APPENDAGES OF SKIN
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Appendages of skin are includes hairs, nails, sebaceous
glands and sweat glands
THE HAIR
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Hair are keratinized threads derived from invagination of epidermal
epithelium
Found over entire skin except palms, soles, glans penis,
dorsal surfaces of distal phalanges, clitoris and labia
minora
It has a shaft which projects above the skin and a root
which is embedded in the skin
Hair root is surrounded by tubular hair follicle consisting
of epidermal and dermal elements
At its deep end hair follicle is expanded into hair bulb
At hair bulb, hair root and epithelial sheath blend with
mass of cells called matrix
Papilla is an indentation at the basal end of the hair
bulb
Associated with each hair follicle are sebaceous gland
and erector pili (arrectores pilorum) muscle.
STRUCTURE OF HAIR
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HAIR SHAFT:
Consists of epithelial cells arranged in three concentric
regions i.e. medulla, cortex and cuticle
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MEDULLA:
Forms central axis of the hair
Consists of two or more layers of shrunken, moderately
keratinized cuboidal cells
The intercellular spaces of medulla usually contain air
CORTEX:
Forms main bulk of hair
It is composed of long, flattened, fusiform, heavily keratinized
closely packed cells
In dark hair the cells contain granules of melanin pigment
Between cells small air filled spaces occur
Cuticle: Consists of single layer of flattened, clear, keratinized
cells which partially overlap each other.
Cells are non nucleated except those present at the lower part of the
root.
Hairs grow at an average rate of 2 mm per week.
After a specific period of growth, each hair ceases to grow
and is lost and replaced
Colour of the hair is due to quantity of the melanin and
amount of the air in the hair substance
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Melanocytes are present between epithelial cells of the hair
root from there melanin is transported to the cortex of hair and
imparts black colour to it.
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Whitening of colour of hair is due to failure of pigment formation and
increase in amount of the air in the intercellular spaces of cortex and
medulla
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Hair follicle:
Surrounds the root of hair and is responsible for production and
growth of hair
Lies in dermis but frequently invades hypodermis
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Parts of hair follicle:
Each hair follicle consists of three parts infundibulum,
isthmus and inferior segment
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Infundibulum: it is segment from the surface opening of the follicle to
the level of opening of the duct of sebaceous gland associated with hair
follicle
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Isthmus:
Is the part of hair follicle that lies between the opening of the
sebaceous gland and attachment of arrector pili muscle
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Inferior segment:
Is the part hair follicle from arrector pili attachment to the
proximal end of the follicle where it expands to form hair bulb
Each hair bulb is indented at its base by dermal papilla consisting of
loose vascularized connective tissue
The capillaries of the dermal papilla provide oxygen and nutrients for
the cells of hair follicle
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COATS OF HAIR FOLLICLE:
Each hair follicle is composed of the following two major coats,
epithelial root sheath and connective tissue sheath
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Epithelial root sheath
Is derived from epidermis
Surrounds hair root immediately next to the hair cuticle
It is further composed of two coats inner and outer epithelial root
sheath.
INNER EPITHELIAL ROOT SHEATH:
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This sheath represents the superficial layers of the epidermis and does
not extend above the entry point of the duct of sebaceous
gland
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From inner part to out it has three layers i.e. cuticle,
huxley’s and henle’s layer
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The cuticle: lies against the cuticle of the hair.
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It is single layer of the clear flat horny cells which over lap
each other
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The huxley’s layer: consists of several rows of
polyhedral cells
The henle’s layer: consists of single row of low columnar cells
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OUTER EPITHELIAL ROOT SHEATH:
corresponds to the malpighian layer of the epidermis
Consists of several layers of irregularly polygonal cells
with processes surrounded by a single layer low
columnar cells
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CONNECTIVE TISSUE ROOT SHEATH:
It is also called dermal root sheath lies outer to epithelial
root sheath
The connective tissue sheath disappears at the level where
the arrector pili muscle is attached to the hair follicle, it
consists of three layers from inside to out
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Vitreous layer (Glassy membrane):
Is a thin band corresponds to the basal lamina of the epidermis
It is composed of an amorphous ground substance in which are
embedded fine reticular fibers
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Middle layer:
Consists of circularly arranged collagen fibers with some elastic fibers it
represents papillary layer of the dermis
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Outer layer:
Is composed of coarse bundles of collagen fibers running longitudinally,
corresponds to reticular layer of the dermis.
ARRECTORES PILORUM:
Are oblique bundles of smooth muscle fibers found in the
dermis in relation to hair follicle
An arrector pili muscle is attached at one end to the papillary
layer of dermis and at other end to the connective tissue root
sheath of a hair follicle at about middle of the follicle
Arrector pilorum is innervated by the sympathetic nerve fibers
It causes errection of hair (slopes normally at an obtuse angle to
skin)
Errection of hair also accompanied with elevation around the shaft
of hair producing tiny bumps on the skin and thus giving the skin
appearance of goose flesh
Function of the hair
Human hair do not perform the insulatory function against the heat loss
as in animals
Play an important role in tactile sensation
Any stimulus causing deformation of hair is transmitted along the shaft
of hair to the free nerve endings that surround the hair follicle
The nails
Are hard translucent roughly rectangular, horny plates that
cover the dorsal surfaces of the terminal phalanges of
fingers and toes
Homologous to the stratum corneum of the epidermis there
fore nail plate consists of extremely compact,
highly keratinized epithelial cells
Each nail plate rests on nail bed that consists of skin under
their nail
Nail plate consists of three parts
Body visible part of nail
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Free edge which projects beyond the skin distally
Root which is proximal part of nail bed that lies beneath the
skin fold.
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Body of the nail is translucent and appears pink
Because it transmits color of blood
Cresenteric whitish area on the proximal part of nail body is
known as lunula it is reflection of partly keratinized cells in this
region
The fold of skin around the proximal and lateral borders of the
nail is known as nail fold the furrow between nail fold and nail bed is
called nail groove
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The epidermis of the nail bed consists of only of stratum
basale and stratum spinosum
The dermis of nail bed does not bear typical papillae but
instead shows longitudinal ridges the epidermis of the nail bed
is very thick
The nail matrix is germinative zone and exhibits high rate of
cell division and is responsible for growth of nail
The epithelial cells of the matrix proliferate constantly move
distally and eventually become keratinized
These keratinized cells are continuously added to the
proximal part of the nail plate so nail plate constantly slides
forwards on the nail bed
The growth of finger nail is about 0.2mm per week but toe nail
grow slower
Nail keratin is a hard keratin that consists of compactly packed
keratin filaments lying in a matrix of amorphous keratin (has a
high sulpher content which imparts hardness to the nail
plate)
Keratinized cells don’t desquamate because contain hard
keratin
Epidermis of the nail bed is continuous distally with
epidermis of finger tip under free edge of the nail (stratum
corneum of the epidermis is thickened and is known as
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hyponychium)
The stratum corneum of the proximal nail fold extends for a short
distance over the free surface of the nail plate as eponychium.
GLANDS OF THE SKIN
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SEBACEOUS GLAND:
Are spherical or ovoid
Enclosed in connective tissue capsule
They lie in the dermis but their duct open in the infundibular
part of hair follicle
The duct and infundibular of the hair form pilosebaceous
canal in some other locations like lips, penis, labia minora it
directly opens to the surface of skin
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Most abundant in face , and scalp but absent in palms and soles
Are simple branched acinar glands of holocrine variety
5 to 10 acini open into duct
Acini are filled completely by the stratified epithelium
Basal cells are low cuboidal and resting on delicate basal
lamina and contain a spherical nucleus and in cytoplasm are
mitochondria, smooth endoplasmic reticulum, and lipid
droplets
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Basal cells produce more basal cells and larger round cells which
contain more smooth endoplasmic reticulum and lipid
droplets
These cells move to centre of acini and accumulate more
lipid and become polyhedral, nuclei becomes pyknotic
and disappear
Finally all centrally located cells break down releasing
fatty mass and cellular debris (sebum) which pass through the duct of
gland into infundibulum of hair follicle then to skin
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Duct is lined by stratified squamous epithelium
Secretions are under control of androgens in both male
and female
In male it is testosterone and in female it is combination of
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ovarian and adrenal androgens
Function of sebaceous gland sebum facilitates the maintenance of
proper skin texture and hair flexibility
It is also proposed that sebum has weak antifungal and anti
bacterial and anti bacterial properties
At time of puberty they have heightened activity under the
influence of androgens
Excessively produced sebum accumulates and is invaded
by bacteria giving rise to acne
SWEAT GLAND (SUDORIFIC GLAND):
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Classified into two types eccrine sweat gland and apocrine sweat glands
ECCRINE SWEAT GLAND:
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Distributed all over the body so term sweat gland is applied
to it until specified
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Simple tubular glands
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0.4mm in diameter
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Release product by merocrine method
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Abundant in palms and sole and not present in glans penis
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Terminal secretory portion is coiled and lies deep in dermis
Duct travels through dermis and epidermis and terminates by a small
opening sweat pore
Secretory portion is lined by cuboidal or low columnar epithelium
Two varieties of cells can be distributed i.e. dark cells and clear cells.
DARK CELLS (MUCOID CELLS)
appear dark in routinely prepared sections for light
microscopy.
Cells have narrow basal area and broad apical part lining the
lumen
Their basal end rarely touches basal lamina
Under electron microscopy they contain prominent golgi apparatus, long
mitochondria, many cisternae of rough endoplasmic reticulum and
abundant free ribosomes.
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Their broad apical portion contains dense secretory granules containing
glycoproteins
Produce mucoid secretions
CLEAR CELLS:
Stain lightly as does not contain secretory granules
Pyramidal in shape with broad basal region lying on basal lamina and
narrow apex which does not reach lumen so communicate lumen
through narrow intercellular canaliculi present between the dark cells
Under electron microscope clear cell cytoplasm
contains organelles similar to those of dark cells
except that rough endoplasmic reticulum and free
ribosomes which are scarce in these cells.
Their basal plasmalemma shows infoldings
characteristic features of those cells which are
engaged in transepithelial fluid and electrolyte
transport
Produce clear watery secretion (serous secretion)
Secretions reaches the lumen of sweat gland through intercellular
canaliculi
A layer of myoepithelial cells is present between the
bases of secretory cells and surrounding basement
membrane and are fusiform in shape and show
cytoplasmic processes .
The myoepithelial cells contain actin and myosin
filaments and are supplied by sympathetic nerve
fibers and their contraction causes discharge of
secretions
Duct of sweat gland is highly coiled and is lined by stratified cuboidal
epithelium consisting of two layers of cuboidal cells
Basal layer cells have darkly staining ovoid nucleus and abundant
mitochondria and ribosomes.
Luminal cells are some what smaller but have nearly the same
organelles as those of basal layer cells but have large number of
tonofilaments in their apical parts because of it these parts are intensely
eosinophilic
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Myoepithelial cells and intercellular canaliculi are absent in
ducts of sweat gland
As ducts join epidermis it losses its own wall and becomes
a specialized passage through stratified squamous
epithelium
Sweat glands are innervated by cholinergic sympathetic
nerves fibers and their secretion is produced in response
to heat and nervous strain
Sweat is clear liquid derived from network of capillaries that
intimately envelop the secretory portion of the sweat glands
Its components include water potassium sodium chloride
urea ammonia uric acid and lactic acid
FUNCTIONS
Temperature regulation by producing a film of moisture for
evaporative cooling
Excretion of waste products like ammonia urea and uric acid and some
drugs
APOCRINE SWEAT GLANDS
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Found in axilla, areola of the breast, circumanal region and
labia minora
Simple coiled tubular glands
Secretory portion is in dermis or hypodermis
Duct opens into canal of hair follicle just distal to the
sebaceous gland’s duct
Lumen of secretory portion is larger than eccrine gland and is lined by
simple epithelium containing a single layer of cuboidal or low columnar
cells.
Lining cells contain round to oval nucleus, abundant
mitochondria, a large golgi apparatus numerous lysosomes
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and many lipofusin pigment granules.
Myoepithelial cells are present between secretory cells and adjacent
basal lamina
Become functional at puberty
Produce viscous fluid which contain proteins,
carbohydrates, ammonia, lipids and certain organic
compounds that may color the secretions.
When released the secretions are odour less but quickly
acquires distinct odour as a result of bacterial decomposition that
generates compounds (short chain fatty acids) having disagreeable
odour
Are innervated by adrenergic sympathetic nerve fibers and they secrete
in emotional and sensory stimuli
Under electron microscopic studies the concept of apocrine secretion
(losing apical portion) is prove wrong and these glands also secrete by
merocrine method
THE END