172
UNIT TWO
(der
mis), is thicker than the epidermis and is made up of
connective tissue containing collagen and elastic fibers,
smooth muscle tissue, nervous tissue, and blood. A
base-
ment membrane
anchored to the dermis by short F
brils sepa-
rates the two skin layers.
Beneath the dermis, masses of areolar and adipose tis-
sues bind the skin to underlying organs. These tissues are not
part of the skin. They form the
subcutaneous layer
(sub
ku-
ta
ne-us la
er), or hypodermis
(fig. 6.2)
. The collagenous
and elastic F
bers of this layer are continuous with those of
the dermis. Most of these F bers run parallel to the surface
of the skin, extending in all directions. As a result, no sharp
boundary separates the dermis and the subcutaneous layer.
The skin, or cutaneous membrane, includes two distinct
layers: epithelial tissue overlying connective tissue. The outer
layer, called the
epidermis
(ep
i-der
mis), is composed of
stratiF
ed squamous epithelium. The inner layer, or
dermis
Skin cells help produce vitamin D, necessary for normal bone and
tooth development. This vitamin is ingested or it forms from a sub-
stance (dehydrocholesterol) synthesized by cells in the digestive
system. When dehydrocholesterol (provitamin D) reaches the skin by
means of the blood and is exposed to ultraviolet light from the sun, it
is converted to vitamin D.
(a)
Hair shaft
Epidermis
Hair follicle
(b)
Sebaceous gland
Dermis
Sweat
Epidermis
Dermis
Hair shaft
Sweat gland pore
Capillary
Stratum corneum
Stratum basale
Dermal papilla
Arrector pili muscle
Lamellated (Pacinian) corpuscle
Basement membrane
Sebaceous gland
Hair follicle
Sweat gland
Nerve cell process
Adipose tissue
Blood vessels
Muscle layer
Sweat gland duct
Subcutaneous
layer
Tactile (Meissner’s) corpuscle
FIGURE 6.2
±Skin.±(
a
) A section of skin and the subcutaneous layer. (
b
) A light micrograph depicting the layered structure of the skin (75×).
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