The Integumentary System
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Main Anatomy Index
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Last updated 30 March 2006
This page was contributed by Ross Keen.
The Integumentary System
- This is the skin and its derivatives.
- It forms the external covering of the body and 15-20% of it's mass.
The Skin
- Skin is composed of 2 main layers:
- The epidermis;
- And the dermis.
- The hypodermis is deep to the dermis and is the superficial fascia
of gross anatomy.
- It is a looser form of connective tissue than the dermis.
- The hypodermis has variable amounts of adipose
tissue.
Epidermal Derivatives
of Skin
- The epidermal derivatives of the skin are:
- Hair and hair follicles;
- Sweat glands;
- Sebaceous glands;
- Nails;
- And mammary glands.
Functions of the Integumentary System
- There are a number of functions of the integumentary system:
- A barrier function, protects against physical,
biological and chemical agents;
- A homeostatic
function, to preserve the internal environment
by regulating body temperature;
- A sensory function, information about the environment,
informative and protective;
- A secretory function;
- And an excretory function, via sweat
glands.
Types of Skin
- There are 2 types of skin:
- Thick skin, which is hairless
and on the palms and the soles
of the feet;
- And thin skin, which is hairy.
The Epidermis
- The epidermis is composed of stratified
squamous epithelium and is composed of 4 layers in thin
skin but 5 in thick skin.
- These layers are:
Stratum Basale
- This is a single layer of cells resting on basal lamina.
- It has stem cells from which keratinocytes
arise.
- The cells are small and cuboidal
to low columnar.
- It has less cytoplasm than cells above.
Stratum Spinosum
- This layer is several cells thick and is larger than in
the stratum basale.
- Cytoplasmic processes attach the cells in this layer to
other cells by desmosomes.
- As they mature, they become flatter and increase in size.
Stratum Granulosum
- This is the most superficial of the non-keratinised
layers.
- The cells contain numerous keratohyalin granules.
Stratum Lucidum
- In this layer, the nucleus disappears and cells fill
with keratin.
- This layer is only visible in thick skin.
Stratum Corneum
- The cells are flattened and coated
with a glycolipid to act as a water
barrier.
Cells of the Epidermis
The Keratinocyte
- This is the predominant cell of the epidermis.
- On leaving the basal layer
has two functions:
- The production of keratin;
- And creation of an extracellular
water barrier.
- The keratinocytes are engaged in intermediate filament production
(tonofilaments).
- This is the protein of keratin production, which are
bundled into tonofibrils.
- In the upper part of the stratum
spinosum they start to make keratohyalin granules and
lamellar bodies.
- Keratinisation is the conversion
of granular cells into cornified
cells.
- It involves the breakdown of the nucleus and the thickening of the plasma membrane.
- The lamellar bodies are secreted
and coat the cell with a glycolipid
forming a water barrier.
- Here it is soft keratin compared with the hard keratin of the nails and hair.
The Melanocyte
- The melanocyte represents a small proportion of the
total epidermal cells.
- They are also present in dermis
where they are stellate cells with long
processes and an elongated nucleus.
- They are derived from the neural
crest and with the keratinocytes form an epidermal-melanin unit.
- The melanin granule is called a melanosome.
- These melanosomes are concentrated near the bases of the cell processes when they are nearly
mature and in the processes or at the ends if they are mature.
- Melanosomes are transferred to the keratinocyte through phagocytosis
of the ends of the processes.
The Langerhan's Cell
- This cell doesn't form desmosomes with the neighbouring keratinocytes.
- Its nucleus is characteristically
indented in many places.
- It has granules that appear as rods
with a striated band.
- It is involved in the initiation of cutaneous contact
hypersensitivity reactions.
The Merkel Cell
- This is a modified epidermal cell located in the stratum basale.
- It is most abundant in skin where sensory perception is acute,
e.g. the fingertips.
- The Merkel cell is bound to nearby keratinocytes by desmosomes.
- They have keratin in the cytoplasm
and their nucleus is lobed.
- These cells are characterised by the presence of dense-cored neurosecretory granules.
- The combination of the neuron and epidermal
cell is called a Merkel's corpuscle, a very sensitive mechanoreceptor.
The Dermis
- This is the layer of skin under the epidermis.
- It is secured to it via dermal papilla, which are
complemented by epidermal pegs.
- At areas of increased mechanical stress the epidermal pegs are much deeper
and the dermal papillae are much longer
and closely spaced.
- Dermal ridges are present in thick
skin along with dermal papillae and form fingerprints.
- A series of attachment sites called hemi-desmosomes
link the basal plasma membrane to the basal
laminae and attach to anchoring filaments.
- Anchoring fibrils connect the dermis
and the basal laminae.
- The dermis is composed of 2 layers:
The Papillary Layer of the
Dermis
- This is the more superficial and cellular
layer.
- The collagen fibres are not as
thick.
- The elastic fibres form an irregular network, including
the substance of the dermal papillae and dermal ridges, blood vessels.
- Nerve endings are concentrated in
the papillae.
The Reticular Layer of the
Dermis
- This layer is considerably thicker and less cellular.
- It has thick irregular bundles of collagen
and more coarse elastic fibres.
- Collagen and elastic fibres
are oriented into Langer's lines.
The Hypodermis
- This layer contains adipose tissue and individual smooth muscle cells or bundles.
- This smooth muscle cells or bundles constitute the arrector pili
muscles of the hair.
Nerve Supply of the Skin.
- The most numerous neuronal endings are free nerve endings in the epidermis
and the papillary dermis.
- They terminate in stratum granulosum.
- Networks of free dermal endings surround most hair follicles attached to their outer
root sheath.
- They are mechanoreceptors and very
sensitive.
Pacinian Corpuscles
- These are deep pressure receptors for mechanical and vibratory pressure.
- It is found in the deeper dermis
and the hypodermis, especially the
fingertips.
- They are composed of myelinated nerve endings
surrounded by a capsule.
- The myelin is retained for one or
two nodes and then lost.
- The unmyelinated section is covered by a series of flattened Schwann cell lamellae that form the inner core.
- The rest of the corpuscle is composed of concentric lamellae.
Meissner's Corpuscles
- These are touch receptors, particularly
responsive in the papillary layer of hairless skin, e.g., lips and palmar and volar surfaces of
the fingers and toes.
- They are present at the tip of the dermal papillae.
- One or two myelinated nerve fibres follow a spiral path in the corpuscle (they are often described as
looking like a "skein of wool").
Ruffini Endings
- These respond to mechanical displacement of adjacent collagen fibres.
- Collagen fibres pass through the capsule and the neural element
is a single myelinated fibre.
Skin Appendages
- Hair follicles and hair;
- Sebaceous glands and their product sebum;
- Eccrine sweat glands and their product sweat;
- Apocrine sweat glands and their product serous secretion.
Hair
- Hairs are composed of keratinised cells that develop
from hair follicles.
- Coloration of the hair is due to the content
and type of melanin that
the hair contains.
- The base of the follicle is called the bulb.
- At the base of the bulb there is an invagination;
a dermal papilla.
- The outermost part of the follicle is the external root sheath.
- This is a down growth of the epidermis.
- Other cells of the bulb are called the matrix and are
called matrix cells.
- The dividing cells of the germinative layer of the matrix
differentiate into the keratin producing cells
of the hair and the internal root sheath (multilayered).
- Both have 3 layers, the hair
having the medulla, cortex
and cuticle and the internal root
sheath having a cuticle, Huxley's
layer and Henle's layer.
- Keratinisation occurs at the keratogenous
zone.
- Hair has hard keratin while the internal
root sheath has soft keratin.
- The arrector pili muscle attaches to outer
sheath.
Sebaceous Glands
- These glands secrete sebum.
- This is a holocrine-type secretion where both the product and the cell are discharged from gland.
- It sebum coats the hair
and skin surface.
- Sebaceous glands develop as outgrowth of the external root sheath.
Sweat Glands
- There are 2 types of sweat glands:
- Eccrine sweat glands, all over body except lips and part of external genitalia;
- Apocrine sweat glands, only in axilla,
areola, nipple of mammary gland, and circumanal region and the external
genitalia. The ceruminous glands of ear and glands of Moll of eyelid are also apocrine.
Eccrine Sweat Glands
- These are simple coiled glands that regulate
body temperature.
- The secretory segment is deep in the dermis
or upper hypodermis.
- Its duct leads to surface.
- In the secretory region there are clear
cells that produce the watery component of sweat and dark cells that
produce a proteinaceous secretion.
- There are also myoepithelial cells that are responsible
for the expression of sweat from the gland.
- Duct cells form the walls
from the secretory portion to the area near the surface where the epidermal cells form the
wall.
- The duct is stratified cuboidal.
- There is both thermoregulatory sweating and emotional sweating.
- Resorption of some minerals
take place in the duct.
- Myoepithelial cells are present in the duct.
Apocrine Sweat Glands
- These are large lumen glands associated with hair follicles.
- They develop from the same down
growths that give rise to hair follicles.
- The connection is retained and they are coiled tubular glands, sometimes branched.
- The secretory portion is in the dermis
or upper hypodermis.
- The secretory product is stored
in the lumen.
- Myoepithelial cells facilitate the expulsion
of the secretory product from the gland.
- The duct has a narrow lumen.
- This duct has a stratified cuboidal epithelium.
- Resorption does not take place in the duct.
- Myoepithelial cells are also not
present in the duct.
- Apocrine secretions contain protein,
carbohydrate, ammonia
and lipid.
Nails
- These are plates of keratinised
cells containing hard keratin.
- The nail plate rests on the nail
bed.
- The proximal part, called the nail root, covers the germinative zone.
- This is called the matrix.
- The crescent shaped white part near the nail called the
lunula.
- The fold of skin near the root of
the nail is the eponychium.
- The hyponychium is the thickened
epidermal layer that secures the free edge of the nail plate at
the fingertip.