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