Biology of Hair and Nails
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Trichinosis
Molecular and Cellular Pathology
Dr Tim Scott-Taylor
Health and Human Sciences
Topics
Topics covered;
 the nature of hair follicles
 structure of keratin
 normal patterns of hair growth
 causes of hair lost
Tutorial;
 review of skin structure and function
 light microscopy
Learning Objectives
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to know the nature of skin follicles and hair growth
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to know the function of hair in animals and humans
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to know the structure of hair fibres
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to understand the causes and treatment of hair
loss
Hair
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Longitudinal filamentous growth from the skin
Composed of keratin
Found mainly in mammals; definition of phylum
Wool, fur, pelage
Function:
insulation
protection
protective coloration
ornamentation / communication
sensory (vibrissae)
Skin and Keratin
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Keratinized stratified
squamous epithelium is
found only in the skin
Keratin accumulation
within maturing cells
effectively waterproofs
the cells, blocking
diffusion of nutrients and
wastes. The cells
subsequently die.
Hair is specialised adaptation of epithelial
production of keratin
Keratocytes
• keratocytes are cells that make keratins
• >40 highly insoluble proteins forming a
network in the cytoplasm of keratocytes.
• transform from undifferentiated basal
cells to cornified skin cells.
• four continuous layers basal
spinous
recognizable
granular
histologically
cornified
In the course of approximately 13 days columnar basal cells
mature through a polygonal spinous cell, a diamond-shaped
granular cell to a flat cornified cell that covers 25 basal cells.
Parts of Hair
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Hair follicle; involution of epidermis
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Root; implanted in skin
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Shaft or scapus; projecting from surface
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hair Bulb; root enlargment, soft, white
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Papilla; dermal, blood and nerves
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Sebaceous glands; open into follicle
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Arrector muscle; attached to epidermis
Nails
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Longitudinal section through
nail and its nail groove
Claws, Nails, Hooves and Spurs
Keratin structures found in all mammals except cetaceans (whales).
Derived from reptile claws.
Consist of dorsal unguis = nail plate
ventral subunguis = sole plate
a) claws - unguis is curved and encloses the subunguis
retractable in some carnivores.
b) hooves - Unguis completely surrounds subunguis.
cloven hooves in even-toed ungulates (Artiodactyls)
c) spurs - in male monotremes a hollow keratin spur on hind legs
poison gland associated with it.
d) nails - only in primates. Flat unguis + subunguis
greater precise manipulation of food or tools.
Some Horns and antlers
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rhino horn - mass of keratin threads produced by epidermis
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bovid horn - bony core, horny sheath, no shedding
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pronghorn - bony core, horny sheath shed annually
Hair is a Defining Feature
Comparison Reptiles - Mammals
Reptiles
Mammals
scales
hair
few epidermal glands
many epidermal glands
no mammary glands
mammary glands
3-chambered heart
4-chambered heart
ectothermy
endothermy
little cerebral development
much cerebral development
indeterminate growth
determinate growth
egg laying
live birth
Hair is characteristic of
the phylla
Temperature regulation
was a fundamental
development in
mammalian evolution
Animal Hair
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Fur, coat, pelage, wool: furry insulating covering
Special adaptations;
Spines;
porcupine
Bristles;
lion’s mane
UV transporter;
polar bear
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dolphins, whales;
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Size and hair;
aquatic adaptation
(otters and seals)
mass  1/ follicle per cm2
eg elephant, rhinoceros
heat loss
Types of Animal Hair
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Vibrissae;
elongated + stiffened
associaed with many nerve fibers
mainly tactile receptors
facial and carpal
especially in carnivores
primates
rodentia
Types of Animal Hair
Underhair; main function is insulation.
a) wool
long soft and curly
angora
b) fur
fine relatively short hair
definitive growth
c) velli
down or fuzz with velvety appearance
Types of Animal Hair
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Guard hair;
a) awns
b) spines
long, straight and pointed overhairs
present on most mammals
stiff intermediate sized hair, protects down
enlarged, stiff guard hairs with finite growth
evolved parallel
hedgehogs
~6 times in mammals
spiny anteaters
tenrecs
barbs
porcupines
poisonous
montremes
c) bristles
long firm hair eg. horse and lion manes#
hair serves to insulate, to conceal, to signal, to protect, and to sense
the immediate surroundings. Insulation serves to conserve heat, but
can protect against excessive heat eg camel
Tenrec
•Tenrec ecaudatus
(tailless tenrec)
•The common tenrec
occurs on Madagascar
and on the Comoro
Islands, between
Madagascar and Africa.
• Pelage is not dense and is a combination of hairs
and blunt spines.
Spiny Anteaters
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Echidnas,
genus
Tachyglossus, subclass
Holotheria.
monotreme
found only in Australia
and New Guinea.
lays a single egg that is
kept in her pouch for
seven to ten days until
spines begin to harden
• Pelage a mixture of spines and quills
Hair on Humans
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Filamentous growth from the skin
Mainly in mammals; genus definition
Function head:
protective, ornamental
eyelashes: protect eyes from dust
eyebrows: shield eyes
axillae;
wicking of sweat
scalp
axillae terminal
pubes
chest hair
beard
Naked ape
palms
everywhere
soles
bare
else covered
penis
in vellus hair
labia
fingers
mucus membranes
Naked Ape
Great apes have sparse and bare areas but none extensive
Why is man different????
1). Adaptation;
cool forest to hot savana
bipedal locomotion
upright stance
but other cursorial apes, eg gorilla, chimpanzee, no loss
ape pelage shields from UV, wind, radiant heat
2). Eco-parasites;
fitness cost high
cohabitation, communal living
but other gregarious apes, eg chimpanzee, no loss
3). Aquatic ape;
5-7 e6 yr BC, Danakil region Ethiopia
extended period wading lifestyle
hair loss and subcut. fat more efficient
theory not account for eccrine gland, density same as gorilla an
chimp but developed for sweat production
Neoteny
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paedomorphism
slowing of certain aspects of physiological development
retention by adults of juvenile physical characteristics
dogs share many physical features of immature wolves
Louis Bolk 1926: "man is a primate fetus that
has become sexually mature"
advocated by;
Desmond Morris: The Naked Ape
Stephen Jay Gould: chimp bone brain
4 fold increase in hominid brain size over past 3e6 years
loss of body hair a side effect
Types of Hair
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Languno:
Vellus:
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Terminal:
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fine hair covers nearly entire embryo
poorly pigmented, short, peach fuzz
growing most areas
fully developed, longer, coarser thicker
and darker
Although vellus hair continues to develop and accounts
for 6% to 25% of scalp hair, it is less noticeable
because of its light pigmentation
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Middle ear hair
Depth
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vellus follicles situated high
in the dermis
large terminal follicles
rooted deep in the
subcutaneous fat
Dermis
Variable Skin
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The density of hair and the
numbers of associated
eccrine glands varies with
different situations in the
body
Hair Growth
Hair
elongates
from the
dermal
papilla
Length And Growth Rate
Length of hair and growth rate per day vary for
each site on the body
hairs on the head
Eyebrows
beards and whiskers
armpit hairs
pubic hairs
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70cm / 0.35mm
3cm / 0.15mm
28cm / 0.4mm
8cm / 0.3mm
10cm / 0.2mm
Normal Hair Growth
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anagen
1000 days
Anagen = growth phase
2.5 – 3 years
Catgen = short
Telogen = resting 100 days
catgen
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Telogen
100 days
Cyclical pattern
asynchronous
Growth Cycle
• 100 to 150,000
follicles on the scalp
• ~95% of them are
in an active growth
phase called Anagen
• ~5% completed
their growth phase
• Telogen hairs typically remain in the scalp for about
three months before they are shed.
• shampooing, combing or brushing typically releases hair
at a rate of about 50 to 80 per day.
Functions of Human Hair
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head hair for insulation and protection;
trauma
UV
eyelashes blink to prevent entry into eye
eyebrows, like awnings, protect the eyes from sunlight
and sweat that might be hazardous
body hair for insulation; goose bumps
traps air
residual?
axilla;
wicking of sweat
long hair; marker of health?
secondary selection
Inner Ear Hair
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Hairs in the ampullae of the semi-circular canals
are responsible for balance
Otoliths, crystals of calcium carbonate, floating
in gelatinous medium of the saccule brush
against the stereocilia of the inner ear. This
movement generates stereotypic awareness
Hair Colour
Two types of melanin account for all possible hair colors:
Eumelanin:
colors hair brown to black
Pheomelanin:
an iron-rich pigment
colours hair yellow-blonde to red
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Both made from amino acid tyrosine, by tyrosinase.
Increased tyrosinase activity results in darker hair color
Melanocytes in the hair bulb contain subcellular organelles
melanosomes; synthesize and store melanin.
Melanocytes deliver the melanosomes to keratinocytes,
where they are incorporated into the growing hair shaft
Long Hair
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anthropologists speculate functional significance of long
head hair may be adornment
byproduct of secondary natural selection once other
somatic hair had been lost
or Fisherian runaway sexual selection;
lustrous hair is a visible marker for a health
waist length hair = ~1 m or 39 inches long
~80 months, 7 years, to grow
long period of nutrition
stability
hygiene
explain why long hair attractive both sexes
Shape Affects Curls
straight hair is round
curly hair is flattened
Hair Structure
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medulla = central core
of cuboidal cells
cortex = highly packed
cells
cuticular scale =
outer layer
Follicle
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The hair follicle consists of two coats—an outer or dermic, and an
inner or epidermic.
The outer or dermic coat is formed mainly of fibrous tissue; it is
continuous with the dermis, is highly vascular, and supplied by
numerous minute nervous filaments.
Dermal coat consists of three layers The most internal is a hyaline
basement membrane, well-marked only in the larger hair follicles,
it is limited to the deeper part of the follicle
Outside this is a compact layer of fibers and spindle-shaped cells
arranged circularly around the follicle; this layer extends from the
bottom of the follicle as high as the entrance of the ducts of the
sebaceous glands.
Externally is a thick layer of connective tissue, arranged in
longitudinal bundles, forming a more open texture and
corresponding to the reticular part of the dermis; in this are
contained the blood vessels and nerves.
Keratins
A long-chain protein of >300 residues
 contain a high proportion
glycine
H
side
smallest amino acids
alanine
methyl chains
 sterically-unhindered hydrogen bonding
tight
between the amino and carboxyl groups
-helices
peptide bonds on adjacent protein chains
 closely aligned and tightly bound fibres
 tertiary structure: fibres twisted in helical filaments
 second in stength to chitin in organic molecules
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Keratin Secondary Structure
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keratin molecules > 300 residues
high proportion of smallest of amino
acids; glycine
H side group
alanine, methyl side group.
sterically-unhindered hydrogen
bonding between the amino and
carboxyl groups of peptide bonds on
adjacent protein chains
close alignment and strong binding
between chains
keratin molecules twist around each
other to form helical intermediate
filaments.
Keratin Tertiary Structure
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keratin molecules twine around
each other in a left-hand  helix
coil structure
two helices in turn form another
left rotating helical fibre, called a
protofibril
eight protofibrils form a circular
or square structure, called a
microfibril, that is the basis of the
structure of hair.
This structure is stretchy and
flexible and can be compared to
a rope containing various threads
that are twined together.
The Arrangement of Fibres
• The a-helix coiled keratin
molecules are wound
together in protofibrils.
• protofibrils are bound
together into microfibrils
• Bundles of microfibrils are stitched
together to make macrofibrils
• Each hair is wrapped in a cuticle
made up of protein scales and
contains macrofibrils embedded in
matrix proteins
Interchain Bonding
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In addition to intra- and intermolecular hydrogen bonds,
keratins have large amounts of the sulfur-containing
amino acid cysteine ~14%
disulfide bridges confer additional strength
rigid, permanent, thermally-stable crosslinking
similar to sulfur bridges in vulcanized rubber
disulfide bonding contributes to insolubility of keratins
pungent smells of burning hair and rubber due to the
sulfur compounds formed
Strength
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Intra- and intermolecular hydrogen bonds
disulfide bridges that confer additional strength and
rigidity by permanent, thermally-stable crosslinking
It can be made to curl ’permanently’ or set temporarily.
The breakage and reseting of the interchain bonds allow
hair to regain its shape. Hair can stretched about 70%.
Cross-linking creates strength. Hair is equivalent in
strenght to iron wire. A wound strand of 5 mm can
withstand a weight of 60kg
Static
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The close connections of
chains is associated with
the hair's ability to
become charged with
static electricity.
keratin is a good
insulator of heat and
electricity
The capacity to hold an
electric charge is refered
to as triboelectric
Styling
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hair heated with steam and
pulled breaks the relatively weak
hydrogen bonds the a-helices
The a-helices can elongate
without breaking the molecules
• matrix proteins form a tangled supporting mechwork linked
by many disulphide bridges. These bridges are also disrupted
with moist heat
• new interchain hydrogen bonds and disulphide bridges
are formed after styling to keep the hair in its new style
Nails Growth
Growth occurs from a nail bed over a keratin matrix layer
Nail Keratin
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Nailbed The nailbed is an essentially parallel epidermal
structure located directly beneath the fingernail formed
with parallel lines spaced at intervals. During normal
growth, the fingernail travels over the nailbed.
The nail is also composed of keratin microfibrils but the
molecules are arranged in flat B-pleated sheets
Nails Arranged in Stacks
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The molecules of b-keratin in finger and toe nails are
organised in sheets stacked in layers. The proteins still fall into
two groups; the helical and matrix, but in nail there is less
sulphur in the matrix than in the hair matrix.
Hair Length
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Sexual selection; visible marker of health
1.25 cm / 0.5 inch per month:
39 cm = 7 years
good hygiene
nutrition
health
stabilty
Dimorphism
• male androgens at puberty
• testosterone
• vellus - terminal hair
• affects
androgen
sensitivity
growth rate
weight
length
Hair and Culture
Much variation in style and length in
different times and places
 Egypt head hair shaved, especially
children long single lock of hair growing
Maasai warrior
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Middle ages:
shaved head + wig
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17 to 18th C:
long hair
eg Cromwell
Napoleon
Washington
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WWI:
lice and fleas brought typhus,
short hair adopted
Body Hair
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attitudes towards hair on the human body also vary
between different cultures and times
some cultures excessive chest hair on men is a symbol
of virility and masculinity, other societies display a
hairless body as a sign of youthfulness
ancient Egypt, people regarded a completely smooth,
hairless body as the standard of beauty
adopted Greeks; hairless body = youth and beauty
(strigyl)
Islam; many hair tenets, five traits of fitrah
Western societies; removal, bikini fashion, ‘manscaping’
Hirsutism
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Hormonal changes alter hair presence and thickness
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Vellus hairs on face of women, invisible/inconspicuous
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Oestrogen, antagonist of DTH
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Menopause; thicker and darker
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Diseases;
Polycystic ovary syndrome
Cushing's disease
ovarian or adrenal gland tumors
Hair Loss
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Whales & porpoises, and walrus use blubber for
insulation.
Elephants, rhinos, hippos - warm climate, favorable
mass/surface ratio -> retention of heat no problem.
molt = continuous or seasonal (1 or 2x /year) hair
replacement
seasonal fur color change: arctic fox, hares, some
weasels
Hair Loss
genetic
aging
skin conditions
diseases
medications, eg anticancer drugs.
mechanical damage
skin infections.
The followings are the most common
Alopecia areata
Telogen effluvium
Alopecia androgenica = Male Pattern Hair Loss
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caused by range of factors;
Alopecia androgenica
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characterized by "receding hairline"
from lateral sides of the forehead
follicular miniaturization, shaft width
decreases becoming fragile
30% males by 50 yrs
50% by 65 years
66% of adult males
largely genetically determined
caused by male hormones or
androgens esp DHT
Male Pattern Baldness
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occur in varying forms in about 66% of adult males at
some point
characteristic
(i) receding hair from lateral forehead
stages;
(ii) bald patch develops on vertex
(iii) coalescence of patches
pattern baldness is classified on the Hamilton-Norwood
scale I-VIII
Process of Balding
In presence of DTH in genetically prone men, hair
undergoes progressive thining;
- the anagen growth phase is shortened so the hair is
shorter when it stops growing
- overseveral life cycles progressive follicular
miniaturization decreases shaft width and hair becomes
thinner and shorter
Treatments
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Finasteride (marketed in the U.S. as Propecia)
Minoxidil (marketed in the U.S. as Rogaine)
Saw Palmetto (Serenoa repens) is an herbal DHT
inhibitor
easier to prevent the aging and falling out of healthy
hairs than to regrow hair
Alopecia Areata
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characterized by patchy scalp hair loss
many as 1 in 1,000 people some time
most common causes are medications,
pregnancy, birth control pills, thyroid
malfunctions, anemia, syphilis, and
arthritis
autoimmune response, immunological
rejection, anti-follicular
Often resolves spontaneously
50% of people will have recurrence
Cyclosporins may have dramatic effects but are usually
temporary
careful review of your medical history
Alopecia Universalis
• complete loss of hair from all
site of body
• some are born with some hair
but begin losing it quickly
• can be inherited as an
autosomal recessive trait.
• traced to mutation in a gene dubbed HR in chromosome
band 8p21.2. human homologue of mouse "hairless" gene
• Several families known where hairlessness passed through
3 generations
Telogen Effluvium
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characterized by sudden
diffuse hair loss
general thinning of the hair
over a period of months
most commonly experienced
by those who have just given
birth, or are undergoing
chemotherapy
Also caused infection, severe
chronic illness, severe
psychological stress, major
surgery, hyperthyroidism, Crash Diets resulting in poor
health or inadequate protein, and medications
Trichotillomania
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characterized by incessant pulling
or plucking of one's own hair
tillein Greek for "to pluck, pull out
children, males most commonly
in adulthood and adolescence,
most patients are females
hair manipulations usually occur
sedentary activities, reading,
writing, or watching television
impulse control disorder
mental state characterized by
tension with gratification or relief
from the hair pulling
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once the behavior is
established, it
becomes habitual,
regardless of the
initial causative
emotional problem
Resolution requires
psychological
intervention
Tinea Capitis
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infection characterized by bare
patches of skin on the scalp
fungal infection of the skin of
the scalp, eyebrows and
eyelashes
most commonly found in
children ages 10 and under,
scaly non-inflamed area of skin
seborrheic dermatitis
person-to-person transmission. The organism remains
viable on combs, brushes, couches, and sheets
Since effective treatment of Tinea Capitis by griseofulvin
in the 1950s incidence dropped from 14% to 1.2%
Aging
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pigment in the hair is lost, hair becomes colourless
time varies from person to person
men tend to become grey at younger ages than women
pale blond hair becomes white instead of grey
red hair becomes a sandy color and later white
gray or white appearance due to air bubbles in medula
scattering light
scalp hair loss or thinning with aging in both sexes
half of all men are affected by male pattern baldness by
the time they are 50
Death
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It is commonly claimed that hair and nails will continue
growing for several days after death.
This is a myth; the appearance of growth is actually
caused by the retraction of skin
as the surrounding tissue dehydrates, nails and hair
become more prominent
Histological Prep
Crossectional View
medulla
cortex
cuticle
epidermal
sheath
dermal
sheath