- 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.
The skin is composed of several layers, which are categorized as follows (from superficial to deep):
- Consists of five layers
- Maintains the skin's barrier function
- For more details, see .
- Dermal-epidermal junction
Dermis: A layer derived from mesoderm that contains blood vessels and provides structural integrity.
- 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)
- 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.
- Papillary dermis
- Also called hypodermis.
- Derived from mesoderm.
- Consists mainly of fat that protects from cold and trauma.
- Contains superficial veins and free nerve endings
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)
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):
Stratum corneum: outer layer of the epidermis
- Consists of dead (anuclear), keratin-filled cells
- This layer is constantly being sloughed off.
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.
Stratum granulosum: also called the granular layer
- Contains keratohyalin.
- This layer has waterproof properties.
- Composed of actively dividing keratinocytes with spinous-like projections (prickle cells).
- This layer produces keratin and induces keratinization.
- Stratum basale: also called the basal cell layer
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The epidermis is mainly composed of four different type of cells:
- Definition: principal cell of the epidermis
- Location: throughout the whole epidermis
- 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:
- Definition: -producing cells derived from neural crest
- Location: stratum basalis
- Function: melanin synthesis
- Clinical significance
- Langerhans cells (macrophages of the skin): Dendritic cells primarily located in the stratum spinosum.
- Merkel cells: mechanoreceptor for light touch and sensation
|Receptor||Type of fibers||Function|
|Ruffini corpuscles|| || |
|Meissner corpuscles|| || |
|Pacinian corpuscles|| || |
| || |
|Free nerve endings|| |
The skin appendages include:
- 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 shaft
- Hair bulb
Internal root sheath
- Composed of the Henle layer, Huxley layer, and cuticle.
- External root sheath
- Arrector pili muscle
- Hair disorders: see learning card on .
|Sebaceous gland||Sweat glands|
|Eccrine sweat gland||Apocrine sweat gland|
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