CHAPTER 13
Integumentary System
245
Figure 13-2.
Overview of the layers of the epidermis.
The
epidermis
is composed of F ve cell layers. (1) The
stratum basale
is composed of a single layer of cuboidal or tall cuboidal cells,
melanocytes
and
Merkel cells
which are also called
Merkel cell neurite complexes
or
Merkel disks
. Many of these cells are actually
stem
cells
; they divide continuously and migrate from the basal layer toward the surface and give rise to keratinocytes in the
other layers. (2) The
stratum spinosum
contains polygon-shaped keratinocytes with many tonoF
lament bundles in their cytoplasm.
These cells are interconnected with each other by desmosomes.
Langerhans cells
are often found in this layer. The stratum basale
and the stratum spinosum are the only layers with mitotically active cells, and, together they are also called the
Malpighian layer
.
(3) The
stratum granulosum
contains three to F ve layers of
keratinocytes
with fl
attened nuclei. The cytoplasm of the cells is F lled
with basophilic keratohyalin granules from which the name derives. The cytoplasm also contains lamellar granules, which can
release their contents into the intercellular spaces to help seal the skin, preventing water loss. This layer is more obvious in thick
skin; only a single cell layer is visible in thin skin. (4) The
stratum lucidum
is a very thin, clear layer that contains keratinocytes with
pycnotic nuclei. This layer is found only in thick skin. (5) The
stratum corneum
is the top layer of the epidermis. It contains many
layers of fl attened cells F
lled with mature keratin. These are
dead cells
with no nuclei or organelles. The cells in this layer, which are
constantly replaced by cells from deeper layers, form a barrier to prevent loss of water and entry of pathogens. This layer is much
thicker in thick skin than in thin skin.
SYNOPSIS 13-1
Functions of the Skin
Protection
of body from invasion of pathogens; prevention of tissue damage by toxic chemicals and ultraviolet light
Prevention
of dehydration and loss of body fl uids (impermeable to water)
Regulation
of body temperature (production and excretion of sweat, vascular shunts)
Sensation
of touch, pain, temperature, pressure, and vibration; important for communication, dexterity, and injury
prevention
Immunological function
of Langerhans cells (antigen-presenting cells) present antigens to lymphocytes in the immune
responses (See Chapter 10, “Lymphoid System.”)
Production
of vitamin D from precursors under the effects of steroids and sunlight
5. Stratum
corneum
3. Stratum
granulosum
Vein
Artery
Nerve
2. Stratum
spinosum
Merkel cell (Merkel disk)
Langerhans cell
Horny cell (dead cell)
Flattened keratinocyte
Keratinocyte with
keratohyalin granules
Keratinocyte
Sensory nervous fiber
1. Stratum
basale
Melanocyte
Nerve fiber
4. Stratum
lucidum
Small blood vessels
Epidermis
Dermis
Epidermis
Dermis
Hypodermis
(subcutaneous layer)
Reticular
layer
Papillary
layer
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