Conditioning Agents for Hair and Skin

Biology of the Hair and Skin 25
feel normal, the water content of this layer must be above 10% (54). Water is
lost through evaporation to the environment under low humidity conditions
and must be replenished by water from the lower epidermal and dermal layers
(55). The stratum comeum must have the ability to maintain this moisture or
the skin will feel rough, scaly, and dry. However, this is indeed a simplistic view,
as there are minimal differences between the amount of water present in the
stratum corneum of dry and normal skin (56). Xerotic skin is due to more than
simply low water content (57). Electron micrographic studies of dry skin demonstrate
a stratum corneum that is thicker, fissured, and disorganized. It appears
that the scaly appearance of xerotic skin is, in part, due to the failure of
corneocytes to desquamate appropriately.
Other disease states, such as psoriasis and atopic dermatitis, also demonstrate
abnormal barrier function due to ceramide distribution (58,59). Interestingly,
xerosis tends to increase with age, due to a lower inherent water
content of the stratum corneum (60). However, this fact does not totally account
for the scaliness and roughness of aged skin; probably an abnormal
desquamatory process is also present (61).
B. Epidermal Barrier Repair
Once the epidermal barrier has been damaged, signals must be transmitted
to the intracellular machinery to initiate repair or reconditioning of the skin.
Remoisturization of the skin occurs in four steps: initiation of barrier repair,
alteration of surface cutaneous moisture partition coefficient, onset of dermal-epidermal
moisture diffusion, and synthesis of intercellular lipids (62). It
is generally thought that a stratum corneum containing between 20% and 35%
water will exhibit the softness and pliability of normal stratum corneum (63).
Perturbations within the barrier must be sensed before the onset of lamellar
body secretion and a cascade of cytokine changes associated with adhesion
molecule expression and growth factor production (64). Thus, if skin with barrier
perturbations is occluded with a vapor-impermeable wrap, the expected
burst in lipid synthesis is blocked. However, occlusion with a vapor-permeable
wrap does not prevent barrier recovery (65). Therefore, transepidermal water
loss is necessary to initiate synthesis of lipids to allow barrier repair and skin
reconditioning to occur (66,67).
C. Mechanisms of Moisturization and Skin Conditioning
Once skin damage has occurred and the barrier damaged, reconditioning can
occur only if the loss of moisture is retarded. This is the goal of moisturizers,
which function temporarily until skin integrity can be reestablished. There are
three physiological mechanisms for rehydrating the stratum corneum: the use
of occlusives, humectants, and hydrophilic matrices (68).