346 JOURNAL OF COSMETIC SCIENCE
The total time for SC on the volar forearm to turn over increased from 19.8 ± 1.4 days in
the under 35 years old group to 28.1 ± 2.7 days in an over 60 age group.56 Similar changes
were seen on the upper inner arm as shown in Table II. Their results agree with the positive
correlation between age and the time to turn over the SC reported by Roberts and Marks.87
Grove found the number of SC cell layers was approximately constant between age groups
(Table II) indicating that the increased SC turnover time results from decreased epidermal
cell proliferation rates with age.24,56 Marks reported a small but statistically significant
decline with age in the number of epidermal cells in the DNA synthesis phase.88 Engelke
et al. investigated differences between young normal skin, young dry skin, aged normal
skin, and aged dry skin89 using an antibody that stains the nuclei of proliferating cells (Ki-
S3).90 Their data also indicate a significant reduction in epidermal proliferation rate in aged
“normal skin” compared to young normal skin. A slower rate of epidermal cell proliferation
is consistent with the significant increase in time to repair a disrupted SC barrier reported
by Ghadially et al.85 Grove also reported that small chemically induced blisters healed
more slowly in subjects between 65–70 years of age than in subjects 18–25 years of age.91
DRY SKIN IN THE ELDERLY
The greater tendency of older subjects to develop dry skin is well known.4,92–94 Elderly dry
skin is especially prone to itching which can sometimes be severe.48–50
It is not entirely clear why older subjects are more prone to developing dry skin. One
possibility is lower levels of NMF with age as NMF levels are reported to be lower in dry
skin.40 Data on the effect of age on NMF levels do not present a consistent picture. One
of the key moisturizing components of NMF is PCA.34,40 Harding et al. reported lower
levels of PCA in older subjects and the difference was more marked deeper in the stratum
corneum.99 Horii et al. reported a general decrease in free amino acids with increasing
levels of dry skin in elderly subjects.100 On the other hand Jacobson et al. found that some
SC amino acids decreased with age while others increased, but no difference was seen in
total NMF free amino acids (FAA) normalized to protein content between young and old
subjects.101 Takahashi and Tezuka102 reported that NMF FAA increased in older subjects.
However, both groups sampled the stratum corneum by scraping skin flakes from the
surface rather than tape stripping to sample from lower levels of the SC as Harding et al.
did. Further, as Tagami103 has pointed out Takahashi and Tezuka reported their results
as FAA/corneocyte. Thus, the higher levels of NMF FAA they report could be due to the
presence of larger corneocytes in older skin25,26,56 as discussed above.
Table II
Measurements of Epidermal Turnover Rate From Grove et. al56
Site Age Transit time
(days)
Number of
cell layers
Turnover rate
(hours/layer)
Volar forearm 35 19.8 ± 1.39 17.0 ± 0.83 28.3 ± 1.2
Volar forearm 60 28.1 ± 2.66 16.8 ± 0.66 40.1** ± 3.8
Upper inner arm 35 17.7 ± 2.02 14.3 ± 0.61 30.0 ± 2.6
Upper inner arm 60 25.5 ± 2.63 13.9 ± 0.81 46.6 ± 6.5
Results are presented as mean ± S.E. Calculated from the data due to a typographical error in
paper.

347 Aging Skin Barrier
Biniek at al.104 investigated the effect of age on the mechanical properties of isolated SC.
They reported that the SC stiffened with age, and that delamination energy increased with
age. These factors could potentially contribute to an increased tendency to form both cracks
and flakes in elderly skin.
Lactate is a component of the NMF that may be derived either from sweat105 or through
anaerobic metabolism in the upper epidermis.106 Nakagawa et al. investigated the correlation
between NMF components and SC stiffness, pH, and hydration in summer and winter with
healthy subjects.42 The only components that correlated significantly to SC stiffness and
hydration were lactate and potassium. One might expect that the slower metabolism of
aging skin reflected by the lower SC turnover rate might lead to lower levels of lactate in
the SC. However, Prahl and coworkers reported that keratinocytes from aging skin produce
higher levels of lactate ex vivo.107 It is well established that sweating rates decrease with
age108 which might lead to lower levels of lactate and thus stiffer, less well hydrated SC.
Another factor may be the decline sebum production that occurs with age109 especially
in post-menopausal women. Fluhr et al. have presented data from studies of asebic
mice, indicating that glycerol derived from the hydrolysis of sebum contributes to skin
hydration.110 Sebum production declines dramatically in women at menopause but does not
decline significantly in men before age 80109 so this may be relevant to post-menopausal
women but not to men before age 80.
It is also possible that the increased cholesterol sulfate observed in older subjects by Starr
et al.58 contributes to skin scaling. Congenital X-linked ichthyosis results from a defect in
the enzyme that convert cholesterol sulfate to cholesterol51 and there is evidence that this is
the main cause of the excessive skin scaling in this disease52 probably by inhibition of the
enzymes that break down SC desmosomes.111
Tagami’s et al. studied the dorsum of the hands of Japanese golfers who only wore a golfing
glove on one hand to compare intrinsic aging to photoaging on properties of the SC.103
Roughness of the skin surface was measured from silicon replicas. Interestingly, there was
a significant negative correlation between the difference in roughness between exposed
and covered hands and the golfer’s handicap. Better golfers (lower handicap) had larger
increases in roughness on their exposed hand. Hydration as measured by higher frequency
conductance was lower for the more exposed site. indicating a drier skin surface but there
was no difference in barrier function as measured by TEWL. Tojahn et al.112 compared sun-
exposed and protected skin on the arms of female subjects and also found no significant
difference between exposed and protected sites.
The tendency for elderly subjects to develop dry skin probably has multiple contributing
factors including lower PCA levels, the slower rate of SC renewal, lower levels of lactic acid
due to reduced sweating, possibly reduced glycerol due to reduced sebum production and
perhaps other factors that have yet to be discovered.
CONCLUSIONS
Aging and photoaging profoundly affect the structure of the dermis leading to well
characterized changes in skin structure and appearance.1–4,113 The effect of age on the
epidermal barrier is less obvious and less pronounced. Somewhat surprisingly there is little
or no decline in SC barrier function as measured by TEWL (see Table I) except for the
décolleté in women68 and one report on the forehead.69 In fact most studies indicate reduced