Skin - types, receptors, appendages

The skin (integumentary system) is the largest organ in the human body, accounting for 15%-20% of the body's weight. Contains 2 main layers:

• Epidermis: the epithelial layer originating from the surface ectoderm.
• Dermis: a layer of dense CT originating from the mesoderm.
• Hypodermis (subcutaneous layer): a layer loose connective tissue containing numerous adipocytes – connecting the skin to underlying CT

Main functions of the skin:

Protection:

• Physical and thermal barrier from the outside environment: pathogens and physical conditions
• UV protection: contains melanin pigment that protects cell nuclei from UV radiation.
• Water regulation: Selectively permeable to water and other substances, preventing an excessive loss or uptake of water.

Sensory: contains numerous sensory receptors of many types that monitor the environmental conditions and body interactions with the environment.

Metabolic: cells in the skin has the ability to produce vitamin D using UV radiation - needed for Ca⁺² and bone formation.

Energy source: The skin's adipose tissue

Thermoregulation: The skin is a thermal insulator from the environment. It also has mechanisms like sweating and microvasculature dilation and constriction to regulate body temperature.

Sexual signaling: important for appearance and sexual attraction. Its sweat glands have the ability to secrete sex pheromones.

Epidermis

The skin can be classified into 2 types according to the epidermis thickness:

• Thick skin: found on the palms and soles and it is hairless, epidermis is 400-1400 micrometer.
• Thin skin: found everywhere else, epidermis is 70-140 micron.

The epidermis is composed of stratified squamous keratinized epithelium and can be classified into 5 layers:

• 

  Layers of the skin
  Stratum Basale: contains simple cuboidal layer of live keratinocytes, melanocytes and Merkel cells
• Stratum spinosum: stratified keratinocytes and Langerhans cells
• Stratum granulosum: 3-5 layers of keratinocytes with distinct kerato-hyaline granules
• Stratum lucidum (only in thick skin): 2-3 layers dead keratinocytes
• Stratum corneum (affects most on skin thickness): 20-30 layers of dead keratinocytes

The epidermis contains a few types of cells:

• Keratinocytes: highly specialized epithelial cells, designed to separate the organism from the external environment. The cells differentiation between the layers is called keratinization. Immature cells are basophilic due to large number of active ribosomes involved with keratins synthesize.
• Melanocytes: pigment producing cells – in stratum basale
• Langerhans cells: involved in immune system signaling – in stratum spinosum
• Merkel cells: are associated with sensory nerve endings – in stratum basale

Dermis

A CT layer that binds to the epidermis, a distinct basement membrane separates between the two. Contains epidermal derivatives (sweat glands, hair follicles and sebaceous glands).

The junction between the epidermis and dermis is characterized by dermal papillae formed by the upper layer of the dermis and epidermal ridges formed by the deep parts of the epidermis.

The dermis is divided into 2 layers:

• Reticular layer: thinner, deeper - loose CT layer containing blood vessels, fibroblasts, and macrophages.
• Papillary layer: thicker, superficial - dense irregular CT (more fibers and fewer cells).

A network of elastic fibers is present, providing elasticity to the skin In old people, the elastic fibers gradually degenerate due exposure to the sun – cause the skin to develop wrinkles.

Blood supply to the skin

Subpapillary plexus (superficial): between the papillary and reticular layers,

Deep plexus: larger vessels between the reticular layer and the subcutaneous tissue.

Sensory receptors

The sensory receptors are divided into 2 groups: encapsulated receptors and unencapsulated receptors

Unencapsulated receptors

• Merkel cells: mechanoreceptors of light touch, contain unmyelinated expansions of nerve fibers.
• Free nerve endings: found in the papillary dermis and extend into lower epidermal layers. Responsible for sensing high/low temperatures, pain and itching.
• Root hair plexuses: surround hair follicles in the reticular dermis. Sense hair movement.

Encapsulated receptors

Meissner corpuscles: elliptical structure - sensory axons spiraling among Schwann cells,

• Location: dermal papillae (numerous in the soles and palms, and their number reduces since puberty)
• Function: Create impulses in response to light touch.

Lamellated (Pacinian) corpuscle: oval structure - Schwann cells surrounding an unmyelinated axon

• Location: Deep in the reticular dermis and hypodermis; CT of internal organs as urinary bladder and rectum
• Function: Sensing pressure and vibrations.

Ruffini corpuscles: have collagenous fusiform capsules anchored by surrounding CT, contains sensory axons.

• Function: twisting and stretching of the skin.

Krause end bulbs: simple ovoid structures with very thin collagenous capsules, penetrated by sensory axons.

• Location: mainly in the skin of penis or clitoris
• Function: sense low frequency vibrations

Skin Appendages

Hair follicles and sebaceous glands:

• Hair cells and sebaceous glands combine to form a pilosebaceous unit (only found on hairy skin)
• Sebaceous gland release theur glandular secretions via a holocrine mechanism into the hair follicle shaft. The hair follicle itself is associated with an arrector pili muscle (which contracts to make the follicle stand up straight).

Hair follicles:

Dermal papillae – Inserts onto base of bulb containing a capillary network sustaining follicle.

Hair root – Epidermal cells covering dermal papilla

Hair shaft – Part of hair protruding beyond skin surface

Internal root sheath – Outermost cells of hair bulb completely surrounding initial part of hair root but degenerating above level of attached sebaceous gland.

External root sheath – Covers internal sheath extending all the way to epidermis where it becomes continuous with basal and spinous layers

Glassy membrane – basement membrane separating hair follicle from dermis and surrounding dermis forms a CT sheath

Arrector pili smooth muscle – attach to sheath contraction pulls hair to more erect position

Nail:

Nail root – proximal part of nail

Proximal skin fold – covers nail root

Cuticle (eponychium) – extension of stratum corneum from proximal nail fold

Nail bed – bed of epidermis that nail body/plate is bound too

Nail matrix – where the nail plate/body arises from

Nail plate – mature part of nail

Sweat glands:

2 main types of sweat glands:

1.) Eccrine glands: Major sweat glands of human boyd. They release a cear, odourless substance, comprised mostly of sodium chloride and water (involved in thermoregulation)

2.) Apocrine glands: Larger sweat glands, located in the axillary and genital regions. Products secreted can be broken down by microbes, producing body odour.