Chapter 1 Structure and function of human skin Introduction of skin structure Human 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 average area of 1.5 m2. The average thickness is 0. 5-4 mm (not includ ing subcutaneous fat tissue). The skin is thickest on the palms and soles. It is very thin on the eyelid 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 enitalia, anus, and areola The skin is composed of epidermis, dermis, subcutaneous tissue and skin appendag including hair follicles, sebaceous glands, ecrrine glands and apocrine glands, nmailsges on its surface by continuously alternating ridges and sulci, in individually unique roved There is no hair on the palms and soles, so called glabrous skin. Glabrous skin is configurations known as dermatoglyphics. It is characterized by the presence of glands. Hair-bearing skin, on the other hand, has both hair follicles and sebaceous gland h encapsulated sense organs within the dermis, and by a lack of hair follicles and sebaceou but lacks encapsulated sense organs Histology of the skin lEpidermis The 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 D)Keratinocyte es 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 througl 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 tonofilamentsChapter 1 Structure and function of human skin Introduction of skin structure Human 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 skin 1.Epidermis The 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, Langerhan’s cells and Merkel cells. 1) Keratinocyte Keratinocytes 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