skin/جلد/皮肤(Pífū)

Skin is a soft outer covering of an animal, in particular a vertebrate.

Skin or skins may also refer to:

• Peel (fruit) or skin
• Skin (aircraft), the outer covering of an aircraft or its wing
• Skinning, removing the outer layer of something's flesh
• Milk skin, a thin layer over the top of heated milk
• Hide (skin), an animal skin processed by humans

 Other animal coverings such as the arthropod exoskeleton or the seashell have different developmental origin, structure and chemical composition. The adjective cutaneous means "of the skin" (from Latincutis, skin). 

In mammals, the skin is the largest organ of the integumentary system made up of multiple layers ofectodermal tissue, and guards the underlying muscles, bones, ligaments and internal organs. Skin of a different nature exists in amphibians, reptiles, and birds. All mammals have some hair on their skin, even marine mammals which appear to be hairless. The skin is one of the most important parts of the body because it interfaces with the environment and is the first line of defense from external factors. For example, the skin plays a key role in protecting the body against pathogensand excessive water loss. Its other functions are insulation, temperature regulation, sensation, and the production ofvitamin D folates. Severely damaged skin may heal by forming scar tissue. This is often discoloured and depigmented. The thickness of skin also varies from location to location on an organism. In humans for example, the skin located under the eyes and around the eyelids is the thinnest skin in the body at 0.5 mm thick, and is one of the first areas to show signs of aging such as "crows feet" and wrinkles. The skin on the palms and the soles of the feet is 4 mm thick and the thickest skin in the body.

Fur is dense hair. Primarily, fur augments the insulation the skin provides but can also serve as a secondary sexual characteristic or as camouflage. On some animals, the skin is very hard and thick, and can be processed to create leather.Reptiles and fish have hard protective scales on their skin for protection, and birds have hard feathers, all made of tough β-keratins. Amphibian skin is not a strong barrier to passage of chemicals and is often subject to osmosis. For example, afrog sitting in an anesthetic solution could quickly go to sleep.

How Does Aging Affect Your Skin?

Your skin is made up of three layers – the epidermis, dermis, and hypodermis.

The top layer – or the epidermis – is where all-important cell renewal takes place. In younger skin, cells divide and replace themselves every 15 to 30 days.

With age, these cells divide more slowly – perhaps twice as long – making your skin less able to repair itself quickly. The total number of epidermal cells decreases by 10 percent each decade of life.

These top moisture-protective cells begin to lose some of their "stickiness", releasing moisture instead of preserving moisture in your skin.

The result is thinner looking and drier skin. Nowhere is this more noticeable than with the fragile skin under your eyes.

The dermal, or second layer, also thins as you age and produces less collagen – the natural protein that keeps your skin looking smooth and firm.

Essential for skin strength and elasticity, a loss in collagen causes your skin to lose its suppleness and begin to wrinkle and sag.

Once active sebaceous glands now produce less skin-lubricating sebum, and the number of sweat glands decrease. Both of these changes lead to skin dryness and the formation of fine lines.

Finally, the third and bottom layer of your skin – the hypodermis or subcutaneous layer – contains fatty tissue that cushions the layers above it. As fat cells shrink with age, these less "plumped" cells can no longer "fill in" damaged upper layers of skin. As a result, skin sags and wrinkles.