EPIDERMIS BASICS OF DERMATOLOGY (1) DERMATOLOGY FOR THE USMLE

DERMATOLOGY FOR THE USMLE

BASICS OF DERMATOLOGY (1)

EPIDERMIS

Skin: Largest and fastest growing organ in the human body. The skin is divided into four layers, beginning from superficial to deep: Epidermis → Dermal-epidermal junction (DEJ) → Dermis (subepidermal) → Subcutaneous tissue

1 EPIDERMIS: Outermost and avascular portion of the skin. This semipermeable barrier is mainly composed of stratified squamous epithelium. The predominant cell type is the keratinocyte Embryonic origin is the surface ectoderm Functions of the Epidermis √ absorption and secretion: Exchange of toxins, medications and sweat through direct interaction with glands and vessels in the dermis. √ Immunosurveillance: Epidermal antigen presenting cells (Langerhans cells) activate the immune system after encountering foreign antigens. Faulty cutaneous immunosurveillance may result in autoimmune diseases, skin infections and cancer √ Pigmentation: Skin pigment (melanin) protects against UV-light damage and provides pigmentation to skin, hair and eyes. Disorders of pigmentation may result in photosensitive, light skin (eg, vitiligo and albinism) √ Protection and repair: Specialized barrier protecting against: Infections, mechanical and chemical injuries, loss of fluids and temperature changes. Provides skin regeneration and repair following damage. Faulty regeneration and repair system may result in xeroderma pigmentosum and keloid formation. layers of the Epidermis √ Stratum corneum (horny layer): Outermost superficial layer of the epidermis; mainly composed of multiple layers of dead, anucleated keratinocytes. Contains a superficial layer of amino acids, fatty acids, sebum and hormones that protects against the environment and external pathogens. USMLE Pearls: Dermatophytes are fungi that cause superficial infection of the skin, hair and nails. They obtain nutrients from keratin in the stratum corneum, thus infection is mainly limited to this cornified layer. Dermatophytes produce metabolic by products that lead to skin inflammation; neutrophils can accumulate beneath the stratum corneum and clinically present as pustules √ Stratum lucidum: Thin layer of anucleated keratinocytes found in thick parts of the skin, such as the palms and soles √ Stratum granulosum (granular layer): Three to five layers of keratinocytes containing prominent keratohyalin granules which appear dark on histology. This layer may be absent in psoriasis and certain types of ichthyosis. √ Stratum spinosum (prickly layer): This layer contains prominent: langerhans cells: Bone marrow-derived dendritic cells; contain Birbeck granules which have a characteristic “tennis racket” shape under electron microscopy. Langerhans cells are CD1a positive and the primary cells involved in Langerhans cell histiocytosis (LCH). Desmosomes: Structure that provide connection between keratinocytes. Destruction of desmosomes by toxins (eg, staphylococcal scalded skin syndrome) or autoantibodies (eg, pemphigus vulgaris) may result in dyscohesion of keratinocytes and intraepidermal blisters √ Stratum basalis (basal layer): Innermost layer of the epidermis located above the dermal-epidermal junction (DEJ). Composed of a single row of columnar basal cells attached to the DEJ by hemidesmosomes Keratinocytes are produced in this layer and move up as they mature to form the other four epidermal layers. The basal layer contains melanocytes and actively dividing stem cells responsible for skin regeneration. Melanocytes: Neural crest-derived cells primarily present in the skin basal layer, retina, uveal tract and leptomeninges In the skin, their main function is to produce pigment (melanin) and store it in melanosomes for transfer to neighboring keratinocytes. Melanin synthesis (melanogenesis) is stimulated by UV­-light, inflammation, melanin stimulating hormone (MSH) and adrenocorticotropic hormone (ACTH), a precursor of MSH. The main steps in melanin synthesis, storage and transfer are summarized below: First step: Conversion of tyrosine → DOPA, followed by DOPA → dopaquinone; both reactions are mediated by the enzyme tyrosinase. Second step: Conversion of dopaquinone → eumelanin and pheomelanin, the two major types of melanin in the skin. Third step: Melanin is stored in melanosomes and transferred to neighboring keratinocytes by melanocyte dendritic processes. Melanin remains permanently inside keratinocytes as pigmented granules. USMLE Pearls: The number of melanocytes is essentially the same in all races. Melanocytes in darker skin types are larger and melanin is degraded slower. Skin complexion is generally classified using the Fitzpatrick skin type scale, which ranges from skin type I (lightest skin) to skin type VI (darkest skin).