1142
U N I T 1 3
Integumentary Function
layers: a superficial and thinner epithelial tissue layer,
called the
epidermis
, and a deeper and thicker connec-
tive tissue layer, called the
dermis
(Fig. 45-1). The base-
ment membrane zone is an interface between the dermis
and epidermis. A layer of subcutaneous tissue, some-
times called the
hypodermis,
serves as a storage site for
fat and contains large blood vessels that supply the skin.
Epidermis
The functions of the skin depend largely on the properties
of its epidermis, which is composed of stratified keratin-
ized epithelium containing four types of cells: keratino-
cytes, melanocytes, Langerhans cells, and Merkel cells.
Keratinocytes
The keratinocytes are the predominant cell type of the
epidermis. They produce a fibrous protein called
keratin,
which is essential to the protective function of skin and
maybe involved inthe immune systemandwoundhealing.
As they divide and mature, the keratinocytes form five
distinct layers, or
strata
: the stratum germinativum, the
stratum spinosum, the stratum granulosum, the stratum
lucidum, and the stratum corneum.
The deepest layer, the
stratum germinativum
or
stra-
tum basale,
consists of a single layer of basal cells that
are attached to the basal lamina in the basement mem-
brane zone (to be discussed). The basal cells are the only
epidermal cells that are mitotically active. All cells of
the epidermis arise from this layer. As new cells form
in the basal layer, the older cells change shape and are
pushed upward toward the skin surface (Fig. 45-2). As
these cells approach the skin surface, their cytoplasm
becomes converted to keratin and they form flattened
plates of dead cells on the skin surface. It normally
takes 3 to 4 weeks for the epidermis to replicate itself.
The rate of cell division in the stratum germinativum is
greatly accelerated when the outer layers of the epider-
mis are stripped away as occurs in abrasions and burns.
The remaining layers of epidermis are formed as cells
from the basal cell layer move upward toward the skin
surface. The second layer, the
stratum spinosum,
is two
to four layers thick. The cells of this layer are commonly
Basement membrane zone
Dermis
Dermis
Arrector pili muscle
Blood vessel
Sebaceous gland
Subcutaneous tissue
Nerve to hair follicle
Sweat gland
Nerve endings
Papillae
Epidermis lifted to reveal
papillae of the dermis
Stratum corneum
Stratum lucidum
Stratum granulosum Epidermis
Stratum germinativum
Stratum spinosum
FIGURE 45-1.
Three-dimensional view of the skin.
