ATLAS OF HEAD AND NECK PATHOLOGY SKIN
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SKIN
Skin is composed of two layers, the epidermis, the outer layer, which is derived
from ectoderm, and the dermis, derived from mesoderm. The two layers are firmly
adherent one to another with the dermis corresponding to the lamina propria of
mucous membrane. Loose connective tissue lies under the dermis and corresponds to
the superficial fascia.
The epidermis is a stratified squamous keratinized epithelium with five layers
or strata. (1) stratum germinativum or stratum basale lies just superficial to the
dermis; (2) stratum spinosum or prickle cell layer; (3) stratum granulosum; (4)
stratum lucidum; (5) stratum corneum.
The stratum germinativum consists of a single layer of columnar or cuboidal cells.
Mitotic figures are found here which produce new cells that eventually will be dis-
placed into the next layer, the stratum spinosum, which is several layers thick and
contains cells that are scale-like or polyhedral and which become flattened toward the
surface. The name spinosum derives from “intercellular bridges” which appear to
project from one cell to another but do not indicate cytoplasmic continuity between
the cells The stratum spinosum and germinativum are grouped together as the
malpighian layer and all cells here are referred to as keratinocytes. The stratum
granulosum rests on top of the stratum spinosum and contains flattened polyhedral
cells with vesicular nuclei. These cells contain granules of keratohyalin giving a
granular appearance to the layer. The stratum lucidum is a clear translucent layer
difficult to identify in many sections. The stratum corneum is the outermost layer of
the epidermis and is composed of dead scale-like cells that become progressively
flattened and joined together. This layer stains pink and is an area for constant loss
of dead cells.
The pigment melanin is responsible for the shades of brown in the skin and is
produced by specialized cells of epidermis, the melanocytes, that are scattered among
keratinocytes of the stratum germinativum and stratum spinosum. They appear as
cells with a clear space surrounding an oval shaped nucleus and the melanin they
produce is transferred to the adjacent basal cells by phagocytosis. There is one
melanocyte for every four to ten basal cells. Melanocytes may be somewhat difficult
to identify in routine histological preparations.
The exact limits of the dermis are often difficult to identify since it merges
into the underlying subcutaneous tissue. It is composed of dense connective tissue
and itself is divided into two layers, the papillary layer, superficially, and the reticular
