The skin is the largest organ of the body and varies in thickness and composition in different regions of the body. It plays five main roles: protection, thermoregulation, sensation, metabolic functions (vitamin D, adipose metabolism), and sexual attraction.
Skin comprises three layers: epidermis, dermis and hypodermis. The epidermis is most superficial and interacts with the external environment. It is a self-regenerating, stratified squamous epithelium that produces a protective layer of keratin. Keratin provides both mechanical support and protection against loss of water. The thickness of the epidermis will vary depending on the its location. The thickest epidermis is usually found in the palms of the hand and soles of the feet.
Underneath the epidermis is the dermis, a layer rich in collagen. The collagen provides mechanical support by resisting tension in multiple directions. The dermis also contains blood vessels, nerves and sensory receptors.
The deepest layer is the hypodermis that is largely composed of adipose tissue but also has large blood vessels. The adipose tissue provides metabolic support and creates an insulating layer to prevent loss of heat.
Note the rete ridges that are down growths of the epidermis into the dermal layer. These generate a stronger bond between the epidermis and dermis and help the skin resist shearing forces. In between rete ridges are dermal papilla, which contain sensory receptors. Also, the stem cells of the epidermis are located in the region above the dermal papilla.
The epidermis is a stratified squamous epithelium that contains discrete layers of cells called keratinocytes that are different states of development and differentiation. All keratinocytes derive from stem cells in the deepest layer, the stratum basale, and undergo structural and functional changes as they migrate upward toward the surface of the epidermis. The epidermis is divided into four layers that have different structural appearances:
This image shows the stratum basale and stratum spinosum. The cells of the stratum basale are cuboidal to columnar and undergo mitosis to give rise to all of the keratinocytes in the layers above. Note the dark staining material in the cells of the basale. This is melanin that gives skin its color and absorbs UV-light. Also note the numerous, prickly connections between keratinocytes. These are the connections made by desmosomes. The desmosomes are linked intracellularly to aggregates of intermediate filaments called tonofilaments. Cytokeratin is the primary component of tonofilaments, and keratinocytes in the spinosum synthesize a large amount of cytokeratin (note the prominent nucleoli in the keratinocytes).
The keratinocytes of the stratum granulosum become more flattened and accumulate numerous dense, dark-staining granules. These are keratohyaline granules that contain proteins which will aggregate cytokeratin to form keratin filaments in the cytoplasm. These cells also produce lipid-rich lamellar bodies that will be secreted to form a lipid layer to prevent water loss. The cells of the stratum corneum are anucleated and have undergone apoptosis. The dead cells leave layers of keratin. The thickness of this layer varies the most and is primary difference between thick and thin skin.
Melanocytes localize to the stratum basale and extend processes between the keratinocytes of the stratum basale and spinosum. Melanocytes produce the pigment melanin that protects cells of the skin against ultraviolet radiation. Melanin is synthesized in membrane-bound organelles called melanosomes that derive from the Golgi. Melanosomes are transported to the ends of the melanocyte processes where neighboring keratinocytes phagocytose the melanosomes. Although melanocytes produce melanin, the cytoplasm of melanocytes is usually pale in comparison to the surrounding basal cells.
Epidermis also contains cells that have roles in generating an immune response. Langerhans cells are antigen-presenting cells in the immune system. They capture antigen and then enter lymphatic vessels to reach a local lymph node where they present antigen to T-cells.
Skin contains a number of important functional structures that derive from epithelial cells in the epidermis. These include hair and different types of sweat glands.
Hair is present over most of the body and plays an important role in regulating body temperature. Hair is composed of concentric layers of epithelial cells that undergo different degrees of keratinaztion. The hair follicle is where hair production and growth of occur. The hair bulb contains stems cells that will differentiate into the different layers of epithelial cells that make up hair.
Eccrine sweat glands occur throughout most of the skin. They arise from downgrowth of the epidermis into the dermis to form of long tubules that lead to secretory glands. The secretory portion of each gland is tightly wound and appears as a collection of cross-sectioned tubules. It consists of of secretory cells and an outer layer of myoepithelial cells that are contractile. The myoepithelial cells receive input from nerve fibers, which cause them to contract and expel sweat from the gland. The secretory cells are a mix of light and dark staining cells. The light staining cells produce a watery substance similar in composition to an ultrafiltrate of blood, whereas the dark staining cells secrete glycoprotein. The ducts stain darker and have a double layer of cuboidal cells which reabsorb ions and water. The ducts spiral to the surface of the epidermis to release sweat. Sweat evaporates on the surface of the skin to help lower body temperature.
Apocrine sweat glands occur in the axilla, the areola of the nipple, the labia majora, and the circumanal region. They are much larger than eccrine glands and produce a thicker secretion, which is rich in protein, lipid, carbohydrate, ammonium and other organic compounds. The glands are characterized by a simple cuboidal epithelium and widely dilated lumen that stores the secretory product.
Sebaceous glands are pear-shaped glands that secrete an oily substance called sebum, which moisturizes and waterproofs hair. They are usually attached to hair follicles near the arrector pili muscle, which causes hair to "stand up". The glands connect with the hair follicle via a short duct called the pilosebaceous canal.