. Lecture [2] By Prof. dr. ashraf mahmoud m Epithelial tissue.
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Transcript of . Lecture [2] By Prof. dr. ashraf mahmoud m Epithelial tissue.
.Lecture [2]
ByProf. dr. ashraf mahmoud m
Epithelial tissue
• Cells are the smallest functional unit of the
organism.
Tissues: 4 main types of tissues are distinguished:
1. Epithelial tissue.
2. Connective tissue.
3. Muscular tissue.
4. Nervous tissue.
Epithelial tissue: is a
tissue that composed
mainly of cells with little
intercellular substance.
These tissues exist in
association with one
another forming
different organs and
systems.
Characterstics of the Epithelial Tissue:
1. Cells are close to each other.
2. Little intercellular substances.
3. Tends to form junctions.
4. Lines surfaces or cavities to form the
parenchyma of the organ.
Characterstics of the Epithelial Tissue:
4. Cells show polarity: apical (free) surfaces, lateral, basal
surfaces.
5. The cells lie on a basement membrane (basal lamina).
6. No blood vessels enter in between the cells (Avascular),
but nerves can pass inbetween the cells.
7. The epithelia are continuously renewed and replaced .
Types of Epithelium
Covering epithelium (surface epithelium)
sheet like tissues that cover or line the surfaces,
cavities and organs of the body
Glandular epithelium : perform secretory
functions
Neuroepithelium
Myoepithelium
Classification of epithelial membranes
Number of cell layers
Simple: one layer
Stratified: 2 layers or more.
Shape of apical surface cells
Squamous
Cuboidal
Columnar
I- Covering epithelium:
Simple Epithelium:
It is formed of one layer of cells resting on the basement
membrane.
Simple squamous
Simple cubical (cuboidal)
Simple columnar
Psudostratified columnar
Simple squamous
epithelium It is formed of one layer of flat cells. Each
cell is formed of a thin cytoplasm and flat
bulging nucleus.
Sites:
Lining the blood vessels endothelium
Lining the serous membranes :pleura,
pericardium and peritoneum
mesothelium
Lining the lung alveoli.
Parietal layer of Bowman’s capsule of the
kidney.
Simple cubical (cuboidal) epithelium
It is formed of one layer of cubical cells
(square cells), resting on the basement
membrane. Each cell has a single
rounded, central nucleus.
Sites:
Thyroid gland.
Anterior surface of the lens of the eye.
Surface of infantile ovary.
Simple columnar epithelium
It is formed of one layer of columnar cells (height of
the cell exceeds its width-rectangular).
Each cell has a single, basal oval nucleus
Types:
Unmodified: in excretory ducts of the glands
Modified:
Secretory: e.g. lining epithelium of the stomach.
Absorptive: e.g. absorptive columnar cells of the small
intestine
Ciliated: the free surface is supplied by cilia e.g.
uterus fallopian tube and lung bronchioles.
The cells are overcrowded, but they all rest on the basement
membrane. Some cells appear short and not reaching the surface. So
appear as if it is formed of more than one layer.
Sites :
Trachea, bronchi (Ciliated with goblet cells).
Large ducts of salivary glands (Non-ciliated).
Pseudostratified columnar epithelium
Stratified epithelium
The cells are arranged in two or more layers.
1. Stratified squamous non-keratinized
The basal cells are columnar and lie on
a basement membrane. This layer is
responsible for generation of other cells.
The intermediate cells are polygonal which gradually flattens to
become thin squamous cells at the surface.
-Sites:
It is found in wet surfaces of all openings leading to the skin:
Oral cavity, Tongue, Oesophagus, Cornea, Anal canal, Vagina,
External auditory meatus.
Types:
Stratified epithelium
Stratified squamous non-keratinized
2. Stratified squamous
keratinized
like stratified squamous non-
keratinized epithelium but
The flat cells of the top layer
are changed into non-living
layer of keratin.
This type is tough and resists
friction and is impermeable to
water.
-Sites: epidermis of the skin.
Stratified epithelium
3. Transitional epithelium
Stratified cells that appear cuboidal
when the organ or tube is not
stretched, and squamous when the
organ or tube is stretched by fluid.
Sites:
Lining the viscera which are
subjected to distension e.g. urinary
passages. The number of layers are
nearly 4-8 layers in the empty bladder
and become 2-3 layers in full bladder
Stratified epithelium
Transitional Epithelium with rounded binucleated surface cells
Stratified epithelium4. Stratified cubical epithelium
The cells in uppermost layer are cubical in shape.
Site: Cells lining the ducts of sweat
glands of the skin.
5. Stratified columnar epithelium:
The cells in the uppermost layer
are columnar cells.
Sites:
- Large ducts of salivary glands.
- Fornix of conjunctiva.
- Penile part of male urethera.
Stratified epithelium
II- Glandular Epithelium
Classification of the glandular epithelium:
According to:
Number of cells: Unicellular & multicellular.
Structure of the gland: Simple & branched.
Mode of secretion: Exocrine & endocrine.
Type of secretion: Serous & mucous …
Cellular changes after secretion (mode of secretion):
Merocrine, apocrine, holocrine.
Epithelium characterized by its secretory function.
Exocrine glands
Unicellular: (Goblet cells)
The gland is formed only of
one cell.
Sites: in tracheal and intestinal epithelium.
L/M: Flask in shape; slender base and expanded
apex due to presence of mucous
granules. These granules
are not stained by H&E.The nucleus is basal and flat.
Multicellular Exocrine glands
Classification:
According to the type of secretion:
Mucous glands: Secretes a viscous
thick secretion containing mucin.
Serous glands: Secretes a watery
secretion containing proteins and
rich with enzymes.
Mixed glands :(Seromucous
acinus): Secretes both mucous and
serous secretions
Mucous glands
Serous glands:
Mucous acini Serous acini
Size
Lumen
In a section
Cytoplasm
Nucleus
Secretion
Relatively large in diameter
Wide
Formed of a few number of cuboidal cells that have well-defined boundaries
Pale basophilic
Each cell contains a single flattened and basal nucleus
Mucous ( a viscous thick secretion)
Relatively smaller than mucous acinus
Narrow
Formed of a relatively larger number of pyramidal cells
Dark Basophilic
Each cell contains a single rounded nucleus , shifted towards the base
Watery whey-like containing enzymes
According to the mode of secretion:
1.Merocrine (eccrine): The secretory cells release their contents by
exocytosis with no loss of the cytoplasm or cell membrane e.g.
salivary glands.
2. Apocrine: Apical Part of the cytoplasm is released with the
secretion e.g. lactating mammary glands.
3. Holocrine: The secretory cell together with its accumulated
secretion is released into the duct e.g. the sebaceous glands of the
skin
III- Neuroepithelium It is a special type of epithelium which is modified to
receive sensations.
Neuroepithelium has three main types of cells;
1) Sensory cells for receiving the stimuli
2) Supporting cells
3) Basal cells ( acts as stem cells)
Sites:
• Taste buds for taste sensation
• Olfactory mucosa
• Organ of hearing
IV- Myoepithelium
• Epithelial cells which have the ability to contract. The cells contain myofilaments.
• They surround the secretory acini of glands
• They help in releasing the secretory products to the lumen of the acini.