CHAPTER 5
INTEGUMENTARY SYSTEM
Organs
are the skin and its derivatives--hair, nails, glands and nerve endings
Skin
is one of the largest organs:
22 square feet
10-11 pounds
16% of body weight
Dermatology
is the branch of medicine.
Skin
consists of:
1.
Outer, thinner epidermis (ET)
2.
Inner, thicker dermis (CT)
Under
these (not part of the skin) is the subcutaneous (subQ layer), also called the hypodermis.
Keratinized
stratified squamous ET (this is a test question)
4
kinds of cells:
1. 90% keratinocytes—4 to 5 layers of
cells that produce keratin, which makes them tough and relatively waterproof.
These cells also synthesize lamellar granules, which add to the waterproofing
effect.
2. 8% melanocytes--produce melanin and
transfer it to keratinocytes, where the melanin clusters between nucleus and
free surface for protection from UV light
3. Langerhans cells--arise from bone
marrow and migrate to the epidermis. They participate in immune responses--easily
damaged by UV light
4. Merkel cells--function in sensation of
touch--located in deepest layer of epidermis
LAYERS
OF THE EPIDERMIS:
4 in most areas
5 in palms & soles
Callus—friction
causes abnormal thickening of
epidermis
1. STRATUM BASALE--single layer of cuboidal to columnar cells. Some
are stem cells which continuously divide and produce new keratinocytes, which
move toward the free surface, forming a part of successive layers all the way.
They are eventually shed off the surface. Keratinocytes in the stratum basale
contain special intermediate filaments called tonofilaments, made of a protein
which will form keratin as cells move upward. Also in this layer are
melanocytes and Merkel's discs (nerve endings for touch) and associated
Merkel’s cells.
2.
STRATUM SPINOSUM--8-10 rows of many-sided keratinocytes that fit closely
together. Desmosomes join them tightly. Melanocytes extend projections among
the keratinocytes.
3.
STRATUM GRANULOSUM--3-5 rows of flattened keratinocytes that contain
keratohyalin, which the tonofilaments convert to keratin. Nuclei begin to break
down in this layer and cells begin to die as they undergo apoptosis. Lamellar
granules release an oily secretion that helps in waterproofing.
4.
STRATUM LUCIDUM--extra layer in palms and soles. 3-5 rows of clear dead
cells.
5.
STRATUM CORNEUM--25-30 rows of flat dead cells completely filled with
keratin. Continuously shed and replaced.
TABLE 5.1 & FIG. 5.3 P. 149
Keratinization
is the process of producing and accumulating keratin as the cells progress from
the stratum basale toward the surface. The whole process normally takes about 4
weeks, but the rate can increase when outer layers are damaged.
Epidermal
growth factor—hormone that stimulates growth of epithelial cells. Needed in
normal amounts, but some oncogenes when activated cause
constant production of too much EGF and this can lead to skin cancer.
CT
with collagen and elastic fibers. Cells are sparse--fibroblasts, macrophages,
adipocytes. Contains blood vessels, nerves, glands and hair
follicles. Thickness varies in different body areas. Layers:
1. PAPILLARY REGION (LAYER)--just
under epidermis--1/5 total dermis--areolar CT with fine elastic fibers. Small
projections called dermal papillae extend up into the epidermis and contain
capillary loops. Also found are Meissner's corpuscles, which are nerve endings
for touch, and free nerve endings for temperature, pain, tickle and itch.
2. RETICULAR REGION (LAYER)--dense
irregular CT with fibroblasts, interlacing bundles of collagen
fibers, and coarse elastic fibers. Contains adipose cells, hair follicles, nerves, oil glands and
sweat glands between bundles. Most variation in
thickness here.
Collagen
and elastic fibers in the reticular layer give the skin strength, extensibility
and elasticity.
These
give us our fingerprints, footprints, etc. Found on skin of the palms, fingers,
soles & toes, their function is to improve the grip. They develop in the
3-4 month in utero due to the pattern of dermal papillae and are unique for
each individual (genetic). Ducts of sweat glands open on the tops of ridges as
sweat pores. Study of fingerprints is dermatoglyphics.
Due
to 3 pigments:
1. Melanin--in epidermis, varies skin
color from pale yellow to tan to black. Melanin comes in 2 types:
Eumelanin—brown to balck
Pheomelanin—yellow to red
All races have the same number of melanin-producing
cells (melanocytes) per square inch but their level of activity varies.
Freckles--patches of active
melanocytes
Liver (age) spots--also clusters of
melanocytes, probably due to damage over the years by UV light
Melanin is produced as a protection
against UV light. Melanocytes have special organelles called melanosomes, where
they use the enzyme tyrosinase to synthesize melanin from an amino acid,
tyrosine.
Albinism--inherited inability to
produce melanin (have melanocytes but they are defective)--white hair,
skin--pink eyes
Vitiligo--loss of melanocytes in
patches of skin--autoimmune
2. Carotene--yellow-orange pigment mostly
in dermis. Asian people also deposit it in the stratum corneum and fatty areas
of dermis and subQ layer (this tendency is in their genes). Precursor of Vit A
and we get it from orange and yellow foods.
3. Hemoglobin--if little of the other 2
pigments are present, the blood in dermal capillaries may give a pink color.
Hemoglobin is the pigment.
P. 151 SKIN COLOR CLUES
TABLE 5.4 P. 157
COMPARES THICK AND THIN SKIN
Develop from the embryonic epidermis
HAIRS
(pili)--are present for protection--from sun, decreases heat loss, etc.
Associated
touch receptors called hair root plexuses are extremely sensitive.
Anatomy of a hair—see FIG. 5.4 P. 153
Hair
consists of columns of dead keratinized cells welded together by extracellular
proteins.
Shaft---superficial portion which projects
from skin surface
Root---portion below the surface--penetrates
into dermis & possibly subQ layer
On
cross-section, 3 layers (both shaft & root):
1. Medulla--inner layer--2 or 3 rows of
cells containing pigment granules and air spaces
2. Cortex--middle--elongated cells with
pigment in dark hair, air in light or white
3. Cuticle--outer--single layer of thin
flat scalelike cells arranged like shingles. These cells are the most heavily
keratinized.
Surrounding
the root is the hair follicle:
1. External root sheath--downward
continuation of the epidermis--has all epidermal layers near surface--taper off
and deep parts have stratum basale only
2. Internal root sheath--tubular sheath of
epithelium between external root sheath and the hair itself
At
the base of the hair follicle is the enlarged bulb which includes:
1. Papilla of the hair--projection of CT
that contains blood vessels and areolar CT
2. Matrix--germinal layer derived from
stratum basale---produces new hairs or growth of old hairs by cell division.
Sebaceous
(oil) glands open into hair follicles.
Arrector
pili muscles--smooth muscle that extends from dermis to side of hair follicle
& pulls hairs upright--cold, fright, emotions—goosebumps
Hair
root plexuses are nerves that surround hair follicles and are very sensitive to
movement of the hair
Hairs
normally grow for a period of time and are shed. The follicle rests and later
produces a new hair. A typical cycle in scalp hair might be growth for 2 - 6
years and then 3 months of rest, but there is wide individual variation. On the
average person, 85% of scalp hairs are usually in the growth phase. 70 - 100
old hairs per day are normally lost.
Alopecia
is the partial or complete loss of hair.
HAIR
COLOR
Melanocytes
in matrix of bulb produce melanin that enters cells of cortex & medulla
Dark—eumelanin—the more melanin, the
darker the hair
Blonde—pheomelanin + sulfur
Red--pheomelanin + iron
Graying—progressive loss of enzyme that
synthesizes melanin
White--no pigment--air bubbles replace
HAIR & HORMONES
P. 154
1. Sebaceous (oil) glands--secreting
portions in dermis & ducts almost all open into a hair follicle
Small oil glands present in most skin
Larger in face, neck, upper chest
Absent in palms & soles
Secretion
is sebum:
Keeps hair and skin moist
Prevents excessive evaporation of water
Inhibits growth of certain bacteria
2. Sudoriferous (sweat) glands
a. Eccrine
(merocirne) sweat glands--almost all skin, most numerous in palms & soles
(3000/sq in) in palms
Secretory portion in deep dermis or
subQ layer, duct ends at a pore on the skin surface
Watery secretion
Function: Cooling
b. Apocrine sweat glands--mainly in
armpit & groin area. In fact, it has recently been decided that these are
really merocrine glands, but the label has not been changed.
Secretory portion in dermis or
subQ--duct opens into a hair follicle
More viscous secretion
Function: Some cooling but contains
pheromones
c. Ceruminous glands--modified sweat
glands in the ear produce cerumen (ear wax)--sticky barrier that traps foreign
materials
Plates
of tightly packed, hard, keratinized cells of the epidermis--form a clear solid
covering over dorsal surfaces of ends of fingers and toes
Nail body--visible portion--clear but
looks pink due to blood in underlying tissue
Nail root---proximal portion buried
under skin
Free edge--white
Lunula--proximal whitish half-moon
due to thickened stratum basale
Hyponychium—secures nail to fingertip
Eponychium (cuticle)--band of stratum corneum of epidermis that extends from the margin
of the nail, adhering to it
Nail matrix--epithelium under the
nail root where growth occurs--superficial cells of the matrix are transformed
into nail cells and pushed forward—average growth rate 1mm/week (.04") in
fingernails
Function: Help handle small objects
Protection for end of digit
FIG. 5.5 P. 156
1.
Thermoregulation---regulation of body temp
a. Activation of sweat glands and
evaporation of perspiration (review negative feedback system)
b. Adjustment of blood flow in dermal
capillaries---capillaries of the dermis also help
control body temperature. Heat is given off if more blood circulates through
these superficial vessels. Heat is conserved if more blood remains in deeper
vessels.
Too warm--dermal capillaries dilate (larger
diameter)--more blood enters & heat is lost
Too cool--dermal capillaries constrict
(smaller diameter)--less blood enters & heat is conserved
2.
Blood reservoir—large amount of blood are usually contained in the vessels of
the dermis. This blood can be diverted into circulation if needed.
3.
Protection
a. From abrasion and
dehydration due to keratin
b. From bacterial invasion—bacteria can't penetrate
normal healthy skin
c. Acid pH of sebum & perspiration
inhibits growth of some bacteria
d. From UV light
due to melanin
e. Langerhans cells recognize invaders and
alert the immune system
f. Macrophages carry out phagocytosis
4.
Cutaneous sensations--temp, touch, pressure, vibration, & pain
5.
Excretion & absorption--small amount of salts and organic compounds are
removed in perspiration; a few things can be absorbed (good include skin
patches of certain drugs—bad include toxic materials such as carbon
tetrachloride & heavy metals)
6.
Synthesis of Vitamin D
Precursor molecule in skin
↓ UV light (1 hour/week on hands, arms,
face)
Activated precursor
↓ Enzymes in liver and kidney
Calcitriol (active form)--aids in
absorption of Ca
With
even a small cut or scrape, the body has deviated from homeostasis and must
return by repairing the damage. 2 types of wounds are considered:
Central
deepest area goes into the dermis, edges involve only superficial cells.
Abrasions (scrapes) and minor burns are examples.
1. Basal epidermal cells break contact
with the basement membrane, enlarge, and migrate across the wound.
2. Stationary basal cells divide to
replace the migrating cells.
3. As cells migrate, they travel in one
direction until they bump into another epidermal cell. They then change
direction and continue this until the entire area is covered. This is called
contact inhibition and results in one smooth layer of epidermal cells
completely covering the wound. Spread of normal cells is controlled by contact
inhibition, but malignant cells lose this characteristic and invade healthy
tissue without control.
4. After resurfacing is accomplished, the
new stratum basale over the wound begins to divide and form the remaining
layers.
5. Epidermal growth factor (EGF), a
protein hormone, is found in wounds and stimulates these events in epidermal
cells.
When
the injury extends below the epidermis, scar formation must be part of the
healing process. Healing begins with inflammation, which is a vascular and
cellular response to tissue injury. The goal is to dispose of microbes and
foreign material, as well as dead and damaged tissue, so the repair process can
begin.
1. Inflammatory phase
a. A blood clot forms and loosely
unites the wound edges.
b. Vasodilation and increased permeability
of blood vessels in the area allow an increased blood flow and also make it
easier for cells and substances to leave the capillaries and migrate to the
damaged area. Phagocytic white blood cells and fibroblasts will be needed.
2. Migratory phase
a. Clot becomes a scab and epithelial
cells migrate beneath the scab, trying to bridge the wound.
b. Fibroblasts migrate along the fibrin
threads of the clot and begin building scar tissue (collagen fibers and
glycoprotein).
c. Damaged blood vessels regrow and
additional capillaries form.
d. This tissue looks puffy and pink and
is called granulation tissue.
3. Proliferative phase
a. Extensive growth of epithelial cells
beneath the scab.
b. Deposition of more collagen fibers
in random patterns.
c. Continued growth of blood vessels.
4. Maturation phase
a. Scab sloughs off.
b. Collagen fibers become more
organized.
c. Fibroblasts decrease in number.
d. Blood vessels are restored to
normal.
Fibrosis---process
of scar tissue formation
Sometimes
this process goes beyond what is necessary to close the wound.
Hypertrophic scar---raised scar due to
excessive scar tissue formation
Keloid scar---scar that becomes much wider
that the original wound and is raised above the skin surface also.
Even
after maturation is complete, scar tissue differs from normal skin in several
ways:
1. Collagen fibers more numerous
2. Less elasticity
3. Fewer blood vessels
4. May not contain hair, glands or sensory
neurons
5. Usually lighter in color
Read about:
Sun damage P. 162
Skin cancer P. 164
Burns P. 164 –165
Pressure (decubitus) ulcers P. 165