Skin Structure Flashcards
What are the layers of skin and their functions?
Epidermis - protect from dehydration
Dermis - tensile strength
Subcutis/hypodermis - energy storage as fat
Describe the embryological development of skin
- one layer of ectoderm overlies a dermis of mesenchyme (mesoderm)
- the ectoderm divides into the basal cell layer/stratum germinativum and the periderm (outer)
- stratum intermedium fills the gap between these layers
- further development leads to the adult structure (with the addition of melanocytes from the neural crest and Langerhans cells from bone marrow)
Describe embryological dermal development
- increase in thickness and number of fibres
- mesenchymal cells become fibroblasts
- collagen fibres precede elastin fibres
- ground substance (proteoglycans and hyaluronan)
- development of nerves and BVs
Describe embryological development of the subcutis
Lipocytes develop in the 2nd half of gestation
What are cells of the epithelium call in skin?
keratinocytes
Label the epithelial layers of skin
Describe the embryological development of hair
- Epidermal basal cells proliferate into dermis -> hair bud
- aggregation of mesenchymal cells (dermal papilla) projects into tip of hair bud
- epidermal cells grow around papilla -> hair bulb from which hair grows
Describe the components of a keratinocye
Adherens junctions (attach cell to cell)
Focal adhesions (attach cells to underlying dermis)
Keratins (intermediate-filament forming proteins that provide mechanical support)
Desmosomes (anchor keratinocytes together and provide intracytoplasmic signalling)
Hemidesmosomes (anchor the epidermis to the dermis)
Actin filaments
Give an example of a condition relating to keratin in the skin
Palmopalmar hyperkeratosis - genetic defect, causes splitting and pain in the epidermis of foot pads
Give an example of condition relating to the desmosomes in skin
Pemphigus foliaceus - immun-mediated disease with antibodies directed at desmosomes - prominent pustules
What are hemidesmosomes?
A strong link between keratin intermediate filaments and the interstitial collagen in the dermis
Give an example of a condition relating to hemidesmosomes
Epidermolysis bullosa acquisita - immune mediated disease with antibodies directed against collagen
Label this keratinocyte
Label the histology of haired skin
What is the function of keratinocytes
Produce extracellular lipids (ceramides, cholesterols, fatty acids) from golgi apparatus in granular layer resulting in lipid lamellae
Immunologically active (not an innert barrier)
Describe the structure and function of the stratum basale
Single layer of cuboidal cells -
proliferative and anchoring
stem cell function (pluripotent)
Anchoring cells:
structural role - hemidesmosomes and desmosomes
Proliferative cells:
mitotically active - one cell remains as germinative cell and other differenitates
What is the stratum basale influenced by?
Growth factors and hormones e.g., epidermal growth factor, cortisol
Inflammatory mediators e.g., IL-1, IL-6
Drugs and vitamins e.g., vit D, retinoids
Describe the structure and function of the stratum spinosum
1-20 cells thick
polyhedral cells
prominent desmosomes
upper layer produced involucrin (part of cornified layer)
Describe the structure and function of then stratum granulosum
variably present in haired skin
slightly flattened cells
shrunken nuclei
various lipids and enzymes secreted extracellularly
Describe the structure and function of the stratum lucidum
Not present in haired skin (variably present in hairless, thickened skin)
Slightly flattened cells, shrunken nuclei
Increased intracellular lipids -> poorly stained
Describe the structure and function of the stratum corneum
Composed of anucleated flattened cells (squames)
Plasma membrane replaced by cornified envelope comprising intracellular proteins (Extensively cross linked)
Hydrophilic binding of lipids forms organised ‘mortar’ between sqaumes
desmosomes now called ‘corneodesmosomes’ (due to addition of corneodesmosin from stratum granulosum)
Describe the formation of the cornified envelope
In the upper granular layer, envoplakin and periplakin are laid on the cell membrane
Involucrin and TGM1 (transglutaminase 1) are added.
Ceramides are incorporated into the wall and fatty acids and cholesterol are released into intracellular space.
The cell membrane is replaced by a ceramide and fatty acid complex.
Give an example of a condition that effects the cornified layer
TGM1 mutation in Jack Russel Terriers -> severe hyperkeratosis
Describe the process of desquamation
Final part of terminal component
Homeostatic process - continual loss of cells
Destruction of the corneodesmosomes
Corneocytes fall off
Describe the structure and functions of the dermis
Connective tissue of mesenchymal origin:
- insoluble fibres (collagen and elastin) to give tensile strength
- soluble polymers (proteoglycans and hyaaluronan) to resist and absorb compressive forces
Contains epidermal appendages, arrector pili muscles, blood and lymph vessels and nerves
What are the cells present in the dermis
Macrophages
dermal dendritic cells
fibroblasts
Sensory nerves in the dermis
Respond to touch, pressure, temp, mociception, pruritus
Mechanoreceptors:
- free nerve endings (also in epidermis)
- rapid adapting: Meisseners and Pacinian corpuscles
- slow adapting: Merkel cells, Ruffini corpuscles
Motor nerves in the dermis
Adrenergic and cholinergic fibres
Arterioles, atrichial and epitrichial sweat glands
Describe the 3 layers of blood supply to the dermis
Deep dermal vascular plexus:
- interface of dermis and subcutis
- supply lower hair follicle and epitrachial sweat glands
Mid-dermal vascular plexus:
- level of sebaceous glands
- supply arrector pili muscles, mid hair follicles and sebaceous glands
Superficial dermal vascular plexus:
- just below epidermis
- capillary loops supply epidermis and upper hair follicle
What is the name for disease of the blood vessels
Vasculitis
Describe the structure and function of sebaceous glands
Differentiated from basal layer of germinative cells
Large, polygonal cells
Lots of vacuoles in the cytoplasm
Produce sebum (holocrine secretion) which is excreted via sqaumous duct to the hair follicle
Describe the components and function of sebum
Contains triglycerides and other lipids, transferrin, IgA, IgG
Functions:
- lubricated hair and skin
- required for normal hair shaft separation
Label this section of skin
Give an example of a condition that effects the sebaceous glands
Sebaceous adenitis - glands destroyed by disease -> scale, hair breakage and follicular casts
What are the 2 types of sweat gland
Epitrichial and Atrichial
Describe the structure and function of epitrichial sweat glands
Associated with a hair follicle
Single layer of flattened cuboidal cells surrounded by myoepithelial cells
Distributed throughout haired skin
Below sebaceous gland
Excrete sweat into piliary canal just above sebaceous gland opening
Not innervated
Describe the structure and function of atrichial sweat glands
Not associated with hair follicles
Single layer of flattened cuboidal cells surrounded by myoepithelial cells
Merocrine secretion
Small, tightly coiled
Innervated by cholinergic fibres
Label this section of skin
Name some specialised ‘atrichial’ glands
Mammary gland
Interdigital glands of small ruminants
External ear canal
Nasolacrimal glands
Apocrine glands of anal sac
What types of hair are there?
Primary (guard hairs)
Secondary (downy hairs)
What are hairs important for?
insulation
signalling
physical protection
Describe the simple anatomy of hair
cortex, medulla and cuticle containing variable pigment
What kind of hair follicles do omnivores and herbivores have?
Simple - each infundibulum has 1 hair shaft
What kind of hair follicles do carnivores have?
Compound follicles
Label this section of skin focusing on the hair
Label this section of skin
How do you distinguish between a growing and resting hair on a histological slide?
Anagen definition
Actively growing hair
Catagen definition
a period of rapid change in which the lower portion undergoes apoptosis
Telogen definition
the hair is resting
How many hairs does a simple follicle have?
1
Describe compound hair follicles
Several hairs meet in follicular orifice
Primary (large hairs):
- bulb in deep dermis
- associated sebaceous, sweat glands and arrector pili muscles
Secondary (smaller hairs):
- not as deep in dermis
- may have sebaceous gland, but no sweat gland or arrector pili muscles
Identify which type of compound hair follicles these are
What is the best drink in the world
Huel daily greens
Describe the process of anagen
New hair produced under previous hair in deep dermis
Distinctive hair bulb containing follicular dermal papilla
Exogen definition
the process of hair being lost
What is the isthmus in hairs
Where the sebaceous gland attaches
And the divide between the infundibulum and the inferior portion
What type of hair growth do dogs, cats, horses and hedgehogs have?
Telogenic
Length is determined by duration of anagen and then they stop growing and rest
How is hair cycle regulated?
photoperiod
temp
hormones (thyroid [+ve], growth hormone [+ve], oestrogen [-ve], corticosteroids [-ve])
Nutrition and general health
growth factors
drugs
Describe the structure of anagen hairs
Found in the deep dermis
dermal papilla provide blood
hair bulb epithelium undergoes mitosis
melanocytes provide pigment
6 layers (inner to outer):
- medulla
- cortex
-cuticle
- huxley layer + henle layer (inner root sheath)
- outer root sheath (continuous with epidermis)
- basement membrane
Label this hair
What structures make up the inner root sheath?
Henle’s layer
Huxley’s layer
Cuticle
Describe the process of catagen
In the mid dermis:
- involution of the hair bulb and dermal papilla
- upward migration of the follicle and papilla
- loss of the internal root sheath
- develops a thick glass membrane (previously basement membrane) above the bulb of the follicle
Fast process mediated by apoptosis
Describe the telogen phase
In mid to upper dermis:
- conical bulb at level of the attachment of the arrector pili muscle
- hair surrounded by external root sheath
- hair bulb separated from dermal papilla by thick basement membrane
- original bulb is dormant
- new bulb and papilla form beneath old follicle
- when stimulated, new bulb forms a new hair and the old hair is lost
What stage of hair growth is this?
Anagen
What stage of hair growth is this?
Telogen
Describe the structure and function of vibrissae (sinus hairs)
Specialised hairs
Simple follicles with blood-filled sinuses between the inner and outer layers of the dermal sheath
nose, above eyes, lips etc.
Mechano/touch receptors
Describe the structure and function of tylotrich hairs
Simple follicles in amongst body hairs
Mechanoreceptors (fast adapting)
Associated with tylotrioch pads (slow adapting)
Label this diagram of the skin and hair follicle
Label this skin histology
A – dermis
B – Stratum corneum
C – follicular osteum
D – sebaceous gland
E – sweat gland
F – hair bulbs
G - subcutis
How does the skin provide photoprotection?
Nuclear capping by melanin prevents damage to basal cell nuclei
Urocanic acid in stratum corneum acts as natural sun screen
Skin thickness
Melanin absorbs UV
How does the skin provide immunosurveillance?
Keratinocytes are immunologically active
Langerhans cells are APCs
T-cells and mast cells in the dermis
Antimicrobial proteins from keratinocytes and in sweat
How does the skin regulate temp?
Altered blood flow in the superficial plexus allows regulation of the skin temp
Sweating
Hair is insulating
What is the predominant cell type in the subcutis/hypodermis?
Lipocytes
What is the function of lipocytes in the subcutis?
Energy reserve
Thermogenesis and insulation
Protective padding and support
Maintaining surface/contour shape
What is the embryological origin of the subcutis?
mesenchymal origin
How does the subcutis protect hair follicles, sweat glands and vessels?
Projects into dermis via papillae adiposae
What are meloncytes?
Cells immersed in the epidermis with close contact with a defined number of keratinocytes via dendritic processes (forms an epidermal melanin unit/EMU)
What are the 2 types of melanin
eumelanin - brown
phaeomelanin - reddish yellow
What is the embryological origin of melanocytes?
Neural crest
How do melanocytes cause pigmentation of skin?
they produce pigment in melanosomes which then transfer the pigment to keratinocytes
Other than pigmentation, what are the functions of melanocytes?
photoprotection
immune function
What is skin colour influenced by?
Genetics
UV light
Hormones/inflammatory mediators e.g., melanocyte stimulating hormone (MSH)
Describe the structure and function of Langerhans cells
Tennis-racket or rod-shaped cytoplasmic organelle
APC
migrate to dermis and regional lymph nodes upon T-lymphocyte activation
What is the relationship between infection and skin pigmentation?
Langerhans cells are markedly reduced in UV light
Increased infection on white areas
Increased melanoma/squamous cell carcinoma in pale skin
Explain the basic structure of hoof
Keratinised epidermis
Arranged as wall (unguis) and sole (subunguis)
Attached to bone by a highly-folded dermis (Corium)
What is the basic structure of scales in birds?
Overlapping plates of heavily keratinsed epidermis
What is the basic structure of fish skin?
Overlapping boney plates derived from dermis
Covered in epidermis
What are the functions of skin?
Protective barrier
Chemical barrier
Normal flora - inhibits infectious agents
Temp regulation
Excretory organ
Sensory organ
Photoprotection
Immunosurveillance
Blood pressure regulation
Produces Vit D
Store nutrients (fat, water, vitamins)
Absorptive surface
How does the skin regulate BP?
the body stores Na+ in the skin
How does skin act as a secretory organ?
Sweat
Small amounts of urea, uric acid, ammonia and lactic acid
Give examples of primary nutritional diseases that affect the skin
Proteins - poor hair growth, increased infection rates
Fatty acids - stark hair growth, greasy or dry skin and hair
Vits and mins
Give examples of secondary nutritional diseases that affect the skin (change the way energy is available for the skin)
Hepatic dysfunction - reduced AA supply, necrosis of the skin
Pancreatic dysfunction - poor fat absorption, dry scurfy skin and pruritus
Bowel disease - poor absorption
Genetic inability to absorb minerals
Explain the endocrine influences on the skin
Melanocyte stimulating hormone (MSH) stimulates melanocytes
Melatonin - infleunces moulting and hair growth
Explain the hormonal influences of hair growth
Thyroid hormone and growth hormone stimulate hair growth
Adrenal cortical hormones and oestrogen decrease hair growth
Explain the clinical signs of atrophic dermatosis
Reduced hair growth - alopecia eventually
Comedone formation due to blocked follicular openings
In what diseases is atrophic dermatosis most common
Common in systemic endocrine disease
e.g., Hyperadrenocorticism (increased cortisol), hypothyroidism (decreased thyroid hormone) and sertoli cell tumour (increased oestrogen)
What does atrophic dermatosis look like histologically?
thin epidermis and dermis
increased dermal ground substance
reduction in hair follicles, sebaceous and sweat glands
telogenic hair
little inflammation
Explain the process of chronic dermatitis
Inflammatory mediators induce skin thickening and hyperpigmentation (blackening)
Increased blood supply
Increased numbers of inflammatory cells seen in the dermis
inflammatory cells crossing the epidermis with secondary infection
Describe the flea lifecycle
Describe tabanidae lifecycle
