379 The Human Stratum Corneum
BACKGROUND
The human SC, the uppermost layer of the skin, consists of about 70% proteins, 15% lipids,
and 15% water. The simplistic view of SC as a “brick and mortar” structure consisting of
corneocyte “bricks” embedded in a continuous lipid matrix made of ceramides, cholesterol,
and fatty acids has been refined over the past three decades, due to more information
on the composition and organization of lipids and proteins in the SC.1–5 The SC, which
was thought to be a dead tissue in the 70s, is now considered to be a hot bed of enzyme
activity.3 The role of the skin microbiome in maintaining a healthy barrier is also receiving
increased attention over the past decade.6–8 Recent advances in our spectroscopic and
imaging techniques including in-vivo techniques9–13 have made it possible to characterize
the structure and function of healthy SC, and in turn help us to understand the changes in
them upon exposure to weather conditions, cosmetic products (such as skin cleansing and
skin care products), and environmental pollutants.
Regular use of cosmetics and personal care products has become part of daily routine in the
modern era. Practices that started off as enhancing daily hygiene and beauty have evolved
into care and protection of skin, scalp, hair, and oral surfaces. Preventive care from harmful
effects arising from UV radiation and environmental pollutants have added another
dimension to the care aspects of personal care products. Furthermore, with the anticipated
increase in the aging population that is expected to exceed over a billion people of 65+
years age, and with skin as a major conduit for infections for this population, protecting
the skin barrier with personal care products for the elderly has become an important part
of the wellness regime. Thus, cosmetic and personal care products have become part of the
wellness-healthcare continuum to ensure an improved quality of life and healthcare for all.
All these trends have increased the exposure of a wide variety of products and associated
chemicals daily, and therefore the safety and efficacy of cosmetic and personal care products
have become even more important than ever before.
While there is an increased use of personal care and cosmetics products for reasons discussed
above, there is also an increased awareness among consumers through social media about
various ingredients that are being used in daily use products. This trend is driving use of
more “naturals” and sustainable ingredients into the product line. The trend also extends to
“free-of” technologies such as sulfate-free, preservative-free, and fragrance-free products. These
changes offer interesting challenges and opportunities for formulators requiring increased
scrutiny of the alternatives for ensuring product efficacy, consumer desired sensory, and safety.
In this changing environment, it is important that the choice of ingredients for cosmetic and
personal care products are made based on a scientific understanding of their safety, efficacy,
and their effects on the SC barrier. In this review, interaction of common classes of ingredients
encountered in personal care products is reviewed and general guidelines for selection of
ingredients that will have minimal impact on the SC barrier are discussed. Specifically, the
discussion here will include surfactants, emulsifiers, emollients, occlusives, humectants, skin
benefit lipids, and penetration enhancers. In areas where recent reviews are available, the
discussion will be limited to general guidelines with appropriate literature references.
THE STRUCTURE OF HUMAN STRATUM CORNEUM
The SC, the uppermost layer of human skin, is the main barrier that prevents excessive
water loss from the body and prevents the penetration of chemicals, pollutants, and germs

380 JOURNAL OF COSMETIC SCIENCE
into the body. The brick and mortar structure of the SC, proposed almost half a century
ago,1,14 has been investigated in detail over the years. See Figure 1 for a schematic of the brick
and mortar structure.5 The SC consists of corneocyte bricks embedded in a lipid matrix.3
Covalently bonded lipids attached to the corneocyte envelope compatibilize the brick with
the surrounding lipids. Corneocytes are further attached together by desmosomal proteins.
The proteins within the corneocyte bricks exist as crosslinked keratin and low molecular
weight natural moisturizing factors (NMFs) that help hold water within the SC.
The SC, which was thought to be a dead layer of skin about half a century ago, is now
recognized to be a hot bed of enzyme activity and biological processes.3,5,15 One such
process is the desquamation of skin layers in a layer-by-layer fashion, losing at least one
layer every day, with a fresh layer being exposed regularly. The overall corneum turn-over
period in normal skin is about four weeks. For this to happen, the desmosomal linkages
need to be degraded as the SC layers move to the surface, and this is accomplished by
proteolytic enzymes in the SC. The other two important enzyme driven processes in the
epidermis include the breakdown of filaggrin protein into NMFs and the conversion of
glucosylceramides into ceramides, a key lipid involved in the matrix bilayer lipids. Both
skin pH and hydration affect these enzymatic processes.
The matrix lipids that surround the corneocytes consist of ceramides, cholesterol, and
fatty acids—approximately in a 1:1:1 molar ratio.4 The composition and the possible
structural organization of these bilayer lipids have been discussed in detail over the past
three decades.4,16–18 Ceramides are two-tailed lipids, and depending upon the nature of the
headgroup and the tails, 20 classes of ceramides have been identified in the human SC.15,19
Similarly, fatty acids have also been found with chain lengths, mainly from C20 to C28,
and even low levels of C32.20 Bouwstra and coworkers have investigated the organization
Figure 1. Refined “bricks and mortar” representation of the structural components of the SC. Reproduced
with permission from A. V. Rawlings and R. Voegeli, Stratum corneum proteases and dry skin conditions,
Cell Tissue Res (2013) 351:217–235, DOI 10.1007/s00441-012-1501-x.5