Biomedical Engineering Reference
In-Depth Information
Up until a decade ago, the classic paradigm of wound healing, involving
stem cells restricted to organ-specific lineages, has never been seriously chal-
lenged.
64,65
Since then, the notion of adult stem cells having 'plasticity' or the
ability to differentiate into non-lineage cells has emerged as an alternative expla-
nation. To be more specific, hematopoietic progenitor cells (which give rise to
mature hematopoietic cells) may have the ability to 'de-differentiate' back into
hematopoietic stem cells and/or 'transdifferentiate' into non-lineage cells, such
as fibroblasts.
4.7.1 Basal stem cells (BSCs) and hair follicular stem cells
(HFSCs)
It is thought that the epidermis contains two (or perhaps three) groups of self-
renewing cells that have the capacity to generate new cells to replenish the
epidermal cell population.
10,17
One group (BSCs) resides at the apex of rete ridges
(of basal cell layer.) These cells are responsible for generating cells residing
outside the hair follicles, sometimes referred to as the interfollicular epidermis
(IFE.)
The other group (or perhaps two groups) is responsible for generating hair
follicle cells and sebocytes (sebaceous gland cells.)
35
Known as hair follicular stem
cells (HFSCs) or bulge stem cells, these cells reside at the hair bulge (region of the
outer root sheath.)
15,17
They appear to be multipotential and may have the capacity
to generate all subtypes of epidermal cells. Although their regenerative potential is
important, HFSCs are non-essential to epidermal homeostasis, as illustrated by the
absence of appendages in scar tissue.
4.7.2
Skin-derived precursors (SKPs): dermal stem cells
Recently, a dermal-derived, multipotential stem cell population has been de-
scribed.
63,66
Known as skin-derived precursors (SKPs), these cells are thought to
reside in the papillary dermis and the dermal sheath of hair follicles.
66
In mice,
SKPs are typically isolated from normal skin, but in humans neonatal foreskin has
been proven to be a superior source.
63
Interestingly, skin-derived precursors can be manipulated to differentiate along
both mesodermal and neural cell lines
in vitro
.
63
SKPs exhibit features similar to
embryonic neurocrest cells
66
and can also give rise to neurons, glial cells (Schwann
cells), smooth muscle cells and adipocytes.
67
In the presence of fibroblast growth
factor and epidermal growth factor, SKPs express nestin (intermediate filament, a
neural stem cell marker), fibronectin and vimentin (intermediate filament), as well
as embryonic-like transcription factors. The presence of TGF-
β
seems to promote
only proliferation and not differentiation or progency.
67
