THE NORMAL EXFOLIATION PROCESS
Exfoliation occurs naturally as outworn stratum
corneum cells detach and are removed from the skin’s
surface.
Replacement of stratum corneum cells occurs
by a self-renewal process as epidermal cells move
upward from the basal layer, the stratum basalis, and
through the subsequent two layers, the stratum
spinosum and stratum granulosum, to reach the stratum
corneum. This process requires about four weeks,
although it can be prolonged in aging and other skin
conditions. Normal desquamation occurs invisibly as
single corneocytes, or very small aggregates of them,
detach from the skin’s surface and are shed.
Desmosomes (see gure 2)
are structural bridges
between corneocytes of the stratum corneum that are
dissolved by naturally occurring skin enzymes secreted
by skin cells and hair follicles into the spaces between
cells (intercellular spaces). Internal clocks within cells
gradually weaken the intercellular connections over time
until cohesion decreases to a critical level. Normal skin
barrier function depends upon maintenance of a healthy
stratum corneum, its proper continuous regeneration,
and appropriate shedding of devitalized cells.
ABNORMALITIES OF EXFOLIATION
The most common derangement to the normal process
of exfoliation and skin renewal is aging. With aging,
metabolism slows and regeneration time of the stratum
corneum becomes prolonged. Also with aging, the
glue-like desmosome cement between cells becomes
denser and cell sloughing is more dif cult. These
changes can also accompany premature aging,
as found with advanced photodamage.
In addition to photoaging, other problems can result in
abnormal skin keratinization and epidermal metabolism.
These include hormonal in uences from androgens or
estrogens, exposure to excess epidermal growth factor,
and de ciencies of vitamins A or D. Dullness, poor tone,
increased or uneven skin thickness, poor texture,
roughness, dryness, and loss of youthful appearance
occur if the desquamation process is impaired.
