C h a p t e r 4 5
Structure and Function of the Integumentum
1143
referred to as
prickle cells
because they develop a spiny
appearance as their cell borders interact.
The third layer, the
stratum granulosum,
consists of
three to five layers of flattened keratinocytes. The nuclei
and organelles of these cells begin to degenerate and
keratin filaments become more apparent. A distinctive
feature of cells in this layer is the appearance of dark
staining granules of a protein called
keratohyalin,
which
serve as a support for the keratin filaments. The
stra-
tum lucidum,
the fourth layer, which lies just superficial
to the stratum granulosum, is a thin, transparent layer
mostly confined to the palms of the hands and soles of
the feet. It consists of transitional cells that retain some
of the functions of living skin cells from the layers below
but otherwise resemble the cells of the stratum corneum.
The top or surface layer of the epidermis is the
stratum
corneum.
The cells in the stratum corneum are made up
of numerous layers of dead keratinized cells. The interi-
ors of the cells contain mostly keratin and keratohyalin.
These cells are continuously shed and replaced by cells
from the deeper strata. The stratum corneum is the layer
that varies most in cell layers and thickness. It ranges from
15 layers of cells in areas such as the face to 25 or more
layers on the arm. Specialized areas, such as the palms of
the hand and soles of the feet, have 100 layers or more.
Melanocytes
Melanocytes are dendritic cells found scattered among
the basal cells in the stratum germinativum. They func-
tion to produce pigment granules called
melanin
, the
substance that is responsible for skin color, tanning, and
protection against ultraviolet radiation.
During embryonic life, melanocyte precursor cells
migrate from the neural crest and enter the develop-
ing epidermis. A specific functional association is then
established in which one melanocyte maintains an asso-
ciation with a given number of keratinocytes. This asso-
ciation is called the
dermal melanin unit
. The ratio of
melanocytes to keratinocytes varies in different parts of
the body and is constant in all races.
As dendritic cells, melanocytes are round to columnar
cells with long undulating processes that extend between
the keratinocytes of the stratum spinosum (Fig. 45-3).
Melanocytes synthesize melanin from the amino acid tyro-
sine in the presence of an enzyme called
tyrosinas
e that is
activated by ultraviolet light. Synthesis of melanin occurs
in membrane-bound structures called
premelanosomes,
which are derived from the Golgi complex. As melanin
accumulates in the premelanosomes, they are transformed
into mature melanin granules, called
melanosomes.
Premelanosomes are concentratednear the golgi apparatus,
and nearly mature and mature melanocytes among the
basal cells of stratum germinativum. Melanosomes and
their melanin content are transferred to neighboring kera-
tinocytes by pigment donation. This process involves the
phagocytosis of the tips of the melanocyte process by kera-
tinocytes. After pigment donation, the melanocyte process
elongates and receives more melanosomes, and the pro-
cess is repeated. Thus, a single melanocyte is able to pro-
vide melanin to all of the associated keratinocytes in its
epidermal melanin unit.
Skin color is determined by the type, number, and size
of the melanosomes transferred into the surrounding
keratinocytes. Dark-skinned and light-skinned people
have approximately the same number of melanocytes, but
the production and packaging of pigment are different.
In dark-skinned people, larger melanin-containing mela-
nosomes are produced and transferred individually to the
keratinocyte, whereas in light-skinned people, smaller
melanosomes are produced and then packaged together in
a membrane before being transferred to the keratinocyte.
There are two major forms of melanin:
eumelanin
and
pheomelanin.
Exposure to the sun’s ultraviolet rays
Melanocyte
FIGURE 45-3.
The melanocytes, which are located in the basal
layer of the skin, produce melanin pigment granules that give
skin its color.The melanocytes have threadlike, cytoplasm-filled
extensions that are used in passing the pigment granules to
the keratinocytes.
Stratum corneum
Stratum lucidum
Stratum granulosum
Stratum spinosum
(prickle cells)
Stratum germinativum
(basal cells)
Keratinized cells
FIGURE 45-2.
Epidermal cells.The basal cells undergo mitosis,
producing keratinocytes that change their size and shape as
they move upward, replacing cells that are lost during normal
cell shedding.
