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Factors affecting colour of skin?
• content of melanin and carotene
• number of blood vessels in dermis
• colour of blood flowing in them
Melanocytes are specialised cell of epidermis found ______ and in _____
beneath or between cells of "stratum basale",
melanocytes have rounded cell bodies from which ___________ (running between cells of strata basale & spinosum)
long irregular extensions branch into epidermis
Tips of extensions (of melanocytes) terminate in ______
invaginations of cells in strata basale and spinosum
Electron microscope of melanocytes?
• pale-staining cells
• numerous small mitochondria
• well-developed golgi
• short cisternae of RER
melanocytes aren't attached to adjacent keratinocytes by desmosomes, but they are ________
bound to basal lamina by hemidesmosomes
Result of tyrosinase activity?
"tyrosine" transformed 1st into --> 3,4-dihydroxy phenylalanine (dopa) --> then into "dopaquinone" --> after a series of transformations, they become "melanin"
"tyrosinase" is made _____, transported in lumen of RER of melanocytes, & _______
accumulated in vesicles formed in Golgi
stage 1 of "mature melanin granule"?
• vesicle surrounded by membrane
• shows beginning of tyrosinase activity
• formation of fine granular materials
• at periphery, electron-dense strands have an orderly arrangement of tyrosinase molecules on protein matrix
stage 2 of "mature melanin granule"?
• vesicle (melanosome) is ovoid
• in inferior of melanosome = there are parallel filaments with periodicity of 10nm
• melanin deposited on protein matrix
stage 3 of "mature melanin granule"?
Increased melanin formation --> makes periodic fine structure less visible
stage 4 of "mature melanin granule"?
• "mature melanin granule" is visible in LM
• melanin completely fills vesicle
• no ultrastructure is visible
• mature granules are ellipsoid (1 μm long, diameter of 0.4μm)
Once formed, melanin granules _______ & transferred to cells of strata germinativum and spinosum
migrate within cytoplasmic extensions of melanocytes
once inside keratinocytes, melanin granules _______ ; thus protecting nuclei from UV radiation
accumulate in supranuclear region of cytoplasm
Although melanocytes make melanin, epithelial cells act as a depot and _______
contain more melanin that melanocytes
Within keratinocytes, melanin granules fuse _______ (reason why melanin disappears in upper epithelial cells)
factors affecting pigmentation of skin?
• rate of formation of melanin granules within melanocytes
• transfer of melanin granules into keratinocytes
•disposition of melanin granules by keratinocytes
melanocytes can be easily seen in _______; this compound converted to _________ (catalysed by tyrosinase)
incubating fragments of epidermis in dopa,
drak brown deposits of melanin in melanocytes
Pattern of distribution of melanocytes per unit area of epidermis is ?
"epidermal-melanin unit" --> i.e. melanocytes aren't distributed randomly
ratio of dopa-positive melanocytes to keratinocytes in "stratum basale"?
• in skin of thigh --> 1000 melanocytes/mm²
• in skin of scrotum --> 2000 melanocytes/mm²
2-step process of skin tanning (darkening after exposure to UV)?
1) physico-chemical rxn darkens pre-existing melanin --> releases it rapidly to keratinocytes
2) rate of melanin synthesis in melanocytes accelerates --> increasing amount of melanin
Lacks of cortisol from adrenal cortex causes ________ which increases skin pigmenation; example is _______
overproduction of adrenocorticotropic hormone,
what are the causes for "albinism"?
• absence of tyrosinase activity, or
• inability of cells to take up tyrosine
result from "albinism"
skin isn't protected from solar radiation by melanin --> greater incidence for BCC or SCC
"vertiligo" is _______ which is caused by _________
a depigmenatation disorder,
degeneration & disapperance of entire melanocytes
"langerhans cells" are ______ found in "stratum spinosum" of epidermis; represent _______
2-8% of epidermal cells
"langerhans cells" derived from ___, carried to skin by blood and capable of ______
binding, processing & presenting antigens to T lymphocytes
"langerhans cells" are ________ & have important role in immunological skin rxns
"merkel's cells" somewhat resemble epidermal epithelial cells but have _______
small dense granules in their cytoplasm (unknown composition)
________ are present at base of "merkel's cells"
free nerve endings (that form expanded terminal disk)
Functions of "merkel's cells"?
• may serve as sensory mechanoreceptors
• related to diffuse neuroendocrine system
Picture 1 - diagram of melanocyte
• Its processes extend into interstices between keratinocytes
• melanin granules are made in melanocyte, migrate to its processes, and transferred into cytoplasm of keratinocytes
Picture 2 - EM of melanocytes & keratinocytes
• greater abundance of melanin granules in keratinocyte (at right) than in adjacent melanocyte.
• clear material at the bottom is dermal collagen
Picture 3 - main features of melanogenesis
• Tyrosinase made in RER --> accumulated in vesicles of Golgi (Free vesicles = melanosomes)
• Melanin synthesis begins in stage 2 melanosomes: melanin accumulated
• go to stage 3 : loses its tyrosinase activity --> becomes melanin granule --> Melanin granules migrate to tips of melanocyte's processes --> transferred to keratinocytes of malpighian layer
Picture 4 - stratum spinosum showing the localised deposits of melanin covering the cell nuclei
• Melanin protects DNA from UV radiation
• highest concentration of melanin occurs in cells that are more deeply localised; they divide more actively. (DNA of cell populations that multiply more actively is esp. sensitive to harmful agents.)
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