The Skin and Its Structure

The skin is the outer covering of vertebrates. It forms a continuous layer over the entire body, making it the largest organ. In mammals, the skin usually bears a dense growth of hairs. Human beings, however, only have a sparse growth of hair over most parts of their bodies.




STRUCTURE OF THE SKIN

The skin is made up of two main layers:

1. The Epidermis
2. The Dermis

1. The Epidermis

 This is the outer layer of the skin. It is made up of three layers:

• horny or cornified layer
• granular layer
• malpighian (germinative) layer

(i) Horny or cornified layer: This is the outermost layer of the epidermis. It is made up of flat, hard, dead and scaly cells. This layer is impregnated with protein called keratin. The keratin makes this layer tough, flexible, and water proof. The cells in the cornified layer are constantly being worn-out replaced by cells from the layers below. It is usually very thick on the soles of the feet and palms and thin on the eyelids.

(ii) Granular layer: This layer consists of living cells that are constantly being pushed up from the malpighian layer. The cells continue to move up towards the surface of the skin through this layer. In this process, they become greatly flattened, accumulate keratin and eventually die.

(iii) Malpighian (Germinative) layer: This layer consists of actively dividing cells. It contains a pigment called melanin which gives the skin its colour. It has also another pigment called keratin, which is responsible for the toughness and flexibility of the skin.

The malpighian layer has many infoldings. Some penetrate deep into the dermis to form hair follicles and sebaceous glands. The cells of this layer get their nutrients and supply of oxygen by diffusion from the blood in the capillaries found in the dermis.

2. The Dermis 

The dermis is a layer of connective tissues containing: 

• blood capillaries, 
• hair follicles, 
• sweat glands, 
• sebaceous glands, 
• sensory nerve endings and 
• fat cells.

(i) Blood capillaries:  These bring food and oxygen to the dermal and epidermal cells and remove their waste. Capillary networks supply hair follicles and sweat glands.

(ii) Hair follicle: This is a deep pit formed by the infolding of the malpighian layer. The malpighian cells at the base of each pit keep dividing to produce a long cylindrical hair. The nerve endings in the follicle respond when the hair is touched. A hair erector muscle is attached to each follicle. Its concentration pulls the hair to a more upright position. i.e., it makes the hair erector stand up. When we feel cold, the hair erector muscles contract and we get 'goose pimples'

(iii) Sebaceous glands: These glands secrete sebum, an oily substance, into the hair follicles. Sebum keeps the hair and the epidermis water-proof and supple. It also keeps out dust and micro-organisms.

(iv) Sweat glands: These are found in the dermis as coiled tubes which continue as sweat ducts that open at the surface of the skin as sweat pores. The cells of the coiled tubes absorb water from the capillaries surrounding them and secrete them into the tubes. This water contains small quantities of wastes such as mineral salts and urea and is known as sweats. When sweat reaches the surface of the skin and evaporates, it cools the body.

(v) Sensory nerve endings: The skin is also a sense organ. It contains sensory cells which can receive stimuli from external surroundings and send them to the appropriate parts of the body for interception and action.

(vi) Sub-cutaneous fat: Below the dermis is a layer of connective tissue which contains variable number of fat cells. This is the subcutaneous fatty (adipose) tissue. It acts as a long term food store as well as an insulating layer that prevents heat loss from the body.

Recommended: Funcions of the skin and care of the skin

Categories: Biology, Excretion, Excretory System, Homeostatis, The skin