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The stratum spinosum (prickle cell layer) is the thickest epidermal layer, composed of desmosome-linked keratinocytes that continue dividing. The basal layer holds the primary stem cells, but the spinous layer contributes substantial proliferative activity and structural bulk.
Langerhans cells are dendritic, bone-marrow-derived antigen-presenting cells that reside in the epidermis and present antigen to T lymphocytes. Melanocytes produce pigment, Merkel cells mediate mechanosensation, and keratinocytes form the structural barrier.
Hemidesmosomes anchor basal keratinocytes to the lamina lucida/basal lamina via integrins and bullous pemphigoid antigens. Desmosomes link keratinocytes to each other, gap junctions allow intercellular communication, and tight junctions seal the intercellular space.
The reticular dermis is the thicker, deeper dermal layer rich in dense collagen bundles and houses adnexal structures such as eccrine coils, hair bulbs, and most vasculature. The papillary dermis is the superficial, looser layer adjacent to the epidermis.
Melanin absorbs and scatters ultraviolet radiation, protecting nuclear DNA from UV-induced damage. It does not produce vitamin D (that is a photochemical reaction in keratinocytes) and is not involved in touch or collagen synthesis.
Splitting at the lamina lucida reflects failure of hemidesmosomal anchoring of basal keratinocytes, as seen in bullous pemphigoid. Desmosomal defects cause intraepidermal acantholysis, and cornified envelope defects affect the stratum corneum barrier.
The stratum granulosum contains keratohyalin granules, and profilaggrin is processed here into filaggrin, which aggregates keratin filaments. Defects in filaggrin are linked to ichthyosis vulgaris and atopic dermatitis.
Eccrine glands are most numerous on the palms, soles, and forehead and secrete a clear, initially odorless sweat that cools the body. Apocrine glands open into hair follicles and are concentrated in the axillae and groin.
Lamellar bodies secrete ceramides, cholesterol, and free fatty acids into the intercellular space, forming lipid sheets that limit transepidermal water loss. Keratin provides mechanical strength but not the main waterproofing.
Type I collagen is the most abundant collagen in the dermis and accounts for most of the skin's tensile strength. Type III is present in smaller amounts, while types IV and VII are basement membrane and anchoring fibril components.
Hemidesmosomes attach basal keratinocytes to the basal lamina, and anchoring fibrils (type VII collagen) secure the basement membrane to the dermis, together resisting shear stress. Melanosomes transfer pigment, lamellar bodies secrete barrier lipids, and Merkel discs are sensory.
Desmosomes are calcium-dependent junctions composed of cadherins (desmoglein, desmocollin) that link to intracellular keratin intermediate filaments, providing mechanical cohesion. Loss of desmoglein function underlies pemphigus.
Subcutaneous adipose tissue provides thermal insulation, energy storage, and mechanical cushioning. Keratin production occurs in the epidermis, melanin synthesis in melanocytes, and antigen presentation largely by Langerhans cells.
Acantholysis is the loss of intercellular cohesion between keratinocytes, reflecting desmosomal dysfunction as in pemphigus vulgaris. Hemidesmosomes and anchoring fibrils are involved in dermoepidermal separation, not intraepidermal acantholysis.
Melanocytes arise from neural crest cells (neuroectoderm) and migrate to the epidermis. Keratinocytes derive from surface ectoderm, fibroblasts from mesoderm, and eccrine gland cells from ectoderm.
Volar skin is glabrous, lacks hair follicles and sebaceous glands, and experiences greater friction, leading to a thick compact stratum corneum. Melanocyte number and dermal vascularity do not determine corneum thickness.
Gap junctions, formed by connexins, create channels that permit direct transfer of ions and small molecules between cells. Desmosomes and adherens junctions provide adhesion, and hemidesmosomes anchor to the matrix.
Dermal papillae are finger-like upward projections of the papillary dermis that interdigitate with the overlying epidermal rete ridges, increasing the surface area for exchange and adhesion. They are not epidermal projections or glandular structures.
The arrector pili is a smooth muscle bundle attached to the hair follicle that contracts in response to cold or emotion, erecting the hair and producing goosebumps. Sebum is secreted by sebaceous glands and sweat by eccrine/apocrine glands.
The hair bulb at the base of the follicle contains the matrix, a region of rapidly dividing keratinocytes that differentiate into the hair shaft. The infundibulum and isthmus are upper follicle segments, and the shaft is the formed hair.
Type VII collagen forms anchoring fibrils that extend from the lamina densa into the papillary dermis, securing the basement membrane to the dermis. Defects cause dermolytic (dystrophic) epidermolysis bullosa. The lamina lucida and hemidesmosome plaque are distinct components.
The papillary dermis is the superficial dermal layer directly beneath the epidermis; it is thinner, looser, more cellular, and has finer collagen fibers than the deeper reticular dermis. The reticular dermis has thicker bundles and fewer cells.
Dermal fibroblasts synthesize collagen, elastin, and ground substance that form the extracellular matrix and provide structural support. Melanin phagocytosis occurs in keratinocytes, antigen presentation is by Langerhans cells, and keratin is made by keratinocytes.
Apocrine glands are located mainly in the axillae, anogenital region, and areolae, and their ducts empty into hair follicles. Eccrine glands open directly onto the skin surface and are the primary thermoregulatory sweat glands, most numerous on palms and soles.
Filaggrin aggregates keratin and contributes to the cornified envelope and natural moisturizing factor; its loss impairs the epidermal barrier and predisposes to atopic dermatitis and ichthyosis vulgaris. Albinism relates to melanin, collagen fragility to dermal matrix defects, and anhidrosis to gland dysfunction.
Langerhans cells are dendritic antigen-presenting cells of the epidermis that capture and present antigens to T lymphocytes, initiating immune responses. Keratinocytes are the main structural cells, melanocytes produce pigment, and Merkel cells are mechanoreceptors.
The stratum basale is the deepest epidermal layer, consisting of a single row of mitotically active keratinocytes attached to the basement membrane; melanocytes and Merkel cells also reside here. The other layers contain cells that are more differentiated and no longer divide.
The subcutaneous layer (hypodermis) is mainly adipose tissue, which insulates the body and cushions underlying structures. The epidermis and dermis are composed of epithelial and connective tissues, respectively.
Linear IgG and C3 along the basement membrane zone is characteristic of bullous pemphigoid, in which autoantibodies target hemidesmosomal proteins BP180 (type XVII collagen) and BP230. Desmogleins are targeted in pemphigus, and type VII collagen is targeted in epidermolysis bullosa acquisita.
Uremic pruritus is primarily associated with the accumulation of uremic toxins due to impaired renal clearance, which triggers systemic inflammation and pruritus. While dry skin and histamine may contribute, they are not the primary cause.
Wrinkling is primarily due to reduced collagen and elastin in the dermis, leading to loss of elasticity and structural support. Other factors like fat loss and epidermal thinning contribute but are not the main cause.
A painless, indurated ulcer (chancre) with a clean base and positive darkfield microscopy for Treponema pallidum is characteristic of primary syphilis. Secondary syphilis presents with a rash, latent syphilis is asymptomatic, and tertiary syphilis involves gummas or cardiovascular/neurologic manifestations.
Basal cell carcinoma is characterized by mutations in the Hedgehog signaling pathway, most often loss-of-function mutations in PTCH1 or activating mutations in SMO. This leads to constitutive activation of the pathway and tumor growth.
Tuberous sclerosis complex is caused by mutations in TSC1 or TSC2 and is characterized by ash-leaf spots, seizures, and hamartomas. NF1 mutations cause neurofibromatosis type 1 with café-au-lait macules, not ash-leaf spots.
The stratum basale is a single layer of columnar cells attached to the basement membrane; these cells divide and supply keratinocytes to the layers above. The upper layers are progressively more differentiated and eventually shed.
Langerhans cells are bone marrow-derived dendritic cells within the epidermis that capture antigen and migrate to lymph nodes to present it to T cells. Keratinocytes, melanocytes, and Merkel cells serve barrier, pigment, and mechanosensory roles respectively.
Desmosomes link the keratin intermediate filament networks of adjacent keratinocytes and are the principal adhesion structure of the spinous layer. Loss of desmosomal adhesion produces intraepidermal acantholysis and blistering.
Melanin, transferred from melanocytes to keratinocytes in melanosomes, absorbs and scatters ultraviolet radiation and limits UV-induced DNA damage. Vitamin D synthesis, barrier reinforcement, and sebum production are functions of other cells.
Hemidesmosomes anchor basal keratinocytes to the lamina lucida of the basement membrane by linking keratin filaments to integrins and bullous pemphigoid antigens. The lamina densa and anchoring fibrils provide additional support but the basal cell attachment itself depends on hemidesmosomes.
The reticular dermis is the thick, dense irregular connective tissue layer that houses most adnexal structures, including hair follicles and glands. The papillary dermis is the thin superficial layer beneath the epidermis.
A plaque is an elevated, flat-topped lesion larger than 1 cm, which matches the described psoriasiform lesion. Papules are smaller than 1 cm, macules are flat, and vesicles are fluid-filled.
A fissure is a linear split through the epidermis that may extend into the dermis and is typical of dry, thickened skin. Erosions are superficial losses of epidermis, ulcers extend deeper, and excoriations are traumatic scratches.
Seborrheic keratoses typically show comedo-like openings, milia-like cysts, and fissures/ridges on dermoscopy. Arborizing vessels and blue-white veils are features of basal cell carcinoma, and a pigment network suggests a melanocytic lesion.
The Koebner phenomenon is the induction of new lesions of a disease at sites of mechanical trauma, classically seen in psoriasis, lichen planus, and vitiligo. It is distinct from photodistributed eruptions and from drug tolerance.
Improvement after withdrawal (dechallenge) and recurrence after re-exposure (rechallenge) supports a causal relationship between an agent and an eruption. Patch, prick, and photopatch tests are specific diagnostic procedures rather than this reasoning principle.
Topical corticosteroids of appropriate potency are first-line for mild, localized atopic dermatitis, together with emollients and trigger avoidance. Systemic immunosuppressants and oral steroids are reserved for severe or refractory disease.
Lotions and solutions are low-viscosity, spread easily through hair, and allow evaporation, making them suitable for hairy and weeping areas. Ointments and pastes are occlusive and difficult to apply in hair-bearing regions.
Prolonged potent topical steroids thin the epidermis and dermis, producing atrophy, telangiectasia, striae, and steroid-induced rosacea, especially on thin-skinned sites such as the face. The other listed findings are unrelated to steroid use.
Unilateral grouped vesicles in a dermatomal distribution are the classic presentation of herpes zoster due to reactivation of varicella-zoster virus. Pityriasis rosea and tinea corporis do not produce dermatomal vesicles, and erythema multiforme shows target lesions.
Beta-hemolytic streptococci, particularly Streptococcus pyogenes, and Staphylococcus aureus are the leading causes of cellulitis in immunocompetent adults. The other organisms cause distinct clinical syndromes in specific settings.