Text Book of DermatologyEDITOR-IN-CHIEFXu JinhuaDepartment of DermatologyHuashan HospitalFudan UniversityCONTENTSChapter1 Structure and function of human skinChapter 2 Diagnosis of Skin DiseaseChapter 3 Therapy in Dermatology and VenereologyChapter 4 Fungal DiseasesChapter 5 Common viral diseases of skinChapter 6 ScabiesChapter 7 Sexually Transmitted DiseasesChapter 8 Contact DermatitisChapter 9 NeurodermatitisChapter 10 EczemaChapter 11 UrticariaChapter 12 Drug EruptionChapter 13 Papulosquamous dermatosesChapter 14 Lupus ErythematosusChapter 15 Bullous DermatosesChapter 16 VitiligoChapter 17 AcneChapter 1 Structure and function of human skinIntroduction of skin structureHuman skin is a uniquely engineered organ covering the body. Being the largest organ, the skin provides around 16% of the body mass of an average person, and it covers an average area of 1.5 m2. The average thickness is 0.5-4 mm (not including subcutaneous fat tissue). The skin is thickest on the palms and soles. It is very thin on the eyelid. It performs many vital roles as both a barrier and a regulating influence between the outside world and the controlled environment within our bodies.Skin color differs by race, age, gender and location. It is darker around external genitalia, anus, and areola. The skin is composed of epidermis, dermis, subcutaneous tissue and skin appendages including hair follicles, sebaceous glands, ecrrine glands and apocrine glands, nails. There is no hair on the palms and soles, so called glabrous skin. Glabrous skin is grooved on its surface by continuously alternating ridges and sulci, in individually unique configurations known as dermatoglyphics. It is characterized by the presence of encapsulated sense organs within the dermis, and by a lack of hair follicles and sebaceous glands. Hair-bearing skin, on the other hand, has both hair follicles and sebaceous glands but lacks encapsulated sense organs.Histology of the skin1.EpidermisThe epidermis is the outmost tissue in human beings. Derived from ectoderm, it can be classified as stratified squamous epithelium. Keratinocyte is the major cell, making up 95% of the total. Other cells include melanocytes, Langerhans cells and Merkel cells.1) KeratinocyteKeratinocytes move progressively from attachment to the epidermal basement membrane towards the skin surface, forming several well-defined layers. The differentiation process is called cornification. The epidermis contains five histologically distinct layers from the inside to the outside: stratum basale, stratum spinosum, stratum granulosum, stratum lucidum and stratum corneum:a.The stratum basale (basal cell layer) is a continuous layer that is generally described as only one cell thick, but may be two to three cells thick in glabrous skin and hyperproliferative epidermis. The cells of the basal layer are similar to those of other tissues within the body; they contain the typical organelles such as mitochondria and ribosomes, and the cells are metabolically active. The keratinocytes of the stratum basale are attached to the basement membrane by hemidesmosomes, which act rather like proteinaceous anchors for these lowest layer cells. Within the stratum basale and the adjacent cell layer, the stratum spinosum, keratinocytes are connected through desmosomes, again highly specialised proteinaceous cellular bridges.b.The stratum spinosum (also known as the spinous layer or prickle cell layer) is found on top of the basal layer, and together these two layers are termed the Malpighian layer. This spinous layer consists of two to six rows of keratinocytes that change morphology from columnar to polygonal cells. Within this layer the keratinocytes begin to differentiate and synthesise keratins that aggregate to form tonofilaments.c.The stratum granulosum (or granular layer) is only one to three cell layers thick, the stratum granulosum contains enzymes that begin degradation of the viable cell components such as the nuclei and organelles. The granular cells are so called because they acquire granular structures. Keratohyalin granules mature the keratins within the cell. d.The stratum lucidum is the layer in which the cell nucleus disintegrates. There is an increase in keratinisation of the cells concomitant with further morphological changes such as cell flattening. The stratum lucidum can be found in soles and palms.e.The stratum corneum (or horny layer ) is the final product of epidermal cell differentiation. Typically, the stratum corneum comprises only 10 to 15 cell layers and is around 10 m thick when dry. This thin layer consists of dead, anucleate, keratinised cells embedded in a lipid matrix. The stratum corneum serves to regulate water loss from the body whilst preventing the entry of harmful materials, including microorganisms. Ty