Skin, the largest organ in the human body, is composed of three main layers: the epidermis, dermis, and hypodermis.
Epidermis:
The epidermis is the outermost layer of the skin, formed by stratified squamous epithelium. It consists of four to five layers:
- Stratum basale: The innermost layer responsible for generating new keratinocytes, the primary cell type in the epidermis. It also contains melanocytes, which produce melanin for skin pigmentation.
- Stratum spinosum: This layer consists of keratinocytes connected by desmosomes, providing structural integrity to the epidermis.
- Stratum granulosum: Characterized by the presence of keratohyalin granules, these granules contribute to the formation of keratin, the protein that provides strength and impermeability to the skin.
- Stratum lucidum: This layer is present only in thick skin, like that found on the palms of the hands and soles of the feet.
- Stratum corneum: The outermost layer, composed of dead keratinocytes, acts as a barrier against external factors like water loss, pathogens, and abrasion.
Dermis:
The dermis lies beneath the epidermis and is a connective tissue layer composed mainly of collagen and elastin fibers. These fibers provide strength and elasticity to the skin. The dermis also contains:
- Blood vessels: Supply oxygen and nutrients to the skin and remove waste products.
- Nerves: Provide sensory input for touch, temperature, and pain.
- Hair follicles: Structures that produce hair.
- Sweat glands: Secrete sweat for thermoregulation.
- Sebaceous glands: Produce sebum, an oily substance that lubricates the skin and hair.
Hypodermis:
The hypodermis, also known as the subcutaneous layer, lies beneath the dermis and is composed primarily of adipose tissue. It acts as an insulator, providing thermal regulation and cushioning the skin.
Functions of the Skin:
The skin plays a vital role in protecting the body from external factors and maintaining homeostasis. Some of its functions include:
- Protection: Acts as a barrier against physical trauma, UV radiation, pathogens, and chemical irritants.
- Thermoregulation: Regulates body temperature through sweating and vasoconstriction/dilation of blood vessels.
- Sensory perception: Provides touch, temperature, and pain sensations.
- Vitamin D synthesis: Converts cholesterol into vitamin D when exposed to sunlight.
- Excretion: Eliminates waste products through sweat.
