Skin: Structure and functions

• Skin is the largest of all the organ of our body that is not uniformly thick.
• It is thick at some places and at some places it is thin.
• In average, the thickness of the skin is about 1 to 2 mm.
• Generally, the skin is thick in the sole of the foot, palm of the hand and inter-scapular region whereas, in other areas of the body it is thin.
• Its size is about 0.5mm only over eyelids and penis and is thinnest of all.
• It is made up of two layers. They are :

A)Epidermis

• The outer layer of skin is called epidermis which is formed by stratified epithelium.
• This layer does not have any blood vessels though the nutrition is provided to this layer by the capillaries of dermis.
• It is made up of five different layers. They are:

i) Stratum Corneum

1. It is the outermost layer of epidermis which is also known as horny layer.
2. It is made up of dead cells called corneocytes.
3. They are called dead cells as they lose their nucleus due to pressure.
4. The cytoplasm is flattened with fibrous protein which is known as keratin.
5. These cells also contain phospholipids and glycogen.

ii) Stratum Lucidum

1. This layer is made up of flattened epithelial cells where many of them contain degenerated nucleus and in some nucleus is absent.
2. This layer looks like a homogeneous translucent zone as they exhibit shiny character for which the name is given to it.

iii) Stratum Granulosum

1. It is a thin layer with two to five rows of flattened rhomboid cells.
2. Its’ cytoplasm contains granules of a protein called keratohyalin which is the precursor of keratin.

iv) Stratum Spinosum

1. 1. It is also called prickle cell layer because the cells of this layer process some spine like protoplasmic projections.
   2. The cells are connected to each other by these projections.

v) Stratum Germinativum

1. It is a thick layer made up of polygonal cells, superficially and columnar or cuboidal epithelial cells in the deeper parts.
2. New cells are constantly formed by mitotic division in this layer.
3. The newly formed cells continuously move towards the stratum corneum.
4. These newly formed cells arise from the stem cells also called keratinocytes.
5. Cells called melanocytes are scattered between the keratinocytes that produce melanin, a pigment that gives color to the skin.
6. From this layer, some projections called rete ridges extend down up to the dermis.
7. These projections provide anchoring and nutritional function.

Image source: Stanford                                                        Image source: Electrolysisbeauty

B) Dermis

• It is the inner layer of the skin under epidermis.
• It is the connective tissue layer made up of dense and stout collagen fibres, fibroblast and histiocytes.
• Collagen fibres exhibit elastic property and are capable of storing or holding water.
• These fibers also contain the enzyme collagenase which is responsible for wound healing.
• There are two layers of dermis. They are:

a) Superficial papillary layer

1. It is the layer containing blood vessels, lymphatics and nerve fibres projecting into the epidermis.
2. It also has some pigment containing cells called chromatophores.
3. Finger like projections called dermal papillae arises from the superficial papillary dermis.
4. Each papilla contains a plexus of capillaries and lymphatics which are oriented perpendicular to the skin surface.
5. The papillae are surrounded by rete ridges extending from the epidermis.

b) Reticular layer

1. It is made up of reticular and elastic fibres which are found around the hair bulbs, sweat glands and sebaceous glands.
2. This layer also contains mast cells, nerve endings, lymphatics, epidermal appendages and fibroblasts.
3. Immediately below the dermis, subcutaneous tissue is present.
4. It is a loose connective tissue connecting the skin with the internal structures of the body.
5. It protects the body from extreme heat and cold of the environment as it serves as an insulator.
6. Lot of smooth muscles called arrector pili are also found in skin around the hair follicles.

Appendages of skin

Hair follicles with hair, nails, sweat glands, sebaceous glands and mammary glands are considered as appendages of the skin.

Color of the skin

Color of the skin depends upon two important factors. They are:

i) Pigmentation of skin

• A brown pigment is present in the cells of the skin which is responsible for the color of the skin called melanin.
• Melanocytes synthesize this pigment, which are present mainly in the stratum germinativum and stratum spinosum of epidermis.
• After synthesis, the pigment spreads to the cells of the other layers.
• Melanin is protein in nature and is synthesized from the amino acid tyrosine via dihydroxyphenylalanine (DOPA).
• Skin becomes dark when melanin content increases.
• If melanin secretion is not proper, it leads to albinism (hypopigmentary congenital disorder).

ii) Hemoglobin in the blood

• The coloration of the skin is also affected by the amount and nature of hemoglobin that circulates in the cutaneous blood vessels.
• Skin becomes pale when hemoglobin content decreases, pink when blood rushes to skin due to cutaneous vasodilation and bluish during cyanosis which is caused by excess amount of reduced hemoglobin.

Functions of Skin

• The primary function of skin is protection of different organs and beside this there are various other functions as well.
• The various important functions of it are:

A) Protective function

• Skin covers all the organs of the body and protects these organs from various factors like bacteria and toxic substances, mechanical blow, ultraviolet rays, etc.

a) Protection from Bacteria and Toxic substances

• Skin covers the organs of the body and protects the organs from having direct contact with external environment. Thus, it prevents the bacterial infection.
• Skin secretes lysozyme which destroys the bacteria. Keratinized stratum corneum of epidermis is responsible for the protective function of the skin.
• This layer also offers resistance against the toxic materials like acids and alkalis.
• Infection occurs due to invasion of bacteria from external environment if the skin gets injured.
• Keratinocytes secretes the following during the skin infection or injury

i)Cytokines like interleukins, α-tumor necrosis factor and γ-interferon which help in inflammation, immunological reactions, tissue repair and wound healing.

ii) Antimicrobial peptides like β defensins which prevent invasion of microbes.

b) Protection from mechanical blow

• Skin is loosely placed over the underlying organs or tissues. So, it moves over the underlying subcutaneous tissues.
• Due to this reason, the mechanical impact of any blow to the skin is not transmitted to the underlying tissues.

c) Protection from ultraviolet rays

• Skin protects the body from ultraviolet radiation of sunlight.
• There is increase in the production of melanin pigment in skin when exposed to sunlight or to any other source of UV rays.
• Melanin absorbs UV rays and also the thickness of stratum corneum increases.
• Stratum corneum also absorbs the ultraviolet rays.

B) Sensory function

• Skin is the largest sense organ in the body which has many nerve endings which form the specialized cutaneous receptors.
• The receptors of skin are stimulated by sensations of touch, pain, pressure or temperature sensation and these sensations are conveyed to the brain via afferent nerves.
• Finally the brain perceives these various sensations.

C) Storage function

• Fat, water, chloride and sugar are stored by the skin.
• It can also store blood by the dilation of the cutaneous blood vessels.

D) Synthetic function

• UV rays from sunlight acts on cholesterol as a result vitamin D3 is synthesized in the skin.

E) Regulation of body temperature

• Skin has got important role in the regulation of body temperature.
• Excess of heat is lost from the body through the skin by radiation, conduction, convection and evaporation.
• Heat loss occurs by secretion of the sweat produced by sweat glands.
• The lipid content of sebum prevents the loss of heat from the body in cold environment.

F) Regulation of water and electrolyte balance

• Water and salts are excreted through sweat, by which skin regulates water balance and electrolyte balance in our body.

G)Excretory function

• Skin excretes small quantities of waste materials like urea, salts and fatty substance.

H) Absorptive function

• Skin absorbs fat soluble substances and some ointments that we use for various purposes.

I)Secretory function

• Sweat glands of skin secrete sweat and sebaceous glands secrete sebum.
• Body temperature gets regulated and water is also balanced through sweat secretion by skin.
• Skin remains moist and smooth due to the activity of sebum.

References

i) https://www.nursingtimes.net/clinical-archive/dermatology/skin-1-the-structure-and-functions-of-the-skin-25-11-2019/

ii) https://link.springer.com/chapter/10.1007/978-1-84882-862-9_1

Skin: Structure and functions