The integumentary system consists of skin and its appendages, which include hair, nails, sebaceous glands and sweat glands.
Functions of Integumentary System
1. Protection. The skin acts as a protective stretchable covering. It also prevents the entry of harmful micro-organisms and foreign material. It also prevents loss of body fluids.
2. Temperature Regulation. In hot surroundings body lose heat through evaporation of sweat. Conversely, in cold environment body lose heat by fat and hair.
3. Excretion. Through perspiration (sweating) small amounts of waste materials such as urea excrete through skin.
4. Synthesis. In the skin the vitamin D3 (cholecalciferol) synthesize from 7-dehydrocholestrol under the influence of ultraviolet rays from the sun. This vitamin is necessary for the normal growth of bones and teeth.
5. Sensory Perception. The skin is also an important sensory organ. It contains sensory receptors for heat, cold, touch, pressure and pain.
SKIN AS INTEGUMENTARY SYSTEM
The skin is the largest organ of the body. It covers all of the external free surfaces. At various orifices (openings) of the body the skin becomes continuous with the mucous membranes.
The skin has two main parts or layers:
- The epidermis is the outer layer consisting of stratified squamous epithelium.
- The dermis is a thicker layer of connective tissue beneath the epidermis.
The epidermis is actually a stratified squamous epithelium consisting of many layers of cells called keratinocytes. When we examine under the microscope, the epidermis is generally consist of five lavers of cells. From deep to superficial, these layers are:
(1) stratum Basale, (2) stratum spinosum, (3) stratum granulosum, (4) stratum lucidum, and (5) stratum corneum.
The surface of the epidermis cover by the stratum corneum, it compose of dead cells. Therefore, this epithelium called as stratified squamous epithelium. The keratinized cells of epidermis offer effective resistance both to passage of fluid through them and to friction.
The most superficial cells of the stratum corneum constantly shed off by cells that gradually move outward from the deepest layer, i.e., stratum Basale, which is stratum germinativum.
A steady mitotic activity in this layer maintains the necessary rate of regeneration of the epidermis.
The epidermis has no blood vessels and obtains its nourishment and oxygen by diffusion of substances from the blood vessels of the underlying dermis. This explains why the deep epidermal layers are living and active, whereas the cells of the outer layers, being away from the blood supply gradually degenerate and the most superficial layer consists of dead cells.
The deeper layers of the epidermis also contain some other varieties of cells. A special type of cells, called melanocytes, produce a brownish black pigment called melanin which distributes to the epidermal cells. Amount of the melanin in the epidermis is the major factor that determines the color of the skin,
The black people having a much higher quantity than the white ones.
Beneath the epidermis lies the dermis or corium, which is a thick layer of dense connective tissue. This connective tissue arranged in two layers. A superficial papillary layer and a deep reticular layer. The papillary layer forms blunt conical projections, called dermal papillae, which fit into reciprocal depressions on the undersurface of epidermis. The reticular layer consists chiefly of interlacing bundles of connective tissue fibers.
The dermis contains abundant blood vessels, nerves and nerve endings. It also lodges hair follicles and the sebaceous and sweat glands extending into the dermis. The dermis is the real skin; when dried it makes green hide and when tanned it makes leather.
Under the skin is present superficial fascia that other name is subcutaneous tissue or hypodermis.
On close observation, faint linear clefts are present in the skin. These clefts are indicative of the direction of collagenous fibers in the dermis. It found that the bundles of collagenous fibers are present in parallel rows, which follow different directional in each region of the body. Generally, they are longitudinal in the limbs and circular in the neck and trunk. These lines of skin tension are cleavage lines.
A knowledge of their direction in different areas of body is very important for a surgeon. A surgical incision made parallel to cleavage lines heals faster and leaves only a fine scar. On the other hand, if an incision is made across the cleavage lines, it results in the formation of a gaping wound that heals slowly and leaves a broad thick scar.
The cleavage lines of skin were first described by Langer in 1861. Later on, it was realized that Langer’s lines, which were mapped out on dead bodies, do not exactly coincide with the lines of greatest skin tension in the living. The Kraissl’s lines (described by Kraissl in 1951) are different from the Langer’s lines and are now considered to be more appropriate cleavage lines for surgical incisions.
APPENDAGES OF SKIN
As mentioned earlier, the skin has four appendages: hair, nails, sebaceous glands, and sweat glands. The appendages are essentially epithelial structures derived from the epidermis.
Hair as Integumentary System
Hair (Latin synonym “pilus” [singular], pili [plural]) are thread-like structures composed of dead cornified cells. Each hair has a shaft that projects above the skin surface and a root that is embedded in the skin. The root is surrounded by a tubular hair follicle that lies in the dermis and is responsible for the growth of the hair. The deep end of the hair follicle is dilated to form the hair bulb. The connective tissue of dermis indents the base of the hair bulb as hair papilla. Associated with each hair follicle is a bundle of smooth muscle fibers called arrector pili. Contraction of arrector pili makes the hair stand erect.
Hair are present over the entire surface of the body except on the palms, soles, lips, glans penis, clitoris, and labia minora.
The hair are constantly shed and replaced by new ones. A hair is shed when its growth is complete. After a short period of rest, the follicle produces a new hair. The life period of hair varies in different regions of the body. Scalp hair have a longer life span (3–5 years), the eyebrow and eyelash hair have much shorter life (3-5 months)
Nails as Integumentary System
The nails are horny plates covering the dorsal surface of the distal segment of fingers and toes. Each nail consists of three parts: the body, the root and the free edge. The body is that part which shows. It consists of cornified dead cells. The root is the hidden part that extends proximally deep to the fold of skin called nail fold. The free edge overhangs the tip of the finger or toe. The nail rests on an epithelial layer of skin called nail bed. The thicker layer of skin beneath the nail root is the matrix, where new cells are generated for the growth of the nail. The fingernails grow at an average rate of 0.5 mm per week; the toenails grow at about half that rate.
Sebaceous Glands as Integumentary System
The sebaceous glands (oil glands) lie in the dermis. Duct of the sebaceous gland opens into the neck of a hair follicle, through which the secretion of the gland reaches the skin. Commonly several sebaceous glands open into a single hair follicle. The sebaceous glands produce an oily secretion called sebum, which lubricates the skin surface. The sebaceous glands not found in the palms and soles.
The sudoriferous or sweat glands are present over most of the body, being most numerous in the skin of the palms and soles. The secretory portion of the gland lies in the subcutaneous tissue and is in the form of a long tube, which is coil to form a ball. The duct of the gland runs tortuously through the dermis, joins the epidermis and spirals through it to reach the skin surface, where it terminates at an opening called sweat pore. The sudoriferous glands produce sweat. Evaporation of sweat from the skin surface helps to cool the body.