Lecture Notes: Unit I: Organization of the Human Body
Chapter Five - The Integumentary System
I. Introduction
A. An organ consists of several different tissues and performs specialized activities.
B. The tissues comprising the skin (integument) are the epithelium of the epidermis and the
connective tissues of the dermis.
C. A system is a group of organs that work together to perform specialized activities.
D. The skin is both an organ and a system; it is the largest organ of the body.
E. Dermatology is the study of the skin and skin disorders.
II. Physiology and Structure of Skin
A. The skin has numerous functions:
1. Regulation of body temperature
2. Protection
3. Reception of stimuli (cutaneous sensations)
4. Excretion and absorption
5. Synthesis of Vitamin D
6. Immunity
B. Structurally, the skin consists of three principal parts: Epidermis, Dermis, and Hypodermis
1. The epidermis is composed of stratified squamous epithelium and contains 4 principal
types of cells: Keratinocytes, Melanocytes, Langerhans cells, and Merkel cells.
(See Figure 5.2, Page 149 of Text or Page 112 of Illustrated Notebook)
a. Keratinocytes produce the protein keratin (waterproofing protein) which protects the
skin from heat, microbes, and chemicals. Keratinization is the 2-4 week process in
which keratinocytes gradually migrate to the surface as more underlying cells are
produced.
b. Melanocytes produce the pigment protein melanin which absorbs damaging ultraviolet
light rays and contributes to skin color.
c. Langerhans cells wander through the dermis and participate in immune responses.
d. Merkel cells contain a sensory structure called a tactile (Merkel) disc which in turn is
associated with a free nerve ending. These units function in the sensation of light touch.
e. Epidermal histologic organization is into four or - in the case of the fingertips/palms of
the hands and soles of the feet - five distinct strata:
.
Stratum basale (“basal layer”)
Stratum spinosum (“prickle-cell layer”)
Stratum granulosum (“granular layer”)
Stratum lucidum (“clear layer”)
Stratum corneum (“keratinized layer”) or (“cornified layer”)
f. It is capable of rapid and extensive regeneration.
g. Epidermal growth factor is a hormone that stimulates growth of both epidermal cells
and fibroblasts of the connective tissue.
2. The dermis consist of two regions:
a. The upper zone, papillary region, is composed of loose connective tissue and
elastic fibers
P. 2
b. The lower zone, reticular region, is composed of dense, irregularly arranged
connective tissue and both collagenous and coarse elastic fibers.
c. Lines of cleavage on the skin's surface overlie dermal regions where collagen fibers
are oriented in a particular direction. (Knowledge of these lines influences a surgeon's
choice of where to make incisions to facilitate healing.)
3. The hypodermis is also known as the subcutaneous layer ("sub-Q") and as the superficial
fascia. Composed of areolar and adipose connective tissues, this component serves as
a storage depot for fat and contains large numbers of blood vessels that supply the skin.
4. There are two types of skin: Thin skin and Thick skin
a. Thin skin covers all parts of the body except for palms/soles; it lacks epidermal ridges;
and has more sparsely distributed sensory receptors than does thick skin.
b. Thick skin covers the fingertips and palms as well as the toes and soles; lacks hair
follicles, arrector pili muscles, and sebaceous glands; and has a greater density of
sudoriferous glands than does thin skin.
C. Skin color is due to melanin (black/brown pigment) in the epidermis, carotene (an orange
pigment) in the dermis, and blood (with its red-pigmented protein, hemoglobin) in the capillaries of the dermis.Differences in shade of the skin are primarily due to the amount of melanin
produced and the extent of its dispersal. (Albinism results from an inherited recessive trait in which the
melanocytes cannot produce tyrosinase and thus cannot convert the amino acid tyrosine to melanin.
Vitiligo is the partial, or complete, loss of melanocytes from patches of skin, resulting in irregular white spots.)
D. About fingerprints: The surfaces of the palms, fingers, soles, and toes are marked by a series of
.
ridges and grooves. These appear either as straight lines or as patterns of loops and whorls as
the epidermis conforms to the contours of the underlying dermal papillae of the papillary region.
(The ridges function to increase the grip of the hand/foot by increasing friction.) Of course, this
ridge pattern (reflecting the dermal papillae pattern) is genetically determined and is thus
unique for each individual
III. Skin Wound Healing: Restoration of Homeostasis
A. Skin wound healing involves the epidermis as well as deeper tissues.
1. Epidermal wounds, such as abrasions or first-degree or second-degree burns, heal through
migration and division of basal cells, which is ultimately halted by contact inhibition; thus
scar formation is limited.
2. Deep wound healing, as in lacerations or surgical incisions, is more complex and usually
produces a scar. The four phases are: inflammatory, migratory (involves granulation tissue), proliferative, and maturation.
B. Use of skin grafts is indicated when the injury destroys the germinal cells of the epidermis.
Grafts, which vary in thickness, may come directly from the patient, from another person or
animal, or be cultured from the patient's own skin over a period of 3 - 4 weeks in the lab.
P. 3
IV. Epidermal Derivatives
A. Epidermal derivatives of the skin include hair, nails, arrector pili muscles, and glands.
B. Hair is distributed variously over the body where it functions in protection; each hair consists
of a shaft and a root. The root is surrounded by a follicle.
C. Nails are hard, keratinized epidermal cells over the dorsal surfaces of the terminal portions of
the fingers and toes.
D. Arrector pili muscles are tiny bundles of smooth muscle cells extending from the superficial
dermis of the skin to the side of a hair follicle, which in normal position lies as an angle to the
surface of the skin. Under physiologic or emotional stress (ie: cold, fright, etc.) these arrector
pili muscles contract, which pulls the follicles (and therefore the hair shafts) perpendicular
to the skin surface, causing "goose-bumps" because the skin around the shaft forms slight elevations on one side alternating with slight depressions on the other.
E. Skin glands include sebaceous (oil), sudoriferous (sweat), and ceruminous (wax) glands.
All glands are derived from epithelial tissues. (Mammary glands are actually modified
sudoriferous glands that are specialized to secrete milk; these glands will be considered with
our discussions of the female reproductive system in Biol. 250)
a. Both sudoriferous and ceruminous glands secrete through ducts which open onto the
skin's surface. Therefore, sweat and wax end up being completely externalized.
b. Sebaceous gland, however, secrete through ducts which open into a hair follicle because
the main purpose of the sebum (oily substance) is to keep the hair more pliable and prevent breakage / splitting. Oil only works its way to the skin surface if produced in excess.
V. Homeostasis of Body Temperature (Thermoregulation)
A. The skin helps to regulate homeostasis of body temperature through a negative feedback
system.
B. Perspiration helps to remove excess heat from the body. (Remember: it is not the sweat itself that cools;
rather it is the evaporation of sweat off the surface of the skin that has the tremendous cooling effect taking into account water's
tremendous heat of vaporization; that is, its removal of heat from the body's surface as it changes from a liquid to a gas.)
C. Dilation of cutaneous blood vessels helps remove excess heat from the body since blood is hotter
than body temp. (100.4o F vs 98.6o F). When cutaneous vessels are dilated, more blood courses
closer to the body surface allowing the excess heat to escape.
D. Conversely, constriction of blood vessels in the dermis and hypodermis of the skin aids in
conserving heat when body temperature drops.