Skin
Epidermis:
o Mainly stratified squamous keratinized epithelium composed of cells called keratinocytes
o Less abundant cells: melanocytes, antigen-presenting Langerhans cells, and Merkel cells.
o Receives oxygen by diffusion from dermis
o From the dermis, the epidermis consists of 4 layers of keratinocytes (5 in thick skin):
Basal layer (Stratum basale):
Single layer of basophilic cuboidal or columnar cells
Joined by hemidesmosomes to the basal lamina and desmosomes bind them to
lateral and upper surfaces.
Intense mitotic activity
Human epidermis is renewed every 15 to 30 days
Has cytoskeletal keratin which are intermediate filaments where cells move
upward on
Spinous layer (stratum spinosum)
Thickest layer
Synthesizes keratins
Keratins assemble to form bundles called tonofibrils that converge and terminate
at the desmosomes holding the cell layers together.
Granular layer (stratum granulosum)
3 to 5 layers of flattened cells undergoing terminal differentiation process of
keratinization.
Cytoplasm is filled with basophilic masses called keratohyaline granules.
o These are dense masses of filaggrin and other proteins associated with
keratins of tonofibrils.
Lamellar granules (derived from Golgi) have various lipids that exocytose these
lipids producing a lipid-rich impermeable layer around the cells.
Stratum Lucidium:
Found ONLY in thick skin
Contains thin layer of eosinophilic keratinocytes held together by desmosomes.
No nuclei or cytoplasm. Just keratin filaments in cytoplasm.
, Stratum Corneum:
15 to 20 layers of squamous keratinized cells filled with keratin filaments.
Has squames which are fully keratinized cells which are continuously shed at the
epidermal surface.
Melanocytes:
o Skin color depends on keratinocytes’ content of melanin and carotene and the number of blood
vessels in the dermis.
o Eumelanins are brown or black pigments produced by the melanocyte.
Melanocytes are found in basal layer and in hair follicles.
o Pheomelanin is a similar pigment found in red hair
o First step in melanin synthesis is catalyzed by tyrosinase, a transmembrane enzyme in golgi-
derived vesicles.
Tyrosinase converts tyrosine into 3,4-dihydroxyphenylalanine (DOPA), which is then
further transformed into the different forms of melanin.
o Melanosomes (granules of accumulated melanin pigments) are transported via kinesin to the
tips of cytoplasmic extensions.
Neighboring keratinocytes phagocytose the tips of these extensions, take in the
melanosomes and transport them by dynein toward their nuclei.
Melanocytes produce melanosomes but the keratinocytes are the melanin depot and
contain more of this pigment than the cells that make it.
o Epidermal-melanin unit = 1 melanocyte + the keratinocytes into which it transfers
melanosomes.
Langerhans cells:
o They are antigen-presenting cells
o Represent 2% to 8% of epidermal cells
Merkel cells:
o Also called epithelial tactile cells
o They are sensitive mechanoreceptors essential for light touch sensation
o Abundant in highly sensitive skin like fingertips and bases of hair follicles
o Has no melanosomes
Dermis:
Epidermis:
o Mainly stratified squamous keratinized epithelium composed of cells called keratinocytes
o Less abundant cells: melanocytes, antigen-presenting Langerhans cells, and Merkel cells.
o Receives oxygen by diffusion from dermis
o From the dermis, the epidermis consists of 4 layers of keratinocytes (5 in thick skin):
Basal layer (Stratum basale):
Single layer of basophilic cuboidal or columnar cells
Joined by hemidesmosomes to the basal lamina and desmosomes bind them to
lateral and upper surfaces.
Intense mitotic activity
Human epidermis is renewed every 15 to 30 days
Has cytoskeletal keratin which are intermediate filaments where cells move
upward on
Spinous layer (stratum spinosum)
Thickest layer
Synthesizes keratins
Keratins assemble to form bundles called tonofibrils that converge and terminate
at the desmosomes holding the cell layers together.
Granular layer (stratum granulosum)
3 to 5 layers of flattened cells undergoing terminal differentiation process of
keratinization.
Cytoplasm is filled with basophilic masses called keratohyaline granules.
o These are dense masses of filaggrin and other proteins associated with
keratins of tonofibrils.
Lamellar granules (derived from Golgi) have various lipids that exocytose these
lipids producing a lipid-rich impermeable layer around the cells.
Stratum Lucidium:
Found ONLY in thick skin
Contains thin layer of eosinophilic keratinocytes held together by desmosomes.
No nuclei or cytoplasm. Just keratin filaments in cytoplasm.
, Stratum Corneum:
15 to 20 layers of squamous keratinized cells filled with keratin filaments.
Has squames which are fully keratinized cells which are continuously shed at the
epidermal surface.
Melanocytes:
o Skin color depends on keratinocytes’ content of melanin and carotene and the number of blood
vessels in the dermis.
o Eumelanins are brown or black pigments produced by the melanocyte.
Melanocytes are found in basal layer and in hair follicles.
o Pheomelanin is a similar pigment found in red hair
o First step in melanin synthesis is catalyzed by tyrosinase, a transmembrane enzyme in golgi-
derived vesicles.
Tyrosinase converts tyrosine into 3,4-dihydroxyphenylalanine (DOPA), which is then
further transformed into the different forms of melanin.
o Melanosomes (granules of accumulated melanin pigments) are transported via kinesin to the
tips of cytoplasmic extensions.
Neighboring keratinocytes phagocytose the tips of these extensions, take in the
melanosomes and transport them by dynein toward their nuclei.
Melanocytes produce melanosomes but the keratinocytes are the melanin depot and
contain more of this pigment than the cells that make it.
o Epidermal-melanin unit = 1 melanocyte + the keratinocytes into which it transfers
melanosomes.
Langerhans cells:
o They are antigen-presenting cells
o Represent 2% to 8% of epidermal cells
Merkel cells:
o Also called epithelial tactile cells
o They are sensitive mechanoreceptors essential for light touch sensation
o Abundant in highly sensitive skin like fingertips and bases of hair follicles
o Has no melanosomes
Dermis:
