The largest organ of human body
The skin is the organ that involves the body and determines its boundaries with the external environment. Corresponds to 16% of body weight and exerts various functions, such as thermal regulation, organic defense, control of blood flow, protection against various environmental agents and sensory functions (heat, cold, pressure, pain and touch). The skin is a vital organ and without it, survival would be impossible.
It is composed of three layers: epidermis, dermis and hypodermis, the outermost to the deepest, respectively.
The epidermis, outermost layer of skin, consists of epithelial cells (keratinocytes) organized like a "brick wall". These cells are produced in the lower layer of the epidermis (basal layer or germinal layer) and during its evolution toward the surface, suffer a process called keratinization, giving rise to the corneal layer, composed mainly of keratin, a protein responsible for skin impermeability. The constant cell renewal of the epidermis causes the cells of the stratum corneum to gradually be eliminated and replaced by new ones.
In addition to the keratinocytes, other cells are also found in the epidermis: melanocytes, which produce the pigment that gives color to the skin (melanin) and immune defense cells (Langerhans cells).
The epidermis gives rise to skin appendages: nails, hair, sweat glands and sebaceous glands. The opening of pilosebaceous follicles (hair + sebaceous gland) and sweat glands in the skin form the tiny holes known as pores.
- Nails are composed of keratin, protein which forms a plate of hard consistency, which provides protection to the finger and toes.
- Hairs are present in almost the entire skin surface except the palms and soles. They may be tiny and thin (lanugo) or thick and strong (terminals). On the scalp, there are from 100,000 to 150,000 strands of hair, which follow a renewal cycle in which, every day, around 70 to 100 hairs strands are replaced.
This renewal cycle has three phases: anagen (growing phase) - lasts about 2-5 years, catagen (stage when growth is interrupted) - lasts about three weeks and telogen (hair loss phase) - lasts about 3 to four months.
- Sweat glands produce sweat and have great importance in regulating body temperature. They are of two types: eccrine, which are more numerous, spread throughout the body and produce sweat eliminating it directly on the skin surface. Apocrine, existing mainly in the armpits, genital areas and around the nipples, are responsible for the characteristic odor of sweat, when its secretion undergoes decomposition by bacteria.
- Sebaceous glands produce oil or sebum. More numerous and larger on the face, scalp and upper chest, they are not present on the palms and soles. These glands eliminate its secretion into the pilosebaceous follicle, through which reaches skin surface.
The dermis layer is located between the epidermis and hypodermis and is responsible for skin strength and elasticity. It consists of connective tissue (collagen and elastic fibers surrounded by extrafibrillar matrix, previously called ground substance), blood and lymph vessels, nerves and nerve endings. The pilosebaceous follicles and sweat glands, which are originated in the epidermis, also are located in the dermis.
The zone in which the epidermis and dermis meet is called the dermal-epidermal junction. In this area, the epidermis protrudes in the shape of fingers in the direction of the dermis, forming the epidermal ridges. These increase the surface contact between the two layers, facilitating the nutrition of the epidermal cells through the blood vessels of the dermis.
The hypodermis, also called the subcutaneous tissue is the deepest portion of the skin. It consists of bands of connective tissue that surround fat cells (adipocytes) and form lobes of fat. Its structure provides protection from physical trauma, besides being a storehouse of calories.
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