outermost region contains many layers of flattened, cornified, dead cells whose cytoplasms are packed with TONOFIBRILS
number of cell layers varies with the thickness of the skin (more layers in thick skin like palms & soles of the feet)
Stratum Lucidum [let's]
thin translucent layer apparent in some sections of thick skin, but not always present (possibly an artifact)
Stratum Granulosum [get]
contains 2-3 layers of flattened squamous epithelial cells whose cytoplasms are filled with coarse basophilic (keratohyalin) granules
granules contain filaggrin (binds tonofilaments into tonofibrils) and loracrin (thickens/stiffens the outer surface of cells)
last layer before the cells "die"
Stratum Spinosum [sun]
THICKEST of the living layers of the epidermis
cells connected by abundant desomosomes
have a spiny appearance
Stratum Basale (Germinativum) [burned]
innermost layer of cells rest on basement membrane & are single layer of cuboidal cells
these cells divide regularly meaning a high number of mitotic figures are visible
they move upward & replenish cells lost at the surface during epidermal turnover
cells of the epidermis that contain abundant water insoluble keratin assembled into 7-8 nm tonofilaments inside their cytoplasm
finger like projections of the dermis into the epidermis which enhance the attachment of the epidermis to the underlying dermis
these structures generate an enormous surface area for strong epidermal-dermal interaction
stem cells are located near the tips of the dermal papilla
primary ridge: encompassing single ridge over two dermal papilla
connective tissue layer that lies deep to the epidermis and consists of a highly vascularized connective tissue with different types of glands, hairs and sensory structures
hypodermis (superficial fascia)
lies deep to the dermis, but is not technically a part of the skin
several structures associated with the true skin (eg. hair follicles, sensory receptors) may be found in this region
hypodermis contains adipose tissue & may contain skeletal muscle
cells of the epidermis
3. Langerhan's Cells
4. Merkel Cells
pigment producing cells derived from the neural crest found in the basal layer
have long processes containing melanosomes that extend among the keratinocytes
melanosomes = membrane-bound organelles that make pigments (melanin and phaeomelanin)
Do keratinocytes synthesize pigment?
NO, melanocytes do
keratinocytes phagocytose the pigment-containing cytoplasm of the melanocyte and ACQUIRE pigment
What might increase melanocyte activity?
exposure to UV light as well as by melanocyte stimulating hormone (MSH)
dendritic cells, functionally related to the monocyte-macrophage group (and thus derived from the bone marrow) found particularly in the upper layers of the stratum spinosum
present antigen to lymphocytes
cells involved in sensory (mechano) reception & contain granules similar to those found in the cells of adrenal medulla
occur in small numbers in the basal part of the epidermis
caused by pathological changes that lead to over-proliferation of the stratum basale layer
can lead to basal cell carcinoma (most common form of skin cancer) if the changes are neoplastic
a much more aggressive form of skin cancer caused by neoplastic changes in the melanocytes
responsible for the majority of skin cancer deaths
Initial diagnoses are usually made based upon visual appearance w/ mnemonic ABCDE:
A - asymmetry
B - Border (irregular)
C - Color (usually multiple)
D - diameter (> 6mm)
E - enlarging
good prognosis w/ excision of early identified tumors
What are the two laybers of the dermis
layer just deep to the epidermis composed of finger-like papilla that project toward the surface, forcing epidermis to acquire an undulating morphology
this greatly increases the surface area for epidermal-dermal adhesion
Why do papilla contain sensory nerve endings & capillary beds?
because they bring the dermis as close to the outside world as possible, an ideal location for two such entities
constitutes the bulk of the dermis & contains large bundles of type I collagen fibrils [Langer lines] + elastic fibers
reticular dermis contains sensory receptors, hairs & several types of glands
What are 3 glands of the skin?
1. Eccrine Sweat Glands
2. Apocrine Sweat Glands
3. Sebaceous Glands
Eccrine SWEAT Glands (merocrine)
a basal coil from which a duct leads through the dermis into the epidermis assists in maintaining a normal body temperature
secretory portions of eccrine sweat glands pump Na+ ions into the lumen to generate an osmotic gradient; water follows by osmosis. In addition to water, several protein components (e.g. dermcidin) and other waste products (urea) are added to the fluid
ductal cells actively resorb sodium & passively reabsorb chloride; the final product to reach the surface is mostly water & a small amount of protein/sodium
are found over most of the body but most concentrated in the forehead, axilla & palms of the hand
What are eccrine sweat gland ducts made of?
stratified cuboidal epithelial cells rich in mitochondria
patients have compromised removal of salts by eccrine sweat gland ducts
--> a high salt content in sweat = marker of CF
Apocrine Sweat Glands
large coiled tubular glands that open into hair follicles rather than directly onto the skin surface
found mainly in the axilla (armpit) & genitals
less numerous than eccrine glands
apocrine glands can store their secretion, & various hormonal/neural stimuli will cause them to secrete
What is the likely primary functions of apocrine glands?
to produce pheromones
they become active at puberty
mode of secretion = eccrine and apocrine (the secretory product as well as a portion of the cells themselves is released)
Which is apocrine and which is eccrine?
left: apocrine - larger lumen for sweat storage
right: eccrine - smaller lumen, no storage
glands found all over the body EXCEPT at palms & soles that secrete sebum, which serves a natural lubricant of the hair and surface of the skin (contains TAGs, FAs, & cholesterol)
sebaceous glands discharge contents via a SINGLE duct into the lumen of hair follicles
mode of secretion = holocrine (entire cell & its product is released)
Where do sebaceous glands secrete sebum directly onto the skin surface?
fordyce spots (penis bumps)
lip vermillion border
caused when sebum production (puberty) exceeds the capacity of the short ducts to drain the glands
can become colonized by surface bacteria
while bacterium is a normal component found in healthy skin, prolonged colonization of sebaceous glands results in the release of lytic enzymes and damages gland & ducts
dead epithelial fibers composed of cross-linked keratin that is harder than that found on the skin, owing in part to the use of trichohyalin rather than filaggrin to bind tonofilaments into tonofibrils
What are the three parts of hair?
1. shaft: part projecting from the skin surface
2. root: the portion embedded in the skin
3. follicle: the tubular invagination of the epidermis
What time of muscle do most hairs have associated with their follicles?
smooth muscle (arrector pili)
contraction causes the hairs to become erect (goose bumps)
arrector pili are absent in axilla, groin, eyelashes & eyebrows
What are the 3 phases of hair growth?
1. anagen (a growth phase that may last for years)
2. catagen (a follicular regression stage which lasts weeks)
3. telogen, (a resting period lasting a few months).
What is responsible for the perception of touch, temperature, pressure and pain?
free nerve endings abundant in the epidermis
skin at every part of the body functions as a sensory receptor
rapidly adapting sensitive organs found in dermal papillae that confer high discrimination
they stop firing after continued stimulation --> why we soon become unaware of the clothes touching our skin
used for tasks such as reading braille
rapidly adapting sensory organs that respond to deep touch pressures (massage) found in the deep dermis and hypodermis
rail roads between cells --> desmosome connections between cells