The Integumentary System
Apocrine secretion contains the same basic components as
true sweat, plus fatty substances and proteins. Consequently, it
is viscous and sometimes has a milky or yellowish color. Te se-
cretion is odorless, but when bacteria on the skin decompose its
organic molecules, it takes on a musky and generally unpleasant
odor, the basis of body odor.
Apocrine glands begin functioning at puberty under the in-
ﬂuence of the male sex hormones (
) and play little
role in maintaining a constant body temperature. Teir precise
function is not yet known, but they are activated by sympathetic
nerve ﬁbers during pain and stress. Because sexual foreplay
increases their activity, and they enlarge and recede with the
phases of a woman’s menstrual cycle, they may be the human
equivalent of other animals’ sexual scent glands.
wax) are modi-
ﬁed apocrine glands found in the lining of the external ear
canal. Teir secretion mixes with sebum produced by nearby
sebaceous glands to form a sticky, bitter substance called
, or earwax, that is thought to deter insects and block entry
of foreign material.
, another variety of specialized sweat
glands, secrete milk. Although they are properly part of the in-
tegumentary system, we will consider the mammary glands in
Chapter 27 with female reproductive organs.
Sebaceous (Oil) Glands
shus; “greasy”), or
(Figure 5.7a), are simple branched alveolar glands that are
(Tese sweat pores are diﬀerent from the so-called pores of a
person’s complexion, which are actually the external outlets of
Eccrine gland secretion, commonly called sweat, is a hypo-
tonic ﬁltrate of the blood that passes through the secretory cells
of the sweat glands and is released by exocytosis. It is 99% wa-
ter, with some salts (mostly sodium chloride), vitamin C, anti-
bodies, a microbe-killing peptide dubbed
, and traces
of metabolic wastes (urea, uric acid, and ammonia). Te exact
composition depends on heredity and diet. Small amounts of
ingested drugs may also be excreted by this route. Normally,
sweat is acidic with a pH between 4 and 6.
Te sympathetic division of the autonomic nervous system,
over which we have little control, regulates sweating. Its ma-
jor role is to prevent the body from overheating. Heat-induced
sweating begins on the forehead and spreads inferiorly over the
remainder of the body. Emotionally induced sweating—the so-
called “cold sweat” brought on by fright or nervousness—begins
on the palms, soles, and axillae (armpits) and then spreads to
other body areas.
Apocrine Sweat Glands
Apocrine sweat glands
o-krin), approximately 2000 of
them, are largely conﬁned to the axillary and anogenital areas.
In spite of their name, they are merocrine glands, which release
their product by exocytosis like the eccrine sweat glands. Larger
than eccrine glands, they lie deeper in the dermis or even in the
hypodermis, and their ducts empty into hair follicles.
(a) Photomicrograph of a sectioned
sebaceous gland (90
(b) Photomicrograph of a sectioned
eccrine gland (140
Skin appendages: Cutaneous glands.