Skin
Introduction
• About 15-20% of total body weight (adult).
• 1.5 - 2 m2 of surface area to the external environment.
• composed of :
1- Epidermis, an epithelial layer of ectodermal origin
2- Dermis, a layer of mesodermal connective tissue
• Beneath the dermis lies hypodermis, a loose connective tissue
that may contain pads of adipocytes.
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Introduction
• Beneath the dermis lies hypodermis, a loose connective tissue
that may contain pads of adipocytes.
• The hypodermis binds skin loosely to the underlying tissues
and corresponds to the superficial fascia of gross anatomy.
• Projections of the dermis called papillae interdigitate with
evaginations of the epidermis known as epidermal ridges.
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Introduction
• Epidermal derivatives include:
1. Hairs
2. Nails
3. Sebaceous
4. Sweat glands
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Functions of the skin
 Protection: physical, biological, against UV
light, from dehydration.
 Sensation: Diff. receptors for touch, pressure,
pain, temp.
 Thermoregulation: Thick hair (in most
mammals) – for heat conservation, Adipose
and sweat glands (humans).
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Functions of the skin
 Metabolic function:
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•
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Synthesis of Vit.D3 with UV absorption – for Ca
metabolism.
Excess electrolytes can be removed in sweat
Adipose (subcutaneous) – major storage of triglycerides for
energy.
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Epidermis
• Primarily made up of keratinized stratified
squamous epithelium(keratinocytes).
• Gives strength to the skin.
• Varies in thickness from thick skin to thin skin.
• It does not have any vascularization, so it
relies on the connective tissues deep to it.
• Also contain melanocytes, merkel’s cells and
Langerhans cell.
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Types of skin
• Thick skin (found on the palms and soles) and thin skin
(found elsewhere on the body) refer to the thickness of
the epidermal layer.
• Total skin thickness (epidermis + dermis) also
varies according to site.
• Skin on the back is about 4 mm thick
• Skin of the scalp is about 1.5 mm thick.
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Layers of epidermis
1) Stratum basale (the
deepest layer)
2) Stratum spinosum
3) Stratum granulosum
4) Stratum lucidum (only in
thick skin)
5) Stratum corneum (most
superficial layer of
epidermis)
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Stratum Basale
•
Single layer of basophilic columnar or
cuboidal cells.
•
Cells are bound to each other by
desmosomes and to basal lamina by
hemidesmosomes.
•
Characterized by intense mitotic activity.
•
is responsible with the initial portion of
the next layer, for constant production of
epidermal cells.
•
keratinocytes in the stratum basale contain
intermediate filaments composed of
keratins.
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Stratum Spinosum
•
The thickest epidermal layer.
•
polyhedral or slightly flattened.
•
Also contain the dividing cells as in
basale.
•
The keratin filaments form visible
bundles called (tonofibrils) converge
and terminate at the numerous
desmosomes .
•
Tonofibrils provide resistant to the
abrasion ( thick skin)
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Stratum Granulosum
• 3–5 layers of polygonal cells.
cytoplasm is filled with intensely
basophilic masses , keratohyaline
granules.
•
The membrane-coated lamellar
granules ovoid structures containing
many lamellae composed of various
lipids. undergo exocytosis, discharging
their contents into the intercellular
spaces of the stratum granulosum,
produces sheets that envelop the cells.
• Form a barrier against the loss of water
from skin.
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Stratum Lucidum
• Is only seen in thick skin.
• A translucent thin layer of
extremely flattened eosinophilic
cells.
• Nuclei and organelles not present
• Desmosomes retained
• The cytoplasm consists almost
only of densely packed keratin
filaments embedded in an
electron-dense matrix.
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Stratum Corneum
• The main difference between thick skin
and thin skin relates to the thickness of
the Stratum corneum.
• 15–20 layers of flattened, nonnucleated
keratinized cells.
• Cytoplasm is filled with filamentous
keratins.
• After keratinization, the cells contain only
fibrillar and amorphous proteins with
thickened plasma membranes and are
called horny, cornified cells.
•
These cells are continuously shed at the
surface of the stratum corneum.
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Epidermal cell types
• Keratinocytes (structure).
• Melanocytes (pigment).
• Merkel cells (sensation).
• Langerhans’ cells (immune defence).
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Keratinocytes
• Most numerous epidermal cell
found in all layers of the
epidermis.
• Chief function is the
production of keratin: a tough
fibrous protein that gives
strength and confers a lot of
protective ability.
• Tightly connected to one
another by desmosomes.
• Provides continuity, strength,
and protection.
• Is the reason skin flakes off in
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sheets rather than as individual
Melanocytes- neural crest
cells
• specialized cell of the epidermis found
beneath or between the cells of the
stratum basale & in the hair follicles.
• Derived from neural crest cells.
• rounded cell body.
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Melanin
• Melanin granules migrates & transferred to the
keratinocytes of the germinativum layer.
• Accumulate as a supranuclear cap which absorbs and
scatters sunlight, protecting nuclear DNA from the
deleterious effects of UV radiation.
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Langerhans cells
• These are star shaped bone
marrow derived cells. forming
a fairly dense network in the
epidermis Migrated to the skin
via blood.
• Present in the S. spinosum of
the skin, represent 2–8% of the
epidermal cells
• These cells are antigen
presenting cells so participate
in immune related reaction.
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Merkel’s cell
• Mechanoreceptors resemble palestaining keratinocytes with keratin
filaments in their cytoplasm but few
if any melanosomes.
• Neural crest in origin.
• Found in the basal layer of the thick
skin & bases of hair follicles.
• Has sensory function.
• have functions related to the diffuse
neuroendocrine system.
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Dermis
• Dermis is connective tissue
–Support epidermis
– Binds epidermis to hypodermis Its
surface
• The thickness : varies (maximum of
4 mm on the back)
• Projections (dermal papillae)
interdigitate with projections
(epidermal pegs or ridges) of the
epidermis
• Contains 2 layers :
– Papillary layer
– Reticular layer
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Dermis
• A basement membrane found between the
stratum basale and papillary layer.
• The basement membrane consisting of the
basal lamina and the reticular lamina and can
usually be seen with the light microscope.
• Nutrients diffuse into the avascular epidermis
from the dermis vasculature through this
basement membrane.
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Papillary layer
• The major part of the dermal
papillae.
• 1/5 of the dermis.
• Is composed of loose
connective tissue, with
fibroblasts and other cells
such as mast cells and
macrophages.
• Anchoring fibrils of type
VII collagen insert into the
basal lamina and bind the
dermis to the epidermis
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Reticular layer
• Irregular dense connective
tissue (type I collagen).
• Reticular dermis is the
lower 4/5.
• More fibers and fewer cells
than the papillary layer.
• A network of elastic fibers is
also present providing
elasticity to the skin.
Blue arrow indicates the reticular dermis
Dermis
• In addition, dermis contains epidermal
derivatives :
– Hair follicle
– Sweat & sebaceous glands
• Rich network of blood & lymph vessels and
nerves supply.
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Subcutaneous Tissue
• Loose connective tissue which
helps in sliding the skin over the
deep structure.
• Also called the hypodermis or
superficial fascia
• Often contains fat cells that vary
in number in different regions of
the body and vary in size
according to nutritional state.
• An extensive vascular supply in
the subcutaneous layer promotes
rapid uptake of insulin or drugs
injected into this tissue.
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Appendages of the skin
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•
•
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Hairs
Sebaceous gland
Sweat gland
Nails
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Hair
• Elongated keratinized structure.
• Develop from epidermis & invade the dermis & hypodermis.
• Everywhere except palms, sole, some part in genitalia.
• Hairs grow discontinuously, have periods of growth ollowed
by period of rest .
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Arrector pili muscle
• The surrounding dermis forms a
connective tissue sheath.
Running from a midpoint on this
sheath and to the dermal
papillary layer is a small bundle
of smooth muscle cells, the
arrector pili muscle.
• Contraction of these muscles
pulls the hair shafts to a more
erect position
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Keratinization of the hair
• Keratin of hair has a harder and more compact
nature than that of stratum corneum, maintaining its
structure much longer.
• Although keratinization in the epidermis occurs
continuously and over the entire surface, it is
intermittent in the hair and occurs only in the hair
root.
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Nail
• Hard, flexible plates of
keratin on the dorsal surface
of each distal phalanx
• The nail root is the most
proximal region of the nail
plate , is formed by a matrix
of proliferating,
differentiating keratinocytes.
• These cells make up the nail
matrix which contribute
keratinized cells to the nail
root.
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Sebaceous gland
• Embedded in the dermis over
most of the body surface, except
the thick, hairless skin of the
palms and soles.
• Branched acinar glands with
several acini converging at a
short duct which usually empties
into the upper portion of a hair
follicle.
• In certain hairless regions, such
as the genital glands, eyelids,
and nipples, sebaceous ducts
open directly onto the epidermal
surface.
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Sebaceous gland
• The acini consist of a basal layer of
undifferentiated flattened epithelial
cells on the basal lamina.
• Their nuclei shrink and undergo
autophagy along with other organelles
and near the duct the cells disintegrate
and release the lipids via holocrine
secretion.
• The product of this process is sebum
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Eccrine sweat gland
• Are widely distributed in the skin.
• Most numerous on the soles of the
feet.
• Sweating is the physiological
response to increased body
temperature and in humans the most
effective means of temperature
regulation.
• The secretory portions and ducts are
coiled and have small lumens.
• Secretory part is generally more
pale-staining than the ducts and has
stratified cuboidal epithelium
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Apocrine gland
• Largely confined to skin of the
axillary and perineal regions
• much larger lumen than
Eccrine gland.
• The secretory portions of
apocrine sweat glands consist
of simple cuboidal, eosinophilic
cells with numerous apical
secretory granules that undergo
exocytosis.
• Their cells show merocrine, not
apocrine, secretion
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