BL L21 Flashcards
Structure of skin
Three ‘main’ layers:
- Epidermis
- Dermis
- Hypodermis
4 ot 5 layers:
5 layers - only palms and soles of feet
Epidermis layers: Come to in more depth later
- St. Corneum (corny layer)
- St. Lucidum
- St. Granulosum (granular layer)
- St. Spinosum (Spiney layer)
- St. Basale (Basal layer)
No St. Lucidum in all other places
Hypodermis - thickness
The thickness of the hypodermis varies in different regions of the body and can vary considerably between different people
Thickness plays an important role in distinguishing between males and females:
- In men-hypodermis is thickest in the abdomen and shoulders
- In women-thickest in the hips, thighs, and buttocks
- Also relatively thick on the palms of the hands –both sexes
Hypodermis - location
- Lowest layer of skin
- Also known as sub-cutaneous layer (so sometimes not considered to be part of the skin)
Hypodermis - structure
- Mainly contains adipose tissue (some neurovascular bundles(nerves and blood vessels)/lymphatics)
- Loose connective tissue –fibroblasts/ macrophages/fibres
Hypodermis - Function
• Provides energy store –generate heat (lots of adipose) this layer passes this energy store to underlying skeletal muscles, these then use it as a fuel and to generate heat)
• Insulator for underlying muscle heat generation
• Shock absorber –cushions impacts/protection for underlying structures
e.g. think about fat on the buttocks when sitting (protects us from damage)
• Connects skin to underlying muscle and bones
• Makes hormones, e.g. leptin to control eating habits (adipose controls hormones like leptin, usually when leptin is low = fat is high)
What happens to the hypodermis as you age?
Thinning of this layer during ageing is responsible for skin wrinkling and hypothermia in old age
Dermis - location
- Between epidermis and hypodermis
- Also known as the corium AND with epidermis -cutis
Dermis - Structure
• Two layers
Papillary -upper (P on pic)
Reticular -lower (R on pic)
Dermal papillae - interdigitating (indignations with the epithelial tissue above - with the epilthelial pegs (E on pic)
• Variable thickness 0.6 mm on eyelid; 3 mm on hands/feet
- Epithelial pegs - holds to eperdmis close to the dermis. These are epidermis extensions. Dermal papilla - projections of the corium or dermis that interlock with the overlaying epidermis.
Epidermis - Location
Outermost layer made of epithelial cells (keratinocytes)
Epidermis - Structure
- Four layers of cells (thin skin)
- Five layers (thick skin)
- Held together (laterally) by adherensjunctions
- Held together (vertically) by desmosomes
- Some terminal nerve endings (up to the bottom basal layer)
- NO blood vessels (no blood vessels in the epithelial tissues)
Epidermis - Functions
- Prevents water loss
- Prevents entry to bacteria and parasites
- Special cells that present pathogens to immune cells (langerhans cells)
- Synthesis of keratin
- Prevents underlying tissue loss due to abrasion (keratin’s function)
- Allows gaseous exchange (skin doesn’t have blood vessels, but epidermis is exposed to the air, this means skin cells in the epidermis can exchange some gas with the otuside of the body (e.g. will absorb some oxygen if needed, and will expel CO2), can’t absorb enough O2 through the skin though !!
Cells of epidermis
- Keratinocytes: Almost 90% of the epidermis is made of cells known as keratinocytes. Keratinocytes develop from stem cells at the base of the epidermis and begin to produce and store the protein keratin. Keratin makes the keratinocytes very tough, scaly and water-resistant.
- Melanocytes: At about 8% of epidermal cells, melanocytes form the second most numerous cell type in the epidermis. Melanocytes produce the pigment melanin to protect the skin from ultraviolet radiation and sunburn.
- Langerhans cells: These are the third most common cells (just over 1% of all epidermal cells). These are star shaped cells dispersed singly throughout the upper part of stratum spinosum. Langerhans cells’ role is to detect and fight pathogens that attempt to enter the body through the skin. Thus they are components of cell mediated immunity.
- Merkel cells: Finally, Merkel cells make up less than 1% of all epidermal cells but have the important function of sensing touch as mechanoreceptors. Merkel cells form a disk along the deepest edge of the epidermis where they connect to nerve endings in the dermis to sense light touch.
Layers in the epidermis and a discription of each
In most of the body, the epidermis is arranged into 4 distinct layers. In the palmar surface of the hands and plantar surface of the feet, the skin is thicker than in the rest of the body and there is a fifth layer of epidermis.
- Stratum basale or stratum germinativum: The deepest region of the epidermis is the stratum basale or stratum germinativum, which contains the stem cells that undergo mitotic activity to form all of the other cells of the epidermis. The cells gradually form all other layers of epidermis. The cells of the stratum basale include cuboidal keratinocytes, melanocytes, and Merkel cells.
-
Stratum spinosum:
Superficial to stratum basale is the stratum spinosum layer where Langerhans cells are found along with many rows of spiny keratinocytes. The spines found here are cellular projections called desmosomes that form between keratinocytes to hold them together and resist friction. -
Stratum granulosum:
Just superficial to the stratum spinosum is the stratum granulosum, where keratinocytes begin to produce waxy lamellar granules to waterproof the skin. The layer is so named because of the granules present in the cell cytoplasm. The keratinocytes in the stratum granulosum are so far removed from the dermis that they begin to die from lack of nutrients. -
Stratum lucidum:
In the thick skin of the hands and feet, there is a layer of skin superficial to the stratum granulosum known as the stratum lucidum. It is made of several rows of clear, dead keratinocytes that protect the underlying layers. -
Stratum corneum:
The outermost layer of skin is the stratum corneum. The stratum corneum is made of many rows of flattened, dead keratinocytes that protect the underlying layers. Dead keratinocytes are constantly being shed from the surface of the stratum corneum and being replaced by cells arriving from the deeper layers.
Epidermis layers - how they are held together
The squames are held together by desmosomes (between the different layers). These desmosomes and the tonofibirls which are between the squames layers act as the water barrier.
pH:
Going up the squames layer the pH changes from pH 6.8-7.5 to 4.5-5.3 (more acidic). As the tonofilbirls and desmosomes are subject to the change of pH, the filaggrin proteins that are holding the desmosomes together are destroyed by proteases. As they do this, they break the filaggrin protein down and release the squames. This is why we are constantly losing the top layer of the skin.
Dust – squames from skin.
Layers in the epidermis
Keratin synthesis
- Basal lamina has the basal cells attached. These basal cells divide, they divide to produce a spinous cell
- Spinous cells produce lamina bodies, which is where the keratin forming factory is. (Tonofilaments are already made, they are sitting in the cytoplasm of the basal cell.)
- The lamellar body starts to package the keratin filaments together. It does this on RER.
- The spinous cells then develop into a granular cell. The granular cell has lots of golgi apparatus which starts to package the keratin and keratin hyaline into granules. Kertain hyaline is a clear substance (can’t see it in H+E staining).
- Keratin and keratin hyaline is released outside of the granular cell (this granular layer of filaments becomes clear). This is why the stratum lucidum is clear.
- As it does this, the granular cell loses it’s nucleus. This causes the cell to ‘die off’ and turn into the squames. At the granular cell level, the cells start to die off.
What are keratins also the main constituent in?
- Hair, nails and animal horns
What is the normal transit time of a keratinocyte from basal layer to stratum cornuem?
28-40 days
What is the transit time of a keratinocyte from basal layer to stratum cornuem for someone with psoriasis?
2-4 days
Psoriasis the transit time is reduced, such that stratum corneum is producedin abundance as silvery scales every 2-3 days (due to low pH)
What is hyperkeratosis?
Too much keratin put on the surface of skin (especially on the hands and feet)
What are keratin horns?
Melanocyte
- Occurat intervals (1:8 of basal layer of epidermis)
- Difficult to see histologically without special stains
- Dark skin produce melanin, the main pigment that gives the skin its colour produce more melanin. But are not increased in numberin dark ortannedskin
- Mature melanosomes containing melanin,
- transferred to neighbouring keratinocytes by pigment donation
- phagocytosis of the tips of the dendritic processes black dots in the photo is melanin
Diagram showing melanocytes
Langerhan cells
Langerhan cells - reponsible for skin reactions
- Difficult to see histologically without special stains
- Highly specialised capacity to present antigens to T lymphocytes mediate immune reactions e.g. in allergic contact dermatitis (appears brown as phagocytosing any spare melanosomes)
Merkel cells
- Difficult to see histologically without special stains
- Mechanoreceptor cells associated with sensory nerve endings (role is to measure pressure and sensations)
- Account for ~6 -10% of the cells in the epidermis
Melanin role - main function
The main function of melanin is to protect the skin against electromagnetic radiation, primarily ultraviolet light, although it also provides a small amount of protect against radionuclides and x-rays too
Variations in skin - response to UV light
- Melanin is laid down on the superior side of the nucleus to protect the DNA inside the nucleus. This helps to stop keratinocytes turning into different forms of skin cancer.
How many types of skin are on one individual?
- Hairy skin
- Non-hairy skin
- Thin skin
- Thick skin
Thickness of different types of skin
Hairy skin tends to be thin skin e.g. external (skin) aspect of the lip (upper lip)
Non-hairy skin tends to be thick skin e.g. oral mucosa (inner lip)
Exceptions to this rule:
lips, back of the ear, some areas of the external genitalia
Examples of hairy thin skin
Scalp, pubic region, eyebrows, axilla, upper and lower limbs
Examples of non-hairy thick skin
Lower layer of the feet (sole)
Palmer of hands (palm of hands)
Example of non-hairy skin (thin skin) - out of the general rule
- Lips
- Eyelids
- Webbing between fingers and toes
Example of the upper lip (external skin aspect), lip and inner lip by the teeth (internal mucous aspect)
- External skin aspect: hairy thin skin
- Vermillion zone (lip): non-hairy thin skin
Non-hairy (thick) skin - Location
- Palmar surface of hand
- Plantar surface of the foot
- Area between fingers and toes (webs)
Non-hairy (thick) skin - Structure
- Same as rest of body except:
- NO hair, arrectorpili muscles, or sebaceous glands
- Thicker strata corneum
- Thinner dermis
- Increased density of mechanoreceptors (as want to have the sensation of picking things up, or stepping on things…)
Non-hairy thick skin - Functions
- Prevent tissue loss due to abrasion (thick strata corneum, protects dermis layer)
- Increased friction between skin and surfaces as no sabeceous glands
- Increased sensation due to lots of mechanoreceptors
Thick (non-hairy) versus thin (hairy) skin
Thick (non-hairy) versus thin (hairy skin)
Thin on the left, thick on the right
What do hair follicles look like?
Epidermis of thick skin from sole (stratified squamous keratinised epithelium)
- can also clearly see dermal papillae
Thick (non-hairy) vs thin (non-hary skin) - pic
Thick (non-hairy skin) on left
Thin (non-hairy skin) on right
(can see less keratin, keratinised layer on the stratum corneum is very thin, and fewer dermal pillae)
List 5 types of skin appendages
Skin appendages are skin-associated structures that serve a particular function
•hairs
•arrectorpili
•sebaceous glands (covered in other lectures)
•sweat glands (covered in other lectures)
•nails
What are the 3 types of hair on skin? Describe them all
There are 3 types of hair:
- Lanugo–covers the developing fetus(sometimes seen at birth) (not always developed)
- Vellus–replaces lanugo: Short, thin, light coloured and soft (notconnected to sebaceous gland), e.g. on fingers or toes
- Terminal–head (scalp and eyebrows, nasal passage), axillae, external genital region. Long, wide, dark coloured and coarse, Produced by the actions of testosterone. Some vellus hair turns to terminal hairs
Hair Functions - Thermoregulation
• Hair lays flat when body temperature is within normal range or elevated
• Hair stands erect when body temperature is lower than normal range
Arrector pili muscle contracts when stimulated by sympathetic fibres from the autonomic nervous system, this makes the hair erect
• Also acts a partial barrier to UV rays (scalp) e.g. hair
Hair functions - Sexual attraction
- Hair style can be alluring!
- Apocrine sweat glands secrete oils (pheromones) that attract the opposite sex Hairs trap these oils and accentuate their pungency
Hair functions - Sensation
• Hairs have sensory nerve endings within the bulb Air movement, physical interaction, vibrations transmitted to the bulb sensed and transmitted to CNS Provides sensory awareness
Hair functions - Protection
- Eyelashes and nasal hair prevent dust and pathogens from entering the body
- Eyebrows reduce the amount of light and sweat from entering the eyes
- Axilla hair (armpit hair) acts as a conduit to conduct sweat away from the body (helps to get the sweat out of the body and reduce body temp)
- Oily hair aids escape when predation is imminent (oil - slippery so predator loses grip)
Role of arrector pili muscle
- Consist of smooth muscle fibres attached to papillary region of the dermis (origin) and hair bulb (insertion)
- ‘Fight or flight response’ or ‘cold’ causes involuntary contraction - ‘goose bumps’
- May also induce sebum release from sebaceous gland (due to the movement of hair bulb against the sabaceous gland)
Contraction of the arrector pili muscle pulls the hair, as shown above, to stand it up. As the bulb squeezes against the sabeacous gland, the sabeacous gland releases it’s contents out onto the hair follicle
Sensation - mechanoreceptors
- End bulbs – thermoreceptor – register temperature
- Free nerve endings have nociceptors – register pain
- Tactile discs – vertical dimpling of skin – attaches to basal layer keratinocyte – senses touch, pressure and texture (Merkel in non-hairy skin)
- Meissner in hairy skin) Meissner corpuscle – tapping and flicker movements detected
- Pacinian corpuscle – vibration and pressure
- Ruffini’s corpuscle – joint movements and tissue stretch
- Root hair plexus – vibrations in the hair shaft
(can see that most are in the dermis)
Nails - Location and Structure
Location:
• Distal surface of phalanges (fingers and toes) Structure:
• Consists solely of alpha keratin:
- nail plate, the nail matrix and the nail bed below it, and grooves surrounding it (all made from alpha keratin)
Nails - Function
Functions:
• Protection of distal phalanx (end of the bone) and surrounding soft tissues from injuries
• Enhances precise delicate movements of the distal digits through counter-pressure exerted on the pulp of the finger
• Enhancing the sensitivity of the fingertip (even though the nail itself has no nerve endings), allows you to get control between thumb and finger when picking up a pin – precision
• As a tool, enabling the so-called “extended precision grip”
- (e.g. pulling out a splinter in one’s finger)
- certain cutting or scraping actions
NB. growth rate of nails is correlated with length of phalanx, this means that middle finger nail grows the fastest
Summary of the Functions of skin
Protection and repair:
- epidermis by keratinocytes and UV protection by melanocytes invasion by Langerhans cells
- papillary dermis by fibroblasts (wound healing) - reticular dermis/hypodermis by fibroblasts/mast cells and macrophages
Temperature regulation and excretion of waste products:
- Sweat glands – urea, sodium chloride and water, some CO2
Lubrication:
- Sebaceous glands release oil onto the surface (also acts as water barrier)
Storage:
- White adipose cells provide lipids for energy
Vitamin D synthesis (covered in detail in the MEH Unit):
- Cholecaliciferol produced in the skin is converted into 25-hydroxyl cholecalciferol (25-OH) by the liver and then into 1, 25-hydroxyl cholecalciferol (1,25 OH) VitD (the active vitamin) by the kidney
Absorption:
- Absorbs small amounts of water and O2 (some drugs – e.g. steroids can be applied and absorbed through the skin)
Aesthetics:
- Skin as a mode of communication or attraction (tattoos, piercing, hair-styles, cosmetics, etc.)
Sensation:
- Specialised structures called mechanoreceptors (pain, pressure, touch, arousal, etc.)
