The Skin

General anatomy.

    Our skin is the envelope that protects us from the outside world, and a membrane that holds our cells, organs and fluids. It is our largest organ.

    The outer layer of the skin, the epidermis, is composed of dead cells originating from underlying layers of epithelial cells. Dead cells of the outer layer are keratinous, like our nails and hair which are teguments. It is the layer that protects us against many pathogens such as bacteria, against strong acids and bases such as hydrochloric acid and caustic soda, and against the physical forces imposed on the body such as the prick of a blunt knife. If the knife is sharp, however, or if the virus is tenacious like a wart, the skin will weaken of disrupt.. Other agents, often small molecules like drugs can be absorbed through the skin and produce local or generalized (via absorption into bloodstream) effects. For example, certain drugs and poisons, such as nicotine will be absorbed and have a widespread effect, while creams or irritants have more of a local effect.

    The piling of plates of keratin on the surface of the skin, makes it virtually waterproof, in both directions, also retaining our body fluid in the most arid environments. Our skin is also very flexible and elastic, allowing movements. It is slim, agile and responsive in some places, and thicker and rougher in other place like the bottom of our feet.

    The living layer of the epidermis, somewhat paradoxically, is actually composed of cells that are slowly dying. The little vascularization of the epidermis contributes to their deaths while minimizing heat loss. Small nerve terminals infiltrate this layer and participate to tactile and thermal sensations, and pain. Epidermal cells originate from the dermis, the truly living part of the skin.

Microscopy of a cross section (left) and the suface (right) of the epidermis.

Microscopic view of a skin section.

Thermoregulation.

    In the dermal layer beneath the epidermis, where the dermal cells multiply and migrate toward the epithelium, a complex network of blood vessels nourishes the skin and helps to control heat loss. In the dermis there is also a variety of nerve receptors, glands, hair roots and fat cells.

    A double circulatory system ensures, on the one hand, nutrition to the skin cells and, on the other hand, thermal regulation. By opening a network of anastomosis (increasing skin circulation), more blood reaches the skin and heat can radiate outward and cool down our body in warm weather. Conversely, when it is cold, the network closes and heat loss is minimized.

    In terms of thermal regulation, another property 'skin' can also contribute. This phenomenon is related to seasonal changes and is especially visible in animals, man is an animal but also increasingly independent of climate change. This phenomenon is the accumulation of fat in the subcutaneous layers of the skin. This occurs when the cold weather approaches and serves both as a nutritional reserve and as an insulation layer.

Cutaneous receptors.

    In the skin, there are a host of receptors that we use to feel things. Whether it is to feel a fine prick, cold, heat or pain, each type of receptor has its own specialty and, together, they contribute to a unique sensation.

    The free endings are unmyelinated (lacking insulation) and infiltrate the dermis. Their functions are to transmit heat and pain information. Other endings can curl around hair roots and participate in mechanoreception (sensation of touch).

    Some free endings could be found in the deeper layers of the epidermis, particularly on the palmar surfaces of our hands and the soles of our feet. These endings are attached to modified epithelial cells called Merkel cells and participate in the mechanoreception.

    Pacinian corpuscles are located in the dermis. They are found especially in the fingers, genitals and breasts. They have from 1 to 4 mm long and 0.5 to 1.0 mm in diameter. At its centre there is a unmyelinated nervous terminal, surrounded by flattened cells and concentrically arranged. These mechanoreceptors provide the subtle sensation left by a small pressure. They also respond to vibrations. They are also found in other locations such as joint capsules and bladder wall where they convey proprioceptive (unconscious) information.

    The Ruffini corpuscles are small and elongated, they are about 1 mm long by 0.1 mm in diameter. They are located deep in the dermis and are quite similar to receptors located in the tendons. They provide information about the stretch state of the skin.

    The bulbs of Krause are small bodies located in the dermis of mucous membrane such as the eye, the eyelids, the tongue and the external genital mucosa. They are also mechanoreceptors.

    Finally, Meissner corpuscles are small mechanoreceptors located mainly in regions of high sensitivity such as the palmar surface of the hand and fingers, the soles of our feet, our eyelids, lips, the external genital areas and nipples of the breasts. These receptors are located near the surface of the skin, at the junction of the dermis and the epidermis.

Integuments.

    Hair, nails and eyelashes are part of the integuments.