Anatomy, Physiology and Immunology of the Skin Flashcards
What is the skin?
The interface between humans and their environment
- One of the largest organs of the human body
- Surface area 1.8m2
- 16% of body weight
- Multiple functions
What are the layers of the skin?
- Outer Epidermis
- which is firmly attached to the dermis - Dermis
- connective tissue layer - Subcutis (loose connective tissue)
- beneath the dermis AKA hypodermis which usually contains abundant fat.
What is the epidermis?
- Cellular external layer
- stratified squamous epithelium
- 0.1 mm thick (0.8-1.4 mm on palms)
- consists of 4 layers
What are the characteristics of the epidermis?
- Many layers of closely packed cells, most superficial layer are flattened and filled with Keratins.
- It adheres to the dermis at the basement membrane where epidermal ridges/pegs interlock with dermal papillae
- Contains no blood vessels
What are the cells of the epidermis?
- Keratinocytes
- main cells - Melanocytes
- synthesize melanin and transfer it via dendritic processes to neighboring keratinocytes - Langerhans cells
- highly specialized macrophages
- possess surface receptors for c3b and for IgE and IgG - Merkels cells
- acts as transducers to fine touch
What are the layers of the epidermis?
- stratum basale (basal layer)
- stratum spinosum (prickle layer)
- stratum granulosum (granular layer)
- stratum corneum (horny layer)
What is the epidermal turnover?
- Cells move from the basal layer (stratum basale) passing through the prickle (spinosum) and granular (granulosum) layers before dying in the horny layer (corneum)
- This journey (epidermal turnover) takes about 30 days (26-42 days)
- During this time the cells’ appearance and functions change in a process known as terminal differentiation.
- Dividing cells replicate every 8-20 day
Describe the stratum basale layer of the epidermis?
- Deepest layer, rests on the basement membrane
- Single layer of columnar cells (mostly keratinocytes)
- Cells contain tonofibrils and are secured to the basement membrane by hemidesmosomes.
- Melanocytes make up 5-10% of the basal cell population.
Describe the stratum spinosum layer of the epidermis?
- Daughter basal cells migrate upwards to form this layer of polyhedral cells.
- The keratinocytes are firmly attached to each other by small interlocking cytoplasmic processes and hemidesmosomes
- Keratin tonofibrils form a supportive mesh in the cytoplasm of the cells.
- Langerhans cells mostly found in this layer.
What is characteristic of the stratum spinosum layer of the epidermis?
Under the light microscope, the desmosomes look like “prickles”
Describe the stratum granulosum of the epidermis?
- Consists of 2/3 layers of flatter cells
- Cells become flat and lose their nuclei
- Keratohyalin granules and membrane coating granules seen in the cytoplasm of the cells
Describe the stratum corneum of the epidermis?
- End result of keratinocyte maturation.
- Sheets of overlapping cornifed cells with no nuclei (corneocytes)
NB: The corneocyte cytoplasm is packed with keratin filaments embedded in a matrix and enclosed by an envelope derived from keratin granule - This gives it the toughness allowing skin to with stand different physical/chemical insults.
What do the 4 layers of the epidermis represent?
represent the stages of maturation of keratin by keratinocytes
What is the stratum lucidum?
part of the stratum corneum in the thick epidermis (palms and soles)
What is the dermis?
- The layer between the epidermis & the subcutaneous fat
- Tough supportive connective tissue matrix
- Composed of: cells, fibers and ground substance
Charateristics of the dermis?
- The dermis varies in thickness from thin 0.6 mm on the eyelids/penis and thicker 3mm/more on the back, palms and soles
- Dermis supports the epidermis structurally and nutritionally
- In old age the dermis thins and loses its elasticity
Layers of the dermis?
- Papillary dermis
- Thin upper layer
- Lies below and interdigitates with the epidermal rete ridges - Reticular dermis
- Composed of coarser and horizontally running bundles of collagen
What are the main cells of the dermis and what do they synthesize?
- fibroblasts (main cells)
- which synthesize collagen, elastin and glycosaminoglycans - Dermal dendritic cells
- mononuclear phagocytes
- mast cells and lymphocytes
Describe the fibers of the dermis?
- Collagen fibers
- impart the toughness and strength to the structure
- make up 70% of the dermis - Elastin fibres
- are loosely arranged in all directions in the dermis and provide elasticity to the skin - The ground substance of the dermis is a semisolid matrix of glycosaminoglycans which allows the dermal structures some movement
What are the specialized structures of the dermis?
- Hair follicles & their sebaceous glands
- Sweat glands
- Nerve bundles and Sensory receptors
- Striated muscle fibres (e.g platysma) and some of the muscles of facial expression are also found in the dermis
What is subcutis?
- Subcutaneous layer is the tissue immediately below the dermis.
- Consists of loose connective tissue and fat traversed by nerves and blood vessels
- Subcutis can be up to 3cm thick on the abdomen
What is hair?
- Keratinized tubes
- Found over entire surface of skin, except palms, soles, glans penis
What are the 3 types of hair?
- Lanugo
- fine, long, formed in the fetus at 20 weeks , shed before birth - Vellus
- short, fine, light colored, cover most body surfaces - Terminal
- longer, thicker, darker
What are sebaceous glands?
- Found associated with hair follicles on
scalp, face, chest and back - Formed from epidermis derived cells
- Produce an oily sebum
- Androgen sensitive
Describe sebum production?
Sebum is produced by holocrine secretion in which the cells disintergrate to release their lipid cytoplasm
Describe acne pathogenesis as it relates to sebacceous glands?
- Increased hormones
- increased sebum
- Blockage of ducts
- comodone formation + inflammation
What are nails?
- Phylogenetic remnant of the Mammalian Claw
- Consists of plate of hardened and densely packed keratin
- Nails protect the fingertips and facilitate grasping and tactile sensitivity in the finger pulp
- Nail growth rate 1mm in 10 days.
What are sweat glands?
- Tube like coiled glands, located within the dermis.
- Their sweat is generated by “decapitation’ secretions of the glands cells and is odourless when produced, an odor develops after skin bacteria have acted on it.
Types of sweat glands?
- eccrine
- apocrine
Describe eccrine sweat glands?
- Develop from down budding of the epidermis
- Open onto skin surface
- Most profuse on palms, soles, axilla forehead
- Under psychological and thermal Control
- Sympathetic cholinergic innervation
Describe apocrine sweat glands?
- Also derived from epidermis
- Open into hair follicles
- Larger
- Most numerous in axillae, perineum and areolae
- Sweating controlled by sympathetic (adrenergic) innervation
- Apocrine glands represent a phylogenetic remnant of the mammalian sexual scent gland.
Muscles in the skin?
- Both smooth and striated muscle are found in the skin.
- Smooth muscle is responsible for ‘goose bumps’ from coldNerv
Nerve supply of the skin?
- Both myelinated and non-myelinated fibres found.
- Skin is richly innervated
- Highest density of nerves : hands, face and genitalia
Nerve fibers found in the skin?
- Free nerve endings
- detect pain, itch and temperature
- seen in dermis - Meissner’s corpuscles
- touch sensitive, mainly in dermal papillae of feet &hands
- seen in dermis - Pacician corpuscles
- detect pressure and vibration
- seen in dermis - Autonomic nerves
- supply the vessels, sweat glands and arrector pili muscles
Blood vessels of the skin?
- Arteries in the subcutis branch upwards, forming a superficial plexus at the papillary/reticular dermal boundary.
- Branches extend to the dermal papillae, each has a single loop of capillary vessels, one arterial and one venous.
- Arteriovenous anastomoses are well innervated and concerned with thermoregulation
Lymphatic vessels of the skin?
Meshes of lymphatics originate in the papillae and assemble into larger vessels that drain into the regional lymph nodes
Functions of the skin?
- protective barrier
- sensation
- thermoregulation
- biochemical reactions
- social and sexual functioning
How is skin a protective barrier?
- From mechanical/thermal/physical injury.
- Prevents water and protein loss
- Immunological protection.
How does skin achieve sensation?
Skin is rich in nerve endings and sensory receptors
Describe the thermoregulation function of the skin?
Alteration of blood flow through the cutaneous vascular bed.
1. Vasodilatation =↑ blood flow= ↑ direct heat loss
2. Vasoconstriction =↓ blood flow = ↓ heat loss
3. Evaporation of sweat from the surface
What are te biochemical reactions of the skin?
Vitamin D synthesis
- 7-dehydrocholesterol found in Keratinocytes is converted by sunlight to cholecalciferol
- The vitamin becomes active after 25-hydroxylation in the kidney
Structural immunological components of skin?
- epidermis
- blood and lymphatic supply to dermis
Epidermis and immunology?
- An important example of innate immunity
- Most microorganisms that have contact with the skin do not penetrate it.
Blood and lymphatics and immunology?
Important channels through which immune cells can pass to/from their sites of action
Cellular immunological components of the skin?
- Keratinocytes
- Langerhans cells
- T lymphocytes
- Mast cells
Keratinocytes?
- Make the protective horny layer
- Synthesize and release antimicrobial peptides
- Produce chemokines that attract cells of the immune system to the skin
produce pro-inflammatory cytokines (especially IL-1) - Their release of IL-1 after injury initiates various immune and inflammatory cascades
- Play a central role in healing after epidermal injury by self regulating epidermal proliferation and differentiation
Langerhans cells?
- langerhans’ cells are on the outlook for antigens in order to trap them and present them to lymphocytes.
- Key players in induction of T-cell response
- characterized ultra structurally by a cytoplasmic organelle known as Birbeck granule
T lymphocytes?
- Like T cells elsewhere they develop and acquire their antigen receptors in the thymus
- T- Lymphocytes that express CD4 On their surfaces: induce immune reactions and elicit inflammation
- CD8: Cytotoxic and can lyse infected and cancerous cells
Mast cells?
- Able to degranulate and release histamine and other vasoactive molecules.
- Also synthesize a wide range of cytokines.
- Normal residents of the dermis, their circulating counterparts are basophils.
- Mast cell numbers increase during inflammatory reactions
What is hair? Where is it usually found?
- Keratinized tubes
- Found over the entire surface of the skin,
- Except palms, soles, glans penis
3 types of hair?
- Lanugo hair - fine, long, formed in the fetus at 20 weeks, shed
before birth - Vellus hair - short, fine, light-colored, covers most body surfaces
- Terminal hair - longer, thicker, darker
How is hair formed?
Hair is formed by rapid division and
differentiation of stem cells which
form keratinocytes that migrate,
flatten, and die, forming keratinized
cells.
3 phases of hair growth?
- anagen(growth)
- catagen(transition)
- telogen (rest)
What are nails?
It consists of a plate of hardened and densely packed keratin
Function of nails?
- Protect the fingertips
- Facilitate grasping and tactile sensitivity in the finger pulp
