Skin is the body’s primary barrier against pathogens, chemicals, toxins, temperature extremes and UV radiation. It also acts as a sensory organ and an immunologic surveillance system.
While seasonal changes may require minor adjustments in your routine, you should always keep your basics simple—a gentle cleanser, moisturizer and sun protection.
The Epidermis
The epidermis is the tough outer layer of skin that protects us from chemicals, pathogens, and ultraviolet radiation. It contains cells that make keratin, which waterproofs and strengthens the skin. It also has cells that produce melanin, the dark pigment that gives skin its color and helps absorb harmful UV rays. It also has cells that allow us to feel touch and provide immunity against bacteria and other germs.
The innermost layer of the epidermis is called the stratum basal. It contains cells that constantly divide and generate new keratinocytes, which then move up toward the surface of the skin, pushing older keratinocytes into the next layer, the stratum granulosum. As they move up, these cells become flattened into a diamond shape and secrete large quantities of keratin and the protein keratohyalin. They are also packed with the dark pigment melanin in small parcels called melanosomes.
In the stratum granulosum, there are no blood vessels to supply nutrients to these keratinocytes, so they have to rely on diffusion through the dermoepidermal junction. This diffusion also allows the penetration of water into the epidermis. Eventually, these cells become so dehydrated that they start to slough off, which is why the epidermis is sometimes referred to as being dry or flaky. This process is known as desquamation. The cells that slough off form the outermost layer of the epidermis, which is called the stratum corneum. These cells have no nucleus and are filled with keratin. This layer is often described as being “cornified” or having a horny appearance.
The Dermis
The dermis is the inner of the two major layers that make up the skin. It consists of dense connective tissue and has a number of functions that include cushioning subcutaneous tissues, regulating temperature, sensing the environment, and excreting wastes. It also contains many glands, including eccrine (sweat) glands and sebaceous glands (“oil glands”), which secrete their substances through ducts to nearby body surfaces, as well as hair follicles and blood vessels.
In the papillary layer, the dermis has a thin arrangement of collagen fibers; in the reticular layer, it is thicker and made up of a grid-like pattern of these fibrils. The dermis contains mechanoreceptors that provide the sense of touch and thermoreceptors that give the sensation of heat. It also contains cells that produce the pigment melanin, which gives skin its color.
In the reticular dermis, there are structures called fibroblasts that lay down a matrix of proteins that provides strength and elasticity to the skin. The dermis also contains hair follicles, which give rise to the long, fine hairs that we see on our bodies. Associated with each follicle is a sebaceous gland, which secretes the oily substance known as sebum that coats and protects the hair. Finally, the reticular dermis contains blood vessels that supply nutrients and remove wastes from both the epidermis and the dermal cells below it.
The Hypodermis
The hypodermis is a layer of fat cells that anchors the skin to the bones and muscles beneath. It also works as an insulator to conserve the body’s heat and protect the organs, tendons, and ligaments. In addition, the subcutis layer stores energy and acts as a shock absorber. Blood vessels, nerves, hair follicles, and sweat glands all pass through this layer.
The dermis consists of two layers, the papillary layer and the reticular layer. The papillary layer consists of loose connective tissue, with numerous blood capillaries extending up into the epidermis. This layer contains elastin fibers that provide elasticity, and it is well vascularized. The reticular layer is thicker, containing dense irregular connective tissue with a coarse network appearance. It has a strong supply of both sensory and sympathetic nerves, and it also contains a number of the skin’s true (eccrine) sweat glands.
As with the epidermis, the cells of the reticular layer have a spiny appearance due to the presence of desmosomes. In contrast to the epidermis, these cells do not have nuclei. The reticular layer also contains macrophage cells, which form a component of the immune system and help keep the body free from invading microorganisms.
The Nervous System
At a basic level the nervous system is a vast network of nerves which send electrical signals, or messages, all over the body. These messages tell the body what to do. It consists of cells called neurons (pronounced no-rih-nons).
Neurones are the building blocks of your nervous system. They come in many shapes and sizes, depending on their function. Each one has a nucleus, finger-like projections called dendrites and a long fibre called an axon. Axons are covered in a fatty substance called myelin to help them carry electrical impulses faster and more efficiently. Neurones communicate with each other via junctions called synapses. Messages jump across these connections, or synapses, using special chemicals called neurotransmitters.
The central nervous system contains your brain and spinal cord. It sends signals back and forth between the rest of the body and your brain, controlling your movement and some involuntary responses like a knee jerk.
The peripheral nervous system contains nerves which end in the skin, organs, glands and muscles of your body. The sensory, or afferent, division of this system detects stimuli from the outside world through our senses – sight, sound, touch, taste and smell. It also conducts signals to the CNS from these peripheral tissues describing their environment. The motor, or efferent, division of the PNS contains motor nerves which control skeletal muscle in response to directives from the brain. This is what we call ‘voluntary’ movement. It also controls involuntary reactions such as the rapid ‘pull away’ of your hand from something hot.