What is the integumentary system?
Answer
In anatomy and physiology, the integumentary system refers to the layers of the skin, glands, and nerves within the skin, nails, and hair. The integumentary system mainly functions to protect and regulate the body. The skin seems simple at the surface level, but it quite literally goes deeper than you would think!
Skin
It may surprise you to find out that skin is considered the largest organ of the human body, or that the skin is even considered an organ at all. Because skin is composed of several tissue types, it is technically an organ. The skin is often broken into three main layers, each having even smaller “sublayers” within them. From superficial to deep (outermost to innermost), the layers of the skin include the epidermis, dermis, and hypodermis.
Epidermis
The epidermis is the outermost layer of the skin that serves as the first layer of protection for the body. It is made up of stratified squamous epithelium, which essentially means it is made up of many extremely thin layers of epithelial cells. Other cells in the epidermis include:
Keratinocytes: cells that produce keratin, the component that provides a water barrier and is also present in nails and hair to make them stronger.
Melanocytes: cells that produce melanin, the component that gives the skin its pigment.
Merkel cells: specialized sensory cells that receive information about touch; this makes them mechanoreceptors. These are mostly found in the fingertips, palms, and soles.
Langerhans cells: immune cells of the skin that play a role in defense and protection.
The epidermis is made up of 5 layers that each have unique compositions of cells and structures. From superficial to deep, the layers of the epidermis include:
Stratum Corneum: The layer of skin that you can see and interact with. This layer is made up of 20-30 layers of flattened epithelial cells, which serve as the first line of protection for the deeper layers of skin and underlying connective tissue. This layer contains corneocytes, which are just dead keratinocytes that have migrated to the surface and protect the skin from harm due to their strength.
Stratum Lucidum: A thin layer of dead skin cells that lies deep to the Stratum Corneum. This layer appears clear under a microscope because it is only made up of 2-3 cell layers, and it is only present in thick skin. This layer contains lipids, which make this layer a barrier against water.
Stratum Granulosum: A thin layer of the skin that lies below the Stratum Corneum (or the Stratum Lucidum in thick skin). This layer contains keratinocytes that are migrating from the deeper Stratum Spinosum layer, so this layer is rich in keratin and has a grainy appearance under a microscope due to the presence of these cells.
Stratum Spinosum: 8-10 layers composed of keratinocytes and Langerhans cells. The presence of keratin in this layer allows for flexibility and strength of the skin. This layer is named for its spiny appearance when stained under a microscope, which is due to intracellular structures called desmosomes. This layer is the second deepest layer of the epidermis.
Stratum Basale: The deepest layer of the epidermis that is composed of a single layer of basal cells. Basal cells are stem cells that differentiate into keratinocytes, so this layer is important for the early stages of keratin production. The Stratum Basale also contains Merkel cells and melanocytes, which contribute to sensory reception and skin pigment, respectively. This layer is attached to the basement membrane, which is a structure made up of collagen that separates the epidermis from the dermis.
If you find it difficult to remember the order of the layers of the epidermis, remember the mnemonic: “Come, Let’s Get Sun Burned!” – Corneum, Lucidum, Granulosum, Spinosum, Basale.
Dermis
The dermis is the layer immediately deep to the epidermis, which it is attached to via dermal papillae. Dermal papillae are projections of the dermis that make up their own layer (appropriately named the Papillary Layer) that also contains fibroblasts, adipocytes, and some blood vessels. This layer mainly serves a protective function, but it also contributes to the formation of fingerprints–as a fetus, the papillae push on the epidermis to cause unique ridges!
The thick, dense layer that underlies the Papillary Layer is called the Reticular Layer, which makes up a majority of the dermis and contributes to skin flexibility, blood supply, and sensory input within the skin. The presence of collagen and elastin in this layer enables strength and skin elasticity.
Hypodermis
The hypodermis, also called the subcutaneous layer, lies below the dermis and connects the dermis to the tissue that exists between the skin and muscle, called fascia. This layer is highly vascularized (has a rich blood supply) and is the location of fat. Fat storage in the hypodermis is important for insulation and protection of the underlying structures.
Hair
Hair is a component of the integumentary system since it is embedded in the dermis and subcutaneous layers of the skin. Although the hair we see on our head and body appears simple on the surface, it has multiple components and layers that are important in understanding the function that hair serves.
The portion of hair that is visible to us is called the hair shaft, which is made up of dead skin cells that have been keratinized. This means that the cells have been converted entirely into the protein keratin, so the hair shafts themselves do not provide any sensory input. The shaft of the hair has three layers: from innermost to outermost, these layers are the medulla, the cortex, and the cuticle. The medulla and cortex contain pigment, which is the color of the hair, and the cuticle is thin and transparent. The medulla extends all the way to the bottom portion of the hair, called the root. The root of the hair contains a hair bulb, which has three layers of its own: the innermost being the hair matrix, then the inner sheath, and the most superficial outer sheath. The bulb also contains the hair papilla, which is the hair's source of blood and nourishment. A diagram of hair can be found at the bottom of this FAQ.
Glands are accessory parts of the integumentary system, but they serve an important purpose. Sebaceous glands, for instance, are present near hair to nourish the hair shaft with oil. These glands ensure that the hair doesn't become dried out by dispensing an oil called sebum, which is the main culprit when blackheads form on the skin. Blackheads are the result of sebum and bacteria buildup, which appears dark on the skin due to melanin buildup.
There are three major functions that hair is responsible for: protection, homeostasis, and sensory input. Hair is in specific places for specific reasons–eyebrows and eyelashes protect the eyes, nostril hair protects us from inhaling debris, and hair on the head protects the scalp from the sun. Hair also plays a role in homeostasis by regulating body temperature with tiny muscles called arrector pili muscles. These bundles of smooth muscle insert into the bulb of the hair follicle, and contract in response to cold temperatures or highly emotional conditions, such as fear. Contraction of the arrector pili causes the appearance of goosebumps because they pull on the hair follicle and make the shaft of the hair stand up. This helps trap body heat by creating an insulating layer of hair that traps air and prevents heat loss. Lastly, hair plays a role in sensory input due to nerves that are located on the bulb of the hair follicle. When the hair shaft is moved, it moves the innervated bulb as well and sends a signal to the brain with sensory information.
Glands
Sebaceous glands, as previously mentioned, mainly function to produce and secrete sebum into hair follicles. In addition to sebaceous glands, the integumentary system also has eccrine glands, apocrine glands, and ceruminous glands. Eccrine and apocrine glands are both classified as sudoriferous glands, more easily known as sweat glands. Eccrine glands are the most common type of gland, which secrete sweat in order to regulate body temperature. These glands are active throughout the lifespan, as opposed to apocrine glands, which only begin to function during puberty. Apocrine glands are located in the armpits, eyelids, and pubic area and are only occasionally active. Ceruminous glands are located in the ears, and these are responsible for producing cerumen, or earwax. Earwax has a protective function due to its stickiness, which traps dirt and other foreign debris from entering the ear.
Nails
Like hair, the nails on our fingers and toes are composed of keratin. The part that is visible and often painted is called the body of the nail, which lies on top of the nail bed. The distal edge of the nail that is cut is called the free edge. There are two folds of the nail, which are located along the sides of the nail (lateral nail folds) and the closer edge of the nail where it emerges (proximal nail fold). This proximal edge of the nail also has the thin fold of skin called the cuticle, and a lightened semicircular portion called the lunula. Under the skin of the fingertip is the nail root, and deeper to the nail root is the nail matrix. A diagram of the nail can be found at the bottom of the FAQ.
The keratinized plate that makes up the nail body protects the fingers and toes, as well as increases dexterity and precision of movements such as grip.
Quizlet is a great resource to use when studying for lab exams. Linked to this FAQ are a couple Quizlets with great pictures to help you identify structures of the integumentary system. Happy studying!