Chapter 3: Epithelial Tissue Flashcards
How many main tissues are found in the body?
4
Where do the main tissues arise from?
3 germs layers of the embryo:
1. Ectoderm, mesoderm, and endoderm.
What are the main tissues of the body?
- Epithelial tissue (ectodermal, mesodermal, endodermal).
- Connective tissue (mesodermal).
- Muscular tissue (mesodermal).
- Nervous tissue (ectodermal).
Why is the Epithelial tissue called by its name?
Because it mostly covers the outer surfaces or lines the surfaces of cavities all over the body.
General characters of epithelial tissue
- Consists of crowded cells with minimal intercellular space between the cells.
- Upper surface is free.
- The basal surface rests on a basement membrane, which may be clear or not.
- Blood vessels and lymph vessels cannot penetrate between epithelial cells but nerves can. So, epithelium is avascular.
- It receives nutrition by diffusion from underlying connective tissue.
- High power of regeneration (renewal).
Origin of epithelial tissue
- Ectodermal: skin.
- Mesodermal: serous membrane.
- Endodermal: Gastrointestinal tract (GIT).
Epithelial tissue is classified into:
- Surface epithelium.
- Glandular epithelium.
- Neuro-epithelium.
- Myo-epithelium.
Surface epithelium
Covers or lines cavities.
Glandular epithelium
Its cells modify to give secretion.
Neuro-epithelium
It’s cells modify to act as receptors.
Myo-epithelium
Its cells have a contractile function.
Surface epithelium is divided into:
- Simple epithelium.
- Stratified epithelium.
Simple epithelium
Called simple because it is formed of one layer of cells. All cells are resting on a basement membrane.
How is simple epithelium divided?
It is divided according to shape of cells into:
1. Simple squamous epithelium.
2. Simple cubical epithelium.
3. Simple columnar epithelium.
4. Simple columnar ciliated epithelium.
5. Pseudo-stratified columnar epithelium.
Simple squamous epithelium
Formed of one layer of flat cells with flat nucleus resting on a basement membrane.
Simple squamous epithelium functions
- Smooth surface: important for easy passage of fluids and movement of organs.
- Thin membrane: helps exchange of gases.
Simple squamous epithelium sites
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1. Blood vessels.
2. Bowman’s capsule (kidney).
3. Heart.
4. Lungs alveoli
5. Serous membrane:
- Pleura.
- Peritoneum.
- Pericardium.
Simple squamous epithelium sites functions
- Blood vessels and heart: forms a smooth surface called endothelium.
- Lung alveoli: forms thin membrane for exchange of gases called pneumocytes.
- Serous membrane: form smooth surface for easy movement of organs called mesothelium.
- Bowman’s capsule: filtration of blood.
Simple cubical epithelium
Formed of one layer of cube-shaped cells with central rounded nuclei.
Simple cubical epithelium functions
- Secretion and reabsorption.
Simple cubical epithelium sites
SfC
1. Secretory acini glands:
- Follicles of thyroid glands.
2. Convoluted tubules of kidney.
Simple columnar epithelium
Formed of one layer of tall columnar cells with basal oval nucleus.
Simple columnar epithelium functions
Secretion and absorption
Simple columnar epithelium sites
Dsls
1. Digestive tract
- Stomach.
- Large intestine.
- Small intestine.
Simple columnar ciliated epithelium
Formed of one layer of tall columnar cells with basal oval nuclei.
The cells are provided with cilia.
Simple columnar ciliated epithelium functions
- Cilia can transport fluid or particles over the epithelial surface in one direction.
Simple column ciliated epithelium sites
LF
1. Lower respiratory tract (lung bronchioles): move secretions upwards.
2. Female genital system: move the ovum.
- Uterus
- Fallopian tube
Pseudostratified columnar epithelium
- A type of simple epithelium because it is formed of one cell layer and all cells rest on the basement membrane.
- Some of the columnar cells can’t reach the surface.
- The cells are very crowded so nuclei are present in more than one level. This gives the epithelium a false appearance of being stratified.
What are types of pseudostratified columnar epithelium
- Pseudostratifued columnar non ciliated epithelium.
- Pseudostratified ciliated with motile cilia and goblet cells.
- Pseudostratified columnar ciliated with non motile cilia (Stereocilia).
Pseudostratifued columnar non ciliated epithelium sites
- Male genital system:
- Vas deferens - Membranous part of male urethra
Pseudostratified ciliated with motile cilia and goblet cells sites
- Upper respiratory epithelium (respiratory epithelium)
- nose
- trachea
- bronchi
Pseudostratified columnar ciliated with non motile cilia (Stereocilia) sites
Epididymis
(Not true cilia but long microvilli)
Stratified Epithelium
Formed of more than one layer.
Stratified epithelium classification is according to?
Shape
Stratified epithelium classifications
- Stratified squamous epithelium.
- Transitional epithelium (stratified cuboidal).
- Stratified columnar epithelium.
- Stratified cubical epithelium.
Stratified epithelium main function
Protection
Stratified squamous epithelium number of layers
5-30
Transitional epithelium number of layers
- 6-8 (empty bladder).
- 3-4 (full bladder).
Stratified squamous epithelium basement membrane
Clear and wavy
Transitional epithelium basement membrane
Non clear non wavy
Stratified squamous epithelium basal cell layer
Columnar cells with basal oval nuclei.
Transitional epithelium basal cell layer
Cuboidal cells with central rounded nuclei.
Stratified squamous epithelium intermediate layers
- Polyhedral cells with central rounded nuclei.
- Cells are joined by desmosomes.
- Cells become smaller towards the surface.
Transitional epithelium intermediate layers
- Polyhedral cells with central rounded nuclei.
- In full bladder, cells are pushed laterally which decreases epithelium thickness to accommodate the increasing urine volume.
Stratified squamous epithelium superficial layer
Flat cells with flat nuclei.
Transitional epithelium superficial layer
- Cells are dome shaped with upper convex surface and have a central rounded nuclei.
- Cells are connected together by junctional complex.
- In full bladder, cells are flat.
- Luminal surface is covered by rigid plaques that act as barriers.
Stratified squamous epithelium subtypes and sites (non keratinized)
Non keratinized: no keratin
1. Oral cavity.
2. Esophagus.
3. Cornea.
4. Anal canal.
5. Tip of urethra.
6. Vagina.
Stratified squamous epithelium subtypes and sites (keratinized)
Keratinized: covered by keratin layer on surface.
1. Epidermis of skin.
Transitional epithelium subtypes and sites
- Lines the urinary tract:
- prostatic urethra.
- pelvis of ureter.
- ureter and urinary bladder.
This type of epithelium is called urothelium.
Stratified squamous epithelium functions
Protection
Transitional epithelium functions
- Distensibility.
- Protective against cytotoxic urine effect.
Stratified cubical epithelium
A rare type of epithelium formed of:
1. Few layers of cells, usually two layers cubical cells.
2. Eg. Ducts of sweat glands.
Stratified columnar epithelium
Similar to stratified squamous epithelium but:
1. The number of layers is few.
2. The superficial cells are columnar.
3. May be ciliated and may not be.
Types of stratified columnar epithelium
Ciliated and non ciliated.
Stratified columnar ciliated epithelium sites
- Fetal oesophagus (rare type).
Stratified columnar non ciliated epithelium sites
- Recto-anal junction.
- Large ducts of glands.
- Male urethra (penile part).
Metaplasia
Under abnormal conditions, one type of epithelium may transform to another type.
Metaplasia examples
- Heavy cigarettes smoker: pseudostratified columnar epithelium (lining bronchi) may TRANSFORM into stratified squamous epithelium.
- Bilhariziasis: epithelium of urinary bladder may change from transitional TO stratified squamous epithelium.
Grandular epithelium
A type of epithelium which modifies to act as a gland a produce secretion.
Classification of grandular epithelium according presence or absence of duct
- Exocrine glands.
- Endocrine glands (ductless gland).
- Mixed glands.
Exocrine glands
Formed of secretory cells (secretory portion) which forms the secretion and ducts (excretory portion) that carries the secretion to outside the gland.
Endocrine gland (ductless gland)
Formed of groups of secretory cells and blood capillaries. But no duct system, their secretion (hormones) is carried by blood to different parts of the body.
Mixed glands
Formed of two parts:
1. Exocrine part: has duct system.
2. Endocrine part: no duct system.
Classification of exocrine glands
- According to number of cells forming the gland.
- According to type (nature) of selection.
- According to mode (mechanism) of secretion.
- According to branching of the duct.
- According to shoe of secretory part.
According to number of cells forming the gland
- Unicellular: formed of one cell.
- Multicellular: formed of more than one cell.
According to type (nature) of secretion
- Water secretion.
- Serous secretion (watery secretion with enzymes).
- Mucus secretion.
According to mode (mechanism) of secretion
- Merocrine secretion.
- Apocrine secretion.
- Holocrine secretion.
Merocrine
- Most common mechanism.
- Secretion is discharged outside the cell by exocytosis so no changes occur in the secretory cell.
Apocrine secretion
The secretion is rereleased and come out with the apex of the cell.
Holocrine secretion
The secretion accumulates in the cell, later on the swollen cell ruptures and whole cell is lost with the secretion.
According to branching of the duct
- Simple.
- Compound.
Simple branching
The gland has a:
1. Single non-branching duct.
2. Non- branching secretory part.
3. The secretory portion is only branched (helps identify simple branched).
Compound branching
The gland has a branching duct like a tree and consequently, each duct has a secretory part.
According to shape of secretory part
- Tubular.
- Alveolar (Acinar).
- Tubuloalveolar.
Tubular
Secretory unit is in the form of a tube.
Alveolar (Acinar)
Secretory unit is rounded.
Tubuloalveolar
Secretory part is flask shaped.
All exocrine glands consist of?
Secretory parts and ducts, which is why they are classified according to their shape.
Neuro-epithelium
Special type of epithelial tissue.
Neuro-epithelium functions
It’s epithelial cells (sensory cells) modify to act as receptors and receive stimuli for some sensation. Eg. Taste buds in tongue for taste sensation.
Myo-epithelium
A special type of epithelium, which is modified to contract.
Myoepithelium site
Present between base of secretory cells and their basement membrane.
What happens with myoepithelium contracts?
It squeezes the secretory cells to discharge their secretion into the ducts.
Eg.
1. Acini of salivary glands.
2. Mammary glands.
3. Sweat glands.
Cell polarity and membranous specializations
- Apical specializations.
- Lateral specializations (cell junctions).
Apical specializations
- Cilia.
- Flagella.
- Stereocilia.
- Microvilli.
And their details..
Lateral specializations
Lateral membrane of epithelial cells are linked via adhesive transmembrane protein molecules.
Types of lateral cell junctions
- Tight our occluding junction (Zonula occludens).
- Adherents junction (zonula adherens).
- Macula adherens (desmosomes).
- Gap junctions (nexus) communicating junction.
- Basal specializations.
Tight or occluding junction (zonula occludens)
- Two adjacent cell membranes fuse at certain points vis transmembrane protein molecules on each cells.
- It completely encircles the apex of the cell like a belt.
Tight or occluding junction (zonula occuludens) function
Restrict passage of substances between adjacent cells.
Adherens junction (zonula adherens)
- Two adjacent cell membranes are separated by wide intercellular space.
- In the intercellular space, extracellular parts of trans membrane protein molecules on two adjacent cell membranes are connected by the help of Ca++.
- At the cytoplasmic side, condensed protein molecules bind transmembrane protein molecules to actin filaments (cytoskeleton).
- Encircles the cell like a belt.
Adherens junctions (zonula adherens) function
Provide lateral adhesion between epithelial cells.
Macula adherens (desmosomes)
- Wide intercellular space shows Ca++ dependent interconnection between extracellular portions of the transmembrane protein molecules.
- The cytoplasmic side is thickened and called attachment plaque that is composed of several protein types which are capable of anchoring the intermediate filaments.
- Does not encircle the cell like a belt, but is scattered as circular spots (which is why it is called macula).
Macula adherens (desmosomes) site
Between epithelial cells, especially in stratified squamous epithelium.
Macula adherens (desmosomes) function
The strongest type of cell junctions that provide firm adhesions among the cells to support the surfaces subjected to friction.
Junctions complex
Includes three types (usually) of junctions:
1. Zonula occludens.
2. Zonula adherens.
3. Desomosomes.
Present between adjacent cells. Eg. Simple columnar cells lining small intestine.
Gap junction (nexus) communicating junction
- Two adjacent cell membranes are separated by a narrow gap but connected by narrow channels that bridge the intercellular space.
- Each bridge is formed of 6 symmetrical molecules of transmembrane protein.
Gap junction (nexus) communicating junction
- Exchange of ions and small molecules between adjacent epithelial cells.
- Passage of impulses between muscle cells.
Basal specializations parts
- Hemidesomosomes.
- Basement membrane.
- Basal infoldings.
Hemidesmosomes
Take the shape of 1/2 a desmosome at the basal part of basal cells.
Hemidesmosomes function
Fix epithelium to underlying basement membrane and connective tissue.
Basement membrane
An amorphous dense layer located next to basal surface of epithelial cells.
Basement membrane LM
- PAS: red line.
- Ag: brown.
- Clear=thick=skin.
- Non clear=thin=transitional epithelium.
Basement membrane EM
2 layers:
1. Basal lamina.
2. Reticular lamina.
Both laminae are attached to the underlying connective tissue by several structures including type VII collagen (anchoring fibrils).
Basal lamina
- Epithelial component.
- Electron dense sheet formed of type IV granules and glycoproteins.
Reticular lamina
- C.T. Component.
- Collagen fibers type III (reticular fibers) and ground substances.
Functions of basement membrane
- Support epithelium and fix it to C.T.
- Control passage of the molecules Eg. Kidney capillaries and lung aveoli.
Basal infoldings
- The basal surface of cell membrane shows invaginations ( to increase surface area) separated by vertically arranged mitochondria to provide energy required for active transport.
- Occurs in ion transporting cells Eg. Kidney tubules.
Zonula
Belt.
Macula
Circular spots.
