The Skin Flashcards
The skin
AKA the integument
largest single organ in the body
has two components: the epidermis (superficial epithelial layer) and the dermis (the underlying CT)
- there is a loose CT hypodermis underneath the dermis, which is not part of the skin)
functions of the skin
protection
sensation
thermoregulation
metabolism
thick vs thin skin
thick skin has a thicker epidermis, and is typically found in areas that are subject to considerable abrasions, such as the palms of hands or the soles of feet
- thick skin has sweat glands, but lacks hair follicles and sebaceous glands
- thin skin has a thin epidermis, and contains hair follicles, sweat glands, and sebaceous glands
epidermis layers
thin skin has 4 distinct layers, thick skin has 5
from deep to superficial:
1. strata basale
2. strata spinosum
3. strata granulosum
4. strata lucidum (thick only)
5. strata corneum
cells of the epidermis
epidermis is stratified squamous epithelium, mostly composed of keratinocytes
- keratinocytes arise in the strata basale, and migrate until they reach the stratum corneum
- epidermis also contains melanocytes, merkel cells (detect sensation), and Langerhan cells (phagocytes)
stratum basale composition
deepest layer, has a single layer of cuboidal to columnar cells that rest on the basement membrane of the epidermis, at the dermal-epidermal junction
stratum basale characteristics
characterized by intense mitotic activity and contains progenitor cells for all the epidermal layers
- division of basal cells replace the more superficial keratinocytes that are lost at the epithelial surface
stratum basale contains melanocytes
Merkel cells
found in basal layer of epidermis, and are most abundant in the fingertips and at the bases of hair follicles
- sensitive mechanoreceptors essential for light touch and sensation
- joined to keratinocytes by desmosomes
basal lamina
cells of the stratum basale rest on the basal lamina, and are anchored there by hemidesmosomes, into which tonofilaments insert
desmosomes bind basal lamina together via lateral and upper surfaces
divisions of the basal lamina
lamina lucida
lamina densa
both of these rest on the reticular lamina of the dermis
stratum spinosum
consists of 3-10 layers of polygonal keratinocytes with euchromatic nuclei
- each time a basal cell divides in the stratum basale, a daughter cell is pushed to the stratum spinosum, where it begins to differentiate into a keratinocyte
keratinocytes of the stratum spinosum
have a centrally located nucleus with nucleoli and cytoplasm that actively synthesizes keratins
- keratin filaments bundle to form tonofibrils
adjacent keratinocytes are attached by desmosomes, creating a spiny appearance
stratum spinosum of thick skin
thick skin is subject to continuous friction and pressure, and as such have a thicker stratum spinosum with more abundant tonofibrils and desmosomes
langerhan cells
APCs and macrophages
most commonly found in stratum spinosum, but present in all epidermis layers
stratum granulosum
3-5 layers thick, composed of flattened keratinocytes filled with prominent keratohyalin granules
- keratohyalin and tonofibrils combine to form soft skin keratin
- simultaneously, the nuclei and other organelles in these cells degenerate, resulting in production of keratinized scales, which form the stratum lucidum and stratum corneum
keratinocytes of the stratum granulosum
contain lamellar granules that release lipid contents by exocytosis into the intercellular space to form the epidermal water barrier
stratum lucidum
only present in thick skin, thin translucent layer
- cells lack organelles and have flattened nuclei, densely packed, and filled with keratin filaments
stratum corneum
most superficial layer of the epidermis, and consists of numerous layers of dead keratinized stratified squamous epithelium
- dry, dead layer helps prevent penetration of microbes and the dehydration of underlying tissues. it also provides protection against abrasion
process of keratinization
- keratinocytes in the stratum basale produce intermediate filaments (tonofilaments) composed of the protein keratin. filaments aggregate to form tonofibrils, which are prominent in the stratum spinosum
- in the stratum granulosum, keratohyalin granules and tonofibrils condense to form keratin; simultaneously, organelles and nuclei are lost. keratin formed by this process is soft keratin of skin
- the resulting flattened, keratin filled squamous forms stratum corneum, which are continuously shed at the cell surface
the dermis
deep to the epidermis, and has 2 major components:
1. the papillary layer: superficial, consists of loose CT. has dermal papillae that project between epidermal ridges of the dermal-epidermal interdigitations
- the reticular layer: composed of dense irregular CT and contains large blood vessels, nerves, and sometimes sweat glands
sensory receptors
the skin contains different sensory receptors located in the epidermis, dermis, and hypodermis, which allow the skin to respond to stimuli like touch, pressure, and vibration
Meissner’s corpuscles
encapsulated sensory receptors located at the apex of the sensory dermal papillae
- provide fine touch discrimination, and are most concentrated in fingertips, palms, soles, and lips
these corpuscles are located in the dermis and project into the epidermis
Pacinian Corpuscles:
encapsulated sensory receptors located in the deep layer of the dermis and in the hypodermis
- respond to pressure and vibration
associated skin structures
sweat glands, hair follicles, and nails
sweat glands
develop of long epidermal invaginations embedded in the dermis
- two types: eccrine sweat glands and apocrine sweat glands
eccrine sweat glands
simple coiled tubular glands widely distributed in the skin, most numerous on soles of feet
- secrete thin and watery secretions onto skin surface to cool and reduce body temperature
- secretory unit is pale staining, and simple cuboidal or columnar epithelium
- the duct is long and unbranched, travels through the dermis to epidermis, opens onto surface of skin. comprised of stratified cuboidal
apocrine sweat glands
simple coiled tubular glands, large in size with a thick lumen
- ducts open into hair follicles, limited to the anal, areolar, and axillary regions
- develop after puberty, produce a thick, protein rich, and odorous secretion
hair follicles
invaginations of the epidermal epithelium, extending through the dermis into the hypodermis
- hair is made of keratinized cells
- all skin has at least minimal hair except palms, soles, lips, penis, clitoris, and labia minor
structures associated with hair
sebaceous gland: secretes sebum
follicle sheath: dense CT sheath that surrounds hair follicle, provides attachment for arrector pili muscle
hair shaft: extends beyond skin surface
arrector pili muscle: smooth muscle, contractions elevate the hair (goosebumps)
dermal papilla: contains capillary network to sustain hair
