The Integumentary System consist of the skin and its derivatives; hair, glands, nails and nerve endings.

THE SKIN
FUNCTIONS OF THE SKIN
1. PROTECTION
– Skin covers the body and provides a physical barrier that protects underlying tissues from physical and bacteria invasion, dehydration and ultraviolet light.
– Certain cells of the epidermis play a role in the immune system, warding off invaders.
2. REGUATION OF BODY TEMPERATURE
– Under conditions of high temperature or strenuous exercise, glands produce sweat and the evaporation of the sweat provides a cooling mechanism for the body.
– Under conditions of low temperature, the production of sweat decreases.
– Sweat also contributes to excretion. Along with heat and some water, it removes a small amount of salts and several organic compounds.
3. BLOOD RESERVIOR
– The dermis contains an extensive network of blood vessels that carry up to 10% of the total blood flow in a resting adult.
4. HOUSING OF NERVE ENDINGS
– Nerve endings that detect temperature, touch, pressure and pain are located in the skin.
5. SYNTHESIS OF VITAMIN D
– Synthesis of vitamin D begins with the activation of a precursor in the skin by ultraviolet light.
– This compound is modified and becomes CALCITRIOL (the most active form of vitamin D).
– Calcitriol aids in the absorption of calcium in food from the digestive tract into the blood.

The skin consists of different tissues that are joined to perform specific functions.

It is the largest organ of the body, in surface and weight.

The skin is composed of 2 parts; the EPIDERMIS and the DERMIS.
1. EPIDERMIS
– The epidermis is the outer, thinner portion of the skin. It is usually 4 to 5 layers thick, depending on its location.
– Its deepest layer is a single layer of stem cells capable of continuous cell division.
– It contains the cells that are sensitive to touch.
– Some of the cells produce KERATINOCYTES that keep moving up the skin layers.
– While these cells are moving up to the skin’s surface, MELANIN is taken in by the cells. The cells lose their nucleus and other organelles, they lose their ability to carry on vital metabolic reactions and eventually die.
– Some of the stem cells move down in the skin and become oil and sweat glands and hair follicles.
– The most superficial layer of the epidermis, the STRATUM CORNEUM, is composed of about 30 rows of flat, dead cells, completely filled with KERATIN. This layer is shed continuously and replaced by cells from the lower layers.
– It serves as an effective layer against light, heat waves, bacteria and many chemicals.
– It takes about 2 to 4 weeks for the epidermal cells to make their migration from the lowest layer to the skin’s surface.
– The epidermis is composed of layers of STRATIFIED SQUAMOUS EPITHELIUM (one of the four main types of tissue in the body).
– The epidermis contains 4 types of cells: KERATINOCYTES, MELANOCYTES, LANGERHANS CELLS and MERKEL CELLS.
1. KERATINOCYTES
– Keratinocytes make up 90% of the epidermal cells.
– They produce the protein KERATIN which is a tough, fibrous protein that helps protect the skin and underlying tissue from heat, microbes and chemicals.
– Keratinocytes also release a waterproofing sealant for the skin.
2. MELANOCYTES
– Melanocytes make up 8% of the epidermal cells.
– They produce the pigment MELANIN, which contributes to skin color and absorbs ultraviolet light.
– Once inside the cells the melanin granules cluster to form a protective veil over the nucleus of the cell on the side of the skin’s surface, this shields the DNA(genetic material) from ultraviolet damage.
3. LANGERHANS CELLS
– Langerhans Cells make up a small portion of the epidermal cells and participate in the immune responses against microorganisms.
4. MERKEL CELLS
– Merkel Cells are the least numerous of the epidermal cells.
– They are located in the deepest layer of the epidermis, where they are in contact with nerve cells and function in the sensation of TOUCH.

2. DERMIS
– The dermis is an inner, thicker layer of connective tissue.
– The connective tissue of the dermis has collagen, elastic fibers, blood vessels, nerves, glands and hair follicles embedded in it.
– The combination of collagen and elastic fibers in the dermis provide the skin with STRENGHT and ELASTICITY(the ability to stretch).
– Nerve endings sensitive to COLD are found in and just below the dermis.
– Nerve endings sensitive to HEAT are located in the middle and outer dermis.
– The dermis is attached by fibers to a layer called the SUBCUTANEOUS LAYER, which isn’t considered a part of the skin.
– The subcutaneous layer serves largely as a storage depot for fat and contains larger blood vessels that supply the skin.
– The subcutaneous layer is attached to the underlying tissue.

SKIN COLOR
– MELANIN, CAROTENE and HEMOGLOBIN are the 3 main pigments responsible for skin color.
– Melanin is a brown-black pigment found mostly in the epidermis and causes variations in skin tone from pale yellow to black.
– Since the number of melanocytes is about the same for all races, differences in skin color are due to the amount of pigment the melanocytes produce and disperse to the keratinocytes.
– ALBINISM is an inherited inability of an individual of any race to produce melanin. Melanin is absent in their skin, hair and eyes, they appear white.
– VITILIGO is the partial or complete loss of melanocytes from patches of skin, it produces irregular white spots.

HAIR
– Hairs are growths of the epidermis.
– Hair occurs all over the body, except on the palms of the hands and the soles of the feet.
– Genetics and hormones determine the thickness and pattern of distribution of hair.
– Normal hair loss is between 70 and 100 hairs per day.
– The main function of hair is PROTECTION. Hair guards the scalp from injury and the sun’s harmful rays.
– Eyebrows and eyelashes protect the eyes from foreign particles. Hairs in the nostrils and ears have a similar function.
– Hair also aids in sensing light touch. Touch receptors associated with hair follicles are activated when a hair moves even slightly.
– Each hair is composed of columns of dead, keratinized cells welded together.
– The SHAFT is the superficial portion of the hair that projects from the surface of the skin.
– The shaft of straight hair is round in cross-section and curly hair is oval.
– The ROOT is the part of the hair that penetrates into the dermis and sometimes the subcutaneous layer.
– The HAIR FOLLICLE surrounds the root.
– Around each hair follicle are nerve endings that are sensitive to touch.
– Smooth muscle (ARRECTOR PILI) is attached to the hair follicle. When these muscles contract under conditions of fright or cold, the hair pulls into an upright position. This is what happens when your “hair stands on end” or you get “Goosebumps”.
– HAIR COLOR is related to the pigment melanin. The melanin is formed by the melanocytes and then passed into the hair.
– Dark colored hair is mostly true melanin.
– Blond or red hair is due to melanin mixing with iron and sulfur.
– Gray hair is the progressive loss of the enzyme responsible for the formation of melanin.
– White hair is the result of the accumulation of air bubbles in the hair shaft.

GLANDS
There are 4 types of glands associated with the skin; oil, sweat, ceruminous and mammary glands.
1. OIL GLANDS
– Oil glands vary in size depending on where they are located.
– They are larger in the skin of the breast, face, neck and upper chest.
– They are smaller in most of the trunk of the body and the extremities.
– They are absent on the palms of the hands and the soles of the feet.
– Oil glands are usually connected to hair follicles. These glands produce an oily substance (SEBUM) that contains fats, cholesterol, proteins, inorganic salts and pheromones.
– Sebum has several functions including, coating the surface of the hair to keep it from becoming dry and brittle, it prevents excessive evaporation of water from the skin, it keeps the skin soft and supple and it inhibits the growth of bacteria.
2. SWEAT GLANDS
– There are 3 to 4 million sweat glands in the human body.
– They empty their secretions on to the skin’s surface
– They fall into 2 categories: ECCRINE and APOCRINE.

ECCRINE
– Eccrine glands are simple, coiled, tubular and more common than apocrine glands.
– They begin functioning soon after birth.
– They mainly produce sweat or perspiration. They are found in the skin of the forehead, palms and soles.
– SWEAT is a mixture of water, salts, urea, uric acid, amino acids, sugar, lactic acid and ascorbic acid.
– Its main function is to help regulate body temperature by providing a cooling mechanism. As sweat evaporates, large quantities of heat energy leave the body’s surface. It also plays a small role in eliminating waste.

APOCRINE
– Apocrine glands begin functioning at puberty.
– They are simple, coiled, tubular glands. They are located in the subcutaneous layer and their excretory ducts open into the hair follicle.
– Their secretion is more thick and sticky than sweat, due to the addition of lipids and proteins.
– These glands are located in the armpits, pubic region and pigmented regions of the breast.

3.CERUMINOUS GLANDS
– Ceruminous glands are modified sweat glands located in the subcutaneous layer of the external ear.
– They produce a waxy substance that acts as a sticky barrier, preventing foreign substances from entering the ear.
4. MAMMARY GLANDS
– Are also modified sweat glands.

NAILS
– Nails are plates of tightly packed, hard keratinized cells of the epidermis.
– The NAIL BODY is the visible portion of the nail.
– The NAIL ROOT is buried in a fold of skin.
– The epithelium underneath the nail root is called the NAIL MATRIX and is made up of cells undergoing mitosis to produce nail growth.
– Nails grow at various rates; the average nail grows about 1 millimeter per week.