histopathology i lecture 12
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HISTOPATHOLOGY I
Lecture 12
Epithelium Tissue Part II
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Stratified Epithelia
Stratified epithelia are defined as epithelia consisting of 2 or more layers of cells
They mainly have a protective function
The degree and nature of the stratification are related to the kinds of physical stresses to which the surface is exposed
In general, stratified epithelia are poorly suited for absorption and secretion by virtue of their thickness
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Stratified Epithelia
Although some stratified surfaces are moderately permeable to water and other small molecules
The classification of stratified epithelia is based on the shape and structure of the surface cells since cells of the basal layer are usually cuboidal in shape
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Stratified Epithelia
Transitional epithelium is a stratified epithelium found only in the outflow tract with special features to make it waterproof as well as expansile
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Stratified Epithelia
1. Stratified squamous epithelium
Stratified squamous epithelium consists of a variable number of cell layers that exhibit maturation from a cuboidal basal layer adherent to the underlying basement to a flattened surface layer
The basal cells include continuously dividing stem cells
Their offspring migrate towards the surface where they are ultimately shed
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Stratified Epithelia
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Stratified Epithelia
Stratified squamous epithelium is well adapted to withstand abrasion since loss of surface cells does not compromise the underlying tissue
It is poorly adapted to withstand desiccation
This type of epithelium lines the oral cavity, pharynx, esophagus, anal canal, uterine cervix and vagina
These sites are subject to mechanical abrasion but which are kept moist by glandular secretions
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Stratified Epithelia
HE200x
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Stratified Epithelia
The previous slide shows section from the vagina
Note the cuboidal basal layer and the maturation through the large polygonal cells of intermediate layers to the flattened superficial squamous cells
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Stratified Epithelia
HE200x
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Stratified Epithelia
The previous slide shows section taken from the skin
Keratinising stratified squamous epithelium makes up the epithelial surface of the skin (epidermis) and is adapted to withstand the constant abrasion and desiccation to which the body surface is exposed
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Stratified Epithelia
During maturation, the epithelial cells accumulate cross-linked cytokeratin intermediate filaments in a process called keratinisation resulting in the formation of a tough, non-living surface layer cells
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Stratified Epithelia
2. Stratified cuboidal epithelium
Stratified cuboidal epithelium is a thin, stratified epithelium that usually consists of only 2 or 3 layers of cuboidal cells
This type of epithelium is usually confined to the lining of the larger excretory ducts of exocrine glands such as the salivary glands
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Stratified Epithelia
They are probably not involved in significant absorptive or secretory activity but merely provides more robust lining than would be given by a simple epithelium
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Stratified Epithelia
HE320x
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Stratified Epithelia
3. Transitional epithelium
Transitional epithelium (or urothelium)
Is a form of stratified epithelium found only in the urinary tract
It is highly specialised to accommodate a great degree of stretch and to withstand the toxicity of urine
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Stratified Epithelia
This epithelial type is so named because it has some features intermediate (transitional) between stratified cuboidal and stratified squamous epithelia
In the non-distended state, transitional epithelium appears to be about 4 - 5 cell layers thick
The basal cells are roughly cuboidal
The intermediate cells are polygonal
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Stratified Epithelia
The surface cells (umbrella or dome cells) are large and rounded, and may contain 2 nuclei
In the stretched state, transitional epithelium often appears only 2 - 3 cells thick (although the actual number of layers remains constant)
The intermediate and surface layers are extremely flattened
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Stratified Epithelia
In the previous slide, the surface cells are large and pale stained with a scalloped surface outline often overlapping 2 or more of the underlying cells (known as umbrella cells)
The luminal surface of the cells appears thickened and more densely stained
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Surface Specialisation of Epithelia
The plasma membranes of epithelial cells exhibit a variety of specialised structures that allow them to perform their function as a barrier with selective permeability
In some cases, the epithelial barrier is very impermeable, e.g. the transitional epithelium of the bladder
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Surface Specialisation of Epithelia
While in other epithelia such as the lining of the small intestine or the convoluted tubules of the kidney promote movement of selected ions and molecules across the epithelium
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Surface Specialisation of Epithelia
There are 3 types of surfaces that have surface specialisation:
1. Intercellular surfaces
2. Luminal surfaces
3. Basal surfaces
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Surface Specialisation of Epithelia
1. Intercellular surfaces
The adjacent or lateral surfaces of epithelial cells are linked by cell junctions so that the epithelium forms a continuous cohesive layer
Cell junctions also operate as communication channels governing such functions as growth and cell division
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Surface Specialisation of Epithelia
The various types of cell junction are composed of transmembrane proteins that interact with similar proteins on adjacent cells and are linked to intercellular structures on the cytoplasmic side
Cell junctions are of 3 functional types:
1. Tight junctions
2. Zonula adherens
3. Communicating junctions
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Surface Specialisation of Epithelia
I. Tight junctions (occluding junctions)
They block the passage of molecules between adjacent cells
These are located immediately beneath the luminal surface of simple columnar epithelium (e.g. intestinal lining) where they seal the intercellular spaces so that luminal contents cannot penetrate between the lining cells
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Surface Specialisation of Epithelia
Each tight junction forms a continuous circumferential band or zonule around the cell and is thus also known as zonula occludens
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Surface Specialisation of Epithelia
II. Zonula adherens (adhering belt)
Zonula adherens and desmosomes give the epithelium strength by linking the cytoskeletons of adjacent cells
Adhering junctions bind to the actin cytoskeleton of the cell
The combination of zonula occludens, zonula adherens and desmosomes is known as a junctional complex
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Surface Specialisation of Epithelia
The junctional complex essentially divides the plasma membrane of the cell into the apical and basolateral surfaces
Desmosomes are also widely scattered elsewhere in epithelial intercellular interfaces
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Surface Specialisation of Epithelia
III. Communicating junctions
Aka gap or nexus junctions
Provide a conduit for the passage of small molecules directly between adjacent cells
These junctions allow signaling by the passage of small molecules between adjacent cells to coordinate and synchronise function of the epithelium
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Surface Specialisation of Epithelia
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Surface Specialisation of Epithelia
2. Luminal surfaces
The luminal or apical surfaces of epithelial cells may incorporate 3 main types of specialisation: cilia, microvilli and stereocilia
Cilia are actively motile structures that are easily resolved by light microscopy
In contrast, microvilli are shorter projections of the plasma membrane that cannot be individually resolved with the light microscope
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Surface Specialisation of Epithelia
A single cell may have thousands of microvilli or only a few
Stereocilia are merely extremely long microvilli usually found only singly or in small numbers
Stereocilia are not motile
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Surface Specialisation of Epithelia
I. Cilia
Cilia are motile structures that project from the apical surfaces of certain epithelial cells, notably in the respiratory and female reproductive tracts
Cilia beat with a wave-like synchronous rhythm propelling surface films of mucus or fluid in a consistent direction over the epithelial surface
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Surface Specialisation of Epithelia
In the airways, mucus traps debris from inspired air and the cilia move the mucus towards the throat where it is swallowed, thus keeping the airways clean
In the Fallopian tubes, ciliary action propels the ovum from the ovary to the uterus
Cilia are up to 10 m long (up to half the height of the cell)
A single epithelial cell may have up to 300 cilia usually of similar length
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Surface Specialisation of Epithelia
II. Microvilli
Microvilli are minute finger-like projections of the luminal plasma membrane found in many epithelia, particularly those specialised for absorption where their presence may increase the surface area as much as 30-fold
Microvilli are only 0.5 - 1.0 m in length and are thus very short in relation to the size of the cell
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Surface Specialisation of Epithelia
Also, individual microvilli are too small to be resolved by light microscopy
Most epithelia have only a small number of irregular microvilli
However, in the small intestine and proximal renal tubules the epithelial cells have up to 3000 regular microvilli per cell and these can be seen with the light microscope as so-called striated or brush borders
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Surface Specialisation of Epithelia
3. Stereocilia
Extremely long microvilli, readily visible with light microscopy, are found in small numbers in parts of the male reproductive tract such as the epididymis
Stereocilia are thought to facilitate absorptive processes in the epididymis
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Surface Specialisation of Epithelia
3. Basal surfaces
The interface between all epithelia and underlying supporting tissues is marked by a non-cellular structure known as the basement membrane
This layer provides structural support for the epithelium and constitutes a selective barrier to the passage of materials between epithelium and supporting tissue
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Surface Specialisation of Epithelia
Hemidesmosomes, a variant of desmosomes, bind the base of the cell to the underlying basement membrane by linking to the cells intermediate filament network
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Glands
Goblet cell
Goblet cells are modified columnar epithelial cells that synthesise and secrete mucus
They are scattered amongst the cells of many simple epithelial linings, particularly those of the respiratory and GI tracts
They can be stained by PAS stain (stains carbohydrates magenta)
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Glands
HE / PAS400x
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Glands
Mucus has a variety of functions:
1. In the upper GI tract, it protects the intestinal lining cells from autodigestion
2. In the lower GI tract, it lubricates the passage of faeces
3. In the respiratory tract, it protects the lining from drying. It also contributes to the humidification of inspired air and acts as a sticky surface trap for fine dust particles and microorganisms
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Glands
Epithelial cells are the major component of all the glands of the body
The simplest glands can be easily recognised as an invagination of a surface epithelium
However, there are also complex glandular structures that have lost contact with the epithelial surface completely
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Glands
Therefore, there are 2 major subdivisions:
1. Exocrine glands
Release their contents onto an epithelial surface either directly or via a duct
2. Endocrine glands
Do not have a duct system
Release their contents into the bloodstream
Can act on distant tissues
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Exocrine Glands
These vary from microscopic sweat glands of the skin to large solid organs such as the liver
The duct system of the liver ramifies throughout the solid gland and empties its secretions (bile) into the duodenum
The simple tubular glands (crypts) of the large bowel consists entirely of the secretory component and empty directly onto the surface of the bowel
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Glands
The simplest exocrine glands are single mucus-secreting cells such as goblet cells
Exocrine glands can be subclassified according to 2 major characteristics:
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Glands
1. The morphology of the gland
Exocrine glands can be divided into the secretory component and the duct
I. The duct system may be branched ( compound gland) or unbranched (simple gland)
II. The secretory component may be tubular or acinar / alveolar (roughly spherical)
III. Both types of secretory component may also be coiled or branched
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Glands
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Glands
2. The means of secretion
Secretion from exocrine glands may occur in one of 3 ways:
I. Merocrine (eccrine) secretion
Most common
Proteins usually the major secretory product
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Glands
II. Apocrine secretion
Unusual mode of secretion
Usually used for lipid secretory products in the breasts and some sweat glands
III. Holocrine secretion
Occurs principally in sebaceous glands
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THE END