250
UNIT 3
Organ Systems
Figure 13-7A.
Special types of cells in the epidermis, thin skin.
H&E,
3
281
Keratinocytes
make up the bulk of the epidermis, but there are
additional cell types:
melanocytes
,
Merkel cells
, and
Langerhans
cells
. All three types of cells have clear cytoplasm and are some-
times called
clear cells
. It is difF cult to distinguish among them
in H&E-stained sections. Melanocytes and Merkel cells are both
located in the stratum basale where they are scattered among the
basal cuboidal cells. In contrast, Langerhans cells are typically
found in the stratum spinosum. The functions of the three cells
are quite different: (1)
Melanocytes
produce melanin granules and
insert them into keratinocytes. (2)
Merkel cells
are receptor cells,
that establish synaptic contacts with sensory nerve terminals;
they have cytoplasmic granules, which contain neurotransmitters
(±ig. 13-2). (3)
Langerhans cells
are monocyte derivatives, which
play an important role in capturing antigens and presenting them
to lymphocytes, thereby participating in the cutaneous immune
response. Langerhans cells make no desmosome junctions with
neighbors; indeed, their function requires that they be motile so
that they can transport any captured antigens from the epidermis
to lymph nodes deep to the skin.
Langerhans
Langerhans
cells
cells
Langerhans
cells
Melanocytes
Melanocytes
Melanocytes
A
Figure 13-7B.
Melanocyte.
EM,
3
8,129
Melanocytes
, along with Merkel cells, are the two clear cell types found in the stratum basale of the epidermis. The two cells cannot
be distinguished in conventionally prepared sections for light microscopy, but each has distinctive ultrastructural features. The mel-
anocyte generates
melanin granules
and injects them into nearby
keratinocytes
. This is a continuous process because keratinocytes
are constantly replaced as they differentiate and move toward the surface. The melanin protects the cells, particularly the nucleus,
from the mutagenic effects of ultraviolet irradiation. As shown here, the melanocyte contacts the
basal lamina
of the epidermis, but
it does not establish desmosome junctions with the keratinocytes.
B
Melanocyte
processes
Melanin granules
Nuclei of keratinocytes
Basal lamina of epidermis
Collagen
fibrils in
the dermis
Nucleus of the
melanocyte
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