Organisation of epithelial tissues Flashcards
Describe the basic structure of the small intestine
The cells are lined up in a single layer, with their nuclei alongside each other. This is known as a simple epithelium.
The base of each cell is attached to a thin fibrous layer which is only just detectable in the light microscope, called the basal lamina or basement membrane. Attachment to a basal lamina is a characteristic feature of epithelia.
The majority of cells are identical with a pink staining cytoplasm. The cells are taller than they are wide and are described as columnar. Their function is absorption of the products of digestion of food (for example, monosaccharides and amino acids) and they are known as instestinal absorptive cells or enterocytes.
Interspersed among them are cells with a very pale cytoplasm and a bulbous shape. They are mucus-secreting cells known as goblet cells.
Describe the additional structures of the small intestine
The apical surface of the enterocytes (the one facing the lumen of the intestine) is specialised for absorption by the presence of a brush border. This can be seen as a zone without organelles at the apical surface of the epithelium. If you have set up your microscope well you should detect the presence of striations (rather like looking at a toothbrush side on, hence the name brush border). It consists of a close packed row of small finger-like projections of membrane which serve to increase the available surface area. Because they resemble villi in shape and function but are about 1000 times smaller they are known as microvilli. There is no brush border on the rest of each enterocyte (what is called the basolateral surface). It is characteristic for epithelial cells to be polarised, that is to have two surfaces with different properties.
No gap is visible between adjacent cells, and if any existed there would be diffusion of undesirable contents of the lumen into the rest of the body. Another characteristic of an epithelium is the presence of junctions between adjacent cells.
What is the epithelium of the small intestine described as
Epithelia can be described concisely in terms of their cell shape, arrangement and specialisation, using the terminology above. Thus the epithelium lining the intestine would be described as a “simple columnar absorptive epithelium with goblet cells”.
What is meant by a lining epithelium
This type of epithelium is a lining epithelium separating two large compartments, the lumen of the intestine and the rest of the body. If the lumen is small, of the same sort of size as a single cell, then the epithelium is forming a gland or duct. Look again at the slide and near the bases of the villi you will find an example of a glandular epithelium.
Describe glands
Glands consist of a tubular structure into which cells secrete substances which diffuse along the tube to their destination. In the intestine these glands are known as crypts, and you should be able to work out from the various cross sectional views that they consist of simple tubes with a closed end. Near the closed end you will find the secretory cells of the small intestine, the Paneth cells. They can be recognised by their secretory granules (stained bright pink) which are positioned asymmetrically in the cell so they release their contents at the cells apical surface, into the crypt.
Describe the crypts
The crypts can be described as simple tubular glands, because each crypt is a single closed tube. If the tubes were branched the gland would be described as compound. If the secretory cells were restricted to regions with a specialised rounded shape at the ends of the tubes the gland would be described as an acinar or alveolar gland (the region of secretory specialisation is an acinus or alveolus and the rest of the tube system would consist of ducts)
What is the mesentery
The mesentery is a thin layer of tissue that attaches parts of the intestines to the rest of the body. It allows the whole length of the intestines to be folded up to fit within the abdominal cavity but still have a connection carrying its blood and nerve supply, and it means that the intestines can be somewhat mobile during peristalsis.
Describe the mesentery
The cells are flat and wide, something that is also reflected in the shape of the nuclei. Cells which are wider than they are tall are known as squamous, and the epithelium surrounding the mesentery is described as a simple squamous epithelium.
Functionally, this epithelium is simply forming a boundary. There is no need for a more elaborate epithelium since the cells do not have other functions like absorption or secretion.
Another example of a simple squamous epithelium can be found on this slide. The epithelium lining blood vessels (alternatively known as endothelium) is of the same pattern.
Describe the oesophagus
The oesophagus has an epithelium lining the central lumen which in this case is stratified, meaning that it is many cells thick. The cells stain differently according to their level with those nearest the basal lamina being almost purple, those higher up having a deep pink cytoplasm, and those nearest the lumen having a very pale cytoplasm. The individual cells have a range of different shapes with basal cells being almost rounded but those closer to the lumen (the majority) being squamous. The structure is referred to as a stratified squamous epithelium.
Relate the structure of the oesophagus to its function
The arrangement is designed to line the lumen of the oesophagus in a way which is resistant to damage. Humans swallow all sorts of things which are not very good for the cells they come in contact with, for example hot coffee or strongly alcoholic drinks. Many animals swallow partly chewed food that contains abrasive bone or vegetable fragments. The stratified arrangement means that if the cells adjacent to the lumen become damaged, the cells underneath maintain the function of the epithelium as a barrier and can divide to replace those that are lost.
Describe the skin as a protective layer
The body uses a stratified squamous epithelium in several locations as a protective strategy, with the skin being a special case. Skin has to be waterproof to prevent the cells below drying out and dying. The surface cells of the epithelium have died (“cornified”) leaving only the dense network of their keratin containing intermediate filaments to form a non-cellular protective layer that absorbs oily secretions to become waterproof.
Describe the skin
The epithelium of the skin is the epidermis. This is a stratified squamous epithelium of similar organisation to that of the oesophagus, but important differences are apparent. The cells of the epidermis are called keratinocytes. The most basal layer is the stratum basale, a single layer of cuboidal cells sitting on the basal lamina. Stem cells in this layer produce new cells that form the upper layers that differentiate and ultimately replace the cells being lost from the surface. The melanocytes, which provide pigment granules to the keratinocytes, are found in this layer.
Describe the other layers of the epidermis
The layer just apical to the stratum basale is the stratum spinosum. In this region, The cells form a few layers of cells which often have a spiky appearance. The region apical to this is the stratum granulosum. The cells in these layers contain numerous granules, which may not be visible in the specimen. The apical layers, the stratum corneum, are layers of dead cells that have been “cornified” (keratinised) during the differentiation and turnover of the epidermis. Nuclei cannot be observed in the cells of the stratum corneum. In thin skin (this specimen), hairs and their follicles are present. Examine the relationship between the surface epidermis and the hair follicles. In thick skin (found on the palms of the hand and soles of the feet) hair is not present, and the stratum corneum is typically many times thicker than in thin skin
What do you see in a longitudinal section
This is part of a longitudinal section through the trachea, so you view just the edge of the tube rather than a complete circle.
Describe the structure of the trachea
You should be able to find a region where the plane of section is approximately right angles to the basal lamina and you can identify individual columnar cells. On their apical surface are fibrous projections that are much larger than the brush border of the intestine: these are cilia. Interspersed among the ciliated cells are goblet cells (with this stain they are purplish). Goblet cells produce mucus which lines the airway and protects it from drying out. The cilia beat to move the mucus up towards the back of the mouth so that the mucus doesn’t accumulate.
Dark purple staining regions are cartilage rings that support the airway when the pressure changes during breathing
