The Skin

Structure of the skin

• The largest organ in the body
• Has a surface area of about 1.5–2 m2 in adults.
• Contains accessory structures: glands, hair and
nails in some areas.
• Made up of 2 major layers;
– Epidermis and
– Dermis.
• Below the dermis is subcutaneous layer
composed of areolar tissue and adipose (fat)
tissue
 Epidermis
• The most superficial layer
• Composed of stratified keratinised
squamous epithelium
• Varies in thickness- thickest on the palms
and soles.
• Does not have blood vessels or nerve
endings
• Deeper layers are bathed in interstitial
fluid from the dermis.
• Made up of several layers (strata) of
cells
• From the surface, they include:
– Stratum corneum
– Stratum lucidum
– Stratum granulosum
– Stratum spinosum
– Stratum basale also called germinative
layer
• Cells of the epidermis
• 1. Basal cells
– Found in the stratum basale
– Undergo mitosis to form keratinocytes
which form majority of the cells in the
epidermis
• 2. Keratinocytes
– Originate from the basal cells
– Make up the majority of cells in the epidermis
– As new cells are formed, the existing cells are
pushed superficially away from the stratum
basale.
– They also undergo keratinization as they are
pushed the surface
– Cells on the surface are flat, thin, non-nucleated,
dead cells, or squames, in which the cytoplasm
has been replaced by the fibrous protein keratin
• 3. Merkel cells
– Found in the stratum basale
– functions as a receptor and is responsible
for stimulating sensory nerves that the
brain perceives as touch.
• 4. Melanocyte,
– Produces the pigment melanin.
– Melanin gives hair and skin its color, and
also helps protect the skin against UV
light
• 5. Langerhans/dendritic cell
– Interspersed among the keratinocytes
– Acts as a macrophage by engulfing
bacteria, foreign particles, and damaged
cells that occur in this layer.
• Healthy epidermis depends upon
three synchronized processes:
– Desquamation (shedding) of the
keratinised cells from the surface
– Effective keratinization of cells
approaching the surface
– Continual cell division in the deeper
layers with newly formed cells being
pushed upwards to the surface
• In areas where the skin is subject to
greater wear and tear, e.g. the palms
and fingers of the hands and soles of the
feet, the epidermis is thicker and hairs
are absent.
• The pattern of ridges on the fingertips is
unique to every individual and the
impression made by them is the
‘fingerprint’
• Factors that affect skin color
– Melanin, a dark pigment secreted by
melanocytes in the deep germinative layer. It
protects the skin from UV light.
– Normal saturation of hemoglobin. When
oxygen saturation is very low, the skin may
appear bluish (cyanosis).
– Excessive levels of bile pigments in blood and
carotenes in subcutaneous fat give the skin a
yellowish color.
 Dermis
• It is tough and elastic.
• Formed from connective tissue and the matrix
containing collagen fibres interlaced with elastic
fibres.
• Rupture of elastic fibres results into stretch marks.
• Collagen fibres bind water and give the skin its
tensile strength, but as this ability declines with
age, wrinkles develop.
• Fibroblasts, macrophages, and mast cells are the
main cells found in the dermis.
• Underlying its deepest layer is the subcutaneous
layer containing areolar tissue and adipose tissue
• The structures in the dermis are:
– Blood and lymph vessels
– Sensory nerve endings
– Sweat glands and their ducts
– Hairs, arrector pili muscles, and
sebaceous glands
Blood and lymph vessels
• Arterioles form a fine network with
capillary branches supplying sweat
glands, sebaceous glands, hair
follicles and the dermis.
• Lymph vessels form a network
throughout the dermis.
 Sensory nerve endings
• Sensory receptors sensitive to touch,
temperature, pressure and pain are
widely distributed in the dermis.
 Sweat glands
• Widely distributed throughout the
skin and are most numerous in the
palms of the hands, soles of the feet,
axillae and groins.
• Formed from epithelial cells.
• There are two types of sweat gland.
• Eccrine sweat glands
– Are the more common type
– Open onto the skin surface through tiny pores.
– Produce sweat that is a clear and watery, that is
important in regulating body temperature.
• Apocrine glands
– Open into hair follicles and become active at
puberty.
– They may play a role in sexual arousal.
– These glands are found, for example, in the axilla.
– Bacterial decomposition of their secretions
causes an unpleasant odour.
 Hairs
• Grow from hair follicles.
• At the base of the follicle is a cluster
of cells called the hair papilla or bulb.
• The part of the hair above the skin is
the shaft and the remainder, the root
• Hair colour is genetically determined
and depends on the amount and type
of melanin present.
• Arrector pili
• Little bundles of smooth muscle fibres attached
to the hair follicles.
• Contraction makes the hair stand erect and
raises the skin around the hair, causing ‘goose
flesh’.
• The muscles are stimulated by sympathetic
nerve fibres in response to fear and cold.
• Erect hairs trap air, which acts as an insulating
layer.
• Sebaceous glands
• Consist of secretory epithelial cells derived
from the same tissue as the hair
follicles.
• Secrete an oily antimicrobial substance,
sebum, into the hair follicles
• Present in the skin of all parts of the body
except the palms of the hands and the soles of
the feet.
• They are most numerous in the scalp, face,
axillae and groins.
• Sebum keeps the hair soft and pliable and
gives it a shiny appearance.
• On the skin it provides some waterproofing
and acts as a bactericidal and fungicidal
agent, preventing infection.
• It also prevents drying and cracking of skin,
especially on exposure to heat and sunlight.
• The activity of these glands increases at
puberty
 Nails
• Derived from the same cells as
epidermis and hair
• They are hard, horny keratin plates
that
protect the tips of the fingers and
toes.
• The root of the nail is embedded in the
skin and covered by the cuticle, which
forms the lunula.
• The nail plate is the exposed part that
has grown out from the nail bed.
• Finger nails grow more quickly than
toe nails and growth is faster when the
environmental temperature is high
 Functions of the Skin
• 1. Protection
• It forms relatively waterproof layer which
protects the deeper, more delicate
structures.
• As an important nonspecific defense.
mechanism it acts as a barrier against:
– Invasion by micro-organisms
– Chemicals
– Physical agents, e.g. mild trauma, ultraviolet
light
– dehydration.
• The epidermis contains dendritic
(Langerhans) cells, which phagocytose
intruding antigens.
• Abundant sensory nerve endings in the
dermis enable perception, discrimination
and location of internal and external
stimuli.
• The pigment melanin protects against
harmful ultraviolet rays in sunlight.
• Regulation of body temperature
• Most heat loss from the body occurs through
the skin
• Heat loss through the skin is affected by the
difference between body and environmental
temperatures, the amount of the body surface
exposed and the type of clothes worn
• Mechanisms of heat loss
• Radiation- exposed parts of the body radiate heat
away from the body.
• Evaporation- the body is cooled as body heat
converts the water in sweat to water vapour.
• Conduction, clothes and other objects in direct
contact
with the skin take up heat.
• Convection-, air passing over the exposed parts
of the body is heated and rises, cool air replaces
it and convection currents are set up.
• When body temperature rises,
– heat loss is increased by dilation of
arterioles in the dermis, increasing blood
flow to the skin, and
– stimulation of the sweat glands causing
sweating.
• 2. Formation of vitamin D
• 7-Dehydrocholesterol is a lipid-based
substance in the skin and is
converted to vitamin D by sunlight.
• Vit D is used with calcium and
phosphate in the formation and
maintenance of bone.
• 3. Cutaneous sensation
• Sensory receptors generates nerve
impulses in sensory nerves that are
transmitted to the cerebral cortex.
• 4. Absorption
• Few substances can be absorbed
through the skin e.g.
– Some drugs, in transdermal patches,
e.g. hormones, nicotine
– Some toxic chemicals, e.g. mercury
• 5. Excretion
• The skin is a minor excretory organ
for some substances including:
– sodium chloride in sweat
– urea, especially when kidney function is
impaired
– Aromatic substances, e.g. garlic and
other spices.