Normal Skin: Introduction
The skin is the most accessible organ of the human body. Its most
basic function is simply a protective one. As a barrier, the skin
holds off desiccation and disease by keeping moisture in and pathogens
out. Nevertheless, characterization of the skin as mere “plastic
wrap” is a gross underestimation of the anatomic and physiologic
complexity of this vital structure.
Unlike parenchymal organs, end-organ dysfunction or failure is
not a prerequisite for the diagnosis of a skin disease, because
all skin diseases can be observed clinically irrespective of their
functional effects. Among the spectacular array of neoplastic, inflammatory,
infectious, and genetic cutaneous disorders, some elicit only trivial
aberrations in skin structure or function, whereas others lead to
profound and morbid consequences.
The integumentary system consists of a layer of tissue, 1–4
mm thick, that covers all exposed surfaces of the body. The skin merges
uninterruptedly with the structurally similar envelope of the mucous
membranes, but skin is distinct from mucosa in that it contains
adnexal structures such as the eccrine units that exude sweat and
the folliculosebaceous units that produce hairs and oils. There
is considerable variation in skin thickness and composition, depending
on the requirements of a particular body site. For example, the
thinnest skin overlies the eyelids, where delicacy and mobility
are essential. The thickest skin is present on the upper trunk,
where sturdiness exceeds mobility in importance. The surfaces of
the palms and soles are characterized by a high density of eccrine
sweat units, reflecting the importance of this region in regulation
of temperature; an absence of hairs, which would interfere with
sensation; and accentuation of the cornified layer (see later discussion), contributing
to the tackiness needed to handle objects deftly. The size of the
structures between sites can also vary greatly, best illustrated
by the contrast between large terminal hair follicles found on the
scalp, bearded areas, and genital skin and the small vellus hair
follicles found at most other sites.
Using a light microscope, two important skin layers are easily identifiable:
a stratified squamous epithelium, the epidermis; and
a layer of connective tissue, the dermis. The subjacent
adipose tissue is considered as a third layer by some and is referred
to as the subcutis.
The epidermis consists of keratinocytes arrayed in four distinct
substrata: the basal, spinous, granular, and cornified layers (Figure 8–1). Basal keratinocytes
include the proliferative pool of keratinocytes. These cells divide,
giving rise to progeny that are displaced toward the skin surface.
As the keratinocytes move outwardly, they progressively flatten
and accumulate keratin filaments within their cytoplasm. Individual
keratinocytes are tightly bound together by intracellular junctions
called desmosomes (Figure 8–2). The desmosomal junctions appear as delicate “spines” between
cells in conventional microscopic sections and are most conspicuous
in the epidermal spinous layer (Figure 8–3). ...