Structure and Functions of the Skin

Image: “Layers of Skin” by Phil Schatz. License: CC BY 4.0

Structure of the Skin

The skin is composed of various layers:

• Epidermis: consists of a corneal layer, a keratinizing layer, and a basal lamina.
• Dermis: consists of the papillary and reticular layers.
• Subcutis (hypodermis): consists of fatty tissue. It also contains Vater-Pacini corpuscles (mechanoreceptors) and hair follicles.

Image: ‘Layers of Skin’ by Phil Schatz. License: CC BY 4.0

Epidermis

The epidermis is a type of epithelial tissue and is only 0.03–0.04 mm thick. It contains no blood vessels and consists of a stratified squamous epithelium with the following layers (from innermost to outermost):

• Basal layer (stratum basale)
• Squamous cell layer (stratum spinosum)
• Granular layer (stratum granulosum)
• Clear layer (stratum lucidum)
• Horny layer (stratum corneum)
  

  Image: ‘Layers of the Epidermis’ by Phil Schatz. License: CC BY 4.0

Image: ‘Epidermis’ by Phil Schatz. License: CC BY 4.0

Together with the squamous cell layer, the basal layer is sometimes called stratum germinativum.

The epidermis consist of 4 types of cells:

1. Keratinocytes – deepest, produce keratin (tough fibrous protein).
2. Melanocytes – these produce the dark skin pigment—melanin.
3. Merkel cells – these cells are linked with sensory nerve endings.
4. Langerhans cells – macrophage-like dendritic cells

The keratinization of the epidermis is a continual process. The cells needed for this constant renewal come from the basal layer where cell division takes place. Since this process is very sensitive to radiation, there are pigments in the stratum germinativum that produce a protective darker skin tone when exposed to strong sunlight.

The stratum corneum, on the other hand, is very thin in those places where higher flexibility is needed, for instance, on the eyelid. In places exposed to intense mechanical stress, like the palms of the hands or the soles of the feet, it is thicker and can form calluses. The formation of calluses is a protective mechanism. As soon as the stress subsides, the formation of the thicker layers stops.

Image: ‘Thin Skin (a) versus Thick Skin (b)’ by Phil Schatz. License: CC BY 4.0

Dermis

The dermis is made up of elastic and very tough fibers. To maintain is elasticity and strength, sebaceous glands keep it smooth. The dermis also supplies the epidermis with important nutrients and moisture, since a healthy skin has a humidity of 10–20%. If this humidity content drops, the skin becomes brittle and cracks. This, in turn, leads to even more humidity loss, and the skin becomes more vulnerable to pathogens.

Image: ‘Layers of the Dermis’ by Phil Schatz. License: CC BY 4.0

The dermis contains blood and lymphatic vessels, nerve endings, collagen, and structures such as sweat glands and hair follicles.

There are 2 basic layers in the dermis:

• Reticular layer: This gives the skin its elasticity and helps to toughen the skin. Sweat glands and hair follicles are found at this layer. The layer is also well vascularized.
• Papillary layer: This layer is made up of areolar connective tissues and elastin fibers. This layer contains the following type of cells:

1. Phagocytes which serve to defend the skin against invading pathogens.
2. Fibroblasts
3. Adipocytes

Subcutis (hypodermis)

The subcutis performs an important task in allowing the skin to move around on top of the underlying tissue. This may be ‘bone skin’ (periosteum) or muscle.

Skin Proliferation

New skin cells form in the basal layer and start their way upward to the skin’s surface. During this process, which lasts 30 days, the cytoplasm, cell nucleus, and organelles are replaced by keratin. The cells also change their shape. They start as a kind of cube, and, upon reaching the upper layer of the skin, have transformed into flat, already dead epithelial cells. This means the skin forms new horny layers down below and then pushes them upward.

Sustenance of the Skin

The metabolism of the skin is rather slow, which can be a great advantage in the case of short-term problems with the blood supply. However, when the blood supply to the skin is interrupted for longer periods of time, the cells die. Delicate blood vessels provide the skin with blood; the drainage flows through the vessels of the subcutis.

Moreover, there is lymph flow through the skin. Regional lymph nodes drain the lymph from the papillary region. Furthermore, some nerve fibers perform an important function in the skin. They build dense bundles in those parts of the body that have to be very sensitive, particularly in the hands, feet, and genitalia. The supply of the nerves is ensured by the dorsal horns of the spinal cord.

Functions of the Skin

The different layers of the skin perform different tasks:

• The horny layer and the dermis provide mechanical protection.
• The fatty tissue of the subcutis is an important energy reserve.
• Blood vessels, hair, the fatty subcutis, and sweat are all protective against heat.
• The horny layer is protective against wetness and mechanical stress.
• The horny layer and the skin’s acid mantle protect the body against bacteria.
• Pigmentation is a potent protection against radiation.

Mechanical protection of the skin

Without the skin, humans would not be able to move. The skin protects the body against external influences, including mechanical stress. For this, the skin’s structure has to ensure that it cannot be harmed even when exposed to strong pressure.

One example is the foot sole, which has to withstand more than the body’s own weight without being blemished. Generally, the skin is most resilient where there are cushions of muscles underneath it. In those parts of the body where the skin lies directly on top of bones, the skin is more tender. The horny layer becomes worn down when it is repeatedly subjected to strain and has to be replaced accordingly.

Temperature regulation through the skin

The skin is also vital in maintaining the body’s temperature. Here, the blood vessels in the papillary layer of the dermis come into play — in reaction to high temperatures, these vessels dilate.

As they are close to the surface, the blood can cool down and the body temperature is lowered. The skin itself cools down by releasing sweat as it transpires. In a reverse manner, the blood vessels react to cold temperatures by contracting and thus, prevent the blood from cooling down any further. This process is called homeostasis

Protection against fluid loss

Two-thirds of our body mass consists of water, and a large loss of body water can be fatal. Therefore, the skin only releases water when needed for regulating the body’s temperature. Over the course of 24 hours, this amounts to 0.5–1 L. Moreover, the skin is also the barrier that prevents fluids from entering the body.

Protection against pathogens

Another important function of the skin is protection against pathogens. If the skin is injured or if the acid mantle is out of balance, pathogens can enter the body. Here, the skin’s sensory receptors play a crucial role: They identify intruding pathogens, which triggers defense responses.

Glands of the skin

There are various glands in the skin. These include sebaceous glands, sweat glands, and scent glands.

Sebaceous glands: Sebaceous glands are located in the dermis. They can be found in almost any part of the human body, except the palms and foot soles. Usually, sebaceous glands are located directly next to a hair follicle. This means that a sebaceous gland merges into the excretory duct of a hair follicle. This junction does not exist on the eyes and eyelids, nor on the lips, penis, or labia minora. Sebum itself is composed of:

• Fat
• Cholesterol
• Protein
• Electrolytes

Earwax (cerumen) is also a product from the sebaceous glands. Sebum waterproofs and lubricates the hair and skin.

Sweat glands: The human body has about 2–3 million sweat glands (eccrine glands), which are located in the dermis. Their excretory ducts follow a winding path, which ends in the skin pores. They are found mostly in the armpits, foot soles, and palms. Sweat does not only contribute to temperature regulation, it also serves to protect the skin. Its pH value is 5–6. Sweat production is influenced by many factors; among others are psychological ones like stress.

Scent glands: Scent glands (also categorized as apocrine sweat glands) are located mostly in the armpits, around the nipples of the breast, and in the genital area. They play an important role in creating the unique body smell of a person, which is further influenced by other factors, such as sweat or bacteria on the skin.

Skin Appendages: Hair and Nails

The hair and nails are also considered part of the skin, i.e., the integumentary system. As such, they are called skin appendages.

Hair: a protective and tactile organ

Hair has various functions. It serves as protection against cold and as an important tactile organ.

Hair growth begins in the dermis, more specifically in the hair papilla. The hair is composed of keratinized cells that emerge and migrate upwards inside the hair follicle. When the hair shaft emerges from the skin surface, it does so at a slightly slanted angle. For every hair follicle, there is an adjacent sebaceous gland and often a scent gland.

Image: “Hair Follicle” by Phil Schatz. License: CC BY 4.0

Hair can be made erect by muscular activity, which is what happens when you have ‘a shiver running down your spine’. It is a very old function of the human body that is invovled in ‘goosebumps’ (cutis anserina). Furthermore, nerve fibers present in the hair make the hair very susceptible to touch.

Image: ‘Hair’ by Phil Schatz. License: CC BY 4.0

Hair growth: In a newborn, the entire body is covered with very fine hair called vellus hair. During puberty, the growth of terminal hair begins, which is much thicker and is predominant in certain areas of the body, such as the genital area or the face (beard). The rest of the body hair is less pronounced, and 4% of the skin’s surface is not covered by any hair including:

• Palms of the hands
• Soles of the feet
• Fingernails
• Toenails
• Lips

These hairless parts of the skin are called the glabrous skin as opposed to the hairy skin.

Hair grows by about 1 cm per month. The growth cycle is divided into 3 phases.

• Phase 1 lasts 2–10 years. It is known as the growth or anagen phase.
• Phase 2 lasts about 2 weeks. It is known as the transitional or catagen phase.
• Phase 3 lasts about 3–8 months. It is called the resting phase or telogen phase.

These phases are determined with respect to a hair follicle which goes through this cycle up to 10 times. After that, it does not form any more hair. A healthy human being loses up to 100 hair strands each day.

Nails: a protective and grasping organ

Nails are made out of hard and dense keratinized cells of the epidermis. They fulfill an important task in making it possible to grasp small objects, similar to the functioning of tweezers. Furthermore, they serve as an important protection of the fingertips and toes against injuries. Nails are translucent. The soft pink color shining through is the nail bed which is very well supplied with blood.

Image: “Nails” by Phil Schatz. License: CC BY 4.0

The whitish crescent-shaped end of the nail (towards the body) is referred to as lunula (‘small moon’). Here, the actual whitish color of the nail can be appreciated because the nail is not transparent in this part. Between lunula and skin, there is a protective layer called the cuticle, which keeps germs from entering. Behind the cuticle (towards the body), there is the nail root. It forms the keratinized cells that push the nail forward. Fingernails grow faster than toenails. The typical growth rate of a fingernail is 1 mm per week; a toenail grows by 0.5 mm per week.

Structure of the nail:

• Nail plate: surface to the opening of the sebaceous gland
• Nail bed: continuous with strata basale and spinosum
• Nail root: covers the germinative zone or matrix
• Eponychium (cuticle)
• Hyponychium

Skin Color

Three skin pigments are responsible for skin color. These are:

• Melanin: This is by far the most important skin pigment
• Hemoglobin
• Carotene

‘Melanocyte. Sunlight increases production and changes chemical characteristics of melanin’ Image created by Lecturio.

Diseases of the Skin

Diseases of the skin are the specialty of dermatologists. As the largest organ of the body, which is also in direct contact with the environment, the skin is especially vulnerable. It can develop diseases from the inside or due to external influences. Skin diseases can be:

• Dangerous or harmless
• Contagious or non-contagious
• Spreading or locally restricted
• Worsened by psychological stress

Causes of skin diseases

Skin diseases and skin changes can be caused by a variety of factors. These include:

• Injuries
• Sunlight
• Allergies
• Fungi
• Parasites
• Bacteria
• Viruses
• Burns
• Poisoning
• Other external influences (e.g., extreme cold)

In addition, many other diseases involve alterations of the skin. In these cases, changes in the skin can be crucial in making a diagnosis, which would be the case, for instance, for scarlet fever, rubella, or German measles.