Dr. heba kalbouneh - Weeblyjumed16.weebly.com/uploads/8/8/5/1/88514776/sheet-13-2017[336].pdfconservation of water inside our body ... a lot of nerve endings that sense pain, touch,

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Dr. heba kalbouneh

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(Integumentary System)

➢ Integument (in Latin): to cover

➢ The skin covers the outer surface of our bodies and it is

continuous with the mucus membranes (the oral cavity or nasal

cavity for example)

➢ The skin (known as “cutaneous

membrane”) is composed of two

layers:

1- Epidermis: keratinized stratified

squamous epithelium

2- Dermis: (Derma = skin) is a

connective tissue

➢ Below the skin is the subcutaneous

layer which is also called hypodermis because it is located

under the dermis

➢ Hypodermis is adipose tissue and in anatomy it is the superficial

fascia and it is NOT part of the skin

➢ Under the hypodermis, you can find the muscles.

Skin appendages (accessory structures associated with the skin):

1- hair follicles: Invaginations from epidermis into dermis where we

can find the hair shaft

2- Sweat glands

3- Sebaceous glands which open on the hair follicles.

4- Nails

➢ Major functions of the skin:

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1- Protection from the outer environment and microorganisms

2- Protection of our DNA from the ultraviolet light

3- Prevention of water loss by forming impermeable layer and

conservation of water inside our body

4- Thermoregulation:

a- Hot weather >> sweating >> cooling body’s temperature

b- Cold weather >> hair terminals will stand up to close pores and

prevent heat loss >> keeping the body warm

5- Sensation: a lot of nerve endings that sense pain, touch,

pressure, and temperature are located in the skin (sensory

receptors)

6- Excretion: sweat (water and other electrolytes)

7- Formation of vitamin D

➢ Skin is an organ, it is NOT a tissue, it is composed of different

tissues (epithelium and connective tissues for example) and

each tissue has a different origin

Epidermis: originates from ectoderm

Dermis: originates from mesoderm

➢ Skin is the largest

organ in our

body; it weighs

about 5 Kg and

covers an area

up to 2 m²

➢ Epidermal-

dermal junction is

the junction

between

epidermis and dermis, we mentioned that the epidermis is

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epithelial tissue and the dermis is connective tissue, so the

epidermal-dermal junction represents the basement

membrane of the epithelial tissue

➢ This basement membrane is a wavy line, not a straight line,

because there are invaginations from epidermis toward the

dermis and we call these epidermal ridges.

➢ And there are invaginations in the opposite direction (toward

the epidermis), the part of the dermis that invaginates inside

the epidermis is called dermal papilla (papilla means nipple like

structure, pl. papillae).

➢ This wavy junction is important to increase surface area for

adhesion, and to increase the surface for diffusion; because

epidermis is avascular so it gets its nutrition from vessels in the

dermis.

➢ These invaginations are more prominent in the palms of hands

and in soles of feet. Also, these invaginations create a unique

pattern for each individual called fingerprint and footprint;

these are so unique and can’t be identical even between

identical twins!

Epidermis:

Layers of epidermis:

• The lowermost layer (right above the basement membrane) is a

single layer of cuboidal cells (or columnar cells), this layer is

called Stratum Basale, and its cells are characterized by mitotic

activity. Cells of Stratum Basale are connected

To each other by desmosomes

To the basement membrane by hemidesmosomes

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• The second layer (above Stratum Basale) is called Stratum

Spinosum and it is

composed of more

than one layer of

cells, the shape of

these cells is

polyhedral, but it

appears spinous in

shape in histological

preparation. The

spinous appearance

in this layer is due to

desmosomes (during

preparation, the cells

shrink and the spines

represent the areas of

desmosomal attachments between the cells)

- Note that the skin cells have high amount of desmosomes

In the lower part of this layer we can find some mitotically

active cells (as in Stratum Basale) and we refer to these two

layers together [Basale and lower part of spinosum] as Stratum

Germinativum.

• The third layer is Stratum Granulosum, it is named “granulosum”

because we can find basophilic granules in its cells’ cytoplasm,

Its cells are flat and connected to each other by desmosomes.

The basophilic granules are keratin granules called

Keratohyalin granules.

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➢ There is another type of granules in these cells, lamellar

granules, they are called lamellar because they show

lamellar pattern (composed of layers) when viewed under

the electron microscope, lamellar granules contain lipids,

and by exocytosis the lipids will be discharged from the cells,

and form a lipid rich layer around the cells (impermeable

layer that prevents water loss or gain)

Keratohyalin granules Lamellar granules

Basophilic

(visible under light

microscope)

Not visible under light

microscope (only under E.M.)

Not surrounded by a

membrane

Surrounded by a membrane

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** Granules start appearing at the upper part of stratum

spinosum **

• The fourth layer is the Stratum Lucidum, it has flat cells which are

dead and full of keratin, and it is 3-4 layers of cells.

• The fifth layer is Stratum Corneum, its cells are dead, filled with

keratin, and surrounded by lipid layer (from lamellar granules)

Why do the cells die as we go upward??

Because they are pushed farther away from blood supply (the

shedding of the dead cells is called desquamation), and the

cytoplasm is filled gradually with keratin.

Again, epidermal cells are connected to each other by

desmosomes, and the desmosomes – as you know – are

connected to intermediate filaments in the cytoplasm, these

filaments are keratin in the skin.

As we go up in epidermis, synthesis of keratin intermediate

filaments increases.

Keratin filaments are called tonofilaments, these tonofilaments

are packed gradually as we go up and cross-linked with each

other by linker proteins like Filaggrin protein to create thick

bundles called tonofibrils.

The process of keratin synthesis and accumulation is called

keratinization (or maturation), and it occurs in all cell stages of

epidermis until the cell is dead; and that is why cells of skin are

called keratinocytes.

Keratinocytes count for 90% of the cells of epidermis, and the

epidermis contains other types of cells (less in number) like:

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- Melanocytes: this type produces melanin, and they are

located in Stratum Basale and connected to the basement

membrane (only to it) by hemidesmosomes, but they are NOT

connected to keratinocytes.

➢ Melanocytes have many processes that run between

keratinocytes.

➢ Melanocytes

synthesize melanin

(melanosomes), but

they don’t store it in

their cytoplasm.

➢ Melanosomes are

transported to the

tips of the

cytoplasmic

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processes of melanocytes to enter the keratinocytes by

phagocytosis.

WHAT?!

Yes, keratinocytes

phagocytose the tips

of melanocytes’

processes to get the

melanin.

➢ When melanin enters

the keratinocyte, it will

form a structure like

an umbrella above

the nucleus to protect

it from ultraviolet light.

➢ Lighter colored people (less melanin) are more prone to

develop skin cancer than darker colored people.

➢ Sun exposure stimulates melanocytes to synthesize more

melanin (sun tanning)

➢ NOTE: melanin is not the only pigment that contributes to the

color of the skin, there are other pigments that participate,

like Carotene pigment (people who eat lots of carrots

become orange in color!!!! Yes, its true)

➢ Carotene accumulates in adipose tissue (hypodermis) and

Stratum Corneum; a disorder

which is called carotenemia.

The blood flow toward the skin

may also contribute in

coloring the skin (become red

like in blushing).

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❖ The number of melanocytes is equal in all humans but

their activity may differ.

❖ The number of melanocytes is 1 cell for each 5-6

keratinocytes of Stratum Basale.

❖ Under the light microscope, you can see these cells

rounded in shape and have a clear cytoplasm. They

have fine processes passing between the keratinocytes

but you can’t see them (under LM).

- Langerhans cells: which are macrophages in the skin

These cells are found mainly in stratum spinosum but you can

also find them in other layers.

They form 2-8% of cells in the skin.

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- Merkel Cells: Found in stratum basale and have many

processes

They are attached to basement membrane by

hemidesmosomes and to other

keratinocytes by desmosomes.

They have the same origin of the

keratinocytes (ectoderm).

They are involved in tactile

sensation (our sense of touch).

They are associated with sensory

neurons, this structure is called

Merkel’s disk (which is a type of

sensory receptors, and it belongs

to mechanoreceptors).

They are more prominent in finger tips.

Dermis:

➢ The upper part of it is loose connective tissue, it is called

papillary layer of dermis.

➢ And the lower part is dense irregular connective tissue, it is

called reticular layer

(but this doesn’t

mean it contains

mainly reticular

fibers!) it is named

reticular because

collagen fibers and

elastic fibers form a

network structure. It

is thicker than loose

connective tissue.

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➢ There are blood vessels exactly under the papillary layer

(subpapillary plexus).

➢ And blood vessels between the dermis and the

hypodermis (Subdermal plexus) >> these give branches to

form subpapillary plexus and they are important in

controlling body heat

(When the weather is cold the diameter of these vessels

decreases (vasoconstriction), decreasing blood flow toward

the skin, preventing heat loss). In hot weather, vasodilation of

these blood vessels takes place resulting in heat loss through

the surface of the skin.

➢ There are two types of skin: thin skin and thick skin

➢ These two types refer to the thickness of the EPIDERMIS not

the whole thickness of the skin.

➢ Thick skin refers only to the skin of the palms of hands and

soles of feet, and skin of the rest of the body is thin skin.

➢ Thick skin is always non-hairy and has no sebaceous

glands (but of course it has eccrine sweat glands)

➢ Remember apocrine sweat glands open into hair follicles,

while eccrine sweat glands open into the surface of the

skin

➢ Epidermis of thick skin has 5 layers (mentioned earlier).

whereas in thin skin, the epidermis lacks stratum lucidum

(so it is only 4 layers)

➢ The skin of the back (which is 4 mm thick) is considered

thin skin same as the skin of the eyelids (which is 0.1 mm

thick) because they have almost the same epidermis (but

the difference is in the thickness of the dermis).

➢ Generally, thin skin has thicker dermis than thick skin

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Sensory receptors:

Sensory receptors which are

surrounded by a capsule

are called encapsulated

sensory receptors, Examples:

1- Meissner’s corpuscles,

which present in dermal

papillae and are

responsible for light touch

reception, more

prominent in fingertips.

2- Pacinian corpuscles,

which present in reticular layer of dermis or sometimes can

reach the hypodermis, they are responsible of transmission of

deep pressure.

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3- Ruffini’s corpuscles, fusiform in shape, which present in

reticular layer of dermis, respond to stretch (tension) or

twisting in the skin

If they are not surrounded by a capsule they are called

unencapsulated sensory receptors or nerve endings. (Examples:

Merkel’s disk, free nerve endings for pain and temperature, root

hair plexus)

At the end of hair follicle, you find root hair plexus surrounding

the hair follicle, which detects the movement of the hair.

Skin

appendages: accessory structures of the skin, we can find inside the

skin the hair and the hair follicles, the nails (which are keratinized

plates of dead cells), sebaceous glands that open into the hair

follicle, we also talked about eccrine and apocrine sweat glands.

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HAIR

It is an elongated keratinized structure that is found inside the hair

follicle (pocket shaped). The hair fiber itself is divided into two parts:

1.the part that is embedded inside the skin is called root of the

hair 2.tha part that is visible

(above the level of the skin) is called shaft of the hair

The epidermis of the skin invaginates inside the dermis and forms a

pocket where you find the hair, this is called hair follicle.

Hair bulb: the most basal swollen area of the hair follicle.

What do we have surrounding the hair follicle? The dermis, we call it

dermal sheath

Structure of the hair follicle

-Inner root sheathmost inner part, terminates at the level of the

opening of the sebaceous glands because we need space or canal

in order for the sebum to be discharged and lubricate the skin and

the hair. Inner root sheath is composed of three layers (not required)

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-Outer root sheathis continuous with the skin epidermis, in particular

it is continuous with stratum Basale and stratum Spinosum of the

epidermis.

-Glassy membrane (thickened basement membrane) also it is

continuous with the basement membrane of skin epidermis. It is

called glassy since it is thick and it appears glassy (transparent)

under the light microscope

-Surrounding dermal sheath

At the base of the hair bulb, the dermis invaginates inside the hair

bulb and forms a dermal papilla (hair papilla), since it’s similar to the

dermal papilla of the epidermis). The dermal hair papilla contains

tiny blood vessels to nourish the hair cells.

Stratum Basale: if we follow the stratum Basale layer, it goes inside

the dermal papilla and lines it, it forms a layer of cells called the

matrix, we previously mentioned that we have mitotic activity inside

the stratum Basale so these matrix cells undergo mitosis and give up

cells that become keratinized as we go up to form the hair shaft. The

hair shaft formation process is similar to the keratinization of the skin,

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only the matrix cells undergo mitosis. They divide and form something

similar to Spinosum layer and granulosum and they accumulate

more and more keratin. The difference between the keratin of the

hair and the keratin of the stratum Corneum is that the keratin of

stratum Corneum of the skin is soft while the keratin of the hair is hard

and it is highly compacted to form the hair shaft.

These matrix cells divide and form the hair shaft and the inner root

sheath.

Arrector pili muscle:

Arrector because it

causes erection to

the hair, pili: hairs. It

is a smooth muscle

which means it’s

involuntary, it is

connected to the

hair follicle from one

side and to the

papillary layer of the dermis from the other side. It is supplied by

sympathetic nervous system-autonomic nervous system. The

contraction of this muscle will cause hair to stand up

(perpendicularly to the skin) and this will close the pore which will

reduce the heat loss, this condition is known as goose bumps (when

you are cold or when you are frightened) under the stimulation of

the sympathetic nervous system. It is more prominent in animals it

reduces the heat loss, in animals covered with fur or hair, the erect

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hairs trap air to create a layer of insulation. The erect hairs also make

the animal appear larger in response to fear.

Structure of the hair shaft; it consists of 3 layers of hard protein

(keratin):

The inner most layer medulla

Intermediate layer cortex- contains the pigment melanin

Outer layer flat squamous cells filled with very hard keratin, called

the cuticle (when the hair is damaged with split ends مقصف those cells

are desquamated from their place)

Between the matrix cells of hair follicle there are melanocytes that

produce the color (same concept as the epidermis)

Hair Growth Cycle

Is all your hair growing at the same time? For example, the hair

follicles on your scalp are they growing at the same rate? No.

The first phase is called anagen: in this phase, the matrix cells of the

dermal papilla undergo mitosis and give new cells to form the hair

root and the hair shaft which gets longer and longer, we call this the

growth state, it lasts from 3-6 years. Some people have difficulty

growing their hair beyond a certain length because they have a

short active phase of growth. On the other hand, people with very

long hair have a long active phase of growth. The hair on the arms,

legs, eyelashes, and eyebrows have a very short active growth

phase, explaining why they are so much shorter than scalp hair

The second phase is called catagen: it is a transitional state, lasts for

10 days, the dermal papilla disintegrates; it separates from the hair

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bulb and once you separate the blood vessels from the hair bulb

what will happen? The hair follicle shrinks

After this period, the dermal papilla is completely detached from the

hair bulb and we call this phase the telogen phase it is a resting

phase; nothing happens. In this case shedding of hair is possible

because it is no longer attached to the dermal papilla it lasts from 5

to 6 weeks.

If hairs enter the resting phase too early, excess shedding of the hair

can occur

After 6 weeks, we go back to anagen; formation of new dermal

papilla within the hair bulb and new formation of hair takes place.

85% of hair follicles are in anagen phase, 15% at telogen phase

which means if we have around 150,000 hair follicles in our scalp

then it is normal to shed up to 100 hairs per day.

Note: we do not generate new hair follicles anytime during our lives.

We have about 5 million hair follicles at the time of birth

Sweat glands

We have about 3 million sweat glands distributed on our skin. There

are two types.

Eccrine are distributed all over our body and the ducts of these

glands open into the surface, they produce true sweat and are the

most numerous of the sweat glands, and are abundant on the palms

of the hands, soles of the feet, and forehead; their secretion is clear

watery secretion with electrolytes and it is filtration from the blood

plasma also it’s Hypotonic (lesser salt concentration)

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What is the main function of sweating? To cool the body

temperature by evaporation

Apocrine: they are in restricted places like the axilla or the genitalia.

In human, their role is not really understood well .. but they are

important in sexual attraction between animals

The secretion of apocrine sweat glands is rich in proteins and lipids

and this is called pheromones

The secretion of apocrine sweat glands is associated with the

presence of the hair follicle. Can we find apocrine sweat glands in

thick skin? No, but we can find the eccrine in the palms and the

soles and that is why we have smelly feet sometimes, not because

sweat has odor but because it is exposed to bacterial

decomposition.

For example, your feet sweat into your shoes all day so they get

wet and bacteria start to grow. The bacteria continue to breed

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once you've taken your shoes off, especially if you put them in a

dark cupboard. Then, when you put your shoes back on the next

day, even if you've just had a shower, putting your feet into still wet

shoes creates the perfect conditions for the bacteria to thrive –

warm, dark and moist

The structure of the sweat gland: simple coiled tubular gland, the

secretory part is lightly stained while the duct is darkly stained.

The duct morphology concerning the eccrine and the apocrine is

the same: stratified cuboidal epithelium. The difference between the

two types lies in the secretory portion; in the apocrine they are very

large (lumen is very wide) and the lining epithelium is simple

cuboidal while in eccrine they have small lumen and The lining

epithelium of eccrine secretory portion is special type of stratified

cuboidal epithelium.

The Nail

Hard plate of keratin, it

is not colored because

there is no activity for

the melanocytes.

Nail is divided to 3

parts

1-free edge; which is

the part we cut.

2-body of the nail; it appears pinkish under the nail and this is very

important clinically as an indicator of distal tissue vascularization of

the patient. When you press on the nail you can see the

vascularization through the nail.

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3-root of the nail; embedded deep inside the skin and covered by a

fold called eponychium or cuticle. Another fold is the hyponychium

(under the free edge) and it is where the dirt accumulates.

Nail bed: under the body of the nail, layer of the epidermis (stratum

Basale and stratum Spinosum)

The nail matrix is the layer of cells at the base of the nail. It consists of

rapidly dividing skin cells that soon fill with the protein keratin. When

matrix cells undergo mitosis, they push the nail plates over the nail

beds and that’s how they grow.

Be like the stem cell;

differentiate yourself from others!