Introduction to Dermatology
Preface
This course covers the topic of dermatology, including the normal histology of skin and the different pathological conditions of the skin. There are numerous pathological changes that represent basic skin diseases without any systemic effects, systemic diseases that affect the skin, and skin diseases with systemic effects. This course will provide a good understanding of major skin diseases, their incidence and prevalence, and the available treatments.
The first chapter of the course discusses normal skin. Skin makes up a large part of the body and has numerous functions from protection against pathogens and extreme temperature to performing many of the sensory functions of the body. In order to identify pathology in the skin, it is important to first identify the normal structures you’ll find in healthy skin. This is the focus of this chapter.
The second chapter of the course involves a discussion of pigment disorders of the skin. These involve certain diseases that cause hypopigmentation, such albinism (which involves a decrease in skin pigmentation), depigmentation (which involves vitiligo, which is the presence of patchy losses of pigment in the skin or albinism, which affects the entirety of the skin and eyes), and hyperpigmentation (which can be post-inflammatory or secondary to hormonal changes, such as is the case in melasma).
Chapter three of the course involves a discussion of the various bullous diseases of the skin. There are many bullous diseases of the skin all of which can be identified by the presence of bullae or fluid-filled lesions of the skin that can affect people of all ages, and with different presentations and etiologies. While bullous diseases often appear similar to one another, they have different etiologies and different modes of treatment.
The fourth chapter of the course will cover the three main types of skin cancer seen in adults. There are numerous types of skin cancer that are extremely rare but only three that make up the vast number of skin cancer cases. In fact, more than 95 percent of cases of skin cancer are one of three types, including basal cell cancer of the skin (BCC), squamous cell cancer of the skin (cSCC or cutaneous squamous cell carcinoma), or melanoma (also called malignant melanoma). The pathological appearance, etiology, and treatment of these cancers are discussed.
Chapter five covers the various acneiform lesions and similar diseases. Acne and related conditions are skin eruptions that are papular in nature and usually involve the face. The most common acne condition is acne vulgaris, although the most common acneiform condition in adults is rosacea. Ocular rosacea is less common, affecting the eyes and midfacial region.
The sixth chapter of the course discusses papulosquamous and eczematoid skin diseases. These two skin diseases represent a wide variety of different dermatological conditions that have similar appearances but different etiologies and slightly different signs and symptoms. The main topics of discussion in this chapter are psoriasis, seborrheic dermatitis, lichen planus, and eczematoid dermatitis of the skin.
The topic of chapter seven is granulomatous diseases of the skin. The most common granulomatous disease is granuloma annulare, of which there are several clinical variants. There are other, less common, granulomatous diseases that look different from granuloma annulare. These diseases are
uncommon and are mainly inflammatory in nature. The different clinical types, manifestations of the various granulomatous diseases, and possible etiologies are covered in this chapter, along with the treatment and complications of these disorders.
The different connective tissue diseases are discussed in chapter eight of the course. Connective tissue diseases may be caused by an excess of connective tissue, such as present in systemic sclerosis, or a defect in the connective tissue, as is seen in Ehlers-Danlos syndrome, pseudoxanthoma elasticum, and cutis laxa. Most of these disorders are inherited or autoimmune and have effects on other tissues of the body, including the connective tissue of the GI tract, blood vessels, heart, lungs, and kidneys.
The ninth chapter covers the cutaneous manifestations of systemic diseases. Many systemic diseases will have manifestations that can be seen on the skin. Things like scleroderma are discussed in the chapter, which has both systemic and skin manifestations. Dermatomyositis is covered in the chapter, which affects both the muscle and the skin. Finally, jaundice is mentioned in the chapter. Jaundice is a manifestation of liver disease or extensive hemolysis with an excess of bilirubin in the body that will be seen as yellowing of the skin and eyes. The treatment and complications of these diseases are covered as part of the chapter.
The tenth chapter covers skin infections, particularly those caused by bacteria. A skin infection can include cellulitis, which is a diffuse infection of the skin caused mainly by Staphylococcus or Streptococcal species. Erysipelas is another skin infection, mainly caused by Group B beta-hemolytic streptococcus. Impetigo is seen mainly in children and can be treated with topical antibiotics. The clinical manifestations, etiology, treatment, and common complications of these diseases are part of the discussion in the chapter.
Chapter eleven covers most of the benign skin lesions a patient might present with. Benign skin lesions are so common that most people have had at least one benign skin lesion in their lifetime. As you will discover, there are benign skin lesions that are mainly seen in childhood, such as verruca vulgaris, and benign skin lesions that are seen mainly in childhood, such as seborrheic keratoses. While some benign skin lesions remain benign, a few have the potential to become cancerous, thus a complete understanding of these skin lesions and ways to manage them is important.
The twelfth chapter of the course is on genetic diseases of the skin. There are numerous hereditary disorders that have skin findings; however, there are a few that primarily involve the skin and other structures. Because inherited diseases tend to cause noticeable changes in the skin that, they are often diagnosed based solely on their clinical signs and symptoms. A few of these diseases cannot easily be identified clinically and require a skin biopsy or a biopsy of other tissue that will show the classic pathologic findings associated with the various disorders. Inherited skin diseases can be autosomal dominant, autosomal recessive, or, less likely, x-lined recessive. Spontaneous mutations can always occur as with any inherited disease.
The repair of lacerations will be the main focus of the thirteenth chapter, although other aspects of wound care will be discussed. Skin wounds can be treated by family doctors, emergency medical specialists, and dermatologists. A skin wound can be any break in the skin, such as a laceration, puncture wound, or abrasion. As you will see in this chapter, tetanus prophylaxis is important in the management of any skin wound. Many skin wounds require only cleansing, while others require a layered closure, which starts with closure of subcutaneous tissues and ends with skin closure.
Chapter 1: Structure and Function of Skin Structures
This chapter discusses normal skin. Skin makes up a large part of the body and has numerous functions from protection against pathogens and extreme temperature to performing many of the sensory functions of the body. In order to identify skin pathology, it is important to understand the normal structures of healthy skin. This is the focus of this chapter.
Skin Basics
The skin can easily be identified as the largest organ of the body. It is approximately 2 millimeters thick and, if separated from the body, would weigh approximately six pounds. It consists of multiple layers that protect the body from external factors, such as heat, pathogens, UV radiation, mechanical trauma, and excesses or deficiencies of humidity in the environment.
Some internal functions of the skin include playing a role in the immune system, the synthesis of vitamin D, the ability to gather sensory information, the storage of water, the regulation of body temperature, and the storage of fat. The skin also has sensory functions, with organs specifically designed to identify the different sensations the body may experiance (including light touch, temperature, pain, and vibration).
There are two different kinds of skin. There is skin that contains hair and skin that does not. Skin varies according to its thickness and texture, with some areas of skin being very thin, and friable and other areas of the skin being calloused and rough. Some areas of skin are highly responsible for sensations and therefore have a large number of sensory fibers within the skin structure. Other areas of skin have very little sensation, are thicker, and resist shear forces.
Epidermis
There are three basic layers to the skin. Each has its own thickness, function, and strength. The outermost layer is known as the epidermis. Its main function is the protection of underlying structures. The epidermis can be as little as 0.05 millimeters thick or as much as 1.5 millimeters thick, depending on where in the body one measures the thickness of this layer. The eyelids have the thinnest skin, while the soles of the feet have the thickest skin.
Important cells in the epidermis include the Langerhans’ cells (important immune cells), Merkel cells (sensory receptors), parts of sensory nerves, and melanocytes (cells that make the pigment known as melanin). There are five different sublayers of the epidermis that slough off and are rebuilt on a regular basis as the body goes through the everyday stresses on the skin.
Epidermal Layers
1. Stratum corneum this is the outermost epidermal layer, which consists of ten to thirty stratified layers of dead keratinocytes that are constantly shedding. It is also referred to as the “horny skin layer” because of the toughness of this layer. The epidermis completely turns itself over every 28-50 days, with slower turnover rates occurring among the elderly.
2. Stratum Lucidum this is deep to the stratum corneum and contains squamous cells that are drying in anticipation of becoming dead keratinocytes as cells continue to turn over. These cells are tightly adherent to one another and help provide a barrier function to the skin.
3. Stratum granulosum it is hard to distinguish this layer from the stratum lucidum. It, too, contains cells that are gradually drying out and migrating toward the stratum corneum in order to provide a major protective function for the skin.
4. Squamous cell layer this is the next deepest layer of the epidermis. It is also referred to as the “spiny layer” or stratum spinosum because the cells have spiny projections that adhere them to one another. These represent basal cells (the deepest layer of cells) that have matured and have been pushed outward as the epidermis is being turned over. This layer is the thickest of all epidermal skin layers. These squamous cells are also referred to as keratinocytes because they make keratin, which is the protective protein that makes up several non-cell skin structures, including the hair and nails. The Langerhans’ cells are in this layer and act as immune cells by attaching to antigens that get into the skin. By doing so, they alert the immune system to become activated to kill the pathogen associated with the antigen.
5. Basal layer this is the innermost layer of the epidermis. Histologically, it consists of multiple small round cells that have mitotic activity and that push up through the epidermis to make new squamous cells/keratinocytes and ultimately the stratum corneum. This layer can also be called the stratum germinativum because it is the “germ cell layer” for the epidermis. The melanocytes can be found in the basal layer and produce melanin, which pigments the skin. This is the layer that protects the body from UV light by making protective melanin to cover deeper structures. The Merkel cells (sensory cells) of the skin are located in the basal layer.
It is important to remember that the oxygen supply must come from diffused oxygen from the surrounding air rather than blood vessels. The keratinocytes, which make up 95 percent of the cells of the epidermis, draw in oxygen through its layers. Other important cells mentioned are Langerhans cells, Merkel cells, melanocytes, and inflammatory cells. There are tiny blood capillaries beneath the dermis that may play a small role in getting blood to the deeper layers of the epidermis.
Between the different cells are tight junctions, referred to as the adherens junctions. Cadherins are transmembrane proteins that are linked to the actin inside the cells, giving cells their structure and connecting two cells to one another. The cell junctions act as a barrier to outside influences.
Sensory Organs in the Cells
There are several sensory organs located in the dermis and epidermis of the skin. They detect different types of sensations and transmit them to the brain for interpretation. The following are some of the nerve structures located in the dermis and epidermis:
Meissner’s Corpuscles. These are located in the dermis and are shaped like an egg. They sense light touch to the skin and help identify objects. Objects placed in the hands can be easily identified by the many Meissner’s corpuscles located on the skin surface of the hand.
Pacinian Corpuscles. These are deep inside the dermis (unlike other sensory receptors). Squeezing or pinching the skin can elicit the sensation of pain.
Thermoreceptors. These receptors sense both hot and cold. Rather than be a specific structure, they are epidermal free nerve endings just beneath the first layers of skin.
Nociceptors. These are also pain receptors but are located in the epidermis as free nerve endings. They pick up sharp pain sensation.
As mentioned, the layers of the epidermis are younger the deeper one goes through the epidermis. The stratum basale or basal layer undergoes mitosis and makes new epidermal cells. The cells undergo mitosis and become displaced upward through many different stages of differentiation until they ultimately lose their nuclei in the stratum corneum, fusing together into sheets of dead squamous cells that eventually slough off during the desquamation process. Keratin is secreted by the living keratinocytes that contribute to the skin barrier and the extracellular matrix. It takes 48 days for cells of the epidermis to go from being made to desquamated.
The differentiation of keratinocytes depends on a specific calcium gradient, with a low calcium level in the basal layer and a higher calcium level in in the stratum granulosum. The level decreases in the stratum corneum because these cells are dead and dry, unable to dissolve these ions. The calcium gradient changes with maturation of the keratinocytes, being a regulating phenomenon in the differentiation process.
The increased extracellular calcium concentrations cause an increase in the increased intracellular calcium concentrations. This comes from a release of intracellular stores and an influx of calcium through a calcium ion channels (of which there are three types). There is believed to be a calciumsensing receptor that detect the extracellular calcium concentration, triggering it to bring calcium into the cells.
The epidermis is responsible for UV light protection, microbial pathogenic protection, mechanical force protection, and protection against chemical compounds. Most of the barrier function happens at the stratum corneum and not the deeper layers of the epidermis. This physical barrier comes from cell to cell tight junctions, chemical barriers (enzymes and antimicrobial peptides), immune cells, the water content of the skin, the low pH of the skin, and symbiotic bacteria (that prevent the overgrowth of pathogens).
There are a couple of things that influence the barrier aspect of the skin. Living with psychological stress will result in an increased glucocorticoid release. This interferes with the barrier function of the stratum corneum. Acute and severe shifts in humidity will change the hydration of the stratum corneum, allowing for an alteration in the ability to fight pathogens by the epidermis. The ability to hold water in the skin comes primarily through the action of the stratum corneum. Lipids line up in an organized way in this layer to from a trans-epidermal water loss barrier.
The pigmentation of the skin comes from the melanin content in the melanocytes of the epidermis. Melanin is found in packets called melanosomes, which are found in the melanocytes mixed in among the melanosomes. The size and number of melanosomes is different in different ethnicities and on different parts of the skin. The number melanosomes increase with UV radiation.
Dermal Layers
The dermis is the second or “middle” layer of skin and is the thickest part, being between 1.5 millimeters to 4 millimeters thick and accounting for ninety percent of the skin’s thickness. The main function of this layer is to regulate the body temperature and supply nutrients to the epidermis (which has no blood vessels for nourishment). The dermis is also a big storage layer for water and contains most of the
special structures of the skin. The two main layers are the papillary area and a deeper layer, called the reticular layer. Basement membrane forms the underside of the dermis. The extracellular matrix consists of collagen, elastin, and extrafibrillar matrix.
The special structures of the dermis include thermoreceptors, mechanoreceptors, sweat glands, hair follicles, sebaceous glands, apocrine glands, blood vessels, and lymph vessels. The capillaries will supply blood to both the dermis and epidermis.
The blood vessels for the dermis and epidermis are located in the dermis. They provide nutrients to the skin and take waste products away from the tissues. Vitamin D produced by the skin is picked up by small capillaries and is then sent to other body areas. There are lymph vessels in the dermis as well, which assist in the immune defense mechanisms in the skin.
Hair follicles and sweat glands are also found in the dermis. Hair follicles surround hairs on parts of the skin that are not hairless. There are about 3 million sweat glands in the dermis, which are of two types. There are apocrine glands (found in the armpits and pubic areas only and which have the capacity to produce body odor). There are also eccrine glands (which are found throughout the body and responsible for the regulation of body temperature by secreting water and electrolytes when the body reaches a certain set temperature). Because these glands secrete mostly water, there is very little body odor associated with these glands.
Sebaceous glands are attached to the hair follicles and are found in all places of the body except for the bottoms of the feet and the palms of the hands. They secrete an oil that lubricates the skin and keeps the skin waterproof. Oil produced by these glands has mild anti-microbial activity.
The dermis contains all of the nerve endings in the skin. There are nerve endings for pain sensation, light touch sensation, pressure sensation, vibratory sensation, temperature sensation, itching sensation, and discriminatory touch sensation. Information from these receptors goes to the brain for interpretation.
The main connective tissue structure of the dermis involves the collagen and elastin proteins, which are made by the fibroblasts. These are specialized skin cells that gives skin its elasticity and strength. Collagen is more resistant and stronger than elastin but elastin allows for the stretching ability of the skin so both proteins are necessary as part of the dermis.
The dermis consists of two sublayers. The upper layer is called the papillary layer, which contains a thin layer of collagen fibers. It is responsible for temperature regulation and supports the dermis. There is an extensive vascular system in this layer that provides nutrients to both the dermis and epidermis. The dilatation or constriction of this vasculature helps control the temperature of the skin.
Beneath the papillary layer is the reticular layer, which is thicker and made of thick parallel collagen fibers. This layer has a higher density than the papillary dermis and provides strength and elasticity to the skin. Because the tissue is so supportive, it also provides a good structure for the various skin components, such as the sebaceous glands, the sweat glands, and the hair follicles.
The dermal papillae or DP are nipple-like extensions of the dermis up into the epidermis. These are what rise above the level of the epidermis to create the fingerprints and fine footprints. Blood vessels in the DP are responsible for the nourishment of all hair follicles as well as the lower epidermal layers.
There is some genetic input in to the location and pattern of dermal papillae so they will stay in the same basic pattern throughout life, although they will get bigger as a child grows.
As their name suggests, the dermal papillae are a part of the upper, papillary layer of the dermis. They form ridges that increase the surface area of the skin. By imbedding into the epidermis, the dermal papillae are perfect for providing nutrients and taking away waste products made by the epidermis. They keep the dermis and epidermis from separating from one another.
The Subcutis Layer
The subcutis layer is the innermost lining of the skin. It is connective tissue laid out in a network of fat cells and collagen-producing cells. This can also be called the subcutaneous layer or the hypodermis. It helps insulate the body and preserve body heat. It is a good shock-absorber and varies in thickness depending on the weight of the individual and the area of the body being assessed. Because this part stores fat, it can provide an energy source for the dermis. Like the dermis, this layer has blood cells, lymph vessels, nerves, and some hair follicles.
The subcutis is also referred to as the subcutaneous or the hypodermis. Some call it the superficial fascia as it bridges the gap between deeper structures and the dermis/epidermis. The cells found in the subcutis layer include fat cells, fibroblasts, and macrophages. This is derived from mesodermal cells but isn’t a part of the dermatome portion of the mesoderm (unlike the dermis). Fat cells also deposit in the subcutis layer.
The main structures that can be found in the subcutis layer include anchoring fibrous bands (which connect the deep fascia to the skin), elastin and collagen (that connect the subcutaneous tissue to the dermal tissue), fat cells, blood vessels being sent to the dermis, lymph vessels sent from the dermis, mammary gland tissue (in the areas of the breast), sweat glands, cutaneous nerve endings, the base/roots of hair follicles, mast cells, Pacinian corpuscles, and Ruffini corpuscles.
The subcutaneous fat is a part of the subcutis most widely found in the skin. It is made almost entirely from adipose tissue (adipocytes), which are lobulated and separated by connective tissue bands. The actual number of adipocytes and their thickness depends on the person and where in the body one looks for these cells. They act to pad the body and provide an energy reservoir. The thickest areas will warm the skin through both insulation and thermoregulation properties. This type of fat is different from visceral fat (found inside the abdomen).
Key Takeaways
The skin has three layers, the epidermis, the dermis, and the subcutis (or hypodermis).
There are multiple layers in the epidermis, which start with mitotic cells and end with surface cells that are dead and without nuclei.
Quiz
There are multiple structures within the skin that have nerve functions, thermoregulatory functions, skin pigmentation functions, and vascular functions.
1. About how thick is the average slice of skin when biopsied?
a. 0.5 mm
b. 1 mm c. 2 mm d. 4 mm
Answer: c. The average slice of skin on a biopsy is about 2 mm thick.
2. Which physiological function is not generally associated with the skin?
a. Synthesis of vitamin D b. Retrieve sensory input c. Regulation of body temperature d. Storage of carbohydrates
Answer: d. The skin serves multiple functions; however, the storage of carbohydrates is not considered a typical physiological function of the skin.
3. What part of the body has the thinnest known skin?
a. Scalp b. Eyelids c. Back of the hand d. Chest wall
Answer: b. The eyelids contain the thinnest skin in all of the body.
4. About what percentage of the skin’s total thickness is made up by the dermis?
a. Fifty percent b. Seventy percent c. Eighty percent d. Ninety percent
Answer: d. The dermis makes up about ninety percent of the total thickness of the skin.
5. What part of the skin’s dermal structures creates malodorous sweat?
a. Sebaceous glands b. Eccrine glands c. Apocrine glands d. Oil glands
Answer: c. The apocrine glands are found in the dermis of the skin, and produce sweat in the armpits and pubic area. They have the ability to make sweat that is malodorous and gives off body odor. The other glands do not give off any odor.
6. What is another name to be used for the basal cell layer of the epidermis?
a. Stratum corneum
b. Stratum lucidum
c. Stratum granulosum
d. Stratum germinativum
Answer: d. The other name for the basal cell layer of the epidermis is called the stratum germinativum.
7. Which skin layer is the least vascular?
a. Subcutis layer b. Dermis c. Basal cell layer d. Stratum corneum
Answer: d. The stratum corneum contains dried out cells that have no vasculature, making it the least vascularized part of the skin.
8. Which part of the body has the thickest skin?
a. Sole of the foot b. Back (trunk)
c. Buttocks d. Upper arm
Answer: a. The sole of the foot has the thickest skin with an especially thick epidermal layer.
9. In what way does the skin control the body temperature in hot conditions?
a. The apocrine glands give off sweat b. The eccrine glands give off sweat c. The hair follicles have muscles that contract d. The elastin relaxes, loosening the skin
Answer: b. The main way that the body controls temperature in hot conditions is to give off sweat by the eccrine glands, which are located throughout the body.
10. Which ion most contributes to the maturation of keratinocytes?
a. Sodium
b. Calcium
c. Potassium
d. Magnesium
Answer: b. It is the calcium gradient that contributes most to the maturation of keratinocytes.
