In this chapter, you learned about the structures and functions of the organs of the integumentary system. Specifically, you learned that:

• The integumentary system consists of the skin, hair, and nails. Functions of the integumentary system include providing a protective covering for the body, sensing the environment, and helping the body maintain homeostasis.
• The skin’s main functions include preventing water loss from the body, serving as a barrier to the entry of microorganisms, synthesizing vitamin D, blocking UV light, and helping to regulate body temperature.
• The skin consists of two distinct layers: a thinner outer layer called the epidermis, and a thicker inner layer called the dermis.

• • The epidermis consists mainly of epithelial cells called keratinocytes, which produce keratin. New keratinocytes form at the bottom of the epidermis. They become filled with keratin and die as they move upward toward the surface of the skin, where they form a protective, waterproof layer.
  • The dermis consists mainly of tough connective tissues that provide strength and stretch, as well as almost all skin structures, including blood vessels, sensory receptors, hair follicles, and oil and sweat glands.
• Cell types in the epidermis include keratinocytes (which make up 90 per cent of epidermal cells), melanocytes that produce melanin, Langerhans cells that fight pathogens in the skin, and Merkel cells that respond to light touch.
• In most parts of the body, the epidermis consists of four distinct layers. A fifth layer occurs only in the epidermis of the palms of the hands and soles of the feet.

• 1. The innermost layer of the epidermis is the stratum basale, which contains stem cells that divide to form new keratinocytes.
  2. The next layer is the stratum spinosum, which is the thickest layer, and contains Langerhans cells and spiny keratinocytes.
  3. This is followed by the stratum granulosum, in which keratinocytes are filling with keratin and beginning to die.
     • The stratum lucidum is next, but only on the palms and soles. It consists of translucent dead keratinocytes.
  4. The outermost layer is the stratum corneum, which consists of flat, dead, tightly packed keratinocytes that form a tough, waterproof barrier for the rest of the epidermis.
• The epidermis protects underlying tissues from physical damage and pathogens. Melanin in the epidermis absorbs and protects underlying tissues from UV light. The epidermis also prevents loss of water from the body and synthesizes vitamin D.

• • Melanin is the main pigment that determines the colour of human skin. However, the pigments carotene and hemoglobin also contribute to skin colour, especially in skin with low levels of melanin.
  • The surface of healthy skin normally is covered by vast numbers of bacteria representing about one thousand species from 19 phyla. Different areas of the body provide diverse habitats for skin microorganisms. Usually, microorganisms on the skin keep each other in check unless their balance is disturbed.
• The thicker inner layer of the skin — the dermis — has two layers. The upper papillary layer has papillae extending upward into the epidermis and loose connective tissues. The lower reticular layer has denser connective tissues and structures, such as glands and hair follicles. Glands in the dermis include eccrine and apocrine sweat glands, as well as sebaceous glands. Hair follicles are structures where hairs originate.
• Functions of the dermis include cushioning subcutaneous tissues, regulating body temperature, sensing the environment, and excreting wastes. The dense connective tissues of the dermis provide cushioning. The dermis regulates body temperature mainly by sweating and by vasodilation or vasoconstriction. The many tactile sensory receptors in the dermis make it the main organ for the sense of touch. Wastes excreted in sweat include excess water, electrolytes, and certain metabolic wastes.
• Hair is a filament that grows from a hair follicle in the dermis of the skin. It consists mainly of tightly packed, dead keratinocytes that are filled with keratin. The human body is almost completely covered with hair follicles.
• Hair helps prevent heat loss from the head and protects its skin from UV light. Hair in the nose filters incoming air, and the eyelashes and eyebrows keep harmful substances out of the eyes. Hair all over the body provides tactile sensory input. The eyebrows also play a role in nonverbal communication.
• The part of a hair that is within the follicle is the hair root. This is the only living part of a hair. The part of a hair that is visible above the skin surface is the hair shaft. It consists of dead cells.

• • Hair growth begins inside a follicle when stem cells within the follicle divide to produce new keratinocytes.
  • A hair shaft has three zones: the outermost zone called the cuticle, the middle zone called the cortex, and the innermost zone called the medulla.
• Genetically controlled, visible characteristics of hair include hair colour, hair texture, and the extent of balding in adult males. Melanin (eumelanin and/or pheomelanin) is the pigment that gives hair its colour. Aspects of hair texture include curl pattern, thickness, and consistency.
• Among mammals, humans are nearly unique in having undergone significant loss of body hair during their evolution, probably because sweat evaporates more quickly from less hairy skin. Curly hair also is thought to have evolved at some point during human evolution, perhaps because it provided better protection from UV light.
• Hair has social significance for human beings, being an indicator of biological sex, age, and ethnic ancestry. Human hair also has cultural significance. For example, hairstyle may be an indicator of social group membership.
• Nails consist of sheets of dead, keratin-filled keratinocytes. The keratin in nails makes them hard but flexible. They help protect the ends of the fingers and toes, enhance the sense of touch in the fingertips, and may be used as tools.
• A nail has three main parts: the nail root, which is under the epidermis; the nail plate, which is the visible part of the nail; and the free margin, which is the distal edge of the nail. Other structures under or around a nail include the nail bed, cuticle, and nail fold.
• A nail grows from a deep layer of living epidermal tissues, called the nail matrix, at the proximal end of the nail. Stem cells in the nail matrix keep dividing to allow nail growth, forming first the nail root and then the nail plate as the nail continues to grow longer and emerges from the epidermis.
• Fingernails grow faster than toenails. Actual rates of growth depend on many factors, such as age, sex, and season.
• The colour of the nail bed can be used to quickly assess oxygen and blood flow in a patient. How the nail plate grows out can reflect recent health problems, such as illness or nutrient deficiency. Nails — and especially toenails — are prone to fungus infections. Nails are more permeable than skin and can absorb several harmful substances, such as herbicides.
• Skin cancer is a disease in which skin cells grow out of control. It is caused mainly by excessive exposure to UV light, which damages DNA.
• There are three common types of skin cancer: basal cell carcinoma, squamous cell carcinoma, and melanoma. Carcinomas are more common and unlikely to metastasize. Melanoma is rare and likely to metastasize. It causes most skin cancer deaths.
• Besides exposure to UV light, risk factors for skin cancer include having light coloured skin, having many moles, and a family history of skin cancer, among several others.

Now that you have learned about the organs on surface of the body, read the next chapter to travel inside and learn about the skeletal system, which protects and supports us internally, among other functions.