CHAPTER 13
Integumentary System
243
as the axilla, nipple, and perianal and genital areas. (2)
Hair
is
found in thin skin: the scalp, pubic region, and armpit (axilla)
in adults have more abundant thick hair than other surfaces
of the skin on the body. Hair growth is discontinuous and is
controlled by various hormones. The hair follicles produce and
maintain hair growth. The cycles of hair growth include three
stages (from early to late): the
anagen phase
(active growth
stage, lasting 2–6 years), the
catagen phase
(regression phase,
lasting about 3 weeks), and the
telogen phase
(resting stage,
lasting about 3 months). The hair shaft is shed as the follicle
goes through the growth cycle and a new hair replaces it. In
cross section, a hair follicle looks rather like an onion, with sev-
eral rings or layers. The central part is the
hair shaft
, which has
a scaly surface called the
cuticle
. The hair shaft is surrounded
by an
inner root sheath
(with its own cuticle) and
outer root
sheath
. The outer root sheath is covered by a
connective tis-
sue sheath
(Fig. 13-10A). The deep end of the hair follicle is
expanded into the
hair bulb
, which is composed of a
dermal
papilla
(
hair papilla
) and a
hair root
composed mostly of the
hair matrix. The
hair matrix
is capable of cell division and gives
rise to the hair shaft (Fig. 13-10B). (3) The
nail
is a translucent
keratinized hard plate resting on the dorsum of the tip of each
digit. The nail plate stems from the base of the nail (
nail matrix
)
and grows over the nail bed toward the tip of the ±
nger or toe.
The components of the nail include the
nail root
(
nail matrix
),
nail plate
,
the eponychium
(
nail cuticle
),
perionychium
(
nail
wall
), and
hyponychium
(Fig. 13-11A,B).
Development of the Skin
The skin develops from
ectoderm
and
mesoderm
. The epithe-
lial cells of the epidermis are
ectodermal derivatives
, whereas
Langerhans cells, the dermis (connective tissue), and subcutis
(hypodermis) develop from the
mesoderm
. Melanocytes and
Merkel cells originate from the neural crest. The basal cells of
the epidermis give rise to the accessory structures (hair follicles,
nails, and glands) of the skin (Fig. 13-12A,B).
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