Skin Structure
Skin is made of 3 layers:
1) epidermis
– stratified squamous epithelium cells
– constantly replaced from inside out
– entirely from ectoderm
2) dermis
– “true skin”
3) hypodermis – subcutaneous layer
– last to develop
– immigration of cells from other body
regions
Epidermis
squamous cells stacked sequentially
by levels of maturity
older cells on the outside
innermost cells are metabolically
active and constantly undergoing
mitosis =
stratum basale
The stratum basale and any newly
dividing cells immediately above the
stratum basale =
stratum germinativum
stratum – layer
basale – innermost portion
germinativum – part that produces
other layers
The cells of the epidermis start out
from the germinativum as cuboidal
or columnar cells, then flattened as
they migrate to the surface.
it takes 60-75 days for cells to reach
the outermost skin from the
germinativum
mitosis in the germinativum occurs 2
ways:
1) cytokinesis in the horizontal plane
→ an outer and inner cell
– new skin layers
– stimulated by vitamin A
– accelerated by continuous pressure or
rubbing
2) cytokinesis in the vertical plane →
growth of skin
– response to growth and stretching
Cells of the stratum germinativum
are located on ridges called dermal
papillae
These projections of the underlying
dermis are bound to the epidermis
cells by a thin layer of fibrous
connective tissue =
basement layer
Interspersed in and on top of the
stratum germinativum are the
melanocytes in a layer called the
Malpighian layer
These cells secrete melanin – a black
to brown colored chemical made of
amino acid building blocks
Some people take the amino acid
tyrosine in an attempt to enhance
melanin production during tanning
no proof of effectiveness
As germinativum cells move up they
form the next layer called the
stratum spinosum (prickly layer)
many desmosomes on the outer
surface of these cells
mixed in with these cells are
Langerhans cells
Stratum spinosum cells give rise to
the next layer of epidermis called the
stratum granulosum
– named for granules in the cytoplasm
accumulation of yellowish sulfur-rich protein
= keratin
cells = keratocytes
keratin
tough protein
gives strength to skin
Keratocytes produce a substance
that acts as a waterproofing to
prevent the free flow of water into
and out of the skin
Stratum granulosum cells flatten out
into a layer of waterproof cells =
stratum compactum
– dying cells filled with keratin
In thick skin areas,
stratum compactum → stratum
lucidum
– breakable layer
The uppermost layer of skin =
stratum corneum
flattened dead cells
shed during desquamation
Dermis
thick layer of connective tissue
attached to the stratum
germinativum
majority of dermis is dense, irregular
connective issue
the dermis also contains loose
connective connective tissue called
areolar connective tissue
areolar connective tissue
used throughout the body to bind
blood vessels, membranes, muscles,
nerves, and skin to other structures
extensive meshwork of protein fibers
secreted by fibroblasts
fibropblasts
body growth
connective tissue maintenance
wound healing
Subcutaneous Layer
inner most layer of skin
thickness and makeup varies
throughout the body and form
person to person
very thin or absent in the areas of
the:
– eyelids
– penis
– scrotum
– nipples
The subcutaneous layer is
responsible for increases in female
hips and breasts after puberty
The subcutaneous layer differs from
the dermis in many ways
1) composed of loosely arranged
elastic fibers that anchor the skin to
underlying fascia
– sometimes the subcutaneous layer is
directly attached to bone and joint
surfaces
2) contains large amounts of adipose
tissue (fat cells)
– thickness of the adipose tissue =
indirect indicator of body fat
3) contains large blood vessels that
branch out into capillaries into the
dermis
lymph vessels are found along with
the blood vessels (part of the
immune system)
the subcutaneous layer is still
susceptible to infection = fasciitis
deep nerves are located in the
subcutaneous layer
desmosome – cellular junction
Langerhan cells – immune system cells
that are important in fighting skin
infections and healing injured skin
fascia – a sheet of fibrous tissue that
covers muscle, skull bones, and
some organs
fasciitis – an irritation of the
subcutaneous layer; can also result
from injuries that leave bone
fragments or other material in skin
that is prone to pressure and
stretching