Biomedical Engineering Reference
In-Depth Information
From a tissue engineering standpoint, a key challenge is to design the environment at
both the micro and macro scales. This problem is characterized by two important
considerations:
The microenvironment (its chemical, geometric, cell architectural, and diffusional
characteristics)
Interactions with other tissues (source of nutrients, exchange of respiratory gases,
removal of waste products, and delivery of soluble protein such as growth factors)
Cellularity
The packing density of cells is on the order of a billion cells per cubic centimeter (cc).
Tissues are typically operating at one-third to one-half of packing density, such that typical
cell densities in tissues are on the order of 100 to 500 million cells per cc. Since the charac-
teristic length scale is about 100
m, the order of magnitude of the number of cells found
in a tissue microenvironment can be estimated: a 100
m
m cube at 500 million cells per cc
contains about 500 cells. Simple multicellular organisms, such as
m
, a much-studied
small worm, that have about 1,000 cells, provide an interesting comparison.
The cellularity of the tissue microenvironment varies among tissues. An example of a
tissue with low cellularity is cartilage. The function of chondrocytes in cartilage is to main-
tain the extracellular matrix. Cartilage is essentially avascular, alymphatic, and aneural.
Thus, many of the cell types found in other tissues are not present, which limits the range
of functions that the tissue can perform. The cellularity of cartilage is about a million cells
per cc, or about one cell per cubic 100
C. elegans
m. Most other tissues, in particular those that are
highly metabolically active and/or remodel at a high rate, are much more cellular, with
hundreds of cells per 100
m
m cube.
The tissue microenvironment is characterized by having a number of cell types
(Table 6.10). In addition to the parenchymal cells (thetissuetypecells—e.g.,hepato-
cytes in liver), a variety of accessory cells are found in all tissues, as described briefly
following.
m
TABLE 6.10
Cells That Contribute to the Tissue Microenvironment
Stromal cells: derivates of a common precursor cell
Mesenchyme
Fibroblasts
Myofibroblasts
Osteogenic/chondrogenic cells
Adipocytes
Stromal-associated cells: histogenically distinct from stromal cells, permanent residents of a tissue
Endothelial cells
Macrophages
