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Skin Histology

Dr.Huda

Slides

By : Qasim M.Al-hussainy

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

1. To recognize the layers of
the skin
2. To list and discuss the types
of cells present in these layers.

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:

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

allow skin to

receptors

. Many types of sensory

Sensory

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 body temperature is

Thermoregulatory

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

vitamin D

. Cells of skin synthesize

Metabolic

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 root

(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

What are the factors

that influence the skin

color?

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

. Dividing

melanocytes

tumor of

invasive

rapidly, malignantly transformed

the basal lamina,

penetrate

melanocytes

the blood and

invade

enter the dermis, 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

stratum spinousum

(2)

Cytoplasmic processes

extend from these cells between

keratinocytes of all the layers.
(3)They are bone marrow–derived.
(4)capable of binding, processing, and

presenting antigens

to T

component to

immunological

lymphocytes, thus providing an
the skin's overall

protective

function

Langerhans

Dendritic Cells

(1)

mechanoreceptors

(sense of touch)

(2)Small

neurosecretory granules

(diffuse neuroendocrine

system)

cells

neural crest

Derived from

)

(

bases of hair

and at the

basale

stratum

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

) It 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 found between the stratum

basale and the dermis.

(4)Nutrients for keratinocytes must diffuse into the

avascular epidermis from the dermis vasculature through

this basement membrane.

Dermis

(1)The thin

papillary layer

, which is located in the dermal

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

mast cells and macrophages. Extravasated leukocytes are also seen.

insert into the

type VII collagen

From this layer, anchoring fibrils of

basal lamina and bind the dermis to the epidermis.
(2)The

reticular layer

is thicker, composed of irregular dense

), and has more

type I collagen

connective tissue (mainly bundles of

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.

Dermis

increase

, Collagen fibers and Elastic fibers

fetal to adult life

) From

(

in number and thickness
(2) In

old age

, collagen fibers and elastic fibers reveal degeneration

due to excessive exposure to the sun. This causes the skin to develop

wrinkles.

(3)In

Ehlers-Danlos syndromes

, there is a considerable increase in

skin and ligament extensibility caused by defective collagen-fibril

processing.

MEDICAL

APPLICATION

Blood vessels

that nourish the cells of skin form two major plexuses :

at the interface between hypodermis and dermis

deep

)

(

between the papillary and reticular dermal layers

superficial

)

(

subpapillary plexus)

sends branches into the dermal papillae and supplies

a rich, nutritive capillary network just below the epidermis.

, dermal vasculature has a

nutritive function

In addition to the

arteriovenous

which involves numerous

thermoregulatory function

anastomoses located between the two major plexuses.

Lymphatic vessels

begin as closed sacs in the dermal papillae and

converge to form two plexuses that run with the blood vessels.

Vessels

sensory receptors are present in skin, including

The

unencapsulated receptors

include the following:

(1)

Tactile discs

function as receptors for light touch.

(2)

Free nerve endings

respond primarily to high and low

receptors.

tactile

also function as

itching

, and

temperatures, pain
(

3)Root hair plexuses

, sensory fibers surrounding the bases of hair

detects movements of the hairs.

follicles in the reticular dermis that

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

A patient presented with patchy lesion of the skin. You observe

sloughing of the surface cell of the epidermis and greater

epidermal thickness . Which of the following would cause this

clinical condition?
1. Loss of tight junctions in upper skin layers
2. Longer cell cycle of cells in deep skin layers
3. Increased mitosis of cells of stratum basale
4. Reduction of keratin synthesis
5. Decreased mitosis of cells of the stratum germinativum

QUIZ

Any Question???