Structure of Skin

Skin is the largest organ of the body. It is the outermost covering of the body. It is streched all over in the form of a layer. There are derived many structures and glands from the skin.

The principal components of the human(mammalian) skin are as follows :- 

Skin Proper :-

 Skin proper can be divided into two parts :-

Epidermis :-  Epidermis is the outer thinner part of the skin. It is formed of stratified epithelium piled up layer after layer. At places, the epidermis becomes thick and hard as on the palms.soles and specially on the heels.

The epidermis shows three regions :-

a) Outermost Cornified layer :- The cornified layer is the outermost layer consisting of several piled up layers of flattened dead cells.

These cells are made up of a horny protein called keratin.

The cornified layer is tough and offers resistance to three things :-

Mechanical damage 

 Bacterial infection 

 Loss of water by evaporation

b) Middle Granular layer :- It is consist of two or three sub - layers of flattened cells.

c) Inner Malpighian Layer :- It is the innermost region of the epidermis. Its cells can actively divide to produce new cells which press and shift outward to replace the worn out cells of the outermost cornified layer.

Colouration of the skin is due to a pigment melanin contained in the cells of the malpighian  layer. Quantity of this pigment is different in different human races which can lead to very light brown to dark colouration of the skin.

Abnormalities related to skin disease :-

Leucoderma  also called "Vitilago" : Skin pigmentation is lost from smaller or larger patches at different regions of the body, exact cause of this disease is not yet known.

Albinism :- Complete loss of pigmentation of the skin all over the body including hair, eyebrows, eyelashes and even the iris. The skin of such persons appears pinkish because of the underlying blood cappillaries. This disease is recessive trait caused due to inheritance.

Dermis :-

The dermis is the inner thick layer of connective tissue made of elastic fibres. It is tough and flexible.It is an elastic fibres, blood vessels, nerves etc.

At certain places the dermis in our skin is very thick as on the palms and soles, and very thin at other places as in the eyelids. 

The dermis contains several other structures - blood vessels, nerve fibres, sensory organs, hair follicles, sweat glands etc.

The outer region of the dermis which lies next to the epidermis is raised into numerous small processes called papillae which contain blood capillaries and nerve endings. The nerve endings and sense organs here are concerned with sensation of touch and pain. The sharp sense of touch in the skin of finger tips enables the blind to read the Braille characters. There are some more sense receptors in the deeper parts, which are concerned with the sensation of pressure, pain, heat, cold etc.

Derivatives of skin :-

Some of the derivatives of skin are Hair, Nails, Mammary glands, sweat glands and sebaceous glands.

Hair :-

A hair is consist of hair root , hair bulb and hair papilla.

Hair root is the part embedded within the dermis.

The expanded lower part of the hair called as hair bulb.

The projections of the dermis called as hair papilla, with capillary blood supply.

The hair follicle is a structure enclosing the hair root. It is composed of an epithelial and a connective tissue sheath. The hair bulb and the hair follicle together are responsible for the growth and elongation of the hair. The growth of hair occurs by addition of cells at the base, which soon die.

The colour of the hair is due to varying quantities of melanin. The gray or silvery colour of the hair is due to minute air spaces formed in the hair when the

Pigment is lost.

Everyone has sometimes experienced "goose flesh" during winter or during some emotion. In this. the hair is lifted called piloerection, (pilo hair) and the surface of the skin presents a somewhat contracted and wrinkled appearance. This is caused by the erector faiso called arrector) muscle of the hair which runs obliquely between the hair follicle and the outer part of the dermis. The contraction of this muscle at one end pulls the hair to a somewhat

Vertical position, and at the other end, depresses the epidermis.

Hair in human beings are continuously lost and regrown. The duration of scalp hair is 2-5 years and that of the eyebrows and eyelashes is 3-5 months.

Hairs from the different parts of the body (head, chest, arm pit, beard, nose, etc.) show subtle differences. These are helpful in forensic (crime detection) investigations.

Hairs also provide a sensation of touch because nerve fibres extend up to their bases.

There are hairs eyelashes along the edges of the eyelids helping to prevent entry of particles, raindrops. Similarly, there are hairs in the nose, again to prevent dust particles from enterring nasal passages. Facial hairs in human males, i.e. moustaches and beard, help in distinguishing the male sex (sexual dimorphism).

Nails :-

Nails are hardened keratinous plate-like structures which grow as dead cells from the nail root, which lies below the skin at the base.

(i) Plate - It is hard and outer part of the nail. It is made up of dead, keratinized cells.

(ii) Bed (root) - It lies below the nail plate.

(iii) Matrix - It lies just below the skin surface at the base of the nail. It is usually visible as whitish half-moon at the base of nail, it produces new cells which on maturation push out the older one towards the tip of the nail and causes growth of the nail.

Sebaceous Glands :-

These branched glands usually open into a hair follicle though, sometimes they even open directly to the outside. They give out an oily secretion (called sebum) which makes the hair and the outer surface of the skin oily and waterproof to keep the epidermis supple and to prevent loss of water by evaporation. In cold and dry weather, the skin may become rough and leave a powdery surface when scratched; this is due to reduced secretion of oil from the sebaceous glands. In hot and humid weather, the skin becomes extra oily due to increased secretion of sebum.

Three common problems related to sebaceous glands: :-

1. Pimples - Sebum accumulation, causes growth of bacteria because it is nutritive, gets infected and results into the formation of boils and pimples.

2. Acne - Sebaceous glands get inflamed due to hormonal influence. It is one of the commonest adolescence problems.

3. Black head - Sebaceous glands of the face get enlarged due to accumulated sebum. On oxidation melanin and sebum give it a black colour named black head.

Sweat Glands :-

Each sweat gland is a simple coiled tube consisting of a deeper secretory part and an excretory part which nuns upwards to open on the surface. The outer openings are called the sweat pores. Their total number in the body is estimated at about two million. Human races belonging to hotter countries usually have more sweat pores than those belonging to colder ones.

The secretory part of a sweat gland absorbs fluid from the surrounding cells and blood capillaries of the dermis and passes it into the excretory sweat duct which pours it out on the surface. Sweating (or perspiration) goes on at all times in minute quantities (incipient or invisible perspiration), or sometimes in large quantities as during strenuous exercise or during hot and humid weather, when a person may lose as much as 1 kg per hour. The major function of sweating is to lose body heat by evaporation.

Sweat consists of about 99 per cent water, 0.2 to 0.5 per cent salts (mainly sodium chloride) and traces of urea (0.08 per cent). The urea lost through the sweat is about 1 per cent of the total urea excreted by the body. "Cold sweat" may be due to psychic influences such as fright and nervousness. Sweating may also accompany nausea and severe pain (due to loss of body salts).

Mammary Gland :-

The mammary (or milk) glands (Fig. 13.3) are modified sweat glands. These glands are present both in males and females. But in males, they persist only in a rudimentary state, whereas in females at puberty, they enlarge in the form of a pair of breasts. Each breast carries a central conical projection called  nipple. 15-20 milk ducts open on the nipple. Each milk duct is continued inward in a branching manner to join a cluster of 15 to 20 lobes of the mammary glands. The activity of the mammary glands is related to the reproductive hormones (prolactin) and pregnancy. The milk secreted by the mammary glands is highly nutritious for the new-born baby.

Meibomian glands : These are modified sebaceous glands which open on the margins of the eyelids. Their secretion is oily and serves to lubricate the margins of the lids and to prevent the overflow of tears.

Ceruminous glands : These are modified sebaceous glands found in the auditory canal and secrete wax like substance called cerumen or earwax which lubricates and protects the delicate eardrum from dust particles and germs.