242
UNIT 3
Organ Systems
Introduction and Key Concepts
for the Integumentary System
The skin and its accessory structures form the
integumentary
system
. The skin covers the entire surface of the body and is
the largest organ of the body in terms of its weight and volume.
The accessory structures of the skin include hair, nails, and three
types of glands:
sebaceous
,
eccrine sweat
, and
apocrine
. The
skin is composed of several types of tissues: epithelium, con-
nective tissue, muscles, blood vessels, and nervous tissue. The
functions of skin include
(1)
Protection
: The skin serves as a
barrier between the internal tissues and the outside world, pre-
venting damage to the internal tissues by physical trauma, toxic
chemicals, radiation, and sunlight.
(2)
Prevention of dehydra-
tion
: The skin forms a waterproof barrier, which prevents the
loss of body fl
uids.
(3)
Regulation of body temperature
: Evapo-
ration of sweat released onto the body surface by the eccrine
glands as well as dilation of the capillary network and arterio-
venous anastomoses (shunts) in the skin help to regulate body
temperature.
(4)
Somatosensory function
: Sensory receptors in
the skin transduce physical energy in an individual’s surround-
ings into action potentials that are carried by peripheral nerves
to the central nervous system where the sensations of touch,
pressure, pain, warmth, cold, vibration, etc. are generated.
(5)
Immunological function
: The Langerhans cells and lympho-
cytes in the skin play roles in the cutaneous immune response.
(6)
Production of vitamin D
: Vitamin D, an essential vitamin,
is synthesized from precursors in the skin under the effects of
steroids and sunlight.
Layers of the Skin
The skin can be divided into two basic layers:
epidermis
and
dermis
. The
epidermis
is a maximally keratinized strati-
F
ed squamous epithelium, which is composed of F
ve named
layers of cells called
keratinocytes
. (1) The
stratum basale
is
the deepest layer of the epidermis and it borders the dermis.
A single layer of cuboidal or tall cuboidal cells lies on the base-
ment membrane. Many of these cells are
stem cells
that actively
divide and give rise to the cells in the other four layers. The epi-
dermal keratinocytes are renewed constantly, with the top layer
of cells continually being shed and new cells from the stratum
basale replacing them. It takes about 3 to 4 weeks for kerati-
nocytes to F
nish their renewal cycle. In addition to the kerati-
nocyte stem cells, two special types of cells,
melanocytes
and
Merkel cells
(
Merkel disks
), are found in the stratum basale.
The melanocytes are melanin producing cells which are in con-
tact with the keratinocytes that are located immediately above
the stratum basale (±ig. 13-7A,B). The Merkel cells (Merkel cell
neurite complexes or Merkel disks) are sensory receptor cells,
which respond to continuous touch stimuli. (2) The
stratum
spinosum
contains polyhedral keratinocytes, which become
more fl
attened in the superF
cial part of this layer. The plasma
membrane of neighboring cells is connected by
desmosomes
(
macula adherens
).
Langerhans cells
(modiF ed macrophages)
are an additional cell type often found in this layer. (3) The
stratum granulosum
contains keratinocytes, which are fl attened
cells with keratohyalin granules in their cytoplasm. These gran-
ules are basophilic in appearance in H&E stained sections (±ig.
13-3B). This layer is more prominent in the thick skin than in
the thin skin. (4) The
stratum lucidum
is a thin layer that is
only found in the thick skin. It contains a few layers of fl
attened
cells, which are densely packed together and lie beneath the
stratum corneum. Their nuclei become pycnotic as they begin
to degenerate. (5) The
stratum corneum
is the most superF cial
layer, which contains numerous extremely fl
attened cells com-
pletely F
lled with keratin. These cells have no nuclei or organ-
elles and are technically dead cells. The cells on the surface are
continuously shed. The
dermis
is a connective tissue layer deep
to the epidermis. It contains the blood vessels, nerves, and affer-
ent sensory receptors, including Meissner corpuscles and free
nerve endings. The
hypodermis
is a transition (subcutaneous)
layer below the dermis of the skin, which contains loose con-
nective tissue, adipose tissue, nerves, arteries, and veins (±igs.
13-2 and 13-4A).
Thick Skin Versus Thin Skin
Thick skin
is found in only a few places in the body, such as
the palms of the hands and soles of the feet. It has a very thick
epidermis. The stratum corneum is particularly prominent,
being about 10 times thicker than that of thin skin. Thick
skin has numerous eccrine sweat glands, but has no sebaceous
glands or apocrine sweat glands. In contrast,
thin skin
, which
covers the rest of the body, has a thin epidermis and its stratum
corneum is much thinner than that of thick skin. The epidermis
of thin skin consists of only four layers; the stratum lucidum is
lacking in thin skin. Thin skin contains all three types of glands
(±ig. 13-9A–C).
Accessory Structures of the Skin
Accessory structures of the skin include
glands
,
hair
, and
nails
:
(1) The
glands
of the skin include
sebaceous glands
,
eccrine
sweat glands
, and
apocrine sweat glands
(±ig. 13-9A–C). The
sebaceous glands
secrete into hair follicles to keep the skin soft
and moist and serve as a barrier to protect the skin. The
eccrine
sweat glands
are important in regulating body temperature; they
are found in both the thin and thick skin. The
apocrine sweat
glands
are also called
sexual scent glands
; their function in
humans is not clear. They may be involved in thermoregulation
and are found only in some special regions of thin skin, such
Figure 13-11B
Nail Root (Matrix) and Nail Bed
Figure 13-11C
Clinical Correlation: Molluscum Contagiosum
Development of the Skin
Figure 13-12A
±etal Skin (5 to 9 Weeks)
Figure 13-12B
±etal Skin (±ifth Month)
Synopsis 13-2
Pathological Terms for the Integumentary System
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