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HISTOPATHOLOGY I
Lecture 11
Epithelium Tissue Part I
Introduction
Epithelium is one of the basic types of tissue
They include both surface epithelia and solid organs
Surface epithelia cover or line all body surfaces, cavities and tubes
They are involved in secretion, selective diffusion, absorption, physical protection, transcellular transport, containment, and detection of sensation
Introduction
Many of these major functions may be exhibited at a single epithelial surface
For example, the epithelial lining of the small intestine is primarily involved in absorption of the products of digestion, but the epithelium also protects itself from noxious intestinal contents by the secretion of a surface coating of mucus
Introduction
Surface epithelial form continuous sheets comprising of one or more layers of cell
Epithelial cells are bound to adjacent cells by a variety of membrane specialisations called cell junctions
Cell junctions provide physical strength and mediate exchange of information and metabolites
Introduction
All epithelia are supported by a basement membrane of variable thickness
Basement membrane separate epithelia from underlying supporting tissues
Epithelia are NEVER penetrated by blood vessels
Thus, they are critically dependent on the diffusion of oxygen and metabolites from adjacent supporting tissues
Introduction
Therefore, epithelial cells are polarised with one side facing the basement membrane and underlying supporting tissues (the basal surface), and the other facing outwards (the apical surface)
Classification of Epithelia
Surface epithelia are traditionally classified according to 3 morphological characteristics:
1. The number of cell layers
A single layer of epithelial cells is called simple epithelium, whereas epithelium composed of several layers is a stratified epithelium
Classification of Epithelia
2. The shape of the component cells
This is based on the appearance in sections taken at right angles to the epithelial surface
Cells are either squamous (flattened), cuboidal or columnar
In stratified epithelia, the shape of the outermost layer of cells determines the descriptive classification
Classification of Epithelia
3. The presence of surface specialisations such as cilia and keratin For example, the epithelial surface of the skin
is classified as stratified squamous keratinising epithelium since it consists of many layers of cells
The surface cells are flattened (squamous) in shape, and it is covered by an outer layer of the proteinaceous material, keratin that is synthesised by the epithelial cells
Glandular Epithelia
Epithelium that is primarily involved in secretion is often arranged into structures called glands
Glands are merely invaginations of epithelial surfaces, which are formed during embryonic development by proliferation of epithelium into the underlying tissues
For example, glandular epithelium is characteristic of the lining of the gastrointestinal tract
Glandular Epithelia
Some solid organs are composed largely of epithelial cells with a supporting tissue framework
Some of these organs are connected to the surface epithelium of the gastrointestinal tract by a branching system of ducts and belong to the category of exocrine glands, e.g. the salivary glands
Glandular Epithelia
Endocrine glands on the other hand have lost their connection to the epithelial surface from which they developed and release their secretions directly into the blood, e.g. the thyroid gland
Simple Epithelia
Simple epithelia are defined as surface epithelia consisting of a single layer of cells
Simple epithelia are almost always found at interfaces involved in selective diffusion, absorption and / or secretion
They provide little protection against mechanical abrasion and thus are not found on surfaces subject to much stresses
Simple Epithelia
The cells comprising simple epithelia range in shape from extremely flattened to tall columnar, depending on their function
For example, flattened simple epithelia are ideally suited to diffusion and are therefore found in the air sacs of the lung (alveoli), the lining of blood vessels (endothelium) and lining body cavities (mesothelium)
Simple Epithelia
In contrast, highly active epithelial cells, such as the cells lining the small intestine are generally tall since they must accommodate the appropriate organelles
Simple epithelia may exhibit a variety of surface specialisations, such as microvilli and cilia, which facilitate their specific surface functions
Simple Epithelia
Simple Epithelia
1. Simple squamous epithelium
They are composed of flattened, irregularly shaped cells forming a continuous surface that is sometimes called pavemented epithelium
The term squamous derives from the comparison of the cells to the scales of a fish
Like all epithelia, this delicate lining is supported by an underlying basement membrane
Simple Epithelia
Simple squamous epithelium is found lining surfaces involved in passive transport (diffusion) of either gases (as in lungs) or fluids (as in walls of blood capillaries)
It also forms the delicate lining of the pleural, pericardial and peritoneal cavities where it allows passage of tissue fluid into and out of these cavities
Simple Epithelia
Silver method /
neutral red
320x
Simple Epithelia
The previous slide shows a mesothelial lining of the peritoneal cavity that has been stripped from the underlying tissues and spread onto a slide
This allows a surface view of simple squamous epithelium
The intercellular substance has been stained with silver thereby outlining the closely interdigitating and highly irregular cell boundaries
Simple Epithelia
The nuclei are stained a dark shade of red
Simple Epithelia
HE800x
Simple Epithelia
The previous slide shows the lining of the lungs (alveoli)
It shows the typical appearance of simple squamous epithelium in section
The epithelial cells are so flattened that they can only be recognised by their nuclei, which bulge
Simple Epithelia
The supporting basement membrane is thin
In HE stained preparations, it has similar staining properties to the endothelial cell cytoplasm
Therefore, it cannot be seen in the slide
Simple Epithelia
2. Simple cuboidal epithelium
Simple cuboidal epithelium represents an intermediate form between simple squamous and simple columnar epithelium
The distinction between tall cuboidal and low columnar is often subjective
Simple cuboidal epithelium usually lines small ducts and tubules that may have excretory, secretory or absorptive functions
Simple Epithelia
For example, the collecting tubules of the kidney and the small excretory ducts of the salivary glands, and pancreas
Simple Epithelia
HE400x
Simple Epithelia
In the previous slide, the epithelial cells appear square, which leads to its traditional description as cuboidal epithelium
On the surface view, however, the cells are actually polygonal in shape
The nucleus is usually round and located in the centre of the cell
The slide shows the cells lining a collecting tubule in the kidney
Simple Epithelia
The boundaries between individual cells are very clear
The underlying basement membrane appears as a prominent darker red
Usually it is indistinguishable from the epithelial cell cytoplasm by HE staining
Simple Epithelia
3. Simple columnar epithelium
Simple columnar epithelium is similar to simple cuboidal epithelium except that the cells are taller and appear columnar in sections at right angles to the basement membrane
The height of the cells may vary from low to tall columnar depending on the site and / or degree of functional activity
Simple Epithelia
The nuclei are elongated and may be located towards the base, the centre or occasionally the apex of the cytoplasm: this is known as polarity of the nucleus
Simple columnar epithelium is most often found on absorptive surfaces such as in the small intestine, although it may constitute the lining of secretory surfaces such as that of the stomach
Simple Epithelia
HE,800x
Simple Epithelia
The previous slide shows simple columnar epithelium taken from the lining of the gall bladder
In this location it has the function of absorbing water, thus concentrating bile
Note the polarity of the nucleus, most are centrally placed while a few are at the apex of the cytoplasm
Simple Epithelia
4. Simple columnar ciliated epithelium
Some simple columnar epithelium have surface cilia on the majority of the cells
Among the ciliated cells are scattered non-ciliated cells that usually have a secretory function
Cilia are much larger than microvilli and are readily visible with the light microscope
Simple Epithelia
Each cilium consists of a finger-like projection of the plasma membrane
Its cytoplasm contains modified microtubules
Each cell may have up to 300 cilia that beat in a wave-like manner synchronised with the adjacent cells
The waving motion of the cilia propels fluid or minute particles over the epithelial surface
Simple columnar ciliated epithelium is found mainly in the female reproductive tract
Simple Epithelia
HE 320x
Simple Epithelia
The previous slide was taken from the Fallopian tube (oviduct)
The predominant cell type in this epithelium is tall columnar and ciliated, the nuclei being located towards the apical aspect of the cells
The less numerous non-ciliated cells have a secretory function
Ciliary action facilitates transport of the ovum from the ovary towards the uterus
Simple Epithelia
5. Pseudostratified columnar ciliated epithelium
Another variant of simple columnar epithelium is described in which the majority of cells are also usually ciliated
The term pseudostratified is derived from the appearance of this epithelium in section
It gives the false impression that there is more than one layer of cells
In fact, this is a true simple epithelium since all the cells rest on the basement membrane
Simple Epithelia
The nuclei of these cells, however, are disposed at different levels
This creates the illusion of cellular stratification
Pseudostratified epithelium is almost exclusively confined to the airways of the respiratory system and is therefore often referred to as respiratory epithelium
Simple Epithelia
HE200x
Simple Epithelia
HE400x
Simple Epithelia
The previous slide shows the lining of a bronchus
In the respiratory tract, the cilia propel a surface layer of mucus containing entrapped particles towards the pharynx in what is often described as the mucociliary escalator
The mucus is secreted by non-ciliated goblet cells found amongst the ciliated cells
Simple Epithelia
Pseudostratified columnar ciliated epithelium may be distinguished from true stratified epithelia by 2 characteristics
1. The individual cells of the pseudostratified epithelium exhibit polarity, with nuclei being mainly confined to the basal two-thirds () of the epithelium
2. Cilia are never present on true stratified epithelia
THE END