Anatomy 101: From Muscles and Bones to Organs and Systems, Your Guide to How the Human Body Works

SKIN, HAIR, AND NAILS

Beauty Is Only Skin-Deep but Skin Is Pretty Deep

SKIN ANATOMY

Skin is the largest organ in the human body. Composed of several layers, it’s the external covering of the body and it serves to protect against infection and dehydration. Also present in skin is hair, which can detect touch and helps keep the body warm. To cool the body, glands in the skin produce sweat that cools the body as it evaporates from the surface.

Epidermis

While skin covers the entire body surface, all skin isn’t created equal. The distinction between the skin on your arm and the skin on the soles of your feet is significant and largely based on the thickness of the epidermal (top) part of the skin.

Up to five individual layers of cells (strata) make up the top layer, or the epidermis, of the skin:

· stratum basale

· stratum spinosum

· stratum granulosum

· stratum lucidum

· stratum corneum

Let’s look at each in turn. Starting at the bottom, adjacent to the basement membrane, is the stratum basale where cells divide and produce a continuous supply of new cells as the old ones are shed from the surface. The next layer as you move up is stratum spinosum, which gets its name because its cells have a spiny shape.

Anatomy of a Word

melanocyte

A melanocyte is one of the pigmented cells that give skin cells their individual color. They occur throughout stratum spinosum layer of the epidermis. They cover over and protect the dividing cells at the bottom from damage from ultraviolet radiation, kind of like an umbrella shading you on a beach.

As skin cells (keratinocytes) move higher and higher (through the process of replacing dead cells with living ones), they become cells of the stratum granulosum. These cells are filled with granules of keratohyalin, which help give skin its structure and give these cells a grainy appearance. This is the last layer of living cells.

The stratum lucidum is next. It’s a thin translucent layer of the skin and is composed of dead skin cells. On top of this layer is the final layer, which is the most variable in its thickness. This stratum corneum is composed of multiple dead cell layers and is the true first-line defense barrier against infection for the body.

Anatomy of a Word

desquamation

Desquamation is the process of shedding dead skin cells. Cells are continually shed from the epidermal surface and new cells are continually made. It takes a new skin cell about 14 days to move from the bottom layer to the surface of the skin.

Thick skin is found on the palms of the hands and soles of the feet. Of the five previously mentioned layers, the stratum corneum is by far the thickest layer. It provides great protection against the friction that occurs when walking or grasping objects; however, unlike the rest of the skin that covers your body, you will not find hair follicles or sebaceous glands in thick skin. Furthermore, you will find fewer sweat glands in thick skin than in thin skin. While your feet and palms may get moist, the volume of sweat produced is much lower than what is produced in other areas of the body.

The majority of the body is covered with thin skin. In fact, the epidermis as a whole is thinner in this type of skin since it lacks two of the five layers that are found in thick skin (stratum granulosum and stratum lucidum are missing). Thin skin possesses many hair follicles and sebaceous glands that support the growing hair. Additionally, while both skin types contain sweat glands, thin skin has a much higher density of these glands to aid in cooling the body.

Dermis

The dermis is the layer beneath the epidermis that forms a transition zone between the underlying connective tissue and the epidermal layer above. It consists of dense irregular connective tissue made up largely of collagen fibers, elastic fibers, and fat tissue. Within the dermis are blood vessels that support the skin, as well as many nerve endings and receptors that detect pressure, pain, and temperature.

Hair and Nails

Human skin may also be modified into the structures known as hair and nails, which are composed of the same material that makes up the surface stratum corneum of the skin. The major difference is how compact these layers are and how they are arranged with the other dead cells in these layers.

Hair

Imagine taking the dead cells of the stratum corneum layer and rolling them into a tightly wound tube of dead cells: that is a hair. The hair follicle is simply a deep pit on the epidermis that projects downward into the deeper dermis of the skin. The other layers of the epidermis surround and support the shaft of the hair as it grows. At the deepest part of the follicle is the hair bulb. This is where the cells divide. This is also where they are nourished by blood vessels that enter into the follicle and support the growth of the hair. New layers are added to the hair and continually extrude the hair out of the follicle and onto the surface of the skin.

Nails

The surface of the skin is relatively soft, as is most of the hair on the body. However, the nails are drastically harder and consist of tightly packed layers of dead cells. What are commonly known as nails are in fact nail plates. This hardened plate of dead cells remains attached to the underlying epidermis via the nail bed, which tightly holds on to the plate to prevent the nail from falling off. At the base of the nail, you’ll find the cuticle (eponychium). The cuticle is a portion of the skin’s epidermis that overlaps the newly forming nail plate as it moves forward. Beneath the cuticle new nail plate material is continuously being produced and pushing the nail forward.

At the end of the nail, the plate extends beyond the tip of the finger forming a crevasse, which is great at collecting dirt, and is technically called the hyponychium. This is the part of the nail that you are most familiar with and must trim on a regular basis to prevent the nails from growing too long.