Summary
Highlights
The epidermis is organized into distinct layers called strata. From bottom to top, these include the stratum basale (site of rapid mitosis and epidermal ridge formation), stratum spinosum (where keratin fibers become visible), stratum granulosum (characterized by keratohyalin protein), stratum lucidum (present only in thick skin, containing eleidin), and stratum corneum (outermost layer, consisting of dead, keratin-filled squamous cells).
The skin is divided into two main parts: the epidermis and the dermis. The hypodermis, while closely associated, is not technically part of the skin. The epidermis, a vascular epithelial tissue, is primarily made of stratified squamous cells—specifically, keratinized stratified squamous epithelium—which are constantly shedding. The dermis, located beneath, contains various accessory structures like hair follicles, glands, nerves, blood vessels, and muscles.
The epidermis contains several cell types: adult stem cells, which facilitate regeneration; keratinocytes, making up the majority of the epidermis and producing keratin for protection; melanocytes, which produce melanin for skin pigmentation and UV protection; Langerhans cells, derived from red bone marrow, acting as phagocytes against microbes; and Merkel cells, sensory receptors found deep within the epidermis.
The integumentary system comprises hair, nails, exocrine glands, and skin; the skin is the body's largest organ, covering an area of approximately 22 square feet. This system is crucial for appearance and performs vital functions like protection, temperature regulation, nutrient synthesis and storage (e.g., Vitamin D production, fat storage), sensory reception, and excretion/secretion. This section will focus primarily on the skin.
Thick skin, found on palms and soles, contains all five epidermal layers, including the stratum lucidum, and has unique dermal papillae arrangements that form fingerprints. Thin skin, covering most of the body, has only four epidermal layers (lacking the stratum lucidum), is more flexible, and is where hair is found.
Skin color is primarily determined by two types of melanin—eumelanin (brown-black) and pheomelanin (red-yellow)—produced by melanocytes. While all races have similar numbers of melanocytes, differences in skin tone arise from the quantity and type of melanin produced, influenced by genetics, hormone levels, and UV light exposure. Melanin plays a crucial role in protecting DNA from harmful UV radiation.
UV light, although dangerous in excess, is essential for vitamin D synthesis. In the skin, 7-dehydrocholesterol is converted into cholecalciferol by UV light. This cholecalciferol then travels to the liver and kidneys, where it is further converted into active vitamin D, highlighting the delicate balance needed for UV exposure.
The dermis consists of two main layers: the papillary layer and the reticular layer. The papillary layer, made of areolar connective tissue, supports the epidermis and forms dermal papillae. The reticular layer, composed of dense irregular connective tissue with abundant collagen, is thicker and more robust. The intermingling collagen fibers make it difficult to distinguish a clear boundary between these two layers.
Burns are classified based on the depth of tissue damage. First-degree burns only affect the epidermis, causing redness and mild swelling, healing within a week. Second-degree burns extend through the epidermis and into the dermis, causing blistering and taking months to heal with potential scarring. Third-degree (full-thickness) burns damage all three layers (epidermis, dermis, and hypodermis), often resulting in no pain in the burned area due to nerve destruction, though surrounding areas may be very painful.