Covering, Support, and Movement of the Body
Te dermis has two layers, the papillary and reticular, which
abut one another along an indistinct boundary
Te thin superﬁcial
er-e) is areolar connective tissue in which ﬁne interlacing collagen
and elastic ﬁbers form a loosely woven mat that is heavily invested
with small blood vessels. Te looseness of this connective tissue
allows phagocytes and other defensive cells to wander freely as
they patrol the area for bacteria that have breached the skin.
Peglike projections from its surface, called
nipple), indent the overlying epidermis (see
Figure 5.1). Many dermal papillae contain capillary loops. Others
house free nerve endings (pain receptors) and touch receptors
that tactile cells and tactile corpuscles are diﬀerent structures.
(We consider them in more detail in Chapter 13.) In thick skin,
such as the palms of the hands and soles of the feet, these papillae
lie atop larger mounds called
, which in turn cause
the overlying epidermis to form
Collectively, these skin ridges, referred to as
assumed to enhance the gripping ability of the ﬁngers and feet
like tire treads help grip the road. Recent studies indicate that they
also contribute to our sense of touch by amplifying vibrations de-
tected by the large lamellar corpuscles (receptors) in the dermis.
Stratum Corneum (Horny Layer)
An abrupt transition occurs
between the nucleated cells of the stratum granulosum and the
ﬂattened anucleate cells of the
Tis outermost epidermal layer is a broad zone 20 to 30 cell
layers thick that accounts for up to three-quarters of the epider-
mal thickness. Keratin and the thickened plasma membranes of
cells in this stratum protect the skin against abrasion and pen-
etration, and the glycolipid between its cells nearly waterproofs
this layer. For these reasons, the stratum corneum provides a
durable “overcoat” for the body, protecting deeper cells from
the hostile external environment (air) and from water loss, and
rendering the body relatively insensitive to biological, chemical,
and physical assaults. It is amazing that even dead cells can still
play so many roles.
Te diﬀerentiation from basal cells to those typical of the
stratum corneum is a specialized form of apoptosis in which the
nucleus and other organelles break down and the plasma mem-
brane thickens. So, the terminal cells do not fragment, but in-
stead eventually slough oﬀ the skin surface. Te shingle-like cell
remnants of the stratum corneum are referred to as
horn). Tey are familiar to everyone as
dandruﬀ, shed from the scalp, and dander, the loose ﬂakes that
slough oﬀ dry skin.
Te average person’s skin sheds some 50,000 dead cells every
minute and 18 kg (40 lb) of these skin ﬂakes in a lifetime, pro-
viding a lot of fodder for the dust mites that inhabit our homes
and bed linens. Te saying “Beauty is only skin deep” is espe-
cially interesting in light of the fact that nearly everything we see
when we look at someone is dead!
Check Your Understanding
While walking barefoot in a barn, Jeremy stepped on a rusty
nail that penetrated the epidermis on the sole of his foot.
Name the layers the nail pierced from the superﬁcial skin
surface to the junction with the dermis.
The stratum basale is also called the stratum germinativum, a
name that refers to its major function. What is that function?
Why are the desmosomes connecting the keratinocytes so
Given that epithelia are avascular, which layer would you
expect to have the best-nourished cells?
For answers, see Appendix H.
skin), the second major skin region, is
strong, ﬂexible connective tissue. Its cells are typical of those
found in any connective tissue proper: ﬁbroblasts, macrophages,
and occasional mast cells and white blood cells. Its semiﬂuid
matrix, embedded with ﬁbers, binds the entire body together
like a body stocking. It is your “hide” and corresponds exactly to
animal hides used to make leather.
Te dermis has a rich supply of nerve ﬁbers, blood vessels,
and lymphatic vessels. Te major portions of hair follicles, as
well as oil and sweat glands, derive from epidermal tissue but
reside in the dermis.
Light micrograph of the dermis identifying the
papillary layer composed of areolar connective tissue and the
reticular layer of dense irregular connective tissue (110