10.0 SKIN: INTRODUCTION
The skin comprises the following structural elements:
10.2 - images: Epidermis
In the embryonal/fetal period a single layer of cuboidal ectodermal cells represents the epidermis, these cells proliferate and form a second layer of squamous cells on top the so-called periderm. Cells of the periderm undergo keratinization and desquamation. The cuboidal cells become the basal layer cells as a source of new proliferating cells for the intermediate layer directly below the periderm layer.
See: 10.2 POJA-L2115 et al.
The adult skin comprises a superficial keratinizing epithelium (epidermis), a deep fibroelastic layer (dermis), together referred as cutis and subsequently a layer of connective tissue and adipose tissue (subcutis) as a connection with the underlying structures. The epidermis consists of slight (in thin skin) or heavily keratinized squamous epithelium (in thick skin): strata basale, spinosum, granulosum, corneum. The keratinized superficial cells of the epidermis are involved in some of the main functions of the skin such as a barrier against bacterial invasion, physical-/chemical-/thermal traumas a.o. They also regulate heat loss and prevent abnormal dehydration as well.
See: 10.2 POJA-L2081; 10.2 POJA-L2124; 10.2 POJA-L2127+2524; 10.2 POJA-L2129 et al; 10.2 POJA-L2007 et al; 10.2 POJA-L2125 et al; 10.2 POJA-L2138 et al.
Within the epidermis three populations of characteristic cells are always present: the melanocytes, Langerhans’ cells and Merkel cells.
See: 10.2 POJA-L2131 et al; 10.2 POJA-L2133 et al; 10.2 POJA-L2129 et al; 10.2 POJA-L2155 et al;
10.2 POJA-L2160+2161.
10.3 - images: Dermis and subcutis
The dermis is divided in the superficial stratum papillare (with sensoric receptors) and the deeper stratum reticulare with thicker connective tissue. The subcutis contains accumulations of univacuolar adipocytes, glands, blood vessels, nerve fibers, sensoric receptors and region-dependent hair follicles.
The dermis is composed of fibroelastic connective tissue and is thick in areas with a thick corneal layer (thick or glabrous skin) or thin in areas with a thin corneal layer (thin or hirsute skin).
It provides mechanical strength to the skin by the arrangement and number of the collagen/elastic fibres.
See: 10.3 POJA-L2053 et al; 10.3 POJA-L2184 et al; 10.3 POJA-L2217 et al; 10.3 POJA-L2234+2238;
10.3 POJA-L2256 et al; 10.3 POJA-L2258 et al; 10.3 POJA-L2940+2529.
Sensoric receptors, See: 10.3 POJA-L2037 et al; 10.3 POJA-L2038 et al; 10.3 POJA-L2274 et al.
10.4 - images: Glands
Characteristic glands usually occur in the subcutis among which the common eccrine sweat glands (sudorific glands) and holocrine sebaceous glands with their excretory ducts draining through the epidermis. Sweat glands develop as solid downgrowths of epidermal cells into the mesenchyme (future dermis). Upon elongation the ends coil and form the secretory portion of the gland. Their secretion is boosted by temperature as well as by emotional impulses due to sympathetic innervation of the glands. The watery secretion product is odourless and clear, rich in sodium, chloride as well as lactate, urea a.o. Under influence of aldosterone the lining cells of the excretory duct modify the composition of the secretion product. Most of the sebaceous glands develop as buds from a developing root sheath of a hair follicle, their outgrowths form alveoli where the centrally the cells degenerate resulting in the so-called sebum. The sebum is a lubricant for skin and hair, protects the skin against hot temperature and is involved in regulation of the body temperature. Solitary sebaceous glands independent of hair follicles similarly form buds directly from the epidermis. The apocrine glands (odoriferous glands) are specialized hormone-driven sweat glands (involved in pheromone metabolism) and are localized in circumscript skin regions of the body such as axilla, scrotum, labia minora, areola mammae. Their secretion products are proteinaceous, under bacterial influence become odorous and contain short-chain fatty acids.
See: 10.4 POJA-L2024 et al; 10.4 POJA-L2026 et al; 10.4 POJA-L2071 et al; 10.4 POJA-L2085 et al;
10.4 POJA-L2197+2198; 10.4 POJA-L2213 et al; 10.5 POJA-L2055 et al.
10.5 - images: Hairs and nails
Hair follicles start as downgrowths of solid epidermal strands into the dermis. The lower end forms a bulb (hair papilla) and dermal structures invade the papilla from beneath and form the dermal papilla.
See: 10.5 POJA-L2090 et al.
In the center of the hair papilla the epidermal cells form layers similar to the strata spinosum, granulosum, corneum in the surface epidermis. At the base of the bulb (the germinative matrix of the hair papilla) the central cells keratinize and form the hair proper while the peripheral cells form within outward cuticle layers, inner root sheath layers and outer root sheath layers. As a result of multiplication of the matrix cells the keratinized centre of the epidermal downgrowth is gradually pushed toward the surface of the skin and eventually emerges as the free, expelled hair or pilus (adapted from Rhodin, 1974). In adulthood generally hair follicles are localized in the subcutis and hair appears to contain non-desquamating cells filled with hard fibrous keratin such in contrast with the neighbouring desquamating cells with soft fibrous keratin. Each hair follicle is surrounded by a connective tissue sheath where the arrector pili muscle inserts as well as peritrichial nerve fibres. Associated sebaceous glands discard their sebum into the distal part of the hair follicle.
See: 10.5 POJA-L2028 et al; 10.5 POJA-L2039+2034; 10.5 POJA-L2055 et al; 10.5 POJA-L2068 et al; 10.5 POJA-L2163 et al; 10.5 POJA-L2170 et al; 10.5 POJA-L2173 et al.
Hair grows continuously over certain periods (anagen phase). After reaching the decisive length the growth process stops (resting phase or telogen phase). The process of hair replacement begins with the arrest of hair growth and subsequently the shedding of that particular hair. The catagen phase starts at the moment cell division stops in the hair papilla matrix. With exception of the matrix cells all other cells in the papilla keratinize resulting in the formation of a so-called club hair. Eventually the matrix cells divide but do not differentiate, together with shrinking processes in the outer root sheath and degeneration of the whole inferior segment of the follicle the club hair is pushed out. Later on cells from the same epithelial base proliferate again to form a strand of epithelial cells and the new hair is formed in the same way as described under downgrowths of hair follicles. The new hair replaces the old club-hair by expelling it.
See: 10.5 POJA-L2172 et al.
The nail (unguis) is a plate of specialized stratum corneum that consist of hard keratin (similar as found in the hair cortex). Its function is protection of the dorsal surface of the distal phalanx. The semitransparent nail plate consists of layers flattened keratinized cells that do not desquamate. It rests on a modified epidermis, the nail bed. The proximal edge is the nail root and is embedded in a fold of the epidermis (nail fold). Beneath the nail root the proximal nail bed appears thicker and is termed nail matrix (with continuous proliferation and growth of matrix cells). It is similar to an active stratum germinativum of the common epidermis. Newly formed cells are added to the nail root by formation of hard keratin without the production of keratohyalin granules.
See: 10.6 POJA-L2074 et al. (outgrowth of keratinized layers of dorsal epidermis over proximal part of nail plate = eponychium or cuticle; epidermal thickening under free edge of nail plate = hyponychium; nail plate above nail matrix = non-translucent lunula).
10.6 - images: Skin areas in human
The adult skin reveals many local variations in thickness of the epidermis, degree of pigmentation, length and amount of hairs, frequency and types of glands, amount of vascularization or innervation etc. etc. For the so-called differential diagnosis of various characteristic skin areas the above-mentioned items has to be taken into account, e.g.:
10.7 - images: Pathology.
Common wart: See: 10.7 POJA-L4341 et al; 10.7 POJA-L4343 et al.
Seborrheic keratosis: See: 10.7 POJA-L4347 et al; 10.7 POJA-L4348 et al.
Basal cell-carcinoma: See: 10.7 POJA-L4354 et al; 10.7 POJA-L4357 et al.
Melanoma: See: 10.7 POJA-L2018 et al; 10.7 POJA-L4367+4368.
The mamma glands are located in the section (7.4) Female Organs/mammary glands.
10.2 - images: Epidermis
In the embryonal/fetal period a single layer of cuboidal ectodermal cells represents the epidermis, these cells proliferate and form a second layer of squamous cells on top the so-called periderm. Cells of the periderm undergo keratinization and desquamation. The cuboidal cells become the basal layer cells as a source of new proliferating cells for the intermediate layer directly below the periderm layer.
See: 10.2 POJA-L2115 et al.
The adult skin comprises a superficial keratinizing epithelium (epidermis), a deep fibroelastic layer (dermis), together referred as cutis and subsequently a layer of connective tissue and adipose tissue (subcutis) as a connection with the underlying structures. The epidermis consists of slight (in thin skin) or heavily keratinized squamous epithelium (in thick skin): strata basale, spinosum, granulosum, corneum. The keratinized superficial cells of the epidermis are involved in some of the main functions of the skin such as a barrier against bacterial invasion, physical-/chemical-/thermal traumas a.o. They also regulate heat loss and prevent abnormal dehydration as well.
See: 10.2 POJA-L2081; 10.2 POJA-L2124; 10.2 POJA-L2127+2524; 10.2 POJA-L2129 et al; 10.2 POJA-L2007 et al; 10.2 POJA-L2125 et al; 10.2 POJA-L2138 et al.
Within the epidermis three populations of characteristic cells are always present: the melanocytes, Langerhans’ cells and Merkel cells.
See: 10.2 POJA-L2131 et al; 10.2 POJA-L2133 et al; 10.2 POJA-L2129 et al; 10.2 POJA-L2155 et al;
10.2 POJA-L2160+2161.
10.3 - images: Dermis and subcutis
The dermis is divided in the superficial stratum papillare (with sensoric receptors) and the deeper stratum reticulare with thicker connective tissue. The subcutis contains accumulations of univacuolar adipocytes, glands, blood vessels, nerve fibers, sensoric receptors and region-dependent hair follicles.
The dermis is composed of fibroelastic connective tissue and is thick in areas with a thick corneal layer (thick or glabrous skin) or thin in areas with a thin corneal layer (thin or hirsute skin).
It provides mechanical strength to the skin by the arrangement and number of the collagen/elastic fibres.
See: 10.3 POJA-L2053 et al; 10.3 POJA-L2184 et al; 10.3 POJA-L2217 et al; 10.3 POJA-L2234+2238;
10.3 POJA-L2256 et al; 10.3 POJA-L2258 et al; 10.3 POJA-L2940+2529.
Sensoric receptors, See: 10.3 POJA-L2037 et al; 10.3 POJA-L2038 et al; 10.3 POJA-L2274 et al.
10.4 - images: Glands
Characteristic glands usually occur in the subcutis among which the common eccrine sweat glands (sudorific glands) and holocrine sebaceous glands with their excretory ducts draining through the epidermis. Sweat glands develop as solid downgrowths of epidermal cells into the mesenchyme (future dermis). Upon elongation the ends coil and form the secretory portion of the gland. Their secretion is boosted by temperature as well as by emotional impulses due to sympathetic innervation of the glands. The watery secretion product is odourless and clear, rich in sodium, chloride as well as lactate, urea a.o. Under influence of aldosterone the lining cells of the excretory duct modify the composition of the secretion product. Most of the sebaceous glands develop as buds from a developing root sheath of a hair follicle, their outgrowths form alveoli where the centrally the cells degenerate resulting in the so-called sebum. The sebum is a lubricant for skin and hair, protects the skin against hot temperature and is involved in regulation of the body temperature. Solitary sebaceous glands independent of hair follicles similarly form buds directly from the epidermis. The apocrine glands (odoriferous glands) are specialized hormone-driven sweat glands (involved in pheromone metabolism) and are localized in circumscript skin regions of the body such as axilla, scrotum, labia minora, areola mammae. Their secretion products are proteinaceous, under bacterial influence become odorous and contain short-chain fatty acids.
See: 10.4 POJA-L2024 et al; 10.4 POJA-L2026 et al; 10.4 POJA-L2071 et al; 10.4 POJA-L2085 et al;
10.4 POJA-L2197+2198; 10.4 POJA-L2213 et al; 10.5 POJA-L2055 et al.
10.5 - images: Hairs and nails
Hair follicles start as downgrowths of solid epidermal strands into the dermis. The lower end forms a bulb (hair papilla) and dermal structures invade the papilla from beneath and form the dermal papilla.
See: 10.5 POJA-L2090 et al.
In the center of the hair papilla the epidermal cells form layers similar to the strata spinosum, granulosum, corneum in the surface epidermis. At the base of the bulb (the germinative matrix of the hair papilla) the central cells keratinize and form the hair proper while the peripheral cells form within outward cuticle layers, inner root sheath layers and outer root sheath layers. As a result of multiplication of the matrix cells the keratinized centre of the epidermal downgrowth is gradually pushed toward the surface of the skin and eventually emerges as the free, expelled hair or pilus (adapted from Rhodin, 1974). In adulthood generally hair follicles are localized in the subcutis and hair appears to contain non-desquamating cells filled with hard fibrous keratin such in contrast with the neighbouring desquamating cells with soft fibrous keratin. Each hair follicle is surrounded by a connective tissue sheath where the arrector pili muscle inserts as well as peritrichial nerve fibres. Associated sebaceous glands discard their sebum into the distal part of the hair follicle.
See: 10.5 POJA-L2028 et al; 10.5 POJA-L2039+2034; 10.5 POJA-L2055 et al; 10.5 POJA-L2068 et al; 10.5 POJA-L2163 et al; 10.5 POJA-L2170 et al; 10.5 POJA-L2173 et al.
Hair grows continuously over certain periods (anagen phase). After reaching the decisive length the growth process stops (resting phase or telogen phase). The process of hair replacement begins with the arrest of hair growth and subsequently the shedding of that particular hair. The catagen phase starts at the moment cell division stops in the hair papilla matrix. With exception of the matrix cells all other cells in the papilla keratinize resulting in the formation of a so-called club hair. Eventually the matrix cells divide but do not differentiate, together with shrinking processes in the outer root sheath and degeneration of the whole inferior segment of the follicle the club hair is pushed out. Later on cells from the same epithelial base proliferate again to form a strand of epithelial cells and the new hair is formed in the same way as described under downgrowths of hair follicles. The new hair replaces the old club-hair by expelling it.
See: 10.5 POJA-L2172 et al.
The nail (unguis) is a plate of specialized stratum corneum that consist of hard keratin (similar as found in the hair cortex). Its function is protection of the dorsal surface of the distal phalanx. The semitransparent nail plate consists of layers flattened keratinized cells that do not desquamate. It rests on a modified epidermis, the nail bed. The proximal edge is the nail root and is embedded in a fold of the epidermis (nail fold). Beneath the nail root the proximal nail bed appears thicker and is termed nail matrix (with continuous proliferation and growth of matrix cells). It is similar to an active stratum germinativum of the common epidermis. Newly formed cells are added to the nail root by formation of hard keratin without the production of keratohyalin granules.
See: 10.6 POJA-L2074 et al. (outgrowth of keratinized layers of dorsal epidermis over proximal part of nail plate = eponychium or cuticle; epidermal thickening under free edge of nail plate = hyponychium; nail plate above nail matrix = non-translucent lunula).
10.6 - images: Skin areas in human
The adult skin reveals many local variations in thickness of the epidermis, degree of pigmentation, length and amount of hairs, frequency and types of glands, amount of vascularization or innervation etc. etc. For the so-called differential diagnosis of various characteristic skin areas the above-mentioned items has to be taken into account, e.g.:
- The axilla is distinguished by a thin stratum corneum, distinct subcutaneous tissue, hairs (hirci) and sweat glands as well as apocrine glands.
- The scalp is distinguished by a thin stratum corneum, distinct subcutaneous adipose tissue, a vast amount of hair (capilli) with associated sebaceous glands and many sweat glands. See 10.6 POJA-L2049 et al.
- The finger is distinguished by a very thick stratum corneum, a well- developed subcutis with adipose tissue and many sweat glands.
10.7 - images: Pathology.
Common wart: See: 10.7 POJA-L4341 et al; 10.7 POJA-L4343 et al.
Seborrheic keratosis: See: 10.7 POJA-L4347 et al; 10.7 POJA-L4348 et al.
Basal cell-carcinoma: See: 10.7 POJA-L4354 et al; 10.7 POJA-L4357 et al.
Melanoma: See: 10.7 POJA-L2018 et al; 10.7 POJA-L4367+4368.
The mamma glands are located in the section (7.4) Female Organs/mammary glands.
COPYRIGHTS
All rights reserves worldwide for the POJA collection are hold by L.G. Poels and P.H.K. Jap and Radboud University Medical Center (Radboud UMC Nijmegen). No one may modify, copy, distribute, transmit, display, or publish any materials contained in the POJA collection without prior written permission of the authors Poels and Jap or the UMC St Radboud. Any commercial use of the POJA collection is forbidden. POJA images are partly deposited in the Health Education Assets Library (HEAL database) as well as in the MedEdPortal/AAMC database).
All rights reserves worldwide for the POJA collection are hold by L.G. Poels and P.H.K. Jap and Radboud University Medical Center (Radboud UMC Nijmegen). No one may modify, copy, distribute, transmit, display, or publish any materials contained in the POJA collection without prior written permission of the authors Poels and Jap or the UMC St Radboud. Any commercial use of the POJA collection is forbidden. POJA images are partly deposited in the Health Education Assets Library (HEAL database) as well as in the MedEdPortal/AAMC database).