MSS - Histology #3 (skin - epidermis "part 2")

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Epidermis = 5 layers of keratinocytes + melanocytes + langerhans cells + merkel's cells

Factors affecting colour of skin?

• content of melanin and carotene
• number of blood vessels in dermis
• colour of blood flowing in them

"eumelanin" is _____ produced by melanocytes

a dark-brown pigment

Melanocytes are specialised cell of epidermis found ______ and in _____

beneath or between cells of "stratum basale",
hair follicles

"pheomelanin" is a pigment found in ______ and contains ______

red hair,
cysteine

melanocytes are derived from ______

neural crest cells

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

"melanin" is made in melanocytes, with ____ playing important role in that process

tyrosinase

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, & _______

on ribosomes,
accumulated in vesicles formed in Golgi

_______ involved in development of "mature melanin granule"

4 stages

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

melanin granules are essentially _______

injected into kerationcytes

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)

with lysosomes

Interaction between keratinocytes and melanocytes _________

creates pigmentation of skin

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

____ mechanism may exist between keratinocytes and melanocytes

feedback

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

_______ doesn't influence skin number of melanocytes per unit area

sex and race

differences in skin colour are mainly due to _______

number of melanin granules in keratinocytes

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,
"addison disease"

what is "albinism"?

a hereditary inability of melanocytes to make melanin

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 _______

star-shaped cells,
2-8% of epidermal cells

"langerhans cells" derived from ___, carried to skin by blood and capable of ______

bone marrow,
binding, processing & presenting antigens to T lymphocytes

"langerhans cells" are ________ & have important role in immunological skin rxns

antigen-presenting cells

"merkel's cells" are generally presen in ______

thick skin of palms and soles

"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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