The largest organ in the body.
about 16% of total body weight (adult)
1.2 - 2.3 m2
of surface area to the external
Variation of skin structure at different sites
classified as thick or thin skin depending on
thickness of epidermis
Thickest – skin of the upper back (~ 5 mm)
Thinnest - upper and lower eyelids (< 1 mm)
Functions of the skin
physical, biological, against UV light, from
Diff. receptors for touch, pressure, pain, temp.
the skin collects infos. about the external environment.
Thick hair (in most mammals) – for heat conservation
Adipose and sweat glands (humans)
Synthesis of Vit.D3 with UV absorption – for Ca
Adipose (subcutaneous) – major storage of
triglycerides for energy.
texture and appearance of skin, hair and nails
Subcutaneous / subcutis SC
The dermis and subcutis
contain an assortment of
skin appendages, i.e. hair
follicles, sebaceous glands,
eccrine glands EG and ducts
ED and, in some areas,
Surface layer in contact with the exterior
stratified squamous keratinizied epithelial tissue
Thickest on the palms and soles.
Epidermal cells are derived from the basal layer of
keratinocyte stem cells that undergo continuous
cell division underneath.
Newly formed cells push to the surface; they
flatten and harden as they accumulate keratin.
Eventually, the keratinized cells die and are
KeratinocytesMost numerous epidermal
cell – found in all layers of
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 sheets rather than
• Spider-shaped epithelial cells
that synthesize the protein
• Melanin is made and then
packaged into membrane-
bound granules called
• Granules are transferred to
the keratinocytes in the 2
deepest layers of the
accumulate on the
“apical part” of the
nucleus of the
protect the DNA
within the nucleus
damaged by the UV
radiation from the
People of the same size have approximately the
same number of melanocytes but may differ in their
level of activity:
Dark skinned - the melanocytes continuously produce
large amounts of melanin.
Light skinned - the melanocytes produce less
presenting cells (APC)
recognize, phagocytose, and
process foreign antigens, and
then present them to T
lymphocytes for an immune
potent stimulators of cell-
responses in the skin.
active and present in increased
numbers in epidermis and upper
dermis in many inflammatory
chemical carcinogens, immunosuppressive agents
and excessive UV light have all been shown to
reduce the number and effectiveness of Langerhans
these are all factors which predispose to the dev’t. of
rare in thin skin
in the stratum basale
contain small dense
may function as sensory
mechanoreceptors or as
1. Thick Skin
Found on soles of feet and
palms of hands and
corresponding parts of
fingers and toes.
Contains 5 epidermal
layers or strata
2. Thin skin
Found everywhere else on
Contains only 4 layers.
(lacks a stratum lucidum).
The remaining 4 layers are
thinner than those of thick
S. basale /
Stratum basale – basal layer
• “stratum germinativum”
• single layer of columnar or
cuboidal keratinocyte stem cells,
which are mitotically active.
• deepest layer
• Responsible for constant
regeneration of other layers
Stratum spinosum - prickle cell layer
deepest layer. Consists of 8-
10 layers of cells.
Cells of the lower layers can still
As cells get pushed upward, they
begin to flatten and begin to
make the precursors of keratin.
A.k.a. the “prickly layer,”
because in tissue sections, they
shrink and pull back. This makes
their exposed desmosomes
connecting adjacent cells appear
to resemble spikes or spines.
Stratum granulosum – granular layer
3 to 5 layers of flattened
cells accumulate keratohyalin
granules with phosphorylated
contain lamellar granules which
are lipid and protein rich
are discharged extracellularly to
produce a “cement” that seals
the skin to foreign objects &
most superficial layer in which
nuclei are present, but no cell
a translucent thin layer of extremely flattened
nuclei and organelles not present
filaments and desmosomes retained
cells contain eleidin, a transformation product of
3-5 layers of
in the light
Stratum corneum - keratin layer
Normally composed of flat flakes and sheets of
keratin, coated with an anti-wetting agent
synthesised by the cells of the granular layer.
Protects against mechanical abrasion – cells can
simply flake off if necessary
Prevents pathogen entry
Prevents desiccation (drying out)
Notice the 4 layers of thin skin in both
the cartoon and the photomicrograph.
• Strong, flexible
• Divided into papillary
dermis and reticular
Papillary dermis is the
upper 1/5 of the dermis
and consists of loose CT
very fine interlacing
collagen and elastic
Projects upward (as
dermal papillae) to
interdigitate and form a
strong connection with
the epidermis. Red arrow indicates the papillary dermis
• Provides an arena for immune cells to fight
• Heavily invested with blood vessels
• Arterioles, capillary loops and venules, lymphatics
• they constrict in cold weather and dilate in warm
• Also contains multiple fine sensory receptors.
• Meissner’s corpucles – touch receptors
• Free nerve endings - pain and itch
• associated w/ Merkel cells in the epidermis.
DermisReticular dermis is
lower 4/5 and consists of
dense irreg. CT.
Fibers are much larger
than in papillary dermis
Collagen – skin’s
strength and resiliency.
Elastin – skin’s ability to
stretch and recoil.
Blood vessels, nerves
and majority of the
appendages of the skin.
(↑in number - skin disease)
Blue arrow indicates the reticular dermis
Hair bulb – bulbous expansion
at the base of a hair follicle.
Hair papilla / dermal papilla
stromal core of CT
Small bld. vessels – nourishment
Nerve endings – for sensation
Germinative cell (GC) layer / Matrix - basal cells in
the hair bulb; mitotically active
Melanocytes – interspersed with germinative layer
level of melanin synthesis determine hair pigment.
Root sheath region – cells produced from GC layer /
matrix being pushed upward.
indistinguishable yet as internal and external root sheaths
As cells in the matrix are pushed toward the skin
surface, the inner 3 layers undergo keratinization.
Medulla – moderately keratinized
Cortex – highly keratinized; forms the bulk of the hair
Cuticle cells – also keratinizing, forming a hard, thin
cuticle (overlapping keratin plates) on surface of hair
Vellus hair - fine and soft hair (body hair)
In infancy, child, females
Terminal hair – coarser hair of scalp, pubic*, axillary*
*Due to male hormone secretion at puberty
Males – vellus hair replaced by terminal hair
Hair and Hair Follicles
Hair root plexus - network of
free nerve endings
Wrapped around the bulb of the
Arrector pili muscle - a bundle
of smooth muscle attached to
In times of fright or cold, these
muscles contract and cause the
hair to stand on end – and
produces goose bumps.
Increases airflow in mammals with
significant hair (i.e., not humans)
and increases the apparent size of
an animal with significant hair.
Vestigial in humans.
… still about hairs
Cross sectional shape of hairs varies bet. races
Straight hair – round (Mongols)
Wavy hair – oval (Europeans)
Curly hair – kidney shaped (Africans)
Structure of hair follicles depends on the type of
hair being produced.
Scalp and other terminal hair – long and straight
Body hair (fine) / vellus – short and plump
The arrow indicates an arrector pili muscle.
Identify the shaft, root, follicle, hair papilla, and sebaceous gland.
Situated at a point about 1/3 length of a hair follicle
from the surface.
Lies within fibrous CT sheath that surround follicle
Glandular epith. – outgrowth of external root sheath
Basal cells – generate the secretory cells
Acinar glands with several sacs
• Most have short ducts that empty into neck of
hair follicle, or onto the skin directly (eyelids,
lips, glans penis and glans clitoridis)
• Sebum is a complex mix of triglycerides, waxes,
cholesterol and esters, with mild anti-bacterial
and anti-fungal activity
• activity controlled by sex-hormones
Eccrine sweat glands
Secrete watery sweat
Frequent on palms,
soles, foreheads and
Sweating – a minor
route of excretion
Apocrine sweat glands
Produce a viscid, milky
Start to be functional
Confined to the areolae
of breasts, axillae and
• Na and Cl ions, urea and small mol. Wt. metabolites,
Coiled secretory portion coiled eccrine duct straight ascending duct
* duct may become coiled again as it pass through epidermis (acrosyringium)
- particularly apparent in the skin of the soles.
• located on dorsal distal phalanx of each finger and toe
• growth due to cells in nail matrix at nail root
• nail bed epidermis has only strata basale and spinosum
• the stratum corneum of the epidermis that overlies the
nail root forms the eponychium (cuticle)
• hyponychium or nail plate consists of the stratum
corneum of the underlying nail bed, and so is a
keratinized epithelial layer
• nail plate composed of hard keratin lying on nail bed
Afferent nervous system
• a specialized structured
• touch receptor
• confined to dermal
• most numerous on hands
• an encapsulated
• pressure and vibration
• found in deep dermis
• In palms and soles
Free nerve endings
detect pain and
system controls blood
flow and hair
are more susceptible to
the effects of ultraviolet
(UV) rays, which may
trigger mutations in
living epidermal cells.
Squamous cell carcinoma
and basal cell carcinoma
- are the most common
forms of skin cancer.
The lesions are visible as
changes in the normal
appearance of the skin, and
a biopsy is used to confirm
These lesions usually do not
metastasize rapidly, and
can be completely removed
using simple procedures
-a more serious form of skin cancer,
which begins in melanocytes.
* Any change in a pigmented spot or
mole (nevus) should prompt a
person to see a doctor.
Melanoma is serious not because of
its growth in the skin, but because it
may metastasize very rapidly to the
lungs, liver, or other vital organ.
Researchers are testing
individualized vaccines for people
who have had melanoma.
Sunscreens contain chemicals such as PABA (para-
amino benzoic acid) that block UV rays and prevent
them from damaging the epidermis.
An SPF (sun protection factor) of 15 or higher is
considered good protection.
Use of a sunscreen on exposed skin not only helps
prevent skin cancer but also prevents sunburn and
its painful effects.
It is especially important to prevent children from
getting severely sunburned, because such burns
have been linked to the development of skin cancer