The Skin

Chapter 19 The Skin



Ivan Damjanov, MD, PhD




1 What are the three main layers of the skin?



image Epidermis, composed of four or five layers of squamous epithelium

image Dermis, composed of connective tissue (Its upper part extending between the rete ridges of the epidermis is called papillary dermis, whereas the lower part is called reticular dermis. In some portions of the body, the dermis also contains hair follicles and sweat and sebaceous glands. Dermis contains blood vessels and nerves.)

image Subcutis, composed of fat tissue and connective tissue arranged to provide support to the upper layers of the skin and link them to the underlying muscles and bones


2 What are the most important features of skin lesions?


Dermatology is a visual discipline, and it relies on accurate macroscopic and microscopic description of skin lesions. Thus it is important to pay attention to the basic characteristics of each lesion, including its:


image Size

image Color

image Location and distribution

image Consistency

image Shape

image Surface characteristics


3 How are skin lesions classified?


In clinical practice, it is customary to classify skin lesions as primary or secondary. Primary skin lesions include macules, papules, patches, plaques, nodules, cysts, wheals, vesicles, bullae, and pustules. Secondary skin lesions are related to the progression of the disease but may also be caused by scratching, trauma, or treatment. This group of lesions includes crusts, erosions, ulcers, fissures, excoriations, scars, and scales (Figs. 19-1 and 19-2).


image image

Figure 19-1 Primary skin lesions.


(From Fitzpatrick JE, Aeling JL: Dermatology Secrets. Philadelphia, Hanley & Belfus, 1996, pp. 9–10.)


image

Figure 19-2 Secondary skin lesions.


(From Fitzpatrick JE, Aeling JL: Dermatology Secrets. Philadelphia, Hanley & Belfus, 1996, pp. 10–11.)



4 What is the difference between a macule and a papule?


Macules are flat lesions presenting as a change in the color of skin, whereas papules are slightly elevated. For example, a freckle (ephelis) is a macule, whereas a congenital mole (intradermal nevus) is a papule. Some childhood exanthemas, such as measles, present in the form of a widespread maculopapular rash—that is, red macules, which are flat, and papules, which are slightly raised. Very large macules are called plaques.



5 What is the difference between a papule and a nodule?


Papules are small, slightly elevated solid skin lesions measuring less than 0.5 cm. In practice, this means they can be covered with the flat end of a pencil. Nodules are deeper seated and larger, measuring more than 0.5 cm in diameter. Most skin tumors present as nodules, but obviously some can be diagnosed early in their development while they are still smaller than 0.5 cm. (Continued on p. 390)



6 What is the difference between a plaque and a papule?


Like papules, plaques are slightly elevated skin lesions, but they measure more than 0.5 cm in diameter. Plaques often arise by a confluence of multiple papules. In contrast to nodules, plaques appear relatively flat and do not have a deeper dermal component. Plaques are typical of eczema, a common skin lesion that may have many mechanisms.



7 What is the difference between a wheal, a papule, a plaque, and a nodule?


Wheals are solitary or grouped, raised, soft skin lesions. Wheals are larger than papules and more elevated than a plaque. In contrast to nodules, which are firm and develop over a longer period of time, wheals are soft, edematous, and of recent onset. Wheals are often pruritic, red or pale, and often tend to disappear without any consequences. Wheals are typically caused by allergies. Dermal edema due to a release of histamine from mast cells is the underlying cause of wheals in dermographism.



8 What is the difference between a vesicle and a bulla?


Vesicles are slightly raised skin lesions filled with fluid. Grouped vesicles are typical of herpes simplex infection. Larger vesicles measuring more than 0.5 cm are called bullae. In common parlance, bullae are called blisters.



9 What is the difference between a vesicle and a pustule?


Vesicles are filled with serous fluid, whereas pustules contain pus. Upon infection, vesicles transform into pustules.



10 What is the difference between a cyst and a bulla?


Cysts are cavities lined by an epithelial layer. Cysts develop from invaginations of surface epithelium or obstruction of excretory ducts of skin appendages. They may contain fluid and thus have some common features with bullae. However, bullae do not have a defined epithelial lining and typically develop fast, whereas cysts develop over a prolonged period of time. Epidermal inclusion cysts filled with desquamated epithelium and keratin may resemble nodules.



11 What is a crust?


Crust, also known as a scab, is the material typically found on the surface of scratches or wounds. It consists of coagulated plasma, cell debris, and extravasated blood cells.



12 What is the difference between erosion and an ulcer?


Erosions are superficial defects of the epidermis above the epidermodermal basement membrane. Ulcers are deeper defects involving both the epidermis and the dermis. Erosions heal without scarring, whereas the healing of ulcers involves granulation tissue and scar formation. Linear erosions caused by scratching are called excoriations.



CLASSIFICATION OF SKIN DISEASES



13 How are skin diseases classified etiologically?


Skin diseases can be classified according to their causation into several categories:


image Congenital (e.g., congenital ichthyosis and congenital nevus)

image Diseases caused by chemical or physical irritants (e.g., burns, frostbite, ulcers caused by exposure to caustic materials, and dust-induced eczema)

image Infectious diseases (e.g., viral exanthemas of childhood, such as rubella or varicella, and impetigo caused by pus-forming cocci)

image Immunologic diseases (e.g., poison ivy and atopic dermatitis)

image Idiopathic diseases (e.g., psoriasis and epidermolysis bullosa; many chronic skin diseases are poorly understood and fall into this category. Some of these diseases have a hereditary component and have a tendency to occur in families.)

image Neoplastic diseases (e.g., squamous cell carcinoma and malignant melanoma)


14 How are skin diseases classified according to their duration?


Like all other diseases, skin disease can be classified as acute or chronic. Acute diseases may heal, recur, or progress to chronic diseases. Chronic diseases can be preceded by an acute stage, but many of them begin insidiously without a distinct acute phase, lasting without an obvious end. The best example of such a chronic disease, psoriasis, is a common chronic disease that has no obvious beginning and rarely, if ever, any end.



15 How are skin diseases classified according to their distribution?


Skin diseases may be localized and limited to an area of the body, or they may involve the entire body. For example, bacterial folliculitis may be limited to the face, whereas viral exanthemas, such as measles, involve the entire body. Many systemic diseases, such as systemic lupus erythematosus (SLE), have skin manifestations and at the same time involve many internal organs.



Key Points: Skin Lesions



1. Various macroscopic and histologic skin lesions must be correlated with clinical data or biopsied for microscopic examination in order to formulate the correct diagnosis.

2. Many viruses, bacteria, fungi, and parasites affect the skin, causing lesions of the epidermis, dermis, or subcutis.

3. Immunologic diseases of the skin include reactions to foreign antigens (as in poison ivy) or autoantigens (as in systemic lupus erythematosus).

4. There are many skin diseases of unknown etiology, but the most common is psoriasis, which affects 1% or 2% of the total population.

5. Skin lesions are found in the course of many metabolic, hormonal, immunologic, circulatory, and neoplastic diseases.


16 How are skin diseases classified microscopically?


Pathologists can classify most skin diseases into several categories, which then must be correlated with clinical data to arrive at the final clinical–pathologic diagnosis. General categories of skin diseases recognized in skin biopsies are too broad. For example, chronic dermatitis may be caused by immune mechanisms, infections, or irritation. It is thus common to classify skin diseases according to the way they involve the skin. These categories include:


image Diseases of the epidermis (e.g., psoriasis, ichthyosis, and pemphigus vulgaris)

image Diseases of the dermal–epidermal interface (e.g., epidermolysis bullosa, bullous pemphigoid, erythema multiforme, and lupus erythematosus)

image Diseases of the superficial and deep vascular bed (e.g., urticaria, cutaneous vasculitis, and contact dermatitis)

image Disorders of the dermal connective tissue (e.g., systemic sclerosis)

image Inflammatory disorders of the panniculus adiposus (e.g., erythema nodosum, erythema induratum, and arthropod infestations)

image Skin neoplasms


17 What is eczema?


The clinical term eczema (from the Greek word meaning to “boil over”) is probably the best example of the vagueness of some dermatologic terms. In acute stages, it presents in the form of erythematous, oozing, or crusted papules, which then become crusted in the chronic stages. Eczema is a reaction pattern and may be a manifestation of:


image Atopic dermatitis

image Allergic dermatitis

image Irritant dermatitis

image Drug reaction

image Photosensitivity reaction

image Idiopathic diseases (e.g., seborrheic keratosis)


INFECTIOUS DISEASES



18 What are the main infectious skin pathogens?



image Viruses, such as herpes simplex virus or human papillomavirus

image Bacteria, such as Staphylococcus aureus and Streptococcus

image Fungi, such as Dermatophytes (e.g., Trichophyton sp.)

image Parasites, such as Sarcoptes scabiei


19 Which skin diseases are caused by herpes viruses?


Herpes viruses (HSVs; Fig. 19-3) cause several skin diseases:


image HSV-1 is the cause of herpetic vesicles on the lips (herpes labialis), also known as cold sores. Histologically, the vesicles contain fluid and detached epithelial cells containing intranuclear viral inclusions.

image HSV-2 is the cause of genital vesicles (herpes genitalis).

image Varicella-zoster virus is the cause of chickenpox in children. Reactivation of the same virus causes shingles (herpes zoster) in adults.

image HSV-8 is the cause of Kaposi sarcoma in AIDS patients.

image

Figure 19-3 Herpetic vesicles. The vesicles (bullae) are intraepidermal. In addition to fluid, these vesicles contain enlarged keratinocytes, which are typically multinucleated and have “ground-glass” nuclei filled with viral particles (inset). Viral particles can be seen only by electron microscopy, but they can also be demonstrated by immunohistochemistry with antibodies to herpes virus.



20 Which skin diseases are caused by human papillomaviruses?


Human papillomaviruses are DNA viruses that are classified into more than 70 subgroups. Some of these viruses have a predilection for certain anatomic sites, causing clinically identifiable skin lesions known as verrucae or warts (Fig. 19-4).


image Verruca vulgaris (common wart): These rough-surfaced, hyperkeratotic papules usually appear on hands. May be solitary or multiple. Histologically, the warts show papillomatosis (formation of papillae that have a central vascular connective tissue core and are covered with thick epithelium), acanthosis (i.e., thickening of the epithelium), granular layer thickening, and hyperkeratosis.

image Verruca plana (flat wart): These small (2–5 mm), flat-topped, multiple hyperpigmented multiple papules appear on the face and hands of children.

image Verruca plantaris (plantar wart): It is a thick, hyperkeratotic, often painful lesion on the soles.

image Condyloma acuminatum (anogenital wart): These moist, cauliflower-like exuberant growths appear on the penis or vulva.

image

Figure 19-4 Human papillomavirus-induced lesions. A, Verruca vulgaris. Histologically, the lesion shows papillomatosis (i.e., formation of papillae that have a central vascular connective tissue core covered with epithelium), acanthosis (i.e., thickening of the epidermis), thick granular layer, and surface hyperkeratosis. B, Plantar wart is flat and shows hyperkeratosis and acanthosis.



21 Do warts ever become malignant?


Although warts contain HPV virions, there is no evidence that the subtypes of HPV causing skin warts are carcinogenic. Hence common warts do not transform into carcinoma.



22 What is impetigo?


Impetigo is a superficial skin infection caused by gram-positive bacteria. Staphylococcus aureus is the most common cause. Most often, the disease presents in the form of superficial pustules on the faces of small children. Histologically, the pus accumulates in the upper part of the epidermis. The pustules rupture easily and heal without scarring.



23 What is folliculitis?


Folliculitis is a bacterial infection of the hair follicles characterized by an accumulation of pus around the hair shaft. Infection is most often caused by S. aureus and involves hairy areas of the skin, such as the buttocks and thighs, bearded part of the face, and the scalp. Suppuration extending into the perifollicular soft tissue may lead to formation of abscesses, which are known as furuncles or boils. Confluent boils form indurated masses that are dark bluish black and are thus known as carbuncles (name derived from a cognate for carbon). See Fig. 19-5.


image

Figure 19-5 Skin lesions caused by purulent bacteria. A, Impetigo is characterized by the formation of intraepidermal pustules. B, Folliculitis is an inflammation of hair follicles. Pus fills the follicle. C, Furuncle is characterized by the extension of the inflammation to the surrounding connective tissue (i.e., formation of an abscess around the hair follicle). D, Carbuncle forms through the confluence of several furuncles.



24 What is acne?


Acne vulgaris is a disorder of the pilosebaceous units of the skin. The disease is multifactorial and involves:


image Overproduction of sebum by the sebaceous glands attached to the hair follicle (Androgenic hormones stimulate the production of sebum, and accordingly the disease is more prominent in men than in women.)

image Obstruction of the pilosebaceous canal by keratin plugs

image Distention of the pilosebaceous canal by sebum and keratin

image Proliferation of anaerobic bacteria such as Propionibacterium acnes

image Secondary infection by pus-forming bacteria, which leads to folliculitis and abscess formation


25 What is cellulitis?


Cellulitis is diffusely spreading suppurative infection of the dermis and soft tissues. Most often, it is caused by Streptococcus pyogenes. This beta-hemolytic group A streptococcus secretes lytic enzymes (streptokinase, DNAse, and hyaluronidase), which facilitate the spreading of the infection through the tissues.



26 What is erysipelas?


Erysipelas is a superficial cellulitis with prominent involvement of lymphatics, usually caused by group A beta-hemolytic Streptococcus pyogenes. Most often, it occurs on the face, arms, and legs. Redness and swelling of the affected area are accompanied by regional lymphadenitis and systemic symptoms, such as high fever and chills.



27 What is hydradenitis suppurativa?


Hydradenitis suppurativa is an infection of apocrine glands in the axilla and the anogenital region, most often caused by Staphylococcus aureus.



28 What are dermatophytes?


Dermatophytes are fungi that live in the keratinized surface layer of the skin. The most common human pathogens belong to the Trichophyton, Microsporum, and Epidermophyton species.


Dermatophytes produce tinea, commonly known as ringworm. Tinea cruris is known as jock itch, and tinea pedis is known as athlete’s foot. Tinea capitis, a fungal infection of the scalp most often found in children, may cause loss of hair (alopecia). Onychophytosis is nail infection caused by fungi of the Trichophyton species.



29 What are deep fungal infections?


Infections of the deeper portions of the skin may occur because of the entry of invasive fungi through damaged skin or hematogenously during a fungemia. Such infections are caused by Blastomyces, Coccidioides, Histoplasma, Cryptococcus, and other fungi that affect internal organs.



30 What is a mycetoma?


Mycetomas are chronic suppurative infections of the subcutis and soft tissues of feet, arms, or back characterized by marked swelling and formation of pus-draining sinus tracts. Despite their name, which implies that these lesions are a manifestation of mycotic infections, only a minority of mycetomas are caused by fungi. The most common causes of mycetomas are filamentous bacteria such as Nocardia and Actinomyces.

< div class='tao-gold-member'>

Related posts:

  1. Cell Pathology
  2. The Pancreas
  3. Immunopathology
  4. The Hematopoietic and Lymphoid Systems

Stay updated, free articles. Join our Telegram channel
Tags:
Jul 23, 2016 | Posted by in PATHOLOGY & LABORATORY MEDICINE | Comments Off on The Skin

Full access? Get Clinical Tree
Get Clinical Tree app for offline access