Epithelia, tissue and growth disorders

Epithelia, tissue and growth disorders

Certain cells of the body are specialised for specific functions. A tissue is defined as a group of cells with a similar structure and function, there are 4 main types of tissue:

1. Epithelium
2. Connective tissue
3. Nervous tissue
4. Muscle

Epithelia

Epithelia are derived from 3 layers, referred to as the germ layers; the ectoderm, mesoderm and endoderm. They line and cover all of the body's surfaces with the exception of articular cartilage, tooth enamel and the anterior iris surface. Its basic functions, dependent on its location are: 

• Protection: for the skin
• Absorption: for the small and large intestines
• Transport: on the surface mediated by cilia cells
• Secretion: for glands
• Excretion: for the tubules of the kidney
• Gas exchange: for lung alveolus
• Gliding: between surfaces, for the mesothelium

Once fertilisation occurs, the sperm and egg form a zygote, then grow into a blastocyst, and then become what is referred to as a gastrula. The ectoderm, the external layer of the gastrula, derives epithelia such as skin cells, brain neurones and cell pigments. The mesoderm, the middle layer of the gastrula, derives cardiac muscle, skeletal muscle, kidney cell tubules, red blood cells and smooth muscle. The endoderm, the internal layer of the gastrula, derives the lung alveolar cells, thyroid cells and pancreatic cells. Germ cells are also derived from the gastrula, these make up sperm cells in males and egg cells in females. Sometimes however these cells can end up in random places, causing disfigurements, such as teratomas. Germ cells are totipotent, meaning they can form anything derived from the germ layers. 

Due to their location and functions, epithelial cells are renewed continuously by mitosis, this however makes them prone to causing cancers. They lack a direct blood or lymphatic supply, thus their nutrients have to be provided via diffusion. There is almost no free intercellular substances. The structural organisation of the epithelium is maintained by cell adhesion molecules and junctional complexes. The cells are anchored to a basal lamina, this and connective tissue components interact in oder to form the basement membrane.  The cells have a structural and functional polarity, apical and basal (top and bottom), however in cancer, this polarity is lost. 

Epithelia classification

This classification is based upon the cellular morphology, not the cellular functional characteristics. It is determined by their shape and arrangement. 

Shape can be:

• Squamous: very flat, almost 2 dimensional squares
• Cuboidal: 3 dimensional squares, cubes
• Columnar: 3 dimensional columns, rectangular

Arrangement can be:

• Simple: single layer
• Stratified: multilayered
• Pseudo-stratified: technically a single layer, but looks double as cells are in a top and tail position
• Transitional: multilayered however can be relaxed and distended

Figure 1

Simple epithelium 

Simple squamous is a single layer of flat cells, and usually forms membranes. It lines body cavities and lungs and capillaries. It is given the name endothelium when it lines the blood and lymphatic vessels, and mesothelium when lining the body cavities. Due to their thin nature, they're useful for gaseous exchange in the alveolar. 

Simple cuboidal is a layer of cube like cells, they're common in glands and ducts. They also form the walls of kidney tubules, and cover the ovaries. This is a large contributor to the incidences of ovarian cancer, a theory suggests that every time an egg is released, it may rupture the outside of the ovary where it will then have to heal, thus the longer the ovulation period, an increase in the chance.

Simple columnar is a single layer of tall, rectangular cells. They often include goblet cells which secrete mucus, they also line the digestive tract. 

Pseudostratified is a single layer of cells, however some are shorter than others giving it the appearance of a double layer, sometimes it can be ciliated, this occurs in the respiratory tract. They may also have a absorption or secretory function. 

Stratified epithelium

Stratified cuboidal cells are multilayered, and they ave an apical surface (the top of the top layer). It is rare if surface cels are cuboidal, this only occurs in sweat gland ducts and the male urethra.

Stratified squamous cells of the free edge are flattened, they're usually found as a protective covering where friction on the area would be common, such as the skin,mouth and oesophagus. These can be keratinised also. 

Transitional epithelium

Shape of the cells will depend upon the amount of stretching required, these kind of cells line the urinary system.

Microvilli

These are finger-like processes which extend from the apical surface of cells, usually around 1-2micrometres in length. Their main component is their core of actin filaments, which are around 6nm thin, these are named microfilaments. They're very extensive in any absorptive epithelium, as they increase the cells surface area, and can be referred to as a striated border or brush border. Bundles of actin filaments extend into a part of the cell which is named the terminal web (see figure 2), these are actually anchored to the cell.

Figure 2.

Cilia

Cilia are long cytoplasmic extensions around 5-10micrometres in length. These do not contain actin filaments. Instead they have a complex  arrangement of microtubules (9 around the edges and 2 in the centre) named "axoneme", which have small appendages named dynein arms which allow for movement, they're essentially molecular motors, thus the main component of cilia is tubulin. Cilia facilitate flow of fluid over an epithelium, for example on the trachea, a tubular organ. The structure is the same as flagellum (see fig 3). 

Figure 3.

Kartagener's Syndrome (KS)

This autosomal recessive disease is essentially immotile-cilia syndrome. It occurs in around 1 out of 32,000 live births in the US. The symptoms include chronic upper and lower respiratory tract disease, as a result of ineffective mucociliary clearance, sterility in males, as the flagella in the sperm doesn't work properly and approximately 50% of those suffering with the syndrome display situs inversus, which is where the visceral internal organs are mirrored (different sides to normal), also referred to as transposed viscera. The biopsy examinations reveal abnormal/non motile cilia. The axoneme is lacking in dynein arms.

Basement membrane

This is an extracellular component which is in direct contact with the basal domain of epithelial cells, it can be made visible under a light microscope with the use of a periodic acid-Schiff (PAS) stain, making it PAS positive. The basement membrane can be defined into 2 layers/laminae:

1. Basal lamina: this contains laminin, fibronectin, type IV collagen, heparan sulfate proteoglycans and nidogen.
2. Reticular lamina: contains type 3 collagen.

The main components of these 2 layers are glycoproteins. They can be witnessed under the electron microscope.

Basal lamina functions

The basal lamina as a variety of functions:

• Adhesion of epithelial cells to underlying CT, in skeletal muscle the BL maintains the integrity of the tissue, and any disruption to it will cause muscular dystrophies.
• Act as a selective permeability barrier, with the filter being based upon the charge of the molecule and its size.
• Good substrate of cellular migration, for example during wound repair or embryonic development.
• As a barrier critical to metastatic potential in epithelial cancer cells referred to as carcinomas. A carcinoma which is in situ means it has not breached the basement membrane. 

The main protein making up the BL is type 4 collagen, this forms flat sheets and has the ability to self polymerise. Perlecan also has the ability to self polymerise, and laminin also has the ability to self polymerise and also can interact with itself.

Secreted protein acidic and rich in cysteine (SPARC)

This is also known as osteonectin.  It is expressing during many stages of development in a variety of organisms, however it is restricted in adult vertebrates primarily to tissue which under consistent turnover or remodelling, such as the bone. The over expression of SPARC is found in various human malignancies. 

Melanoma cells with suppressed expression of SPARC could no longer generate tumours, however melanoma cells which induced expression of SPARC has 100% tumorigenicity. High SPARC levels are also associated with metastatic tumours, as it induces the expression of MMPs, MMP1, MMP2 and MMP9, which are involved in the degradation of basement membranes and extra cellular matrix. This is crucial process for invasion and metastasis in human malignancies. 

Metastatic cascade

Tumour cells coat themselves in the host platelets in order to avoid detection from the immune system. Collagen is an example of an angiogenesis inhibitor. Arrestin, canstatin, turnstatin and endostatin, all type 4 collagen have specific anti-angigenic properties. Without angiogenesis, a metastatic deposit will only grow around 2mm before dying. If the integrin binding it is inhibited, then so it growth of the tumour. 

Cadherins 

The name is derive from the term "calcium-dependent adherence protein", the family consists of around 90 members, whom al participate in the interactions between cells of the same type, which is ideal for epithelial layers. These interactions which connect the plasma membranes of adjacent cells, forming 2 cellular junction types:

1. Zonula adherens: these are small, spot like junctions located near the apical surface of epithelial cells 
2. Desomosomes: stronger and more extensive junctions, present in epithelial and muscle cells. 

Linkage of cadherins with the cytoskeleton occurs through 2 classes of catenins; beta-catenin links cadherins with alpha-catenin which in tern connects to actin, thus completing the connection with the cytoskeleton. 

E cadherins

These mediate homotypic adhesions in epithelial tissue, thus serving to keep the epithelial cells together, and to relay signals between the cells. In several epithelial tumours, e.g adenocarcinomas (gland cancer) of colon and breast, E-cadherin is down regulated, this reduces the cells ability to adhere to one another and promotes their detachment from the primary tumour, and also their advance into other tissues. In some tumours, E-cadherin is normal, but its expression is reduced due to mutations for one of the catenin genes. 

Epithelial-mesenchymal transition (EMT)

This is the name given to the process in which epithelial cells lose their polarity and cell to cell adhesion, and gain invasive and migratory properties, turning them into mesenchymal stem cells, a multi-potent type of cell which has the ability to differentiate into many cell types. 

EMT is crucial in a number of developmental processes, including mesoderm formation, neural tube formation, and foetal development (embyrogenesis). It is also found to occur in wound healing, and in the initiation of metastasis for cancer progression. Normal cells have contact inhibition, or spatial awareness, however cancer cells do not. The cells have an amoeba shape, they detach and become invasive and motile, they break through the basement membrane and reach the blood vessels in order to move to another location. Once they have found a place to form and grow, the process is reversed by a process called mesenchymal-epithelial transition. In order to check for this, a stain can be carried out which will show that any mesenchymal cells will not contain E-cadherin. 

When the above markers are switched on, the epithelial marks seen above in green are turned off, and the mesenchymal markers seen above in red, are turned on. Zeb 1 and 2 should also be in the list, as they are recently discovered to be down regulators of E-cadherin. These can technically be considered as growth factors as they trigger a signalling mechanism. 

Cellular adaptation

Adaptive responses result in an increased tissue mass, the increased functional demand or endocrine stimulation are what usually cause hypertrophy (increase in volume) and hyperplasia (increase in cell number). These new patterns of growth are stable whilst the causative stimulus persists, however once it is removed, the tissue will return to normal growth patterns. 

Size

Acquired changes may be physiological or pathological, this is due to external influences, however these can potentially be reversible.  

Atrophy: this is a reduction in size or activity. Examples of a physiological atrophy are the thymus as it ages, a thymus in a child is much more larger and dense in comparison to a thymus in an adult, ovaries after menopause and immobilisation of a broken limb. An example of a pathological atrophy are denervation, loss of trophic hormone, reduced blood supply and excessive pressure. 

Hypertrophy: this is an increase in cell size. Examples of a physiological hypertrophy are increased skeletal muscle bulk due to training, or the uterus during pregnancy. A pathological example of hypertrophy is cardiac muscle due to an abnormal increase in workload, e.g. aortic valve stenosis. Mechanism sensors appear to be the major triggers for physiological hypertrophy, and agonists and growth factors are more involves in the pathological hypertrophies. 

Hyperplasia: this is an increase in the number of cells. Physiological examples, which are usually hormone induced, include adrenal cortex due to stress, breast enlargement due to puberty and pregnancy, may coexist with hypertrophy such as the uterus in pregnancy. Pathological examples which can also be hormone induced, include the adrenal cortex due to a pituitary tumour secreting ACTH, enlargement of the prostate and endometrial hyperplasia, this could be considered as pre neoplastic is glands are atypical. There is also compensatory hyperplasia, for example in liver and kidney resection. 

On the left, is a normal uterus and on the right is an enlarged uterus during pregnancy. 

Epidermodysplasia verruciformis

This is an example of hyperplasia driven by a viral infection, in this case, HPV is the cause. There are obver 150 types of HPV, the most commonly known are 16 and 18 which cause cervical cancer, and 6 and 11 which cause warts. HPV 5 and 8 cause growths (seen in the photo below), others do also but these are the main contributors, the way they have affected 

this man does suggest he was in an immunocompromised state. It is currently being researched how zinc and EVR1 and EVR2, genes which code for a zinc transport mechanism, affect HPV.  

Metaplasia

This is a reversible change in which one cell type, epithelial or mesenchymal, is replaced by another cell type. This represents an adaptive substitution of cell types which are sensitive to stress with cells that are better to be able to withstand the different environment, which would cause stress or trauma on the original cell. This does not result from a change in the phenotype of a differentiated cell type, instead it is the result of a reprogramming of stem cells which are known to exist in normal tissues, or of undifferentiated mesenchymal cells present in connective tissue. In metaplastic change, these precursor cells differentiate along a new pathway. For example, the metaplastic transformation of oesophageal stratified squamous epithelium into mature columnar epithelium, which can be found in intestinal cells that cause acid reflux this is a pre neoplastic condition, this metaplastic change can develop into cancer if the stress/trauma to the cells is not removed. A risk factor towards this is smoking, although it is primarily caused by HPV, HPV itself can cause a variety of cancers, including cancer of the vulva, penile, oral, anal and skin cancer. HPV requires the presence of stratified squamous cells in order to grow. 

Dysplasia

This is the term used in order to describe cells which have:

• An increased rate of cellular division often with abnormal mitotic figures
• Incomplete maturation
• Loss of normal cellular architecture
• Tend to have a high nuclear to cytoplasmic ratio

The presence and severity of dysplasia can be regarded as an indicator of the risk status of a precancerous lesion, and can be graded however this is very subjective, and so is basically the opinion of the the scientist observing it. Severe dysplasia indicates a very high risk of the subsequent development of cancer. Pre-neoplastic cells however can sometimes be reversible. Dysplasia is defined by 19 parameters.

Scrapes can be taken of the cells, however these are usually only on the surface of the epithelial layer. In cervical cancer, they can be categorised into cervical intraepithelial neoplasia 1, 2 and 3. 

Histological changes in dysplasia diagnosis

• Loss of polarity of basal cells
• The presence of more than one layer having a basaloid appearance
• Drop-shaped rete-ridges (epidermal thickenings)
• Increased nuclear-cytoplasmic ratio
• Nuclear hyperchromatism
• Enlarged nucleoli
• Increased number of mitotic figures
• Mitotic figures that are abnormal in form
• The presence of mitotic figures in the superficial half of the epithelium
• Cellular and nuclear pleomorphism (variability in size and shape)
• Irregular epithelial stratification
• Loss of intercellular adherence 
• Keratinisation of single cells or cellular groups in the prickle cellular layer
• Mild and severe dysplasia is easier diagnose than moderate
• If any of these changes become extreme, it can lead to cancer
• In cancer you get more than one nucleoli

There are 3 major problems attached to the importance of epithelial dysplasia in predicting malignancy development;

1. There is a set of criteria for assessing epithelial dysplasia however the overall diagnosis is very subjective 
2. Not all lesions showing dysplasia will become malignant, some regress
3. Carcinomas can develop from lesions in which epithelial dysplasia has not been diagnosed in previous biopsies.

In this case, there is a substantial requirement to improve the histological assessment of epithelial dysplasia, or, since it does not seem to be invariably associated with or even a necessity for malignant development, it may be necessary to develop other methods for predicting the malignant potential for premalignant lesions. 

DNA ploidy analysis is a good prognostic marker, as it is objective, in comparison to histological grading, which is subjective.