Biomedical Engineering Reference
In-Depth Information
their increased expression has been associated with cellular senescence. This
demonstrates that stem cells require mechanisms to prevent premature senescence
in order to self-renew throughout adult life. In contrast, restricted neural progenitors
from the enteric nervous system and forebrain proliferate normally in the absence of
Bmi-1 [
2
]. Thus Bmi-1 dependence is conserved between stem cells and distin-
guishes the cell cycle regulation of stem cells from the cell cycle regulation of at
least some types of restricted progenitors. Using similar approaches, studying of
additional pathways could be done that as hypothesized, that will also regulate stem
cell self-renewal, and that will contribute to understanding the molecular basis of
self-renewal [
3
] .
Stem Cell Aging
Aging involves a slow deterioration of tissue function, including an elimination of
new growth and decreased capacity for repair. Aging is also associated with increased
cancer incidence in all tissues that contain stem cells. These observations suggest a
link between aging and stem cell function because stem cells drive growth and
regeneration in most tissues, and because at least some cancers are thought to arise
from the transformation of special stem cells—cancer stem cells [
4
]. It could be that
much of age-related morbidity in mammals is determined by the influence of aging
on stem cell function. Hematopoietic and nervous system stem cells were found to
undergo strikingly conserved changes in their properties with aging [
4
] . Therefore,
testing the hypothesis that there are conserved changes in gene expression within
stem cells that regulate these age-related changes in function is of vital importance.
It was hypothesized that stem cell aging is influenced by genes regulating the prolif-
erative activity of stem cells during development, as well as by genes protecting
stem cells from the wear and tear of adult life [
5
] . The identi fi cation of these genes
might significantly contribute to the better understanding of the aging process.
Organogenesis from Stem Cells and Nature's
Tissue Engineering Designs
The most fundamental question in organogenesis is: How do a small number of stem
cells give rise to a complex three-dimensional tissue with different types of mature
cells in different locations? The hematopoietic and nervous systems employ very
different strategies for generating diversity from stem cells. The hematopoietic sys-
tem assiduously avoids regional specialization by stem cells. Hematopoietic stem
cells are distributed in different hematopoietic compartments throughout the body
during fetal and adult life, and yet these spatially distinct stem cells do not exhibit
intrinsic differences in the types of cells they generate [
5
] . This contrasts with the
nervous system, where even small differences in position are associated with the
