Skin and skin appendage
Overview
- The skin is the largest organ of the body, covering an area of roughly 2 m2.
- The skin is composed of the cutis (including the dermis, epidermis, subcutaneous tissue), and skin appendages.
- The epidermis is derived from ectoderm. It is the outermost layer of the skin, and it is mainly composed of keratinocytes.
- The dermis is derived from mesoderm. It is located underneath the epidermis and is mainly composed of elastic fibers, type I collagen, and connective tissue. It is formed by the papillary dermis and the reticular dermis.
- The main functions of the skin are protection (barrier against ultraviolet radiation, microorganisms, and water loss), synthesis of vitamin D, sensation (e.g., touch, temperature, pain), and regulation of body temperature.
Structure of the skin
Microscopic skin structure
The skin is composed of several layers, which are categorized as follows (from superficial to deep):
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Cutis
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Epidermis
- Nonvascularized
- Consists of five layers
- Maintains the skin's barrier function
- For more details, see layers of the epidermis.
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Dermal-epidermal junction
- Consists of a basement membrane that anchors the epidermis to the dermis
- The basement membrane consists of two layers:
- Lamina lucida (superficial)
- Lamina densa (deeper)
- The basement membrane consists of two layers:
- Hemidesmosomes: connect epidermis to basal cell layer
- Collagen fibrils: connect basal cell layer to papillary layer
- Consists of a basement membrane that anchors the epidermis to the dermis
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Dermis: A layer derived from mesoderm that contains blood vessels and provides structural integrity.
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Papillary dermis
- Consists of fine, loosely arranged collagen fibers
- Supplies the epidermis with nutrients
- Important role in temperature regulation
- Form dermal ridges, which connect dermal papillae with downgrowths of epidermis called epidermal ridges, or rete ridges.
- Contains Meissner corpuscles (for fine-touch, and two-point discrimination receptors), and free nerve endings
- Contains immune cells (mast cells and macrophages)
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Reticular dermis
- Consists of thick, densely packed fibers (e.g., reticular, elastic, and collagenous) that provide structure and support to the skin, and its components
- Contains elastic fibers that provide elasticity
- Contains Pacinian corpuscles (pressure receptors)
- Contains roots of hair follicles, sweat glands, and sebaceous glands.
- Langer's lines: Topographic lines that correlate with natural orientation of reticular fibers of the reticular dermis. Are of importance in wound healing, and to guide surgical incisions.
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Papillary dermis
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Epidermis
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Subcutaneous tissue
- Also called hypodermis.
- Derived from mesoderm.
- Consists mainly of fat that protects from cold and trauma.
- Contains superficial veins and free nerve endings
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Lamellar corpuscles primarily located in this layer
- Responsible for vibration and pressure sensation.
- Contains Ruffini corpuscle (mechanoreceptors).
- Responsible for mechanical pressure and distortion sensation
- Collagenous and elastic fibers in this area function to anchor the skin (epidermis, dermis) to the deep fascia.
- Allows for subcutaneous administration of medications.
- Skin appendage: hair, nails, glands (e.g., sweat glands, sebaceous glands)
Layers of the epidermis
The epidermis is the outer-most layer of the skin. The epidermis is derived from ectoderm; . It is primarily composed of keratinocytes, which constantly regenerate approximately every 30 days. The epidermis is composed of 4–5 layers, which are categorized as follows (from superficial to deep):
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Stratum corneum: outer layer of the epidermis
- Consists of dead (anuclear), keratin-filled cells
- This layer is constantly being sloughed off.
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Stratum lucidum: thin, translucent layer
- Found only on thick skin (palms and soles).
- Composed of a homogeneous layer of keratinocytes with no nuclei or organelles.
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Stratum granulosum: also called the granular layer
- Contains keratohyalin.
- This layer has waterproof properties.
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Stratum spinosum
- Composed of actively dividing keratinocytes with spinous-like projections (prickle cells).
- This layer produces keratin and induces keratinization.
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Stratum basale: also called the basal cell layer
- Stem cells of the epidermis (daughter cells migrate upwards and differentiate into other cells).
Carolina Loves Guys that are Super Billionaires! (Corneum, Lucidum, Granulosum, Spinosum, Basalis)
References:[1][2][3][4][5]
Cells of the skin
Cells of the epidermis
The epidermis is mainly composed of four different type of cells:
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Keratinocytes
- Definition: principal cell of the epidermis
- Location: throughout the whole epidermis
- Mitotically active in the basal layer (basal cells)
- Connected to the basement membrane via hemidesmosomes
- Function
- Primarily protection against environmental damage and to form a barrier for pathogens
- Protects against ultraviolet (UV) radiation by storing melanin
- Produce proinflammatory mediators to activate the immune system when exposed to pathogens
- Aid in the process of wound healing: the stratum basale is regenerative, it proliferates to fill skin defects.
- Form tight junctions with nerve endings, and hold lymphocytes and Langerhans cells in place.
- Clinical significance:
- Abnormal keratinocyte desquamation contributes to development of acne
- Sunburn: apoptosis of keratinocytes
- Blistering skin disorders (e.g, pemphigus vulgaris, staphylococcal scalded skin syndrome)
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Melanocytes
- Definition: melanin-producing cells derived from neural crest
- Location: stratum basalis
- Function: melanin synthesis
- Melanin is stored in melanosomes (organelles present within melanocytes).
- Dendritic processes transfer melanosomes to adjacent and distant keratinocytes.
- Melanocytes are stimulated by MCH, ACTH, and by sunlight (UV-A).
- Clinical significance
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Langerhans cells (macrophages of the skin): Dendritic cells primarily located in the stratum spinosum.
- Langerhans cells act as antigen-presenting cells (APC).
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Cells express:
- MHC class II
- Fc receptors
- S-100
- CD1a
- Mesodermal origin
- Stain positive for S-100 and CD
- Contain Birbeck granules
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Clinical significance
- Langerhans cell histiocytosis
- HIV transmission
- HPV transmission
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Merkel cells: mechanoreceptor for light touch and sensation
- Cells rich in keratin filaments, desmosomes, and small dense blue granules
- Mainly located in the stratum basale
- Surround richly innervated areas of connective tissue such as fingertips.
Sensory receptors of the skin
Receptor | Type of fibers | Function |
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Ruffini corpuscles |
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Meissner corpuscles |
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Pacinian corpuscles |
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Merkel discs |
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Free nerve endings |
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Skin appendage
Overview
The skin appendages include:
Nails
- Perionychium: epidermal tissue surrounding the root and base of the nail
- Eponychium: proximal layer of epidermis extending over the nail base
- Hyponychium: epidermal tissue immediately underlying the free distal edge of the nail
- Nail plate (nail body)
- Nail fold: depression proximal to nail plate from which the nail grows
- Vascular supply
- Innervation: trifurcation of the dorsal volar digital nerve (supplying the nail fold, pulp, and distal tip of the finger)
Hair follicle
Invaginations of the epidermis and hair shaft into the deep dermis. It is composed of:
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Hair
- A skin appendage that grows from follicles in the dermis
- Divided into medulla, cortex, and cuticle
- Functions include conservation of body heat, sensation, and protection of the skin.
- Most prominent on the scalp, pubis, axilla, extremities, and face
- See also phases of hair growth.
- Hair shaft
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Hair bulb
- Root of the hair follicle, located deep in the dermal papilla
- Receives blood and nutrients from capillary network of vessels to sustain hair growth
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Internal root sheath
- Composed of the Henle layer, Huxley layer, and cuticle.
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External root sheath
- Contains the stratum malpighii (composed of the stratum basale and stratum spinosum).
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Arrector pili muscle
- Description: Obliquely directed smooth muscle fibers that attach to the dermal sheath surrounding hair follicles.
- Function
- Contraction is responsible for piloerection (goosebumps).
- Conservation of body heat.
- Innervated by the sympathetic autonomic nervous system.
- Hair disorders: see learning card on alopecia.
Glands
Sebaceous gland | Sweat glands | ||
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Eccrine sweat gland | Apocrine sweat gland | ||
Description |
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Distribution |
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Function |
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References:[6]
Functions of the skin
The main functions of the skin include:
- Barrier and protection against ultraviolet (UV) radiation and the external environment.
- Thermoregulation (e.g., perspiration, regulation of blood circulation)
- Sensory function: Sensation of pressure, vibration, touch, pain, and temperature.
- Hormone synthesis of vitamin D
- Melanin synthesis
Clinical significance
Common skin disorders
Vascular tumors of the skin
Viral infections of the skin
Bacterial infections of the skin
Fungal infections of the skin
Bullous skin disorders
- Bullous pemphigoid
- Dermatitis herpetiformis
- Pemphigus vulgaris
- Stevens-Johnson syndrome
- Toxic epidermal necrolysis
Malignant and premalignant skin lesions
- Actinic keratosis
- Melanoma
- Squamous cell carcinoma
- Basal cell carcinoma
Disorders of pigmentation
Other skin disorders
- Acanthosis nigricans
- Erythema nodosum
- Lichen planus