Essentials of Anatomy and Physiology

DERMIS

The dermisis made of an irregular type of fibrous
connective tissue, irregular meaning that the fibers are
not parallel, but run in all directions. Fibroblasts pro-
duce both collagenand elastinfibers. Recall that col-
lagen fibers are strong, and elastin fibers are able to
recoil after being stretched. Strength and elasticity are
two characteristics of the dermis. With increasing age,
however, the deterioration of the elastin fibers causes
the skin to lose its elasticity. We can all look forward
to at least a few wrinkles as we get older.
The uneven junction of the dermis with the epider-
mis is called the papillary layer(see Fig. 5–1). Capil-
laries are abundant here to nourish not only the
dermis but also the stratum germinativum. The epi-
dermis has no capillaries of its own, and the lower, liv-
ing cells depend on the blood supply in the dermis for
oxygen and nutrients.
Within the dermis are the accessory skin structures:
hair and nail follicles, sensory receptors, and several
types of glands. Some of these project through the
epidermis to the skin surface, but their active portions
are in the dermis.

Hair Follicles

Hair folliclesare made of epidermal tissue, and the
growth process of hair is very similar to growth of the

epidermis. At the base of a follicle is the hair root,

which contains cells called the matrix, where mitosis

takes place (Fig. 5–3). The new cells produce keratin,

get their color from melanin, then die and become

incorporated into the hair shaft, which is pushed

toward the surface of the skin. The hair that we comb

and brush every day consists of dead, keratinized cells.

The rate of hair growth averages 0.3 to 0.4 in./month

(8 to 10 mm).

Compared to some other mammals, humans do not

have very much hair. The actual functions of human

hair are quite few. Eyelashes and eyebrows help to

keep dust and perspiration out of the eyes, and the

hairs just inside the nostrils help to keep dust out of

the nasal cavities. Hair of the scalp does provide insu-

lation from cold for the head. The hair on our bodies,

however, no longer serves this function, but we have

the evolutionary remnants of it. Attached to each hair

follicle is a small, smooth muscle called the pilomotor

The Integumentary System 95

Hair shaft

Follicle

Hair root

B

Capillary

Venule

Fat cells

A

Figure 5–3. Structure of a hair follicle. (A) Longitudinal

section. (B) Cross-section.

QUESTION:What is the hair shaft made of?

Table 5–1 EPIDERMIS

Part Function

Stratum corneum

(keratin)

Stratum germina-

tivum (stratum

basale)

Langerhans cells

Merkel cells

Melanocytes

Melanin

• Prevents loss or entry of water


• If unbroken, prevents entry of
  pathogens and most chemi-
  cals


• Continuous mitosis produces
  new cells to replace worn-off
  surface cells


• Produces antimicrobial
  defensins


• Cholesterol is changed to vita-
  min D on exposure to UV rays


• Phagocytize foreign material
  and stimulate an immune
  response by lymphocytes


• Receptors for sense of touch


• Produce melanin on exposure
  to UV rays


• Protects living skin layers from
  further exposure to UV rays