The Integumentary System
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Te skin, or integument, is composed of two discrete tissue layers,
an outer epidermis and a deeper dermis, resting on subcutaneous
tissue, the hypodermis.
Te epidermis is an avascular, keratinized sheet of stratiﬁed
squamous epithelium. Most epidermal cells are keratinocytes.
Scattered among the keratinocytes in the deepest epidermal layers
are melanocytes, dendritic cells, and tactile cells.
From deep to superﬁcial, the strata, or layers of the epidermis,
are the basale, spinosum, granulosum, lucidum, and corneum.
Te stratum lucidum is absent in thin skin. Te mitotically active
stratum basale is the source of new cells for epidermal growth. Te
most superﬁcial layers are increasingly keratinized and less viable.
Te dermis, composed mainly of dense, irregular connective
tissue, is well supplied with blood vessels, lymphatic vessels, and
nerves. Cutaneous receptors, glands, and hair follicles reside
within the dermis.
Te more superﬁcial papillary layer exhibits dermal papillae
that protrude into the epidermis above, as well as dermal ridges.
Dermal ridges and epidermal ridges together form the friction
ridges that produce ﬁngerprints.
In the deeper, thicker reticular layer, the connective tissue ﬁbers
are much more densely interwoven. Less dense regions between
the collagen bundles produce cleavage, or tension, lines in the
skin. Points of tight dermal attachment to the hypodermis
produce dermal folds, or ﬂexure lines.
Skin color reﬂects the amount of pigments (melanin and carotene)
in the skin and the oxygenation level of hemoglobin in blood.
Melanin production is stimulated by exposure to ultraviolet
radiation in sunlight. Melanin, produced by melanocytes and
transferred to keratinocytes, protects the keratinocyte nuclei from
the damaging eﬀects of UV radiation.
Skin color is aﬀected by emotional state. Alterations in normal
skin color (jaundice, bronzing, erythema, and others) may
indicate certain disease states.
Appendages of the Skin
Skin appendages, which derive from the epidermis, include hairs
and hair follicles, nails, and glands (sweat and sebaceous).
Hairs and Hair Follicles
A hair, produced by a hair follicle, consists of heavily keratinized
cells. A typical hair has a central medulla, a cortex, and an outer
cuticle and root and sha± portions. Hair color reﬂects the amount
and kind of melanin present.
A hair follicle consists of an inner epithelial root sheath and
an outer peripheral connective tissue sheath derived from the
dermis. Te base of the hair follicle is a hair bulb with a matrix
that produces the hair. A hair follicle is richly vascularized and
well supplied with nerve ﬁbers. Arrector pili muscles pull the
follicles into an upright position, producing goose bumps, and
propel sebum to the skin surface when they contract.
Except for hairs of the scalp and around the eyes, hairs formed
initially are ﬁne vellus hairs; at puberty, under the inﬂuence of
androgens, coarser, darker terminal hairs appear in the axillae
and the genital region.
Te rate of hair growth varies in diﬀerent body regions and
with sex and age. Diﬀerences in life span of hairs account for
diﬀerences in length on diﬀerent body regions. Hair thinning
reﬂects factors that lengthen follicular resting periods, age-related
atrophy of hair follicles, and a delayed-action gene.
A nail is a scalelike modiﬁcation of the epidermis that covers the
dorsum of a ﬁnger (or toe) tip. Te actively growing region is the
Sweat (Sudoriferous) Glands
Eccrine (merocrine) sweat glands, with a few exceptions, are
distributed over the entire body surface. Teir primary function
is thermoregulation. Tey are simple coiled tubular glands that
secrete a salt solution containing small amounts of other solutes.
Teir ducts usually empty to the skin surface via pores.
Apocrine sweat glands, which may function as scent glands,
are found primarily in the axillary and anogenital areas. Teir
secretion is similar to eccrine secretion, but it also contains
proteins and fatty substances on which bacteria thrive.
Sebaceous (Oil) Glands
Sebaceous glands occur all over the body surface except for
the palms and soles. Tey are simple alveolar glands; their oily
holocrine secretion is called sebum. Sebaceous gland ducts
usually empty into hair follicles.
Sebum lubricates the skin and hair, prevents water loss from
the skin, and acts as a bactericidal agent. Sebaceous glands are
activated (at puberty) and controlled by androgens.
Functions of the Integumentary System
Protection. Te skin protects by chemical barriers (the
antibacterial nature of sebum, defensins, cathelicidins, the acid
mantle, and the UV shield of melanin), physical barriers (the
hardened keratinized and lipid-rich surface), and biological
barriers (dendritic cells, macrophages, and DNA).
Body temperature regulation. Te skin vasculature and sweat
glands, regulated by the nervous system, play an important role
in maintaining body temperature homeostasis.
Cutaneous sensation. Cutaneous sensory receptors respond to
temperature, touch, pressure, and pain stimuli.