General Anatomy (Skin) Flashcards
What are the functions of the skin?
Protection
• Impermeable to water (glycolipid layer present between the cells of stratum granulosum) ‒ preventing loss of body fluids
• Physical barrier to pathogens
• Synthesis of melanin by melanocytes ‒ protecting against UV radiation
Temperature regulation
• Sweating
• Dilation and constriction of superficial blood vessels
Sensory perception
• Sensing the environment (touch, temperature, pain, presurre)
Excretion
• Water, sodium salts, urea and nitrogenous wastes excreted through the sweat
Vitamin D synthesis
• During exposure to sunlight, ultraviolet radiation penetrates into the epidermis and photolyzes provitamin D3 to previtaminD3. Previtamin D3 can isomerize to vitamin D3.
• Vitamin is essential for calcium absorption from the intestinal mucosa.
What is the skin?
• Two distinct regions
• Avascular epidermis (5 layers in
thick skin) and vascular dermis
• Hypodermis (subcutaneous layer of connective tissue and adipose tissue) which forms the superficial fascia in gross anatomy
What is the epidermis (stratum Basale)?
• Mitotically active cells (the stem cells of the epidermis)
• Single layer of columnar to cuboidal cells resting on a basement membrane
• Separating the dermis from the epidermis
• Cells attached to one another by cell junctions called desmosomes and to basement membrane called hemidesmosomes
• Cells continually divide and mature, migrating up toward the superficial layers.
• Cells contain intermediate keratin filaments which increase in number as they move up.
• Keratin fliaments form the component of keratin in the uppermost layer.
What is epidermis (stratum spinosum)?
• Characteristic microscopic appearance of short processes extending from cell to cell
• Keratinocytes from the deepest layer move upward and form the second layer.
• 4-6 rows of cells
• Routine histologic preparations cause the cells to shrink, with the resulting intercellular spaces appearing to form numerous cytoplasmic extensions of spines.
• Spines represent the sites where desmosomes are anchored to keratin filaments and to adjacent cells.
• Keratin synthesis continues, assembling into bundles called tonofilaments.
• Resistance to abrasion
What is epidermis (stratum Granulosum)?
• Numerous intensely staining granules
• Maturing cells moving above the SS contain dense keratohyalin granules
• 3-5 layers of flattened cells
• Granules contain a protein called filaggrin which cross-links with tonofilaments
• Combination of tonofilaments with filaggrin produces keratin through a process called keratinization—soft keratin is formed.
• Cells also contain granules formed by lipid bilayers, which are then discharged into the intercellular spaces between the SG and the next layer, as a lipid that forms a waterproof barrier and seals the skin.
What is the epidermis (Stratum Lucidum)?
• Present in the thick skin (considered a subdivision of SC)
• Translucent and barely visible
• Tightly packed dead cells without nuclei or organelles
• Flattened cells with densely packed keratin filaments
What is the epidermis (stratum corneum)
• Most superficial
• Cells with no nuclei or organelles
• Flattened dead cells with soft keratin filaments
• Keratinized superficial cells are continually shed or desquamated, being replaced by new cells.
• Keratinization process ‒ hydrolytic enzymes disrupt the nuclei and cytoplasmic organelles that disappear as the cells are filled with keratin.
What is an epidermis?
- Stratified keratinized squamous epithelium
- 4 cell types
- Keratinocytes (most dominant)
Divide, grow, migrate up and undergo keratinization or cornification (forming the protective layer for the skin) - Melanocytes, Langerhans cells and Merkel cells : less abundant cells, interspersed among the keratinocytes in the epidermis (normally not distinguishable unless skin is prepared with special stains)
What are melanocytes?
• Long irregular cytoplasmic extensions which branch into the epidermis
• Located between the stratum basale and the stratum spinosum
• Syntesize the dark brown pigment (melanin) from aminoacid tyrosine
• Melanin granules migrate to the cytoplasmic extensions, then are transferred to keratinocytes.
• To protect from damaging UV
• Exposure to sunlight increases its synthesis.
• Imparts a dark colour to the skin
• Melanoma (serious type of skin cancer)
What are Langerhans cells?
• Originate from bone marrow
• Migrate via the blood stream
• Reside in the skin (mainly in the stratum spinosum)
• Also dendritic-type cells
• Recognise, phagocytose and process foreign antigens and then present them to T lymphocytes for an immune response
• Antigen-presenting cells
• Langerhans cell histiocytosis (abnormal proliferation)
What are Merkel cells?
• Specialized cells in the skin that are important for proper neural encoding of light touch stimuli
• In the stratum basale layer
• Most abundant in the fingertips
• Closely associated with sensory nerve fibres (with the expanded terminal of the neuron terminal)àMerkelʼs corpuscle ‒ sensitive mechanoreceptor
• Function as mechanoreceptors for cutaneous sensation
• Also dendritic
• Bound to adjoining keratinocytes by desmosomes
• Contains neurosecretory granules
• MCC (Merkel cell carcinoma) ‒ uncontrolled proliferation of Merkel cells, rare but aggressive, quickly grow and metastasize at an early stage
What is the Dermis?
• Skin appendages are developed from the epidermis and are located in the dermis.
• Directly below the epidermis, containing connective tissue fibres and cellular components of epidermal origin
• Deeper reticular layer ‒ bulk of the dermis ‒ dense irregular connective tissue ‒ more fibres - Sebaceous (oil) glands, Hair follicles, Ducts of sweat glands, Meissnerʼs corpuscles and Paccinian corpuscles
What is the Hypodermis?
- Attaches the reticular layer to the underlying organs
- Loose connective tissue and adipose tissue
- Known as superficial fascia, subcutaneous tissue
- Major blood vessels ‒ rete cutaneum (A vascular network parallel to the skin surface and lying between the dermis and the superficial fascia)
What is papillary Dermis?
• Superficial one fifth of dermis.
• Composed of loose (areolar) collagenous connective tissue, mixed with thick elastic fibers.
• It contains the capillary loops that support, but do not penetrate the epidermis.
• It also contains Meissner’s corpuscles , which are very sensitive mechanoreceptors.
• More cellular component compared to the lower layer
• Indents the base of the epidermis to form dermal papillae (upward finger-like protrusions which produce fingerprints)
• Downward projections of the epidermis is called epidermal pegs/ ridges (interdigitations strengthening the attachment)
What are the functions of the dermis?
• Rich in blood vessels, sensory nerves, postganglionic sympathetic and afferent nerve endings, lymph capillaries and arteriovenous anastomosis
• Thermoregulation and vascular supply to the avascular epidermis with nutrients.
• The dermis is composed of collagen and elastic fibres, and is responsible for the tone and texture of the skin.
• Sweat glands and hair follicles originate from here and hypodermis.
What is the function of the thin skin?
• Surfaces of the body not exposed to increased abrasion and wear and tear—covered by thin skin
• Thinner epidermis ‒ much thinner SC (only the thickness of the epidermis is thin, the layers below it can vary in thickness depending on the location of the body)
• Skin appendages are developed from the epidermis and are located in the dermis.
• Hair follicles, sebaceous glands, different types of sweat glands, and arrector pili smooth muscle
What is the function of thick skin?
• Palms and soles are covered with the thick skin due to wear and tear.
• Thicker SC
• Lacking hair follicles, sebaceous glands, and smooth muscle
• Containing sweat glands
What is Albinism?
a congenital disorder ‒ lack of melanin pigments in skin, hair and eyes due to absence or defect of an enzyme involved in the production of melanin ‒ more susceptible to sunburn and skin cancers
What is vitiligo?
Like alopecia, vitiligo is an autoimmune disease, where melanocytes are targeted and destroyed. Areas of symmetrical depigmentation appear, which are more apparent in darker-skinned individuals.
What is Alopecia Areata?
alopecia is marked by autoimmune destruction of hair follicles, causing hair loss.
What is psoriasis?
the mitosis of keratinocytes in the stratum basale is drastically increased, producing a thickened stratum spinosum. This is clinically apparent as “scaly” skin, typically on the knees and elbows.
What are skin mechanoreceptors?
o Cutaneous mechanoreceptors provide information to the central nervous system (CNS) about touch, pressure, vibration, and skin stretch.
o Inside the dermal papillae, close up under the epidermis
What is the tactile (meissner’s) corpuscles?
o Most numerous in the parts of the skin where touch is most sensitive (the fingertips)
o Mechanical signal of pressure, slow vibration à neuronal impulse
o Touchànerve fiber physically distortedàfiber sends a signal to its neuronàrelays the information to the CNS for further action
What is lamellated (pacinian) corpuscles?
Very large and looks much like a sliced onion in sections
The lamellated corpuscle works the same way as the tactile corpuscle does but on a much grosser and coarser scale.
Lamellated corpuscles are gross pressure receptors, and fast vibration receptors.
Present deep in the skin
