The Integumentary System

The integumentary system includes the skin and its associated structures. It is the largest organ system in the human body. It helps protect the body and maintain homeostasis, which means keeping internal conditions stable.

Skin has three main layers: the epidermis, dermis, and hypodermis, also called subcutaneous tissue. The epidermis is the outermost layer. It forms a protective barrier against bacteria, UV radiation, and chemicals. It also contains melanocytes, the cells that make melanin,...

The epidermis is the outer skin layer and is built from four or five layers of epithelial cells, depending on where it is found in the body. From deep to superficial, these layers are the stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum. Most of the cells in these layers are keratinocytes, which make and store keratin, a protective protein.

The stratum basale is a single cell layer made mostly of basal cells. A basal cell is a cuboidal stem cell that...

The epidermis is the outermost layer of skin, and it is built to protect the body. It contains several distinct layers that change as cells move upward. From the deepest layer to the surface, these layers are the stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum.

The stratum basale, also called the stratum germinativum, is the deepest layer of the epidermis. It has a single layer of actively dividing basal cells, or basal keratinocytes. These cells keep...

The papillary layer is the outer part of the dermis. It is made of loose, areolar connective tissue, so its collagen and elastin fibers form a loose mesh. This layer sits next to the epidermis and helps show how the skin is organized into distinct layers.

The papillary layer projects into the stratum basale of the epidermis. These finger-like extensions are called dermal papillae. Inside this layer are fibroblasts, a small number of adipocytes, and many small blood vessels. It also contains...

The reticular dermis is the thicker inner layer of the dermis. It lies beneath the papillary layer, and both layers are made of connective tissue. Collagen fibers extend from one layer to the other, so the border between them is not very clear.

The papillary layer includes the dermal papillae that extend into the epidermis. Below that, the reticular layer is made of dense, irregular connective tissue. It has a tight meshwork of fibers, which gives it a net-like, or reticulated, appearance.

The hypodermis is the layer beneath the dermis. It is also called the subcutaneous layer or superficial fascia. It links the skin to the fascia, which is fibrous tissue over the bones and muscles. The border between the hypodermis and dermis can be hard to see, and the hypodermis is not strictly part of the skin.

The hypodermis contains loose areolar connective tissue and adipose tissue. Adipose tissue is fat tissue, and its cells are called adipocytes. This layer stores much of the body fat...

Skin color comes from melanin, carotene, and hemoglobin. Melanin is made by melanocytes, which are scattered through the stratum basale of the epidermis. The pigment moves to keratinocytes through melanosomes, which help carry the color into the skin cells.

Melanin appears in two main forms. Eumelanin gives black and brown color, while pheomelanin gives red color. People with darker skin make more melanin than people with pale skin.

Sun exposure changes melanin production. UV rays from the...

Skin color changes can be an important clue about health. A careful skin exam is part of a full physical examination. Most skin disorders are harmless, but some, including melanoma, can be fatal if they are not treated. Two visible disorders, albinism and vitiligo, change the color of the skin and its accessory organs.

Albinism is a genetic disorder that affects the color of the skin, hair, and eyes. It happens when melanocytes, the cells that make melanin, cannot produce enough or any melanin.

Hair and hair follicles are accessory structures of the skin. Hair is a keratin-filled filament that grows from the epidermis. It is mainly made of dead, keratinized cells. Hair is found on most body surfaces, but not on areas such as the palms of the hands and the soles of the feet.

Each hair strand begins in a hair follicle, which is an epidermal opening that extends into the skin. The part above the skin surface is the hair shaft. The part below the surface is the hair root. Deep in the...

Hair is an accessory structure of the skin that grows from a hair bulb inside a follicle. Basal cells in the hair bulb make keratinocytes, and new cells are added there as the hair shaft is pushed upward toward the surface. As the cells move up, keratinization finishes, and the visible part of the hair becomes a dead structure made mostly of keratin.

Because only the surface of the hair is cut or shaved, these methods do not damage the hair structure. Most chemical hair removers also work only...

Nails are hard accessory structures of the skin on the tips of fingers and toes. They cover the dorsal surface of the distal phalanges. Nails are made of specialized keratinized cells. They help protect the fingertips and toes, support sensation, and aid manual dexterity.

The nail plate is the visible part of the nail that extends beyond the fingertips or toes. It is hard and translucent, but it often looks pink because blood vessels underneath show through. The nail plate is made of layers of...

Sweat glands help the body control temperature and remove waste. These sudoriferous glands are small, coiled tubes in the dermis, which is the middle layer of the skin. They release sweat, a watery fluid that supports homeostasis, the body’s stable internal balance.

Sweat glands are classified as merocrine glands. In merocrine secretion, the gland releases its product by exocytosis through a duct, and the gland cells stay intact. This process allows sweat to move out of the gland without...

Sebaceous glands are oil glands in the skin that release sebum. They are found almost everywhere on the body, except on the palms and soles. Most of them are linked to hair follicles, where they help keep the skin and hair lubricated and waterproofed.

These glands make a lipid-rich mixture called sebum and release it onto the skin surface. Sebum softens the dry, dead cells of the stratum corneum, the outer keratinized layer of the skin, and helps keep that layer pliable. Sebaceous glands are...

The skin is the largest organ of the human body, and it helps us sense touch, temperature, and pain. It contains many sensory receptors that detect physical, biological, and environmental cues. These signals help protect the body and guide fast responses.

Skin receptors are grouped into two main categories: capsulated and non-capsulated. The non-capsulated receptors are mainly pain receptors. The capsulated receptors respond to specific kinds of stimuli, such as pressure, vibration, and...

The skin helps make vitamin D when it is exposed to sunlight. UVB radiation from the sun, with wavelengths of 290 to 315 nm, starts a chemical reaction in the skin. This makes vitamin D unique because it can be synthesized in the body through the skin.

In this process, UVB rays are absorbed by the skin and convert 7-dehydrocholesterol, also called provitamin D3, into previtamin D3. Previtamin D3 then changes quickly into vitamin D3, also known as cholecalciferol. Once vitamin D3 forms, it...

Skin infections often start when microbes affect the body’s first line of defense. Many skin conditions can affect people of all ages. They may come from genetic factors, infections, autoimmune reactions, environmental factors, or lifestyle choices.

Many of the most common skin infections are caused by gram-positive Staphylococcus spp. and Streptococcus spp. However, many skin problems are not linked to just one pathogen. Opportunistic pathogens can infect skin wounds, and similar symptoms may...

Skin cancer develops when skin cells grow in an abnormal way because their DNA has been damaged. Ultraviolet, or UV, radiation from the sun and from tanning beds is the main cause. The number of skin cancer cases continues to rise, and it is the most common type of cancer worldwide.

Basal cell carcinoma, or BCC, is the most common form and makes up about 80% of cases. It usually appears on skin that gets a lot of sun, such as the face, scalp, neck, and hands. A BCC may look like a small raised...

Burn injuries damage the skin and the tissues below it. They can result from heat, electricity, chemicals, radiation, or friction. These injuries range from minor superficial burns to deep burns that can be life-threatening.

Severe burns can kill skin cells and cause a major loss of fluid. That fluid loss can lead to dehydration, electrolyte imbalance, and failure of the kidneys and circulatory system. Burn patients often receive intravenous fluids to replace water loss and intravenous...