Histopathology I Lecture 12

HISTOPATHOLOGY I

Lecture 12

Epithelium Tissue Part II

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


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


Transitional epithelium is a stratified epithelium found only in the outflow tract with special features to make it waterproof as well as expansile


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


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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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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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


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


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


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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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


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


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


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

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


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


There are 3 types of surfaces that have surface specialisation:

1. Intercellular surfaces

2. Luminal surfaces

3. Basal surfaces


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


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


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


Each tight junction forms a continuous circumferential band or zonule around the cell and is thus also known as zonula occludens


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


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


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


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


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


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


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


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


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


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


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


Hemidesmosomes, a variant of desmosomes, bind the base of the cell to the underlying basement membrane by linking to the cells intermediate filament network

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)

Glands

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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

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

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

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

Glands

The simplest exocrine glands are single mucus-secreting cells such as goblet cells

Exocrine glands can be subclassified according to 2 major characteristics:

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

Glands

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

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

THE END

Histopathology I Lecture 12

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