Integumentary System

Structure and functions of skin

The skin is the human body’s its largest organ, covering 1.6 M² of surface area and accounting for approximately 16% of an adult’s body weight.

The skin is the largest organ of the body, accounting for about 16% of the total adult body weight. It performs many vital functions, including protection against external physical, chemical, and biologic assailants, as well as prevention of excess water loss from the body and a role in thermoregulation. The skin is continuous, with the mucous membranes lining the body’s surface. The integumentary system is formed by the skin and its derivative structures. The skin is composed of three layers: the epidermis, the dermis, and subcutaneous tissue. The outermost level, the epidermis, consists of a specific constellation of cells known as keratinocytes, which function to synthesize keratin, a long, threadlike protein with a protective role. The middle layer, the dermis, is fundamentally made up of the fibrillar structural protein known as collagen. The dermis lies on the subcutaneous tissue, or panniculus, which contains small lobes of fat cells known as lipocytes. The thickness of these layers varies considerably, depending on the geographic location on the anatomy of the body. The eyelid, for example, has the thinnest layer of the epidermis, measuring less than 0.1 mm, whereas the palms and soles of the feet have the thickest epidermal layer, measuring approximately 1.5 mm. The dermis is thickest on the back, where it is 30–40 times as thick as the overlying epidermis.

Embryology of the skin:

The skin is mainly mesodermal in its embryonic derivation. Specialized skin cells and structures are formed from 3-6 months of gestation.

Types of skin:

1. Non-hairy (glabrous)—a skin type on the palms and soles, it has thicker epidermis and lacks hair follicles.

2. Hairy—a type of skin having hair follicles and sebaceous glands.

Layers of the skin:

1.    Epidermis:- the outer most layer of the skin that is divided into the following five layers from top to bottom. These layers can be microscopically identified:

A.    Stratum corneum:-  Also known as the horny cell layer, consisting mainly of keratinocytes (flat squamous cells) containing a protein known as keratin. The thick layer prevents water loss and prevents the entry of bacteria. The thickness can vary regionally. For example, the stratum corneum of the hands and feet are thick as they are more prone to injury. This layer is continuously shed but is replaced by new cells from the stratum basale (basal cell layer). The stratum corneum constitutes about 15 to 20 layers.

B.     Stratum lucidum:- This layer is present in the thick skin of palms and soles and consists of a transparent layer of dead cells. It functions as a barrier and also has water proof properties.

C.     Stratum granulosum:- Also known as the granular layer, consisting mainly of stratified squamous cells arranged in 1 to 3 rows containing lamellar granules and tonofibrils. It is important to note that besides the palms and soles, skin lacks a well-defined stratum lucidum and stratum granulosum.

D.    Stratum spinosum:- Also known as the spinous layer consisting mainly of a cuboidal cell arranged in multiple layers and synthesizes keratins that function to sup- port structures. The cells are adherent by specialized cells known as desmosomes.

E.     Stratum basale or Stratum germinavatum:- also known as the basal cell layer, is the deepest layer of the epidermis. The layer consists of tall columnar cells that are constantly undergoing cell division and help form new keratinocytes (keratinization) that will replace the lost ones from stratum corneum. This process takes about 27 days. Further down the stratum basale the cell layer is attached to a basement membrane which serves as a demarcation or a boundary between the epidermis and dermis. The layer also contains melano- cytes containing melanin, Langerhans cells which recognize antigens and present them to the immune system and Merkel discs which detect pressure on skin.

2.    Dermo-epidermal junction: - A well demarcated junction that lies between the epidermis and dermis.

3.    Dermis:- Lies between the epidermis and subcutaneous layer. This middle  layer  of  skin  contains  connective  tissue  in  the  form  of  collagen in bulk and elastin in minimal quantities with a  rich intertwining blood supply. The types of cells located in the dermis are fibroblasts, mast cells and histocytes. Hair follicles, nerves, lymphatic vessels and sweat glands also reside in the dermal layer of the skin.

4.    Subcutaneous tissue:- Also known as the subcutis or hypodermis is the lower most layer comprising mainly of fat (adipose) which provides protection from injury, produces heat and serves as a cushion for the body.

Innervation of the skin

• Parasympathetic nerves- consisting of cholinergic neurons that release acetylcholine to the sweat glands.

• Sympathetic- consisting of adrenergic neurons that release norepinephrine to the sweat glands, arteriolar smooth muscle and erector pili muscle.

 Functions of the Skin:

1. Protection and repair which is provided mainly by keratinocytes while UV protection is offered by melanocytes. The subcutaneous layer protects the deeper body organs. Gentle stroking of the skin with a blunt object can result in white line response caused mainly by capillary constriction. A deeper stroke using a tongue blade will lead to the triple cell response, resulting in a red line, flare and wheal. The wheal is caused by the release of histamine that acts as a vasodilator in local response to injury. The eliciting of the red wheal is known as dermographism that is more pronounced in patients who suffer from hives (urticaria).

2. Skin color is given by melanocytes that contain melanin.

3. Temperature regulation and excretion of waste products- sweat glands produce sweat containing urea and water and play a role in temperature regulation. To facilitate heat loss in hot temperatures, the blood vessels in the skin dilate and sweat glands become active. Alternatively, in cold temperatures skin blood vessels constrict to conserve heat and the body burns fat stored in the adipose tissue. The burning of brown fat under sympathetic stimulation is common in infants. In colder temperatures, the sweat glands become inactive and the erector pili muscles become functional to pro- mote trapping of air for insulation of skin. The adrenergic receptors like a_1, innervating the skin blood vessels are responsible for vasoconstriction under sympathetic stimulation.

4. Lubrication of the skin is provided by sebaceous glands, which produce an oily substance known as sebum. Occlusion and infection of these glands and lead to conditions such as acne.

5. Immunity- Langerhans cells in the skin are dendritic cells that take up microbial antigens in the skin  to transform into antigen presenting cells and provide immunity by interacting with T cells. The name Langerhans comes from the German physician and anatomist that discovered these cells in the skin when he was a medical student.

6. Storage - the skin is an organ which stores fats to provide insulation. This is mainly in the subcutaneous layer.

7. Sensation—sensation occurs through specialized structures known as mechanoreceptors:

Pacinian corpuscle—vibration.

Meissner’s corpuscle—tapping and flicker, point discrimination.

Ruffini’s corpuscle—joint movements and stretch.

Hair follicle receptor—speed and direction of movement.

Merkel’s discs—vertical dimpling of the non-hairy skin.

Tactile discs—vertical dimpling of the hairy skin.

Nociceptors—detection of pain.

8. Vitamin  D  synthesis —skin is a rich source of 7-dehydrocholesterol and under the effect of UV light is converted into Vitamin D (cholecalciferol) that is ingested mainly from diet such as milk and dairy products. Cholecalciferol is converted into 25-hy- droxycholecalciferol (25-OH) in the liver and finally to activated 1, 25 hydroxychole- calciferol (1,25 OH) in the kidneys. The activated 1, 25 hydroxycholecalciferol plays a vital role in calcium absorption from the intestine and kidneys.

9. Aesthetic—skin can be seen as a mode of communication or attraction.

10. Absorption—The skin has the ability to absorb oxygen and water. Certain drugs such as topical steroids that are applied topically

Essential skin facts:

Largest organ of the body. Accounts for 16% body weight.

The organ of the body that is most exposed to bacteria, UV light, toxins, dust and other environmental stressors.

Every 24 hours the surface of skin sheds dead layer of cells, and on average 40 kg of skin is shed during lifetime.

Dead skin cells can become a component of household dust.