154
UNIT 2
Covering, Support, and Movement of the Body
5
Te dermis has two layers, the papillary and reticular, which
abut one another along an indistinct boundary
(Figure 5.3)
.
Papillary Layer
Te thin superficial
papillary layer
(pap
9
il-
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
dermal
papillae
(pah-pil
9
e;
papill
5
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
called
tactile
or
Meissner’s corpuscles
(mīs
9
nerz kor
9
pus-lz). Note
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
dermal ridges
, which in turn cause
the overlying epidermis to form
epidermal ridges
(Figure 5.4a)
.
Collectively, these skin ridges, referred to as
friction ridges
, are
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
stratum corneum
(kor
9
ne-um).
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
cornified,
or
horny, cells
(
cornu
5
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
1.
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.
2.
The stratum basale is also called the stratum germinativum, a
name that refers to its major function. What is that function?
3.
Why are the desmosomes connecting the keratinocytes so
important?
4.
Given that epithelia are avascular, which layer would you
expect to have the best-nourished cells?
For answers, see Appendix H.
Dermis
Te
dermis
(
derm
5
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.
Dermis
Epidermis
Papillary
layer
Reticular
layer
Figure 5.3
Light micrograph of the dermis identifying the
papillary layer composed of areolar connective tissue and the
reticular layer of dense irregular connective tissue (110
3
).
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