Skin Layers
The outer layer vs the innermost layer of the skin
Skin is the largest organ of the human body and this is an unbelievable fact. Skin is present all over the body and acts as a protective sheath for the delicate internal organs against the environmental agents like wind, sun, water etc. The skin is composed of three layers from outside to inside- the epidermis, dermis and the hypodermis.
Epidermis is the outermost layer of the skin and very different compared to the innermost layer called the dermis. The epidermis is a tough layer of skin as it is the one which is continuously exposed to environmental trauma. The epidermis has no blood vessels and is composed of many layers stacked one above the other. It is made up of keratinocytes, melanocytes and other cells. The melanocytes present in this layer give skin the color that we see. The epidermis is nourished by diffuse oxygen from the environment whereas the dermis is nourished by blood vessels supplying it. The dermis is full of blood vessels that supply it oxygen and is rich in nerve supply. The dermis is full of connective tissues which provide a cushioning effect from stress and strain caused to the skin.
In the epidermis, out of the multiple layers present, the basal-most layer of cells divide by a process called mitosis. The older, dead cells move upward as new ones are formed beneath. These mature cells are dead and have keratin protein. This process renews the outermost layer of cells and as the older, dead cells are shed off, a completely new layer is formed at the top every three weeks. This process of renewal and desquamation (old dead cells replaced by new young cells) is called as keratinization and occurs exclusively in the upper layer. Keratinization helps in protection from harmful chemical agents as well as keeps the water content optimum in the skin. It also wards off bacteria and other organisms away from the skin.
The dermis has a wholly different set of functions to perform. The dermis contains hair follicles, sweat glands, blood vessels, lymphatic vessels, apocrine (scent) glands and sebaceous (oil) glands. Almost half the content of body fat is present in this layer. It insulates the body and thus, helps in regulating temperature of the body. The blood vessels present in this layer dilate when the body temperature is higher than normal and helps dissipate body heat to the environment to cool down the temperature. When the core body temperature falls below normal, the blood vessels in the dermis contract to keep the body heat from being lost and thus, maintain core temperature. The nerve endings in the dermis help to sense touch, pain,and pressure which is not a feature of the epidermis layer. The epidermis is the visible outer skin that we see whereas dermis is the inner layer of the skin that helps us feel touch and pain. There are dense elastin fibers present in the dermis which provide elasticity and strength to the skin. Based on the presence of these elastin fibers, one can feel the sensation of pressure over skin. The ink engraved into the skin during the process of tattooing is present in the dermis layer and so are the stretch marks. These marks remain lifelong as they are deeply seated in the dermis.
Summary: The epidermis is the tough, outer layer of the skin that has to protect skin from external agents whereas the dermis is the innermost layer that protects the skin from the temperature variations, acts as insulation for the body and gives sensations of touch, pain and pressure.