Biomedical Engineering Reference
In-Depth Information
acid (HA), are more common. These molecules are covalently linked to peptide
chains to form high-molecular-weight complexes called proteoglycans. Along
with collagen, GAGs are commonly utilized to make bioactive scaffolds used to
induce regeneration.
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4.2.4 Appendages
Skin appendages (such as hair follicles) are derived from invaginated epidermal
tissue and are often found projecting deep into the dermis.
8
In certain regions, such
as the scalp, these invaginations actually project into the subcutis (subcutaneous
tissue.)
Skin appendages include the pilosebaceous apparatus, eccrine sweat glands and
apocrine glands. The pilosebaceous apparatus is composed of a hair follicle,
sebaceous gland and associated arrector pili muscle. The lower most portion of a
mature hair follicle (bulb) contains mitotically active germinative cells (which
produce the hair shaft in a manner somewhat analogous to epidermal keratiniza-
tion).
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Eccrine sweat glands main function is thermoregulation, which is
accomplished through excreting sweat. Apocrine glands which are found only in
the axilla and anogenital regions, produce scents (known in some vertebrate
species to function as pheromones.)
A critical issue in wound healing and skin regeneration involves the presence of
cells capable of self-renewal and differentiation plasticity, or so-called tissue stem
cells. For a number of years, epidermal stem cells were believed to reside
exclusively at the tip of rete ridges within the basal cell layer. Recently, an
additional epithelial stem cell niche has been identified in the bulge region of the
hair follicle, near the insertion site of the arrector pili muscle.
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There has been
active interest in these populations, with respect to their contributions to re-
epithelialization, as well as to their potential to differentiate along adnexal lines.
The former is a prerequisite for wound repair and the latter is a fundamental
component of authentic skin regeneration.
Hair follicle (replete with its stem cell niche) is not a trivial structure in
Homo
sapiens
. It is one of few human tissues that retains the ability to involute cyclically,
from a mature and structurally complex anagen (growing) follicle, to a rudimentary
telogen (quiescent) follicle, and then regenerate back into the anagen form.
16,17
Teleologically and evolutionarily in lower vertebrates, this characteristic may
represent the residue of a survival necessity, favoring shorter hair for efficient
locomotion or adaptation to temperature variations (as follicular cycling ultimately
determines hair length.) However, its persistence in humans provides a unique
opportunity to probe the mysteries of how complex structures regenerate in the adult.
4.2.5
Subcutaneous tissue
Lying subjacently deep with respect to the dermis, the subcutaneous tissue has dual
