Download 2- Langerhans Cells

Skin (Integumentary system)
Skin and its derivatives and appendages form the
integumantary system
It is composed of :
1- EPIDERMIS; Epithelium of ectodermal origin
2- DERMIS; Layer of CT of mesodermal origin
3- HYPODERMIS; Or subcutaneous tissue; it is not a
component of the skin.
1. EPIDERMIS is composed of stratified squamous
keratinized epithelium, containing keratinocytes and nonkeratinocytes.
2. DERMIS is dense irregular CT rich in B.V., collagen and
elastic fibers.
Functions
1- It serves as a barrier against infection.
2- It is impermeable prevents pass of fluids from the
external environment.
3- It is a hydrophobic layer prevents loss of body fluids.
4- It regulates body temperature by regulating the blood
flow through the capillary beds and by sweating.
5-sensory perception the skin is a large sensory organ of the
external
environment
,temperature
(heat
and
cold),tuch,pain and pressure
6- formation of vitmin D by of skin to the ultraviolet rays
from the sun
Types of Skin
Two types based on the comparative thickness of the
epidermis into; thin and thick skin.
- The epidermal layer of thin skin is about 75-150 μm, but it
is about 400- 600μm in thick skin.
- Thick skin is smooth, non-hairy, and is found in the palms
and soles.
- Thin skin is hairy, and is found elsewhere on the body,the
epidermis is thinner and its cellular composition simpler
than that of thick skin ,present in the thin skin are hair
follicles s,sebaceous glands,and sweat glands. Attach to the
connective tissue sheath of hair follicles and the connective
tissue of the dermis are smooth muscle fibers called
arrector pili
1- Epidermis contains two types of cells
A – Keratinocyts
- They are keratin forming cells.
- They are organized into layers and become mature in about
4 weeks.
- Each layer represents a dynamic stage of cell division and
cell maturation.
- Cell renewal (Mitosis).
- Cell differentiation (Keratinization)
- Cell death
- Exfoliation (Sloughing off the dead cells OFF)
B - Non-Keratinocytes:
1- Melanocytes
2- Langerhans cells
3- Merkel`s cells
Layers of Epidermis
1- Stratum Basale
- Single layer of basophilic columnar or cuboidal cells resting
on a basement membrane at the dermal -epidermal junction.
- They contain stem cells (have intense mitotic activity),
which are responsible for renewal of epidermal cells.
- All cells in stratum basale contain intermediate keratin
filaments.
- As cells progress upward, the number of keratin filaments
increases until they represent half the total proteins in the
stratum corneum.
- Desmosomes bind cells of this layer in their lateral and
upper surfaces.
- Hemidesmosomes bind cells to the basal lamina.
2- Stratum Spinosum
- It is composed of cuboidal or slightly flattened cells, with
central nuclei.
- The cytoplasm filled with bundles of keratin . filaments
(TONOFILAMENTS)
- These bundles end at and insert into the cytoplasmic
densities of the desmosomes.
- Cells of this layer are firmly bound together by the
filaments filled the cytoplasmic spines and desmosomes that
punctate the cell surface, giving them spine-like appearance.
- The first two layers are called MALPIGIAN LAYER, which
show mitotic activity and contain stem cells.
3- Stratum Granulosum
- It is consisted of 3 -5 layers of flattened polygonal cells,
whose cytoplasm is filled with non-membranous basophilic
granules called Keratohyalin Granules.
- Another type of granules that can be seen by E/M are
membranous Lamellar granules.
- The lamellar granules are 0.1 – 0.3 μm ovoid or rod-like
structure containing Lamellar Disks of Lipid Bilayers.
- These granules fuse with the cell membrane and release
their lipid content into the intercellular spaces forming a
Sheet of Lipid Deposit.
- These sheet acts as a barrier sealing the skin.
4- Stratum Lucidum
- More common in thick skin, as a translucent thin layer of
flattened eosinophilic cells.
- The cytoplasm and cell organoids are no longer present,
but desmosomes are still present.
- The cytoplasm consists of densely packed Keratin Filamints
embedded in electron dense matrix.
5- Stratum Corneum
- - It is consisted of 15-20 layers of flattened non-nucleated
keratinized cells, whose cytoplasm is filled with Keratin.
- - After keratinization, the horny cells contain only fibrillar
and amorphous proteins and thickened plasma membrane.
- During keratinization, the lysosomal hydrolytic enzymes
play a role in disappearance of cytoplasmic organelles.
Non- Keratinized Cells
1- Melanocytes
- The color of skin is the result of several factors such as
melanin, carotene and the hemoglobin in the dermal
capillaries.
- There are two types of melanin:
- Eumelanin: dark brown pigment produced by melanocytes.
- Pheomelanin: reddish pigment present in red hair.
- Melanocytes have long cytoplasmic processes that branch
into the epidermis, running between cells of stratum basale
and spinosum.
- They are located beneath or between cells of stratum
basale.
- They are ectodermal in origin, derived from the neural
crest and migrate to the skin.
- We cannot see melanocytes in skin sections stained with
Hx&E, but they can be seen if the sections are stained with
DOPA REACTION (when fresh sections of skin are incubated
with DOPA, the tyrosenase enzyme within the melanocytes
converts DOPA into dark melanin pigment).
Mechanism Of Melanin Formation
Melanin synthesis starts in rER; then melanin is
concentrated, accumulated and stored in Golgy bodies.
The pigments are found in secretory vesicles called
Melanosomes.
Then the pigment granules leave melanocytes by exocytosis.
Lastly, the pigments are phagocytosed by keratinocytes.
In Summary
Tyrosine -------- tyrosinase enzyme------ (dopa) 3,4
dihydroxy-phenyl-alanin ------ dopa quinon------ melanin
2- Langerhans Cells
- They are phagocytic skin cells present mainly within the
cells of Stratum Spinosum; and represent 2- 8% of the cells.
- They are mesodermal in origin.
- They are star shaped cells, with indented nucleus.
- The cytoplasm contain special granules called BIRBECKS
GRANULES.
- They are demonstrated by gold chloride impregnation.
3- Merkel’s Cells
- Sensory skin cells.
- Located mainly in thick skin.
- Act as mechanoreceptors.
- Other evidence suggests that they have functions related
diffuse neuro-endocrine cells.
DERMIS
- It is a CT layer rich in BV, lymphatics, and CT cells.
- It is formed of two layers:- Papillary layer
- Reticular layer
Papillary Layer
- Vascular C.T. layer separated from the epidermis by
basement membrane.
- It contains fine axonal connections of free sensory nerve
endings (sensory receptors ) such as :
- Superficial touch receptor or corpuscle of touch
(Meissner's corpuscles). - Lamellated corpuscle or deep
pressure receptor (Pacinian corpuscle) .
Dermal Papillary
They are the junction of the dermis and epidermis. They are
irregular projections of the dermis (papillae) interdigitate
with evaginations of the epidermis ( epidermal ridges). They
contain fine interlacing collagen and elastic fibers, BV, and
lymphatics.
Reticular layer
- It is thicker than the papillary layer.
- It is formed of reticular, collagen and elastic fibers.
- It contains many sensory receptors like Pacinian
Corpuscles.
Skin derivatives or appendages
1-hairs , develop from surface epithelium of the epidermis
and reside deep in the dermis,smooth muscles (arrector pili)
attach to papillary layer of dermis and to sheath of hiar
follicle
2- sebaceous glands , associated with each hair follicle
3- sweat glands , widely distributed in skin and are of two
types
a- Eccrine sweat glands, are simple coiled glands located
deep in dermis , consist of clear and dark secretory
cells , clear cells secrete watery product whereas dark
cells secrete mucus ,this excrete water ,sodium saltes ,
ammonia , uric acid , and uria
b- Apocrine sweat glands , found in deep dermis of axilla
,anus and areolar regions of the breast , become
functional at puberty when sex hormones are present
and secretion has unpleasant odor after bacterial
decomposition
4- feathers, avian skin consist of an epidermis which is
generally thinner than that of mammls and a dermis which
in feathered skin lacks papillae and is non glandular