Skin The skin is the first heaviest organ in the - PowerPoint Presentation

Slide1

Skin

The skin is the first heaviest organ in the

body

16

% of body weight,

1.2-2.3 square meter.

It

is composed from the

epidermis

and

dermis

.

Based

on the thickness of the epidermis there are thick and thin skin.

The

junction of dermis and epidermis is irregular, the projection of the dermis

(papillae)

interdigitate

with epidermal

evagination

(epidermal ridge).

The skin functions are:

1

.

provide continuous communication with the environment (as receptor organ

)

2

. has protective action against ultraviolet rays (UV) by the action of Melanin pigment

3.

Thermoregulator

by the action of blood vessels ,glands of the skin and the adipose tissue

4

.

Execretion

of waste product and

5

.

Vit.D

synthesis

.

Upon close observation, human skin show ridge and groove in the tips of the finger and

volar

surfaces of the hands and feet (palm and sole) called

Dermatoglyphics

, they are unique for each individual, appearing as loops, arches etc--- which are used for personal identification.

Epidermis:

The

epidermis consists of

a

stratified

squamous

keratinized

epith

with three less abundant cell types:

Melanocytes

,

Langerhans

cells and

Merkel's cells

.

The epidermis consists of five layers:

1- Stratum

Basale

(

St.Germinativum

)

:

Is

a single layer of columnar OR

cuboidal

basophilic cells resting on the basal lamina.

Desmosomes

and

Hemidesmosomes

High mitotic

activity, this layer with the next layer responsible for constant renewal of epidermal cells (which occur every 15-30 days).

The

cells contain

keratin intermediate

filaments. As the cells progress upward, the number of filaments increases until it represent half of the protein in last layer.

2-St.

Spinosum

(prickle layer)

:

St.

spinosum

consist of

cuboidal

cells with centrally located nuclei, the cytoplasm filled with bundles of keratin filaments (

Tonofilament), these bundles terminated in the spiny cytoplasmic projection, the cells of this layer firmly bound together by cytoplasmic projections and Desmosomes. St. Basale and St. spinosum also called (malpigian layer)

St.

spinosum

3-St.

Granulosum

:

Consist

of 3-5 layers of polygonal cells,

Their

cytoplasm are filled with basophilic granules called

Keratohyalline granules which account for their basophilia. Presence of lamellar granules which are surrounded by membrane, these granules fuse with the cell membrane and discharge their content into the intercellular spaces to form the intercellular cement which act as a barrier to prevent the penetration of foreign materials and provide important sealing effect in the skin.

4-St

.

lucidum

:

Is

more apparent in thick skin, is translucent consist of

eosinophilic

flattened cells,

The organelles and nuclei are no longer evident The cytoplasm consist of densly packed keratin filaments.

5- St.

corneum

:

Consist

of 15-20 layers of flattened non-nucleated keratinized cells their cytoplasm are filled with filamentous

scleroprotein

called keratin

.

Tonofilaments are packed together in a matrix contributed by the keratohyalline granules. After keratinization, the cells consist of fibrillar proteins and thickened plasma membrane, they are called Horny cells. During keratinization, lysosomal hydrolytic enzymes digest the organelles.

In psoriasis,

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 result in greater epidermal thickness and more rapid renewal of epidermis.

Melanocytes

:

The

colour

of the skin is depends on the pigment called Melanin, the number of blood vessels and the color of the blood flowing in

them.

Melanin

is a dark brown pigment produced by the

melanocyte found between the cells of St. basale and the hair follicles. They have rounded cells bodies with long processes extend between St. basale and spinosum Melanin is synthesized in the melanocytes. Melanin granules once formed migrate by the cytoplasmic processes of melanocytes into the cells of St. basale

and

spinosum

, then the melanin granules are injected into

keratinocytes

,

Melanin granules accumulate

in

supranuclear

area of the cytoplasm, to protect the nuclei from the deleterious effect of solar UV radiation

.

Melanocytes

are not distributed in a random way, they are about 1000 melnocytes/square mm in the skin of the thigh ,and about 2000/square mm in the skin of scrotum. Sex and race does not influence the number of melanocytes/square mm, difference in skin color is due to the difference in the number of melanin granules in the keratinocytes. Darkening of the skin color after exposure to sun light (Tanning) is due to 2 steps:1- Rapid release of pigment into the keratinocytes 2- Acceleration of the rate of melanin pigment synthesis

In human lack of

cortisol

from the adrenal cortex causes overproduction of

adrenocorticotropic

hormone, which increases the pigmentation of skin. An example of this is

Addison disease

, which is caused by dysfunction of the adrenal glands.

Albinism

, a hereditary inability of the

melanocyte

to synthesize melanin, is caused by the absence of

tyrosinase

activity. As a result the skin is not protected from solar radiation by melanin, and there is a greater incidence of basal and

squamous

cell carcinoma (skin cancer). The degeneration and disappearance of entire

melanocytes

result in a depigmentation disorder called Vitiligo

Langerhans's

cells:

Are star shaped cells found in the St.

spinosum

, they are bone marrow derived, they are capable of binding, processing and presenting antigens to T lymphocytes to stimulate them. So these cells are

Ag-presenting cells

.

Merkel's cells:

They are present in thick skin of palm and soles, there are free nerve endings are present at the base of these cells, so, these cells serve as sensory

mechanoreceptors.

Thank you