Skin structure: what is it composed of?

A true interface between the body and the external environment, the skin has numerous functions: it regulates temperature, perceives the sensory stimuli and contributes to the synthesis of certain vitamins and hormones. This complexity is based on a well-defined organisation, composed of three distinct layers which work in synergy to ensure protection, regeneration and the proper functioning of the skin. The epidermis, the outermost layer, provides protection against external aggressions and limits water loss. Just beneath, the dermis contains an extracellular matrix rich in support fibres, as well as a vascular, nervous and glandular network. Finally, the hypodermis, the deepest layer, houses adipose tissue that cushions impacts and contributes to thermal insulation.

The epidermis forms the outermost layer of the skin and constitutes the first barrier between the body and the external environment. Thin and semi-permeable, it protects against dehydration, pathogens, allergens and UV radiation. Its composition is predominantly cellular: approximately 80% keratinocytes, together with melanocytes, Langerhans cells and Merkel cells. Devoid of blood and lymphatic vessels, the epidermis is nourished by diffusion from the dermis. Its thickness varies considerably across different body regions, and it undergoes continuous renewal, thereby maintaining its integrity and protective function.

The epidermis comprises five superimposed layers, from the deepest to the most superficial: the basal layer, the spinous layer, the granular layer, the clear layer and the horny layer.

• The basal layer : Adjacent to the basement membrane, the basal layer houses stem keratinocytes, actively dividing cells. They continually generate new keratinocytes that migrate towards the surface. It is also within this layer that themelanocytes, responsible for melanin synthesis, and the Merkel cells, sensitive to pressure and touch.

• The spinous layer : Composed of keratinocytes interconnected by desmosomes, this layer provides cohesion to the epidermis. It is also at this level that Langerhans cells, immune sentinels capable of capturing and presenting antigens to lymphocytes, members of the white blood cell family, are located.

• The granular layer : It contains flattened keratinocytes housing keratohyalin granules and lipid-rich lamellar bodies, essential for the formation of the skin barrier.

• The stratum lucidum : Present only in the palms of the hands and the soles of the feet, this layer is composed of dead cells containing eleidin, a translucent derivative of keratohyalin.

• The stratum corneum : The stratum corneum consists of anucleate corneocytes, i.e. cells that have lost their nucleus, rich in keratin and embedded within a lipid matrix. This outermost layer provides the skin’s primary barrier function, limiting water loss whilst protecting it from external aggressions.

Between the epidermis and the dermis lies the basement membrane, which serves both anchorage and communication functions. Composed of laminin, type IV and type VII collagen, perlecan (a proteoglycan) and nidogen (a glycoprotein that links laminin to collagen), it ensures the cohesion between the two skin layers whilst allowing the exchange of nutrients, oxygen and molecular signals. The basement membrane also supports the hemidesmosomes of basal keratinocytes, which bind the epidermis firmly to the dermis.

The dermis constitutes the intermediate layer of the skin, located between the epidermis and the hypodermis. Thick, elastic and highly vascularised, it forms a complex connective tissue whose principal role is to mechanically and functionally support the epidermis. The dermis also participates in thermoregulation, supplies nutrients to epidermal cells, aids in wound healing and enables sensory perception thanks to a dense network of blood vessels, nerve endings and sensory receptors. Its thickness varies across different regions of the body, and it represents the largest water reservoir of the integument, thus contributing to the skin’s suppleness and resilience.

The dermis is divided into two distinct regions: the superficial papillary dermis and the deeper reticular dermis.

• The papillary dermis : Located just beneath the dermo-epidermal junction, the papillary dermis consists of loose connective tissue composed of fine collagen and elastin fibres and an extracellular matrix rich in glycosaminoglycans. It forms small projections, the dermal papillae, which interlock with the epidermis to facilitate nutrient exchange. These structures, particularly prominent on the fingertips or soles of the feet, give rise to fingerprints. Highly vascularised and abundant in sensory receptors, the papillary dermis contributes to the perception of touch, temperature and vibrations.

• The reticular dermis : Thicker, the reticular dermis forms the deeper portion of the dermis. It is composed of dense connective tissue made up of interlaced bundles of fibres of collagen and elastin that provide the skin with its strength, elasticity and extensibility. The main cells of the dermis are fibroblasts, the true architects of the extracellular matrix: they synthesise collagen, elastin, fibronectin and proteoglycans, such as hyaluronic acid, essential for the hydration and viscosity of the skin tissue. The reticular dermis also houses hair follicles, the sebaceous glands and sweat glands, as well as an extensive network of lymphatic and blood vessels, ensuring thermal regulation and immune defence.

The dermis is a key compartment from an immunological perspective.

Indeed, the dermis contains the majority of the skin’s immunocompetent cells : macrophages, mast cells, dendritic cells, eosinophils, neutrophils and T and B lymphocytes, including regulatory T cells and natural killer cells. These cells form a complex immune surveillance network capable of detecting any pathogen or local imbalance. In the event of microbial invasion, they release inflammatory mediators that attract further defence cells and orchestrate an appropriate immune response. This defence system is not limited to combating infections: it also contributes to maintaining cutaneous homeostasis and to tissue repair following injury by stimulating fibroblast proliferation and the production of collagen to restore the dermal structure.

The hypodermis is the deepest layer of the skin. Located beneath the dermis, it serves as the junction between the skin and deeper structures such as muscles, tendons and bones. This loose connective tissue is richly vascularised and traversed by large blood vessels and nerves, ensuring metabolic exchanges between the skin and internal organs. The hypodermis plays a role in support, protection and energy storage, constituting both a mechanical shock absorber and a thermal regulator for the entire organism.

Its structure is primarily based on white adipose tissue, comprising adipocytes, fibroblasts and cells of the vasculo-stromal compartment. The latter contains mesenchymal stem cells, endothelial cells and immune cells such as T lymphocytes and macrophages. Mature adipocytes store energy in the form of large lipid droplets, which serve as a mobilisable metabolic reserve when required. This energy storage also contributes to thermoregulation, as subcutaneous fat acts as a thermal insulator.

In addition to its mechanical and energy-related functions, the hypodermis is also distinguished by its endocrine role. Adipocytes indeed secrete a wide range of hormones and bioactive mediators, such as leptin, adiponectin and certain cytokines, which contribute to the regulation of food intake, blood glucose levels, inflammation and hair follicle growth. The composition of the hypodermis varies among individuals and across body regions: it is thicker over the hips, buttocks and abdomen, and thinner on the eyelids or forehead.

• Société Française de Dermatologie. Structure de la peau. Annales de Dermatologie et de Vénéréologie (2005).

• RAWLINGS A. V. Ethnic skin types: Are there differences in skin structure and function? International Journal of Cosmetic Science (2006).

• FAHRIYE K. & al. Measurement of epidermis, dermis, and total skin thicknesses from six different body regions with a new ethical histometric technique. Turkish Journal of Plastic Surgery (2018).

• LINDER K. E. & al. Structure and function of the skin. Paediatric Dermatology (2020).

• CHAUHAN R. & al. Transdermal patch: A novel approach for transdermal drug delivery. Journal of Drug Delivery and Therapeutics (2022).

• KOTZBECK P. & al. Modelling the complexity of human skin in vitro. Biomedicines (2023).

• SHARMA S. & al. Anatomy, skin (integument), epidermis. StatPearls (2024).