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Hair follicles and their keratinized product, hair, are skin appendages present on nearly every part of the body. Areas of the body typically devoid of hair include the palmar and plantar surfaces, lips, and urogenital orifices. Sex hormones influence the distribution, texture, and color of hair. Hair follicles and hair are both multifunctional. Hair follicles generate hair and help to provide epithelial stem cells used for wound repair. Besides the cosmetic concerns of hair color, growth, and distribution, hair plays a vital role in thermoregulation, tactile sensation, and ultraviolet (UV) radiation protection. Diseases of hair follicles and hair are an important concern to the general population and have been an active topic of research in recent years., (Copyright © 2022, StatPearls Publishing LLC.)

Nevus anemicus is an uncommon, congenital vascular malformation resulting in hypopigmented cutaneous macules and/or patches that characteristically do not create erythema in response to trauma, heat, or cold., (Copyright © 2022, StatPearls Publishing LLC.)

Blastomycosis is a fungal infection caused by the organism  Blastomyces dermatitidis ,  which is endemic in the soils of the Ohio and Mississippi River Valleys, Great Lakes region, and the southeastern United States. It most commonly presents as a pulmonary infection following the inhalation of spores, which may be asymptomatic and, therefore, undetectable, though severe, life-threatening complications like acute respiratory distress syndrome can occur. Extrapulmonary disease occurs in approximately 25% to 30% of patients after hematogenous dissemination from the lungs, with the skin being the most common site of extrapulmonary disease. Primary cutaneous blastomycosis, though rare, can occur due to direct inoculation after trauma to the skin. Unlike other deep fungal infections that occur predominantly in immunocompromised patients, blastomycosis also occurs in immunocompetent hosts. , (Copyright © 2022, StatPearls Publishing LLC.)

The sebaceous gland is integral to the structure and function of the skin, providing 90% of its surface lipids. While much of the focus relating to the sebaceous gland comes from its central role in acne vulgaris, several new functions have come to light that highlights this versatile cellular unit’s complex role in skin homeostasis. The sebaceous gland is special in at least two ways. Firstly, the product of this gland is synthesized via holocrine secretion, a unique method characterized by the purposeful self-destruction of its primary cellular unit, the sebocyte. Secondly, despite being of epithelial origin and possessing numerous hormone receptors, sebocytes engage in lipid synthesis and metabolism, a job normally reserved for adipocytes. Thus, the sebaceous gland can be considered both a hormonal target as well as an endocrine organ. The production of sebum is the most important function of the sebaceous gland in humans. Unique to the sebaceous gland is the production of squalene and certain fatty acids. Sebum production is critical for maintaining skin homeostasis, lubrication, and physiological defense against environmental and infectious insults., (Copyright © 2022, StatPearls Publishing LLC.)

Previously known as Lyell syndrome, Stevens-Johnson syndrome (SJS), and toxic epidermal necrolysis (TEN) are variants of the same condition and are distinct from erythema multiforme major staphylococcal scalded skin syndrome­, and other drug eruptions. Stevens-Johnson syndrome/toxic epidermal necrolysis is a rare, acute, serious, and potentially fatal skin reaction in which there are sheet-like skin and mucosal loss accompanied by systemic symptoms. Medications are causative in over 80% of cases. Stevens-Johnson syndrome/toxic epidermal necrolysis is classified by the extent of the detached skin surface area. Stevens-Johnson syndrome: less than 10% body surface area. Overlap Stevens-Johnson syndrome/toxic epidermal necrolysis: 10% to 30% body surface area. Toxic epidermal necrolysis more than 30% body surface area., (Copyright © 2022, StatPearls Publishing LLC.)

The dermis is a connective tissue layer sandwiched between the epidermis and subcutaneous tissue. The dermis is a fibrous structure composed of collagen, elastic tissue, and other extracellular components that includes vasculature, nerve endings, hair follicles, and glands. The role of the dermis is to support and protect the skin and deeper layers, assist in thermoregulation, and aid in sensation. Fibroblasts are the primary cells within the dermis, but histiocytes, mast cells, and adipocytes also play important roles in maintaining the normal structure and function of the dermis.  , (Copyright © 2022, StatPearls Publishing LLC.)

Lichen planus (LP) is an inflammatory disorder of the skin and mucous membranes with no known cause. It appears as pruritic, violaceous papules and plaques most commonly found on the wrists, lower back, and ankles. A lattice-like network of white lines called Wickham striae overlies the lesions but is most easily observed on the buccal mucosa where erosions can also be present. Drug-induced lichen planus, or lichenoid drug eruption, is frequently photo distributed but may be indistinguishable from idiopathic LP. The natural history of LP varies significantly. The majority of patients with cutaneous lesions spontaneously clear within 1 to 2 years after initial presentation. However, recurrences are common, and residual hyperpigmentation of the skin frequently results. By contrast, oral LP is a chronic disease that may or may not remit. Drug-induced LP gradually resolves after removal of the causative medication., (Copyright © 2022, StatPearls Publishing LLC.)

Neutrophilic eccrine hidradenitis (NEH) is a benign, rare condition that was initially used to describe the skin eruptions observed in acute myelogenous leukemia (AML) patients receiving systemic chemotherapy. It can be considered a reactive disorder and is commonly associated with malignancies that include other leukemias, Hodgkin lymphoma, and solid tumors. Since its introduction in 1982, neutrophilic eccrine hidradenitis is frequently described with other neutrophilic dermatoses. these are a group of skin disorders characterized by normal neutrophilic infiltrates in the predominantly dermal portion of the skin, with no identifiable source of infection. , (Copyright © 2022, StatPearls Publishing LLC.)

Roseola infantum is a common disease of childhood caused by a primary infection with human herpesvirus 6 (HHV-6) and less frequently, by human herpesvirus 7 (HHV-7). This disease, also known as exanthema subitum and sixth disease, presents in children ages six to 12 months with 90% of cases occurring in children younger than two years. Caused by the B variant of HHV-6, patients with the virus classically present with an acute onset of a high-grade fever up to 40 C (104 F) for three to five days. The child will experience a rapid defervescence of the fever with accompanying nonpruritic, pink papular rash that begins on the trunk. It is found universally and has been discovered to be the cause of 10% to 45% of febrile illness in infants. Due to the high fever and the ability of the virus to cross the blood-brain barrier, 15% of children will also experience an acute febrile seizure during the febrile phase of the illness.  Roseola infantum is a clinically diagnosed, self-limited illness that can be treated symptomatically. HHV-6 will likely remain latent in immunocompetent patients but can be a major cause of morbidity and mortality in patients who are immunosuppressed. , (Copyright © 2022, StatPearls Publishing LLC.)

Cradle cap, or pityriasis capitis, is a subset of infantile seborrheic dermatitis (ISD). It is a very common, mostly self-limiting, chronic non-inflammatory scaling skin condition that presents between the third week and first couple months of life. The peak incidence is at 3 months of life. The skin manifestations are marked by erythematous plaques with greasy-appearing yellowish scale. Cradle cap's name is analogous to its anatomical location. It can be found in areas dense in sebaceous gland activity, such as the scalp, T line of the face and external ears. Parents should be educated properly on the management and benign nature of this condition., (Copyright © 2022, StatPearls Publishing LLC.)

Albinism, from the Latin albus , meaning "white," is a group of heritable conditions associated with decreased or absent melanin in ectoderm-derived tissues (most notably the skin, hair, and eyes), yielding a characteristic pallor. The most commonly thought of presentation is that of oculocutaneous albinism (OCA). OCA is a group of phenotypically similar genetic disorders derived from errors in melanin synthesis. As the name implies, the most dramatic effects are in the eyes and skin. The skin manifestations are more heterogeneous and appear along with a spectrum of severity depending upon the subtype of OCA. The ocular structures rely upon melanin for signaling as they develop, in utero; thus, misrouted optic nerve fibers yield more uniform ocular manifestations of the disorder. To date, seven types of nonsyndromic albinism (OCA1 to OCA7) have been described. These are all due to isolated genetic mutations whose constellation of signs and symptoms do not manifest so broadly that they can be classified as syndromic. A discussion on albinism, however, would be incomplete without the mention of isolated ocular albinism (OA1) and the syndromic albinisms: Hermansky-Pudlak syndrome (HPS) and Chediak-Higashi syndrome (CHS). The syndromic albinisms have the same hallmark lack of dermal and ocular pigment as OCA. They, however, involve genes that encode for proteins that have more extensive applications to cellular function. Loss-of-function mutations in these genes, therefore, yield predictable systemic consequences associated with the syndromes mentioned. Examples include inactivation of genes involved in lysosomal synthesis (and not simply melanin synthesis) that lead to bleeding diathesis in HPS and propinquity to infection in CHS. Other conditions may present like albinism with congenital nystagmus and/or generalized hypopigmentation. Most of these are included in the Differential Diagnosis section. Of special mention is a pair of syndromes that derive their albino-like features because of deletions in the same genes that are mutated in OCA type 2: Angelman (AS) and Prader-Willi (PWS) syndromes., (Copyright © 2022, StatPearls Publishing LLC.)

Pyoderma gangrenosum is an ulcerative disorder that falls into the category of neutrophilic dermatoses. Pyoderma gangrenosum should not be confused with pyogenic granuloma, a completely separate entity but with an equally ill-fitting name. Despite its name, pyoderma gangrenosum is not caused by infection or gangrene. Pyogenic granuloma is often associated with systemic disease.  The diagnosis is made clinically after excluding other similar skin disorders., (Copyright © 2022, StatPearls Publishing LLC.)

Onychomycosis is a fungal infection of the nail unit. When onychomycosis is caused by dermatophytes, it is called tinea unguium. The term onychomycosis encompasses not only the dermatophytes but the yeasts and saprophytic molds infections as well. An abnormal nail that is not caused by a fungal infection is a type of dystrophic nail. Onychomycosis can infect both fingernails and toenails, but onychomycosis of the toenail is much more prevalent. The disease burden, clinical types, staging, diagnosis, and management of toenail onychomycosis will be discussed., (Copyright © 2022, StatPearls Publishing LLC.)

Peutz-Jeghers syndrome (PJS) is a hereditary cancer syndrome characterized by gastrointestinal (GI) polyposis, mucocutaneous pigmented macules, and cancer predisposition. Patients with Peutz-Jeghers syndrome are at an increased risk for developing GI cancers of the colorectal, pancreatic, and gastric organs, in addition to a wide variety of non-GI epithelial malignancies such as breast, uterine, and cervical cancers, lung cancer, and tumors of the ovaries and testicles. Among the various malignancies, colorectal is most common, with a lifetime risk of 39%. Followed by breast cancer in females with a lifetime risk of 32% to 54%. Females with Peutz-Jeghers syndrome are also at risk for gynecological cancers such as benign ovarian sex cord tumors with annular tubules (SCTATs) and mucinous tumors of the ovaries and fallopian tubes. Males with Peutz-Jeghers syndrome are at risk of Sertoli cell tumors of the testes, which are often hormonally active, secreting estrogen. Males present with gynecomastia, advanced bone age, and rapid growth with short stature. Due to the increased risk for various malignancies, diligent surveillance is recommended., (Copyright © 2022, StatPearls Publishing LLC.)

The procerus muscle is a pyramidal-shaped muscle of facial expression part of the glabellar complex. The procerus muscle is derived from the second pharyngeal arch, innervated by branches of the facial nerve, and supplied by the facial artery via the external carotid artery. The structure and functionality of the procerus muscle and the associated glabellar complex are of clinical importance in cosmetic and surgical procedures, including botulinum toxin injection, injectable fillers, and blepharoplasty. , (Copyright © 2022, StatPearls Publishing LLC.)

Crohn disease, a well-known subtype of inflammatory bowel disease, is an inflammatory condition of the gastrointestinal tract with numerous possible extra-intestinal manifestations. Approximately 40% of all patients affected by Crohn disease experience at least one extra-intestinal manifestation of the disease, with the skin being the most common site of extra-intestinal involvement. Other common locations of extra-intestinal involvement include the eyes, joints, and hepatobiliary system. Interestingly, an extra-intestinal manifestation may precede the formal diagnosis of Crohn disease in about 25% of patients., (Copyright © 2022, StatPearls Publishing LLC.)

Skin is part of the integumentary system and considered to be the largest organ of the human body. There are three main layers of skin: the epidermis, the dermis, and the hypodermis (subcutaneous fat). The focus of this topic is on the epidermal and dermal layers of skin. Skin appendages such as sweat glands, hair follicles,  and sebaceous glands are reviewed in-depth elsewhere.[1], (Copyright © 2022, StatPearls Publishing LLC.)