Skin

Anatomy of the Skin

The skin is made up of three layers—epidermis, dermis, and hypodermis—and contains anatomical features including hair shafts, sweat pores, nerves, and capillaries.

The skin consists of three layers that together provide protective, sensory, regulatory, and excretory functions characteristic of the integumentary system. An integument is a tough outer covering that in humans consists of skin and related structures such as nails and hair.

The outermost layer of the skin is the epidermis, which consists of epithelial cells. Most epidermal cells produce keratin, a structural protein that lends durability and water impermeability to skin, hair, and nails. Epidermal cells form deep within the epidermis and are pushed toward the surface as subsequent cells form. These cells become keratinized, a characteristic of epidermal cells that have changed into a tough, resilient, keratin-containing form as they mature and eventually die. Keratinized cells undergo desquamation, shedding of the outermost layer of skin to protect the body from microbes bound to the skin. Melanocyte cells give the skin its color through the production of the pigment melanin. Nerve endings innervate epidermal cells, providing them their sensory functions. Langerhans cells (a type of antigen presenting cell) are found within the epithelium (most prominent in the stratum spinosum) and assist in the body's immune response by processing and presenting microbial antigens.

The epidermis rests on the dermis, the middle layer of skin, and is separated from the dermis by a membrane. The dermis consists of connective tissue that gives the skin strength. The dermis in turn connects to the subcutaneous layer, or hypodermis, of the skin. Composed of loose connective tissue that includes elastin fibers and collagen, the subcutaneous layer connects skin to bone and muscle. It also supplies nerves and blood vessels to the dermis and epidermis.

Skin contains several characteristic anatomical features, some of which are known as accessory structures. Hairs consist of hair shafts above the skin's surface and roots beneath it. Hairs are usually found on all skin except that of the lips, palms of the hands, distal end of the penis, soles of the feet, nipples, and tips of the toes and fingers. Each hair root is contained within a hair follicle, a small cavity formed by an infolding of the epidermis into the dermis. Ducts of oil-producing sebaceous glands and apocrine sweat glands, which produce sweat containing lipids and proteins, also open into hair follicles. In humans apocrine sweat glands are only present in the armpits and genitalia and become active with the onset of puberty. Eccrine (or merocrine) pores are found in specific places like the palms of hands and soles of feet, and are much less prominent along the torso. Capillaries in the outer portion of the dermis are blood vessels that provide nutrients to the epidermis through diffusion.

Layers and Structures of the Skin

The skin consists of three layers containing many structures, some of which function in the skin's natural defenses, for example, keratinized cells, various glands, antimicrobial peptides, sebum, and lysozymes.

Natural Defenses of the Skin

Keratin, sebaceous glands, antimicrobial peptides, sebum, and lysozymes are all components of the skin's natural defenses.

Components within the skin contribute to its natural defenses against pathogens. Cells at the skin's surface, the top of the epidermis, are dead and consist primarily of keratin. Keratin is a tough, insoluble protein, so this outer layer of cells forms a protective and impermeable barrier. Microbial pathogens can typically only cross the epidermal barrier where it is damaged, such as at wound sites or insect bites. Desquamation (shedding) of keratinized cells prevents infection by removing potential pathogens attached to skin.

A sebaceous gland is a gland that is joined to a hair follicle and produces oil to keep the skin supple and prevent the growth of microorganisms. Sebaceous glands secrete sebum, a lipid-rich oily substance that lubricates and helps waterproof the skin, protecting against invasion of some bacteria.

Although the epidermis is a tough barrier, microbes live successfully on the skin's outer surface. In addition to physically blocking invasion by these microbes, the skin employs chemical responses. One example is the presence of a class of diverse antimicrobial peptides. An antimicrobial peptide is any one of many naturally occurring molecules at the skin's surface that provide a chemical defense against the infection of microbes across the skin. Lysozymes also participate in the skin's chemical defenses. A lysozyme is an enzyme produced by many types of cells in the body that breaks down the bonds that make up bacterial cell walls. Lysozymes are also produced by and give antimicrobial properties to secretions such as tears, mucus, saliva, and human milk.

Skin Microbiota

The skin is colonized by a diverse community of microorganisms, including common strains of Staphylococcus.

Normal healthy skin hosts diverse microbial communities made up of hundreds of different species. The skin contains more microbes than anywhere else in the body except the alimentary canal. Bacterial species are the most numerous, but mites, viruses, and fungi are all found on the skin of healthy adults. A person’s native microbe community initially comes from its mother during childbirth and later through contact with other individuals. Biota is the collective term given to all species of living things in a particular region. While these communities are complex, they can often be characterized by the type of skin microenvironment—(dry, moist, or oily) in which they live.

Skin microbes are often harmless but can sometimes occur in harmful or pathogenic forms that infect humans. Bacterial resident microbiota found in the upper epidermis of the skin fall primarily under four different phyla: Firmicutes (Staphylococcus, Micrococcus), Actinobacteria (includes Corynebacterium, Propionibacterium, Brevibacterium, and Dermabacter), Bacteroidetes, and Proteobacteria. Firmicutes inhabit areas that tend to be dry, such as the soles of the feet. Actinobacteria tend to inhabit those areas of the body that are oily, such as the groin area and forehead. Transient bacteria, or those isolated on occasion, include Clostridia in the perineal area and Acinetobacter. Fungi of the skin include Malassezia species. Little information is known about the prevalence of viruses on skin.

Microbiota of the Skin

"Many organisms are regular and benign residents of human skin, including a wide range of bacteria, some fungi and mites, and viruses. (C. matruchotii: light microscope, 1,115X; M. furfur: light microscope; Human papilloma virus: light microscope; D. folliculorum: light microscope)
Credit: CDC (top), CDC/Dr. Lucille K. George (top-middle), Wellcome Collection (bottom-middle), Tarkowsi et al. (bottom)License: CC BY 3.0 (bottom)
One prevalent organism that is part of the skin biota is Demodex, a genus of mites that are common commensal parasites of humans. This means that these mites neither harm nor benefit their human (or mammalian) host. Only two species of Demodex live on humans, D. brevis and D. folliculorum, and they are contracted either through direct contact or from dust carrying their eggs. They live in hair follicles, including the lashes of the eyes, and feed off the sebum, or oil, produced by the glands around the hair follicles. They are ubiquitous on healthy adults and typically do not cause symptoms in their hosts unless growing numbers of mites cause an immune imbalance. This could result in inflammatory skin conditions such as acne and rosacea. A common commensal bacterium of skin is Propionibacterium acnes. Gene sequencing of P. acnes has revealed that it has genes encoding lipases that break down skin lipids, or fats, and sebum. P. acnes further hydrolyzes, or degrades, the triglycerides present in sebum, which releases fatty acids into the skin. The release of fatty acids onto the skin surface has two effects. The P. acnes bacterium adheres to these free acids, aiding in their colonization of the sebaceous glands, and the fatty acids lower the skin pH to approximately 5, thereby inhibiting growth of the potentially harmful pathogens Staphylococcus aureus and Streptococcus pyogenes.
Two common microorganisms found on the skin are Staphylococcus aureus (scanning electron microscope, 20,000x) and Streptococcus pyogenes (scanning electron microscope). Both can cause dangerous infections under the right circumstances.
Credit: CDC/Matthew J. Arduino, DRPH (left), NIAID (right)