Structure and function of the skin
The functions of the skin
include:
1 Physical protection
2 Protection against UV light
3 Protection against
microbiological invasion
4 Prevention of fluid loss
5 Regulation of body temperature
6 Sensation
7 Immunological surveillance.
The epidermis
• The epidermis is composed of
stratified squamous epithelium.
• It is derived from ectoderm.
• Epidermal cells undergo
keratinization in which their cytoplasm is replaced with
keratin as the cell dies and
becomes more superficial.
The epidermis is composed of the
following five layers, from deep to
superficial.
1 Stratum germinativum
• This is also known as the basal
layer.
• The cells within this layer have
cytoplasmic projections, which firmly link them
to the underlying basal lamina.
• This is the only actively
proliferating layer of skin.
• The stratum germinativum contains
melanocytes.
2 Stratum spinosum
• The stratum spinosum is also
known as the prickle cell layer.
• This layer contains large
keratinocytes which produce keratin.
• The cells within this layer are
joined to each other by tonofibrils (prickles).
3 Stratum granulosum
• The stratum granulosum contains
mature keratinocytes, which possess cytoplasmic
granules of keratohyalin.
• This layer is called the stratum
granulosum because of these granules.
• The stratum granulosum is the
predominant site of protein synthesis.
4 Stratum lucidum
• This is a clear layer.
• The stratum lucidum is only
present in the thick skin of the palms and feet.
5 Stratum corneum
• The stratum corneum contains
non-viable keratinized cells.
• The thick cells of this layer
protect against trauma.
• The stratum corneum:
• Insulates against fluid loss
• Protects against bacterial
invasion.
• Sebum produced by the sebaceous
glands of the stratum corneum is bactericidal
to both streptococci and
staphylococci.
Cellular composition of the
epidermis
• Keratinocytes are the predominant
cell type in the epidermis.
• Langerhans cells form part of the
immune system and function as antigenpresenting
cells.
• Merkel cells are mechanoreceptors
of neural crest origin.
• Melanocytes:
• Are neural crest derivatives
• Are usually located in the
stratum germinativum
• Produce melanin, which protects
the surrounding skin by absorbing UV
light.
The dermis
• The dermis accounts for 95% of
the thickness of the skin.
• The papillary dermis is
superficial and contains more cells and finer collagen
fibres.
• The reticular dermis is deeper
and contains fewer cells and coarser collagen
fibres.
The dermis is composed of the following.
Collagen fibres
• These fibres are produced by
fibroblasts.
• They are responsible for much of
the strength of the skin.
• The normal ratio of type 1 to
type 3 collagen is 5 : 1.
Elastin fibres
• These are secreted by
fibroblasts.
• They are responsible for the
elastic recoil of the skin.
Ground substance
• This consists of the
glycosaminoglycans (GAGs), hyaluronic acid, dermatan
sulfate and chondroitin sulfate.
• GAGs are secreted by fibroblasts
and become ground substance when hydrated.
Vascular plexus
• This separates the denser
reticular dermis from the overlying papillary dermis.
Skin appendages
The skin contains the following
appendages.
Hair follicles
• Each hair is composed of a
medulla, cortex and outer cuticle.
• The hair follicle consists of an
inner root sheath, derived from the epidermis, and
an outer root sheath, derived
from the dermis.
• Several sebaceous glands drain
into each follicle. Discharge from these glands is
aided by the contraction of
erector pili muscles.
• Velus hairs are fine and downy.
• Terminal hairs are coarse.
• Hairs are in either the telogen
or the anogen phase.
• 75% of hairs are in the anogen
(growth) phase at any one time.
• The remaining 25% of hairs are in
the telogen (resting) phase.
Eccrine glands
• These sweat glands secrete an
odourless hypotonic fluid.
• They are present in all sites of
the body.
• Eccrine glands occur more
frequently in the eyelids, palms, feet and axilla.
Apocrine glands
• These are located in the axilla
and groin.
• They emit a thicker secretion
than eccrine glands.
• They are responsible for body
odour.
• Hidradenitis suppurativa is an
infection of the apocrine glands.
Sebaceous glands
• These are holocrine glands that
usually drain into the pilosebaceous unit.
1• They drain directly onto the
skin in the labia, penis and tarsus (meibomian
glands).
• They occur more frequently on the
forehead, nose and cheek.
• Sebaceous glands are not the
sole cause of so-called sebaceous cysts. These cysts
are in fact of epidermal origin
and contain all of the substances secreted by the skin
(predominantly keratin).
• Some authorities maintain that
they should therefore be called epidermoid
cysts.
Types of secretion from glands
• Eccrine or merocrine glands
secrete opened vesicles via exocytosis.
• Apocrine glands secrete
unbroken vesicles which later discharge.
• Holocrine glands secrete whole cells which then
disintegrate.
Histological terms
• Acanthosisahyperplasia of the
epithelium.
• Papillomatosisaan increase in the
depth of the corrugations at the junction
between epidermis and dermis.
• Hyperkeratosisaan increase in the
thickness of the keratin layer.
• Parakeratosisathe presence of nucleated cells at the
skin surface.
