Ceramides are predominant lipid components of the stratum corneum and comprise 30–40% of the stratum corneum lipids by mass. They are composed of long-chain sphingoid bases (dihydrosphingosine, sphingosine, phytosphingosine or 6-hydroxy sphingosine), which are linked to long-chain free fatty acids (non-hydroxy fatty acids, α-hydroxy fatty acids or ester-linked ω-hydroxy fatty acids) via amide bonds. Hence, the head groups in ceramides include hydroxyl groups that can form inter- and intramolecular hydrogen bonds. The number of hydroxyl groups in the head group of ceramides is essential for the integrity of the stratum corneum’s barrier function.

Ceramides are the major lipid constituent of lamellar sheets present in the intercellular spaces of the stratum corneum. These lamellar sheets are thought to provide the barrier property of the epidermis. The intercellular lipid domain comprises approximately equimolar (having equal molar concentration) concentrations of free fatty acids, cholesterol, and ceramides. (Luisa Coderch, 2012)

The mammalian skin is a multilayered tissue designed to protect the body against undesired influences from the external environment. The main barrier, which is in the upper layer of the skin, the stratum corneum (SC), consists of dead cells enclosed by hydrophobic crystalline lipid lamellar regions since the lipids form the only continuous part of the stratum corneum and are essential for the skin barrier function. (J.A Bouwstra, 1999)

Variations in Ceramides species within this layer have been linked to several skin diseases associated with compromised permeability barriers functions, such as atopic dermatitis, contact dermatitis, and some genetic disorders, psoriasis and xerosis. (Qihong Zhang, 2022) The link between skin disorders and changes in barrier lipid composition, especially in ceramides, is challenging to prove because of the many variables involved. However, most skin disorders with a diminished barrier function present a decrease in total ceramide content with some differences in the ceramide pattern.

The researchers thought these disorders could be improved by replacing decreased ceramide levels. Therefore, topical applications, including conventional and novel carrier systems, have been formulated by diverse researchers. (Emine Kahraman, 2019)

Research stated that all the phytoceramides ("Phyto" means plants) improved the recovery rate of the damaged stratum corneum and enhanced hydration as they have a similar lipid structure to the ceramides found in our skin and may help replenish ceramide stores. The Natural oil-derived phytoceramides could represent a novel class of ceramides for cosmetic applications in developing an ideal skin barrier moisturiser. (Myoung Jin Oh, 2017)

Natural ceramides are found at trace levels in animals and plants, and their isolation for use in skin care products. Several investigators have attempted to prepare synthetic pseudo-ceramides as it is tedious and expensive. These compounds are pseudo-ceramides because their molecules do not contain a sphingol structure. Studies show that synthetic pseudo-ceramide exhibits water-retaining properties like those of natural lipids. (Osamu Morita, 2009)

A properly functioning stratum corneum (SC) is essential for healthy skin. The stratum corneum employs several natural systems to maintain integrity to keep water in or on the skin. One of these systems is the natural moisturising factor (NMF), which predominates free amino acids, pyrrolidone carboxylic acid (PCA), urocanic acid, lactic acid and urea. The NMF components are highly efficient humectants that attract and bind water from the atmosphere, drawing it into the corneocytes. (Fabrizio Spada, 2018:)

The structure of the stratum corneum is arranged of flattened corneocytes embedded in a lipid-enriched extracellular matrix, organised into parallel stacks of lamellar bilayers. These water-repellent lamellar bilayers are composed of a blend of ceramides, free fatty acids and cholesterol and restrict not only the outward flow of water and electrolytes but the inward absorption of chemical substances, allergens and microbial pathogens. Further, sebum is produced by the sebaceous glands, forming a film over the top of the skin.  (Fabrizio Spada, 2018:)

The skin is an effective barrier mainly due to the structure of the stratum corneum, its outermost layer. Ceramides are the main lipids in the stratum corneum, which play a key role in the barrier function. Alterations of ceramides or stratum corneum lipid composition result in skin disorders with barrier defects, such as atopic dermatitis, psoriasis, acne, dry skin etc.

The lipid envelope and other organisations keep the moisturising balance under control and reduce TEWL. Although supplementary products do not moisturise the skin, hydration content is preserved in skin layers. With decreasing ceramide levels in the skin, the barrier function of lipid envelopes becomes incapacitated. Compared to dry and normal skin, ceramide I, II, III, IV, V, and VI diminish with dryness, while Ceramides are at a high level in normal skin.

Experimental evidence says that oxidative stress plays a major role in aging. With age, the production of free radicals increases while the endogenous defense mechanisms decrease. This imbalance leads to the progressive damage of cellular structures, presumably resulting in the ageing phenotype. Ageing of hair manifests as a decrease in melanocyte function or greying and a reduction in hair production or alopecia. There is circumstantial evidence that oxidative stress may be a key mechanism contributing to hair grey and loss. (Trüeb, Jan)

A recent study has revealed that ceramide potentially affects hair growth in a mouse model. However, the role of ceramide in human dermal papilla cells, known to play an essential role in hair growth, has yet to be well understood.

Ceramide is the most important intracellular lipid that constitutes the lipid layer of hair cuticle along with fatty acid and cholesterol. Ceramide is a component of intercellular lipids present in hair cuticles to protect and strengthen internal components of hair. A study has shown that ceramide antagonistically affects cell growth and cell survival. However, it has recently been reported that ceramide can affect hair loss prevention and growth enhancement. Ceramide synthase 4 is vital in maintaining epidermal stem cell stability and regulating the hair cycle. Research suggests synthetic ceramides stimulate hair growth by induction proliferation of human dermal papilla cells.  (Jee Hye Oh, 2019 )

Human skin is the body’s largest organ, accounting for 16% of total weight and having an area of 1.8 m². It prevents excessive water loss from the body. It helps to maintain the immune system and protects the organism against microorganisms, ultraviolet radiation, toxic substances and mechanical damage, creating a physical barrier between environmental conditions and the body. The human skin is a multi-layered membrane. They are from deep to superficial Hypodermis (subcutaneous or subcutis), Dermis, and Epidermis (upper skin). The epidermis is composed of five layers. From deep to superficial, these layers are:

The skin acts as a primary barrier protecting the body from the external environment and preventing water loss from the body. In particular, the stratum corneum (SC, the uppermost layer of the skin) is crucial for the skin barrier function. The structure of the Stratum Corneum is generally described by the “brick and mortar” model, in which corneocytes and intercellular lipids represent bricks and mortar, respectively. The Stratum corneum contains a complex mixture of low-molecular-weight, water-soluble compounds termed natural moisturising factors (NMFs). (Kyoungmi Jung, 2021)

NMF consists of hydrophilic amino acids (AAs) and their derivatives (AADs), which contribute to establishing a rise in water in the stratum corneum. The intercellular lipids in the stratum corneum play an essential role in regulating the water content. The major skin lipids are ceramides, cholesterol, and free fatty acids (FAs). Ceramides are the most important lipids that form this permeability barrier. Reductions in the NMF and ceramide contents lead to dry skin due to an increase in transepidermal water loss (TEWL) and a decrease in the flexibility of the stratum corneum.  (Kyoungmi Jung, 2021)