Skin (integumentary system) Flashcards
To learn about the skin...
What are the functions of the integumentary system?
- Maintain internal environment
- Protection from invaders and gross physical units
- Receive stimuli from environment
4 Thermoregulation via sweat glands, fat deposits, blood vessels - Excretory and photochemical functions
- Good source of diagnostic info
What is the embryonic origin of the epidermis?
Surface ectoderm.
- Initially a single layer, the epidermis reaches full thickness at birth
What is the embryonic origin of the dermis?
mesoderm
What is the embryonic origin of accessory structures (hair, sweat glands, mammary glands, etc)?
They are derivatives of epidermal cells and the epidermis originates in the surface ectoderm
What are the layers of the skin?
From the top:
- Epidermis
- Dermis
- Hypodermis
Describe the structure of the epidermis?
- Composed of keratinized, stratified squamous epithelium
- forms outer layer of skin
- predominantly keratinocytes and melanocytes (pigment cells)
- Langerhans and Merkel cells also found there
What are Melanocytes?
- Melanocytes are cells that create skin and hair pigmentation.
What are Langerhans Cells?
epidermal dendritic cells involved in signaling the the immune system
What are Merkel cells?
Cells associated with sensory nerve endings
- They contain dense core neurosecretory granules
- believed to be sensitive to tactile stimuli
From the dermis outward, what are the 5 layers of epidermis?
- Stratum basale (germinativum)
- Stratum spinosum
- Stratum granulosum
- Stratum lucidum (thick skin only)
- Stratum corneum
Describe the cells of the stratum basale (germinativum)
- Basopihlic keratinocytes that are cuboidal to columnar
- Cells have hemidesmosomal junctions with underlying basement membrane
- Desmosomes bind cells to lateral and upper surfaces
- mitotic figures are common as these cells are continuously renewing epidermal cells
Describe the cells of the stratum spinosum
- polygonal or slightly flattened cells
- prominent desmosomal attachments to adjacent cells
- appear “spiny” in light microscope
- cells contain abundant keratin filaments
Describe the cells of the stratum granulosum
- Cells contain keratohyalin granules (not membrane bound)
- these are basophilic granules, not to be confused with melanin granules (mostly found in basal layers of epidermis)
- Cells have lamellar bodies
What are lamellar bodies?
Contribute to the formation of the intercellular epidermal water barrier
- ovid-shaped membrane bound organelles that hold a mixture of pro-barrier lipids and proteases used to form the epidermal water barier
Describe the cells of the stratum lucidum
- thin layer of weakly eosinophilic, flattened cells specific to thick skin only
- very few organelles or nuclei
- abundant filaments in an electron dense amorphous matrix with no keratohyalin granules
Describe the cells of the stratum corneum
- flattened, keratinized cells
- plasma membranes are thickened by deposition of secretions from membrane-coating granules
- strong barrier to movement of material across skin
Where is thin and thick skin present?
Thick skin: palms and soles of feet
Thin skin: everywhere else
What are the layers of the dermis?
Papillary layer
Reticular layer
Where are melanocytes found in the epidermis?
Beneath or between epithelial cells of the stratum basale and in hair follicles attached to basement membrane
What is the origin of melanocytes?
They come from the neural crest and are responsible for synthesizing melanin granules
What is albinism?
no melanin produced, usually due to absence of the enzyme tyrosinase
How is melanin produced?
by oxidation of dihydroxy-phenylalanine (DOPA). DOPA is made from tyrosine by the enzyme tyrosinase
How are melanin granules produced?
By the deposition of melanin onto a fibrous lattice, called the melanosome
Why does pigment vary in individuals?
Pigment is determined by melanosome size, distribution, and density of pigment.
What is the process of melanin distribution throughout epidermal cells?
Melanin granules, made in melanocytes, are “injected” into keratinocytes.
- melanocytes don’t accumulate melanosomes, they continually transfer them. This process is known as cytocrine secretion.
What are free nerve endings?
Terminals of thinly myelinated or unmyelinated nerve fibers that penetrate the epidermis
- responsible for sensing pain and temp changes
Describe the papillary layer
More cellular connective tissue than the reticular layer
- contains fibroblasts, macrophages, mast cells, fine collagen fibers
- Outstanding features: vascular papillae and Meissner’s corpuscles
What are vascular papillae
In dermis, complementary to epidermal projections (rete ridges)
What are Meissner’s corpuscles?
In dermis, encapsulated sensory receptors for light touch
Describe the reticular layer
In dermis, thick layer containing irregular dense connective tissue which has more fibers (and thicker ones) and fewer cells than papillary layer
Describe the subcutaneous tissue
Known as the hypodermis:
- binds skin to subjacent organs
- often contains adipose tissue, blood vessels, lymph vessels, and pacinian corpuscles
What are pacinian corpuscles?
endcapsulated, ovoid sensory receptors
- sensitive to deep pressure
- together with Meissner’s corpuscles, they are responsible for vibratory sensation
What are the functions of blood supply in the skin?
- nourish epidermal structures
- termp regulation
Where are blood vessels found in the skin?
In dermis and hypodermis (few vessels found in reticular layer of dermis)
What are the structures of hair follicles?
Layers from inside out:
- Matrix cells (including melanocytes)
- Inner root sheath
- Outer root sheath (source of new epidermal cells in wound healing)
- Connective tissue sheath
What are the structures of a hair shaft after emerging from follicle?
- Medulla - forms central axis
- Cortex - multilayered structure containing pigment granules
- Cuticle - thin protective covering
What are arrector pili?
bundle of smooth muscle fibers obliquely attached to hair follicle
What are sebaeous glands?
alveolar glands with excretory duct opening into hair shaft
- secretes sebum (made of lipid rich cell debris)
- these glands have no myoepithelial cells
What are nails composed of?
Keratinized plates which form a protective covering over the distal ends of digits
What is the nail bed?
the nail plate rests on the nail bed which consists of epithelial cells continuous with the epidermis
What is the nail matrix?
These cells produce the nail plate
What are myeoepithelial cells?
Cells that surround secretory cells and their contractions aid in movement of secretions toward the duct
What are dark cells?
Cells that border the lumen of secretory tubules and produce a mucoid substance
Have abundant rER
What are clear cells?
Cells that rest on the basal lamina of secretory tubules and are involved in secretion of water and electrolytes
Have abundant smooth ER
Describe sweat gland ducts.
Lined by stratified cuboidal epithelium, they recover sodium as sweat passes through the duct
Where does new growth occur when epidermis is removed by trauma (blisters, burns, scrapes…)?
From hair follicles and sweat glands
Where are stem cells located for the epidermis?
Midway along the hair follicle
