversian canals. This picture is supported by electron
microscopy studies [26] that showed that the cortical capillaries of the growing rat
were similar to those found in the skeletal muscle, although a basement membrane
surrounding the capillaries could not be demonstrated. Thus, it appears that the
capillaries of the bone are a closed tube formed from a single layer of endothelial
cells. It has been suggested that transendothelial passage of substances involves two
separate pathways: one through the intercellular clefts for hydrophilic substances
and another across the endothelial cells themselves for lipophilic substances. If the
intercellular capillary clefts are present, they are probably filled with material that
makes their permeability low. This is suggested by the work of Cooper
et al
. [25],
