Structure and Function of Skin: Skin Layer and Diagram

Skin is the largest organ of the body covering an area of 20 sq feet. Skin is the outer tissue covering of the body. It helps us to regulate body temperature and allows the sensation of heat, cold, and touch. The study of skin is known as dermatology.

What is Skin?

Skin is the outermost layer covering the entire external surface of the body. There are many structures and glands derived from the skin. It protects us from microbes and the elements and regulates body temperature.

The skin is not just a simple covering of our body but one of the most active organs, functioning in numerous ways. It belongs to the integumentary systems of our body. It contains hair follicles that anchor the hair strands into the skin.

Human Skin Diagram

The diagram of human skin is given below:

Layers of Skin

The structure of the human skin consists of three layers:

• Epidermis
• Dermis
• Hypodermis

Epidermis

The epidermis is the outermost layer of the skin. In some places, the epidermis becomes thick and hard like on the palms, soles, and especially on heels. It is devoid of blood vessels in all places.

The epidermis prevents germs and bacteria from entering and causing infections inside our bodies. It forms new skin cells. It contains melanin which provides color to the skin.

Epidermis is sub-divided into following layers:

• Basal cell layer
• Spinous cell layer
• Granular cell layer
• Stratum Corneum

Basal cell Layer

It is the innermost layer of the skin epidermis, and contains small round cells called basal cells. Here, new cells are formed continuously by active cell division, and push older cells towards the skin surface, where they eventually shed. Basal cell layer is richly supplied with blood vessels which promotes active growth of cell.

Spinous and Granular Layer

As older cells are pushed away towards the surface, there shape and size starts to vary and they form next layer called spinous layer. Above this layer lies granular layer. Since the cells moved away from the blood supply, they start to die resulting in the formation of a protein called Keratin.

Stratum Corneum

This consists of the outermost layer of the epidermis. Stratum Corneum, consisting of dead cells, is also called “horny layer” as the cells are toughened like an animal’s horn. These cells contain keratin in abundance and gives rigidity to the skin.

Epidermal Cells Types

The Epidermis cell contains 3 main cell types:

• Keratinocytes: These are the main cell types of epidermis and originate in the basal layer. They produce keratin, and forms water barrier by secreting lipids.
• Melanocytes: These cell produce melanin pigment which provide protection against harmful sun rays. Melanocytes are located in the basal layer.
• Langerhans: These specialized cells forms part of the body’s immune system and provide protection against foreign particles

Dermis

The dermis is the inner thick middle layer of connective tissue. It is tough and flexible. The dermis in our skin is very thick on the palms and soles, and very thin in other places as in the eyelids. The dermis layer of the skin contains other structures- blood vessels, sensory organs, sweat glands, etc.

There are some more sense receptors in the deeper parts, which are concerned with the sensations of heat, cold, pain, and pressure. In the dermis blood vessels provide nutrients to the epidermis, keeping layers of skin healthy. The sweat glands present in the dermis releases sweat through skin pores. Thus, regulating the body temperature.

Hypodermis

The hypodermis is the innermost layer of the skin. The fats in the hypodermis protect the bones and muscles from injuries when we fall. It is made of subcutaneous fats, blood vessels, tissues, and nerve cells. It provides contouring and shaping.

The fat in hypodermis acts like a shock absorber or padding which protects the body’s muscles from hot, cold, and trauma. It is also important for body temperature regulation. The layer becomes thin with age. This layer is also used for injections in some medications.

Functions of Human Skin

The number of important functions skin serves as follows:

• Protection against microorganisms: Skin prevents pathogens from entering our body and causing diseases.
• Prevents dehydration: Thick skin reduce water loss and hence prevents from dehydration.
• Sensation:Our skin serves as a sense organ for pain, touch, pressure, heat, etc.
• Temperature regulation: The skin prevents loss of heat in cold weather and loss of heat in hot weather.
• Storage : The skin stores reserve food in the form of a layer of fat contained in the main cells
• Excretion: The skin assists in the process of excretion of water, salts, and to a very limited extent, urea. Since sweating from the skin occurs mainly in temperature regulation, we cannot really consider the skin as an excretory organ.
• Synthesis of Vitamin D: The skin can synthesize 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.

Skin Derivatives

1. Hair
2. Nails
3. Sebaceous glands
4. Mammary glands
5. Sweat glands

Hair

Hair consists of three parts: 

• Hair shaft is the part that projects from the skin and may extend slightly below the surface of the epidermis, 
• Hair root is the part embedded within the dermis, 
• Hair bulb contains a projection of the dermis called the hair papilla, with a capillary of blood supply.

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. A nail has following parts –

• Root: It is the hard and outer part of the nail. It is made up of dead, keratinized cells.
• Plate: It is the visible external part of the nail.
• Free margin: is the area that protrudes out and is generally trimmed to maintain hygiene
• Nail Bed: part under nail plate which is pink in colour due to presence of capillaries in dermis.

Sebaceous Glands

These are microscopic glands found in hair follicle that secretes sebum. It is an oily secretion that makes the hair and the outer surface of the skin oily and waterproof to keep the epidermis supple and prevent loss of moisture by evaporation. In cold and dry weather, the skin may become rough and leave a powdery surface when scratched this is due to a reduced secretion of oil from the sebaceous glands. In hot and humid weather, the skin becomes extra oily due to increased secretion of sebum.

Mammary Glands

The mammary (or milk) glands are modified sweat glands. Each breast carries a central conical projection called a 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. 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 pair of breasts. The activity of the mammary glands is related to reproductive hormones and pregnancy. 

Sweat Glands

Each sweat gland consists of a deeper secretory part and an excretory part that runs upwards to open on the surface. 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. Their total number in the body is estimated at about two million. Sweat consists of about 99 percent water, and 0.2 to 0.5 per cents salts. The urea lost through sweat is about 1 percent of the total urea excreted by the body.

Skin Disorders

These are some of the following disorders that affect the skin:

• Leukoderma: It is also known as ‘vitiligo’. Skin pigmentation (melanin) is lost from smaller or larger patches at different regions of the body. The exact cause of the disease is still unknown.
• Albinism: Causes loss of pigmentation of the skin all over the body including eyebrows, hair, eyelashes, and even the iris. The skin of affected individuals appears pinkish because of the underlying blood capillaries. Albinism is a recessive trait caused due to inheritance; an albino couple would produce all albino children.
• Acne: It commonly occurs on the face, neck, chest, and shoulders. The symptoms are redness, blackheads, pimples, or deep cysts. It may leave scars on the skin if untreated.
• Measles: It causes red rashes which spread from the face to down the body within 3 to 5 days. The symptoms include fever, red or watery eyes, cough, and runny nose. 
• Melanoma: It is the most serious form of skin cancer. It can also appear as a mole that has changed color or gotten bigger over time. Melanoma occurs in areas that are less exposed to the sun and it is common in people with light skin. 
• Lupus: The symptoms include fever, headaches, fatigue, and painful joints. It spreads across the cheeks and nose.
• Wart: It is caused due to different type of viruses called Human papillomavirus (HPV).  

Conclusion – Human Skin

The skin is the outermost and most vital organ of the body. The human skin consists of 3 layers – Epidermis, Dermis, and Hypodermis. Skin provides protection against microorganisms, dehydration, and UV light. It regulates bodily temperature and stores reserve food in the form of a layer of fat. The derivatives of skin includes – hair, nails, sebaceous glands, mammary glands, and sweat glands.

FAQs – Structure and Functions of Human Skin

What do you mean by Epidermis?

The epidermis, the outer layer of skin, consists of stratified epithelium organized into three sublayers: cornified, granular, and inner. Epidermis is devoid of blood vessels.

What is Dermis?

The dermis, a thick inner layer of connective tissue, varies in thickness across the body, with thick regions like the palms and soles. It contains papillae, small raised processes near the epidermis, rich in blood capillaries and nerve endings.

How does the Skin get its Colour?

The skin get its colour from a pigment – melanin – which is synthesised in the epidermis. The amount of melanin varies the skin colour from dark to pale.

How is Skin Important?

Skin is important because it acts as a protection barrier against physical injuries, prevents moisture loss, reduces harmful effects of UV radiation and helps in regulating body temperature.

What are the 3 Layers of Skin?

The skin consists of 3 layers – Epidermis (top layer), Dermis (middle layer), and Hypodermis (innermost layer).

What are the Functions of the Skin?

Skin has many functions including – protection against microorganisms, dehydration, UV light. Skin regulates body temperature and stores fat which is used as food reserve.