Structure of the skin

1. Structure of the skin Prof.Dr. Gehan Mosaad

2. At the end of this lecture, the student should be able to: • Understand the properties of the skin • Identify the different layers of skin and its function • Know the circulation and sensation of skin • Understand the biophysical properties of the skin • List the functions of the skin

3. The skin • It is the largest human organ, compromising about 16% of the total body weight. • Skin thickness varies for different body parts, as it is 10 times as thick on the palm of the hand as compared with other body areas. • It is the organ which is of the main concern with water application as it lies between the body and environment.

4. It protects the body from extreme heat and cold and from injuries because: • It is waterproof, elastic and durable. • It transmits stimuli to the central nervous system (CNS). • It mirrors the general bodily functions and gives clues to the state of person’s health.

6. Skin structure 1- Epidermis: • It is the outer layer of the skin. • It is composed of: -keratinocytes, constitute the majority of cells (95%), which produce a tough protein called keratin. - Melanocytes which are responsible for the surface color of the skin. They produce melanin that protect the body from UV radiation. - Langerhans cells are parts of skin’s immune response and engulf foreign material.

7. 1- Epidermis (cont.) • It doesn’t contain blood vessels but is nourished by the capillaries in the dermis below. • Its function: • Epidermis layer is responsible for the continual replenishing of the skin. New skin cells are produced at the basal membrane (deepest epidermal layer) pushing the older cells towards the surface. • It resists friction. • It is waterproof and prevents water loss.

8. 2- Dermis • It is much thicker than the epidermis and lies underneath it. • It is composed of collagen rich connective tissue that contains fibroblast cells that produce collagen and elastin which are responsible for the pliability and strength of skin. • Two main glands are embedded in the dermis a-Sweat glandsthat produce a watery substance to cool the body and excrete waste products. b-Sebaceous glandsthat produce an oily secretion which prevents the hair and skin becoming dry.

9. 2- Dermis (cont.) • It contains sensory nerve endings, hair follicles, lymphatic, and capillaries. • Its function: • It is responsible for the elasticity and mechanical support of the skin • It supplies the epidermis with nutrients • It is important in thermoregulation

10. 3- Hypodermis • It is not a skin layer but lies below the dermis, and is a subcutaneous tissue. • It contains fat, blood vessels and sensory receptors. • Its function: • It acts as protective cushion and insulator.

11. Circulation of the skin Circulation through the skin serves two functions: -Nutrition of the skin tissues - Regulation of body temperature by conducting heat from the internal structures of the body to the skin as it is lost by exchange with environment.

12. Circulatory apparatus of the skin It comprises two types of vessels: 1- The usual nutritive vessels ( arteries, veins and capillaries). 2- Vascular structures concerned with heat regulation which include extensive subcutaneous venous plexus and arteriovenous anastomoses.

14. Sensation of the skin • The skin is innervated with around one million afferent nerve fibers, which terminate in the epidermis. Most of them innervate the face and extremities; relatively few supply the back. • The skin has a special cutaneous nerve endings or receptors for Touch Pressure Temperature Pain

15. Some terms related to sensations 1- Anesthesia Complete loss of sensation 2- Hypoesthesia Diminished sensation 3- Hyperesthesia Increased sensation 4- Analegesia Loss of pain sensation 5- Hypothermaesthesia Diminished temperature sensation

16. Biophysical properties of the skin 1- Electrical properties • It is a function of its water and electrolyte content. • Moist skin conducts electricity, but dry skin resists it - Resistance of dry skin = 1,000,000 ohms - Resistance of moist skin = 20,000 ohms • Factors increase skin resistance a-Dry skin b- Ischemia c- Edema

17. Biophysical properties of the skin (cont.) • Factors decrease skin resistance a-Moist heated skin b-Exercises as it causes vasodilatation 2-Thermoregulatory sweating • It is the function of the autonomic nervous system. The surface electrical resistance of the skin is an index of its sweating. • Sympathetic nerves control the sweat glands, hair follicles and walls of blood vessels.

18. Skin Functions • The skin protects our bodies from trauma • It acts as a barrier to bacteria and viruses • It prevents us absorbing and losing excess water. • It secretes waste products . • It regulates our body temperature • It senses what is happening in the external environment (touch, pressure , heat). • Its pigments as well as hair protect us from sun.