ICSE Class 9 Biology Chapter 13 Skin The Jack of All Trades

Skin - "The Jack Of All Trades"

Various parts of the skin and their functions. Special derivatives of the skin with reference to sweat glands sebaceous glands, hair, nails and mammary gland. Heat regulation - vasodilation and vasoconstriction.

The skin is not just a simple covering to hold the body substance inside. It is one of the most active organs of the body, functioning in numerous ways. Many of the recognizable features come from the complexion, hair patterns, etc. The most apparent feature of an aged person, a healthy youth or a tender infant is the texture of the skin (whether loose and wrinkled, or tight and smooth). You will learn several interesting things about the skin - the "Jack of all trades" as named by some while some others would even call it "master of many tasks".

The skin is the largest organ of our body. It accounts for about 15% of an adult human's total body weight. Primarily the skin is an almost impermeable barrier.

What Is Skin?

The skin is the outermost covering of the body. It is stretched all over in the form of a layer. There are many structures and glands derived from the skin.

Functions Of The Skin

The skin (or the integument) serves a number of important functions as follows:

Protection

Protection is the primary function of the skin. It affords protection in four different ways:

(i) It protects the underlying tissues from mechanical shocks.

(ii) It holds the body fluids inside and prevents excessive loss of water by evaporation.

(iii) It prevents the entry of harmful substances or disease - causing germs.

(iv) It protects the body against excess ultraviolet light which is potentially very harmful.

Sensation

Our skin serves as a sense organ for touch, pain, pressure, heat, etc.

Temperature Regulation

The skin prevents loss of heat in cold weather and facilitates loss of heat in hot weather. You will read about the actual mechanism of temperature regulation later in this chapter.

Storage Of Food

The skin stores reserve food in the form of a layer of fat contained in special cells.

Excretion

The skin assists in the process of excretion (through sweating) eliminating water, salts and, to a very limited extent, urea. (Since sweating from the skin occurs mainly for temperature regulation, we cannot really consider the skin as an excretory organ).

Synthesis Of Vitamin D

The skin can synthesise vitamin D when exposed to sunshine. This is a minor function and its mechanism is not yet fully understood. In fact, too much sunshine may cause tanning and other ill-effects including even skin cancers.

Additional Function

The skin on our fingers and palms forms ridges and grooves which provide a more efficient grip.

Progress Check

1. List the four ways in which the skin serves as a protective layer for our body.

2. Cross out the items in the following list which are NOT the functions of the human skin: Sensation, respiration, storage of glycogen, excretion, digestion, temperature regulation, synthesis of vitamin D, secretion of hormone

Structure Of The Skin

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

A. Skin Proper

Epidermis - cornified, granular and germinative layers.

Dermis - elastic fibres, blood vessels, nerves, etc.

B. Derivatives Of The Skin

Hair

Nails

Mammary glands

Sweat glands

Sebaceous glands

The Skin Proper

Microscopically, the skin proper is composed of two layers - the outer epidermis and the inner dermis.

Epidermis

The 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. It is devoid of blood vessels at all places. The epidermis shows three regions (or sublayers) depthwise: (a) outermost cornified layer, (b) middle granular layer and (c) inner malpighian layer.

The Cornified Layer (Stratum Corneum)

The cornified layer (stratum corneum) is the outermost layer consisting of several piled up layers of flattened dead cells. These cells are made of a horny protein called keratin (also found in nails, hairs, horns, hoofs and silk). Its cells are continually worn away or shed and are replaced from beneath by those arising from the deeper malpighian layer. The cornified layer is tough and offers resistance to three things: (i) mechanical damage, (ii) bacterial infection and (iii) loss of water by evaporation.

The Granular Layer

The granular layer is a very thin middle layer consisting of two or three sub-layers of flattened cells. Gradually, it gives way to the outermost cornified layer.

The Malpighian Layer (Stratum Malpighi, Also Called Germinative Layer)

The malpighian layer (stratum malpighi, also called germinative layer) 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 (e.g. the complexion of the face) is due to a pigment melanin contained in the cells of the malpighian layer. The different quantities of this pigment in different human races lead to form very light-brown to dark colouration of the skin. African negroes have genetically dark coloured skin, the Europeans light coloured (whitish) and most Indians an intermediate coloured (wheatish) skin. In fair-coloured people also, on continued exposure to sunlight, the face or other parts of the body may get tanned, but revert to original condition when the exposure is cut down.

The skin pigment acts like an "umbrella" to protect inner parts of the body from the harmful effects of the ultraviolet rays of sunlight.

Two abnormal conditions of skin pigmentation.

Leucoderma also called "vitiligo": Skin pigmentation (melanin) 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 capillaries. Albinism is a recessive trait caused due to inheritance; an albino couple would get all albino children.

Dermis

The dermis is the inner thick layer of connective tissue made of elastic fibres. It is tough and flexible. 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. (Leather obtained from the hides of animals is actually the dermis part of the skin).

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 sensations 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 sensations of pressure, pain, heat, cold, etc.

Fat

The layers beneath the dermis contain numerous fat cells (adipose tissue). This subcutaneous fat not only serves as a food reserve but also as a heat-insulating layer, as well as a shock-absorber.

Progress Check

1. Name the three sub-layers of the epidermis from outer to inner:

2. Write True (T) or False (F) for the following statements:

(i) Stratum corneum is made of dead cells having keratin.

(ii) The malpighian layer is hard and resistant to bacterial invasion.

(iii) The pigment melanin which imparts colouration to the skin is found in the malpighian layer.

(iv) The dermis is very thick on palms and soles.

Teacher's Note

Your skin reveals your health status and lifestyle choices - from the glow of good nutrition and hydration to the damage from sun exposure and stress, making skincare a window into overall wellness.

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