Skin pdf - د.ملاك الياور د.فردوس محمد د.هدى الخطيب د.زهراء قاسم

بسم اهلل الرحمن الرحيم

Prof. Dr. Huda Al-khateeb

Head of Histology Section/Dept. of Anatomy
Head of Quality Assurance & Academic Performance
College of Medicine/Univ. of Baghdad

SKIN

By the end of this lecture, the student should be able to:

List the layers of skin

Describe the types of cells and tissues present in
epidermis and dermis

Discuss the types of keratinocytes and non-
keratinocytes

Recognize some related pathological conditions

objectives

15–20% of total body weight

What is the heaviest organ in the

body?

1.5–2 m

What is the largest organ in the

body?

Composed of:

, an

epidermis

) the

(
epithelial layer of

ectodermal

origin

, a layer of

dermis

) the

(

mesodermal

connective tissue

origin

the integument (L. integumentum,

covering) or cutaneous layer

The boundries between dermis and epidermis is
irregular

papillae

called

dermis

* projections of the

epidermis

of the

evaginations

with

digitate

* inter
known as

epidermal ridges

Dermo-epidermal

Junction

Beneath the dermis lies the

superficial fascia (of gross anatomy)

under, +

hypo,

(Gr.

hypodermis

subcutaneous tissue

derma, skin), a loose connective tissue that may contain
pads of adipocytes.

Hypodermis binds skin loosely to the underlying tissues

hypodermis

Thick skin

(Palm & Sole)

Thin skin

(Rest of the body)

Skin

Protective

(1)

physical barrier

against thermal and mechanical insults such as

frictional forces and against most potential pathogens.

(2)Microorganisms that do penetrate skin alert resident

lymphocytes and

antigen-presenting cells

of skin and an immune response is mounted.

(3)The dark pigment

melanin

in epidermis protects cells against ultraviolet

radiation.

(4)

permeability barrier

against excessive loss or uptake of water.

drugs such as certain

lipophilic

allows some

selective permeability

Skin's

steroid hormones and medications to be administered via skin patches.

Functions of

skin

. Many types of sensory

Sensory

receptors

allow skin to constantly monitor

the environment and various
mechanoreceptors with specific locations in
skin are important for the body's
interactions with physical objects.

Functions of the

skin

. A constant

Thermoregulatory

body temperature is normally more easily
maintained thanks to the skin's

insulating

components

(eg, the fatty layer and hair on

the head) and its mechanisms for
accelerating

heat loss

(sweat production

and a dense superficial microvasculature). §

Functions of the skin

. Cells of skin

Metabolic

synthesize

vitamin D

, needed in calcium

metabolism and proper bone formation,
through the local action of UV light on the
vitamin's precursor. Excess

electrolytes

can

be removed in sweat and the subcutaneous
layer stores a significant amount of

energy

in the form of fat.

Functions of the skin

keratinized epithelium

squamous

stratified

keratinocytes

Major cells :

keratinocytes

Non

Other cells are

(1)pigment-producing

melanocytes

(2)antigen-presenting

Langerhans cells

(3) tactile epithelial cells

Merkel cells

Epidermis

Composed of 5 layers
(Keratinocytes):

Stratum basale
Stratum spinosum
Stratum granulosum
Stratum lucidum
Stratum corneum

Thick skin

epidermis

layer of basophilic columnar cells on the basement membrane at

single

)

(

the dermo-epidermal junction.

(2)Hemidesmosomes

in the basal plasma lemma bind these cells to the

basal lamina

(3)

Desmosomes

bind the cells together from their lateral wall .

(4)This layer is characterized by intense

mitotic activity

and is responsible

for constant production of epidermal cells.

, depending on

days

–

)The human epidermis is renewed about every

(

age, the region of the body, and other factors.

(6)All keratinocytes in the stratum basale contain

intermediate filaments

composed of keratins.

Stratum basale

(1)the thickest epidermal layer, consists of polyhedral cells having central nuclei.

stratum

are called the

spinosum

+ lower layers of stratum

basale

)Stratum

(

germinativum

(3)The keratin filaments form bundles called

tonofibrils

which terminate at

desmosomes.

(4)Cytoplasm has short cellular extensions around the tonofibrils, leading to the
appearance of many short

spines

prickles

at the cell surfaces.

(5)It is thicker in areas subjected to continuous friction and pressure (such as the
soles of the feet)

stratum spinosum

are of the skin.

cancers

of all

one third

In adults,

Most of these derive from cells of the basal or spinous

layers, producing, respectively,

basal cell carcinomas

and

squamous cell carcinomas

fair

Skin cancer shows an increased incidence in

individuals residing in regions with high

skinned

solar radiation

amounts of

MEDICAL APPLICATION

of flattened polygonal cells.

layers

–

) consists of

(

(2)Their cytoplasm is filled with intensely basophilic

keratohyaline

granules

. These structures are

not membrane-bound

and consist of

dense masses.

(3)Other characteristic features are the

membrane-coated

lamellar

granules

, (seen by TEM)composed of various lipids. Lamellar

granules undergo exocytosis, discharging their contents into the

intercellular spaces of the stratum granulosum. this lipid-rich

epidermal barrier

material produces sheets that envelop the cells (

against the loss of water from skin and barrier to penetration by most

foreign materials)

stratum granulosum

(1)is only seen in

thick skin

(2) it consists of a thin, translucent layer of extremely flattened
eosinophilic cells.

nuclei and organelles have been lost

)The

(

(4) the cytoplasm consists almost only of densely packed

keratin filaments

embedded in an electron-dense matrix.

are still evident between adjacent cells.

Desmosomes

)

(

Stratum lucidum

(1)consists of 15–20 layers

keratinized cells whose cytoplasm is

nonnucleated

) flattened,

(
filled with filamentous

keratins

thickened plasma membranes

)

(

cells

cornified

horny

squames

are called

*These cells

* These cells are continuously shed at the surface of the stratum
corneum

Stratum corneum

(1)No stratum lucidum (4 layers only)
(2) Stratum spinosum, granulosum and corneum are thinner

Thin skin epidermis

Psoriasis

, a common skin disease, there is an increase in

the number of proliferating cells in the stratum

basale

and the

stratum

spinosum

as well as a decrease in the cycle time of these

cells. This results in

(1)greater epidermal thickness

(2) more rapid renewal of epidermis

(3)abnormal keratinization with a defective skin barrier.

MEDICAL

APPLICATION

(1)specialized cells of the epidermis found among the cells of

stratum

basale

and in the

hair follicles

(brownish black

melanin

they synthesize

)From tyrosine,

(

melanosomes

pigment ) and store it in vesicles called

(3)They are

neural crest

derivatives which migrate into the

developing epidermis' stratum basale.

Melanocytes

(4)They have

rounded cell bodies

and long irregular

dendritic extensions

from each melanocyte branch into the

epidermis, running between the cells of the basal and

spinous layers.

(5)They form

hemidesmosomes

with the basal lamina, but no

desmosomes with adjacent keratinocytes.

(6)Ultrastructurally, a melanocyte is a pale-staining cell with

numerous small mitochondria, short cisternae of rough

endoplasmic reticulum (RER), and a well-developed Golgi

apparatus.

Melanocytes

melanocytes

converts tyrosine into melanin within

Tyrosinase

Melanin accumulates in vesicles until they form mature granules

called

melanosomes

Once formed, melanin granules are transported to tips of the

melanocyte's dendrites.

The associated keratinocytes in both the basal and spinous layers

the tips of these dendrites.

phagocytose

Within each keratinocyte they accumulate as a

supranuclear cap

which absorbs and scatters sunlight, protecting nuclear DNA from

the deleterious effects of UV radiation.

Melanin

the result of several factors,

(1)the most important of which are

melanin

' content of

keratinocytes

the

carotene

)

(

in the

blood vessels

) the number of

(
dermis

Color of the skin

Malignant melanoma is an invasive

tumor of melanocytes. Dividing rapidly,

malignantly transformed melanocytes

penetrate the basal lamina, enter the dermis,

and invade the blood and lymphatic vessels to

gain wide distribution throughout the body.

MEDICAL APPLICATION

Albinism

, a hereditary inability of the

melanocytes to synthesize melanin, is caused by the

absence of tyrosinase

activity or the inability of cells to

take up tyrosine. As a result, the skin is not protected

from solar radiation by melanin, and there is a greater

incidence of (skin cancers).

The

degeneration and disappearance of entire

causes a patchy loss of pigment in the skin

melanocytes

disorder called

vitiligo.

MEDICAL APPLICATION

(1)usually most clearly seen in the

spinous

layer.

(2)

Cytoplasmic processes

extend from these dendritic cells

between keratinocytes of all the layers, forming a fairly dense
network in the epidermis .

(3)They are bone marrow–derived.

(4)capable of binding, processing, and

presenting antigens

to T

lymphocytes, thus providing an immunological component to
the skin's overall

protective

function

Langerhans

Dendritic Cells

(1)

mechanoreceptors

(sense of touch)

(2)Small

neurosecretory granules

(related to diffuse neuroendocrine

system)

cells

neural crest

)Derived from

(

bases of hair

and at the

basal epidermal layer

) located in the

(

follicles.

(5)The basolateral surfaces of the cells contact expanded terminal

discs of

unmyelinated sensory fibers

that penetrate the basal lamina.

Tactile (Merkel)

Cells

the epidermis

supports

, that

connective tissue

)The dermis is the

(

it to the subcutaneous tissue (hypodermis).

binds

and

(2)The thickness of the dermis varies according to the region of the

body, and reaches its maximum of 4 mm on the back.

(3)A

basement membrane

is always found between the stratum basale

and the dermis.

(4)The basement membrane consists of the

basal lamina

and the

reticular lamina

and can usually be seen with the light microscope.

(5)Nutrients for keratinocytes must diffuse into the avascular

epidermis from the dermis vasculature through this basement

membrane.

Dermis

The dermis contains two layers with rather indistinct boundaries:

(1)The thin

papillary layer

, which constitutes the major part of the

dermal papillae, is composed of loose connective tissue, with fibroblasts

and other connective tissue cells, such as mast cells and macrophages.

Extravasated leukocytes are also seen. From this layer, anchoring fibrils of

type VII collagen insert into the basal lamina and bind the dermis to the

epidermis.

(2)The

reticular layer

is thicker, composed of irregular dense

connective tissue (mainly bundles of type I collagen), and has more fibers

and fewer cells than the papillary layer. A network of elastic fibers is also

present, providing elasticity to the skin. Spaces between the collagen and

elastic fibers are filled with proteoglycans rich in dermatan sulfate.

Dermis

, collagen fibers thicken and collagen synthesis decreases. Elastic fibers

age

)With

(

steadily increase in number and thickness, so the elastin content of human skin

increases approximately fivefold from fetal to

adult life

old age

, extensive cross-linking of collagen fibers, the loss of elastic fibers, and

degeneration of these fibers due to excessive exposure to the sun (solar elastosis)

cause the skin to become more fragile, lose its suppleness, and develop wrinkles.

(2)In

Ehlers-Danlos syndromes

, there is a considerable increase in skin and

ligament extensibility caused by defective collagen-fibril processing.

MEDICAL

APPLICATION

The connective tissue of the skin contains a rich network of blood and lymphatic

vessels.

Blood vessels

that nourish the cells of skin form two major plexuses :

(1) deep at the interface between hypodermis and dermis

(2)superficially between the papillary and reticular dermal layers (

subpapillary

plexus)

sends branches into the dermal papillae and supplies a rich, nutritive

capillary network just below the epidermis.

thermoregulatory

, dermal vasculature has a

nutritive function

In addition to the

located between

anastomoses

arteriovenous

which involves numerous

function

the two major plexuses.

Lymphatic vessels

begin as closed sacs in the dermal papillae and converge

to form two plexuses located with the blood vessels.

Vessels

sensory receptors are present in skin, including

The

unencapsulated receptors

include the following:

(1)

Tactile discs

associated with the epidermal tactile cells, which

light touch

function as receptors for

(2)

Free nerve endings

in the papillary dermis and extending into

lower epidermal layers, which respond primarily to high and low

receptors.

tactile

, but also function as

itching

, and

temperatures, pain

(

3)Root hair plexuses

, a web of sensory fibers surrounding the bases

detects movements of the

of hair follicles in the reticular dermis that

hairs.

Sensory Receptors

The following

encapsulated receptors

are tactile mechanoreceptors:

(1)

Tactile corpuscles (Meissner corpuscles

) are elliptical structures, in the

light touch

fingertips, palms and soles. They detect

(

2)Lamellated (Pacinian)

corpuscles are large oval structures, found deep in

the reticular dermis or hypodermis, with an outer capsule and 15 to 50 thin,

concentric lamellae of flat Schwann-type cells and collagen surrounding a

highly branched, unmyelinated axon. They are specialized for sensing

vibrations.

(sustained touch), and

pressure

coarse touch

(

3)Krause corpuscles

and

Ruffini corpuscles

are other encapsulated,

in dermis, but are more poorly

sensing mechanoreceptors

pressure

characterized structurally

Sensory Receptors

Any Question???

Nails