Zoology Assignment Help >> Integumentary-System (Skin), Epidermis, Dermis, Derivatives of Epidermis

Keywords: Integumentary System (Skin) Assignment Help, Zoology Homework Help, Theory of Epidermis, Derivatives of Epidermis, Glands of Epidermis, Functions of Epidermis, Dermis, Functions of Dermis, Functions of Skin, Zoology Homework Help, Biology Projects Assistance, Coursework Help, Defining Epidermis and Dermis, Biology Help: Expertsmind.com

Integumentary System (Skin)

The skin (or integument) forms the outer protective covering of the body. It maintains the shape of the body. In mammals, it is covered with characteristic hairs. It is waterproof, thick and made up of two layers.

EPIDERMIS

It derives from ectoderm and forms the outer layer of skin. It is stratified. It is provided with sensory nerves (nerve endings) which are tactile in function. It has several layers.

Epidermis gives rise to the appendages of skin viz. - hairs, nails, sweat glands, sebaceous glands etc. It is made up of a superficial cornified zone and a deep germinative zone. The cornified zone includes three strata viz.-

1. Stratum corneum

2. Stratum lucidum

3. Stratum granulosum

The germinative zone includes two strata viz.

1. Stratum spinosum

2. Stratum germinativum

(1) Stratum corneum-It is outermost layer which is keratinised completely. It prevents the entry of water in the body. The cells of this layer are devoid of nuclei. This layer casts off regularly from the body.

The formation of keratin is called cornification. It is very thick in sales and palms. It is generally eight to ten layers thick.

(2) Stratum lucidum-It is outer to the granulosum layer. It forms the barrier layer. It is formed of transparent, hard and compact cells. It is two to three layers thick. Eleidin is the basic feature of this layer.

(3) Stratum granulosum-It is outer to the spiny layer. It contains the granular cytoplasm. It is six to seven layers thick. Its cells are flattened. Main feature of the cells of this layer is keratohyalin granules.

(4) Stratum spinosum-It is also called prickle cell layer. It is also six to seven layers thick. Its cells are polyhedral and having finger like cytoplasmic processes. It provides firmness and rigidity to the epidermis.

(5) Stratum germinativum or stratum malpighi-It consists of columnar cells. It is the only living and dividing cell layer. It forms all cells of upper layers.

It absorbs nutrition from the blood vessels of dermis and make good the loss of surface layer of stratum corneum. It is also called stratum cylindricum or stratum basale.

Colouration of skin: In the stratum basale, there are melanocytes of neural crest origin. These cells synthesise their own pigment "melanin". This melanin, due to its fractional concentration provides colour to the skin. The synthesis of melanin is regulated by M.S.H. of pituitary origin.

Colour is also affected by the pigment carotene and respiratory pigments haemoglobin and oxyhaemoglobin.

DERMIS

It develops from mesodermal layer and is present below the epidermis. It is tough flexible and forms the elastic covering over the body wall. This layer contains the hair follicle, connective tissue, unstriped muscles, nerves, fat cells, tactile corpuscles, integumentary glands etc.

It is made up of following two layers¬

(1) Stratum spongiosum-It is elastic layer present just below the epidermis and contains blood vessels, nerves and glands mainly. It is thrown into folds to form wavy dermal papillae.

(2) Stratum compactum- This is the innermost layer of the skin and is made up of connective tissue and adipose tissue which forms the heat insulating layer over the body.

The dermis is used for making the leather. The formation of leather from dermis is called tanning. It is done by treating it with tanning agents viz- tannin, alum, chromium salts etc. Taxidermy is the process in which whole skin is preserved.

Often in man stratum compactum accumulates enormous amount of fat and forms a heat insulating and contour forming layer. This is called paniculus adiposus.

DERIVATIVES OF EPIDERMIS

Hair-It develops from stratum malpighi and is made up of two parts-

(1) Root-It is deeply situated in the dermis and is surrounded by the hair follicle. Hair follicle is the invagination of stratum malpighi. The follicle is situated above the dermal or hair papilla.

Fine blood vessels and small nerve fibres enter the hair follicle through the root of hair. It gives the nourishment for its proper growth and function.

Unstriped arrector muscles are attached to the hair follicle which causes the erection of hair due to excessive cold or fear.

(2) Shaft-This part is projected out of the epidermis and is attached by root.

Wall of hair follicle-Having two strata-

1. Outer sheath

2. Inner sheath

Inner sheath is composed of three layers viz.

1. Cuticle- of dead cells

2. Huxley's layer

3. Henle's layer

Hair has two layers-

(a) The central part is medulla which contains air.

(b) The outer covering part is cortex which is pigmented and covered by the cuticle.

Colour of hair is due to the presence of melanin pigment. When the melanin formation stops, there is accumulation of air bubble in the cortex of hair. Thus hair reflects light and gives the appearance of white. Arrector pilli muscles are responsible for the various movements of hair.

Hard derivatives of skin: Claws, nails, horns and hooves are keratinised structure derived from epidermis.

GLANDS OF EPIDERMIS

(1) Sebaceous glands-These open into the hair follicle and are also known as oil glands. These are glandular structure of the wall of follicle. These are provided with alveoli of branching sacs.

Their secretion is known as sebum. This sebum keeps the skin and hair waterproof.

(2) Sweat glands-It is also known as sudorific glands. It is thin, long, tubular and coiled at its blind end. These glands open on the surface of epidermis and their secretion is known as sweat. The composition of sweat is very much similar to the composition of urine.

Functions:-

(i) It plays an important role in maintaining the body temperature constant. Rise in body temperature stimulates the sweat glands to take water from the surrounding blood capillaries. The secretion is poured over the surface of skin. In evaporation of sweat, the heat of the body is utilized thus causing the lowering of the body temperature.

(ii) The excess body salts are also removed in the form of sweat.

The sweat glands are absent in the dogs.

(3) Mammary glands-These are modified sweat glands. These are alveolar glands. Their ducts open into raised nipples or teats. In female these glands secrete the milk for feeding and nourishing the young ones after birth.

(4) Perinaeal glands -These are also known as scent glands. These are situated very close to the anus and open into depressions known as perinaeal pouches. The secretion of these glands has a specific and characteristic smell of body (not found in human).

(5) Meibomian glands-They are situated on the margin of eyelids outside the eyelashes. These glands are also modified sebaceous glands. These glands are provided with several alveoli. The secretion of these glands is an oily liquid for the lubrication of eyeball.

(6) Glands of Moll- They are present in the eyelids of human being. They are modified sweat glands.

(7) Ceruminous glands-They are present in external auditory meatus. They are also modified sweat glands.

(8) Glands of Zeis- They are also found in eyelids and keep eyelashes smooth and supple.

FUNCTIONS OF SKIN

1. Maintenance of shape of the animal.

2. Protects the organs from the mechanical injuries.

3. Prevents the direct entry of bacteria in the body.

4. Checks the undue evaporation of body's water.

5. It is a storehouse of fat and reserved food.

6. It conserves the heat.

7. It acts as body thermostat.

8. It is a sensory organ.

9. It acts as an accessory excretory organ.

10. It acts as oil and milk producing organ.

11. It is an external skeleton.